CENTRAL NERVOUS SYSTEM

. . CENTRAL NERVOUS SYSTEM (CNS) INTRODUCTION central nervous sytem (CSS) ts the part of the nervous system that funcuons to coordinate the actH1ty of all parts or the bodies of multicellular organisms In vertebrates, the w•ntral nervous »stem IS enclosed in the meninges It contains the maJority of the nervous system and conststs of the bram and the spinal cord Together With the peripheral nervous system It has a fundamental role tn the control of behavior ne CNS •s contained within the dorsal cavity, wth the bram in the cranial cavity and the spinal cord in the spinal cavity The bram is protected by the skull, while the spinal cord is protected by the vertebrae The three memnges or cell layers of the CNS are made of Fibroblasts which are: (i) The outermost layer called Dura matter (for (ii) •Ihe middle layer called Arachnoid matter (for protection). (iii) *Ihe inner layer called Pia matter (for protection and nutrition). "Ihe space between the dura and arachnoid is called sub-dural space and it contains interstitial fluid. 'Ihe space below the arachnoid is called sub-arachnoid space and together with the spaces in the ventricles and spinal canal contains cerebro-spinal fluid (CSF). The bram contains four fluid filled cmities called VENTRICLES, CSF is formed in the Choroid plexus (capillary network at the roof of the ventricles) within all 4 of the ventricle and between them, flowing down a pressure gradient to the sub-arachnoid space from where it enters the venous system. Fu nctions of cerebrospinal fluid (CSF) + To cushions brain - acts as a shock absorber, brain "floats" in it within cranial cavity Transport - of materials such as glucose, nutrients and metabolites. CELLS' FOUND IN THE CNS A) NEURONS (NERVE CELLS) The CNS contains lots of neurons (about 100 billion in man) with many different shapes and functions. Collection of cell bodies of neurons in the CNS is called nucleus (nuclei). Few neurorrs and especially the postganglionic autonomic neurons have their cell tx»dies outside the CNS in structure called ganglion (ganglia), Neuronal pools and circuits Neurons never function in isolation; they are organind into ensembles or circuits that specific kinds of information Although the arrangement Of neural circuits variB greatly.

. . accord,ng to the intended some reatures are character,sttc of all such ensembles •1b. napuc connections that define a are made tn a dense tangle of dendntes. axo„s terminals. and glial cell processes that together constitute (the sufTIX -pl comes from the Greek ssord Pilos, meaning "felt') Thus. the neuropil between nerve cell IS the regton where most syaptic connectivity occurs The direction or Information flow in any particular circuit essential to understanding its function. Nerve cells that carry Information touard the central nervous system (or farther centrally within the spinal cord and bratn) are called afferent neurons: nerve cells that carry Information away from the brain or spinal cord (or aua_v from the circuit in question) are called efferent neurons. Nerve cells that only participate In the locnl mspects of a circuit are called or circuit neurons 'These three classes—afferent neurons, efferent neurons. and interneurons—are the bxsic constituents of all neural circuits. Neurons are interconnected with one another to form circuits, much as electronic cornm»nents are wired together to form a functional circuit Many neural circuits together form a neural system, just as many electronic circuits together form a device such as a computer or a TV. Therefore areas in the CNS with large collection of neurons catenng for a variety of functions are called neuronal ptH)ls. connectiorrs between the neurons in the pool form what is known neuronal circuits. Examples of neuronal pools are the cerebral cyortex, and the specific nucle,' in the thalamus. basal ganglta, cerebellum. gxms and medulla and the entire grey matter in the spinal cord fie connections In neural circuits can be classified into a few basic patterns (i) Convergence circuits where signal(s) from various pathways or fibers converge on one common pathway. For example, the interneuron in the spinal cord that synapse with the alpha (a) motor neuron to the muscles receive inputs from the cerebral cortex, Type Ia afferent fibers from the muscle spindles, type lb affarent fibers from the golgi tendon organ and motor neurons from midbrmn..

. . (ii) Divergence circuits occur where excitation jn a Single neuron spreads to many other neurons either m the same tract (amplification) or In dl fferent tracts For example stimulation of one motor neuron tn the cerebral cortex can cause sttmulauon of many motor neurons and the stimulation of the motor neuron leads to stimulation of each branch of the axon that tnnenate all muscle fibers D", ergence Into different tracts the same Information from a gnen source to be sent to seseral parts of the CNS For example. transnussmns in the dorsal columns of the spinal cord go to the cerebellum, thalamus and the cerebral cortex (iii) Parallel circuits occur where a neuron forms several that contain varying number of synapses in between before converging to a common pathway. As there is a synaptic delay Of 0.5 milliseconds for the impulse to cross the synapse, the output in the common pathway recetves impulses one by one after varying period of delay with the one crossing one synapse arriving first, followed by two, three and so forth (iv) Reverberatory (Oscillatory) circuits in which a neuron(s) can form a collateral nerve fiber back to its oval dendrite or axon such that there can be self-excitation. An initial stimulation takes a long time before the stimulus disappears (after discharge). enle duration of the after discharge on the number of the collateral branches and the synapses involved. Sometimcs, the collateral fiber(s) can synapse with an inhibitory fiber that keep the brain alert throughout the day (wakefulness) and the limbic lobe/system resrx»nsible for behaviour and emotional responses contain many reverberatory circuits..

. . (v) Inhibitory circuits contain one or several inhibitory neurons that covert an excitatory signal to inhibitory one through secretion of an inhibitory neurotransmitter substance (e.g. GABA or glycine secreting neurons). Inhibitory circuits play a key role in filtering of information where only the most important ones are allowed to pass and also in ensuring orderly contraction and relaxation of flexors and extensors during movements such as walking, withdrawal and crossed extensor reflexes. During withdrawal reflex, the causing contraction of the flexors of the affected limb '.såll at the same time cause relaxation of the extensors of the same limb (reciprocal innervation) due to action of an inhibitory interneuron in the spinal cord. NERVES Nerves contain several axons of nerve cells (neurons) enclosed in a sheath epineurium. The nerves that connect with the CNS in the brain stem and midbrain are called cranial nerves while those that connect through the spinal cord are called spinal nervß. Some nerves carry purely sensory or motor impulses while others contain a mixture of sensory and motor neurons. Major differences between a nerve and a nerve cell Nerve Processes of neurons (axons and dendrites) enclosed in a tough sheath called epineurium Forms the connection between CNS and semsory receptors and effectors Action potential recorded at a distance from the point of stimulation is characterised with multiple peaks A nerve can cl)ntain axons or dendrites that carry sensory or motor impulses (mixed) Nerve cell Basic functional cell in nervous system with the processes and soma Exist in many varieties in the CNS and also in the PNS Action potential recorded at a distance from the point of stimulation of the cell has a single peak A given neuron is ether sensory or motor or interneuron.

. . Cranial nerves •nu.•reare 12 cranial nerves number mth roman letters. Except for the Olfactory and optic crantal the remaining 10 cramal ner•ses connect to the CNS at the nud-bram level Cranial nerves Number Name Impulses carried Sensor,' Sensory Motor Motor Both sensory and motor Motor Motor Sensory Both sensory and motor Both sensory and motor Motor Motor Function VIII x XII ()lfactorv Optic Oculomotor Trochlear T ngeminal A bducens Facial Vestibulocochl Glosopharyng Vagus Spinal accesso Hypoglossal .Smell V iS10n I) Contain sympatbeUc fibers respomsible for pupillary light reflex 2) Contain fibers to levator palpebrae, dorsal medial, ventral rectus and ventral olbltgue muscles of the eye Innervates the dorsal Obligue muscle of the eve Mandibular branch is 1M)tor to muscles of mastication Opthalmic and maxillary branches are sensory to skin of the head, mucous membrane of the mouth cavity and eyeball Motor to lateral rectus and bulbi of the Muscles of fapi@gypression Vestibular branch - normal posture, muscle tone and equilibrium Cochlear branch- Hearin Carries both sensory and motor impulses from and to the Carries both sensory and motor impulses from and to the Larynx, pharynx and visceral organs such as the intestines and the heart Impulscs to the spinocephalicus muscle Impulses to muscles of the tongue and enioh oid muscles II) Spinal nerves For the spinal cord nerves, sensory neurorus enter ua the dorsal root while the motor neurons leave via the ventral root The neurons in a nerve differ in threshold values as well as in therr speed of conduction B) GLIAL (NEUROGLIA) CELLS Apart from the neurons. there are also other cells collectively called Ghal (Neuroglia) cells For each neuron. there are about 10-50 glial cells Ihe most common glial cells are.

. . I) Schwann cell'. that peripheral Sheath 2) Oligodendrogliocytes or Oligodendrocytes that rnvel/n sheath 'n thc ( 'SS Astrocytes are supporting celll% that play a Oile jn neuronal nutntjon and care send manv procev.es to the blood wsse19 that are Ili'.oi',ed Jn the formation of tight responsible the blood brain barner (BBB) 4) Microglia that are trnntune-llke cells resembling tissue macrophages for protection aganst foreign antigens S) Ependymal cells that forms the lining of the cavities tn the CNS 'Ihe unmyelinated neurons in the CNS and cell bodies of the neurons Impart a grey like coloration (grey matter) and myelinated neurons impart a whitish colour (Whitematter) In the bram the grey matter is found on the outside while the white matter is found rn the middle layers In the sprnaJ cord, white matter is found on the outside while the grey matter is located in the middle SPINAL CORD AND SPINAL NERVES The spinal cord forms the connection between the brain and the peripheral nervous system In addition it serves as an important relay station for many reflexes. Housed in vertebral canal. the spinal cord is wrapped in spinal cord meninges and cushioned in cerebrospinal fluids. Nerve roots (pisnl rcxn.s) emerge from each side of the spinal cord. The nerve rc.x»tsjoin and form the 31 s:pmal nerves which exit from the vertebrae anal through the intervertebral foramena Each spinal nerve hæs a dorsal root sensory neurons enter (AFFERENT) and a ventral root where motor neurons leave (EFFERENT). In mammals, the spinal cord ends in the lumber region and caudal to this the vertev.brae column contains nerve roots called cauda equine arachnoid and subarachnoid space extend beyond the level where the spinal cord terminates and form a sac (lumbar cistem) surrounding the cauda equine. lumber cistern is an appropriate site for sampling cerebrospinal fluid In avian the spinal cord fiils the entire vertebral canal and birds thus have no cauda equ:nc Ihe spinal cord acts as a relay station with many nerve tracts that connect the spinal cord and brain centres with the various sensory receptors and effector organs via spinal nerves Also, the spinal cord is an integratory centre for simple reflexes (eg knee jerk, mthdrawal reflex) rn lower organisms almost all-important responss are mediated at the spinal cord level, but rn higher animals, the brain assumes more control of body activities and therefore the hemispheres are more developed i e higher degree of ENCEPHALIZATION..

. . SEGMENTAL DIVISION OF THE SPINAI, CORD AND SPINAL NERVES ne spinal ne.r'.B take thetr names from the corresponding level of the vertebral column The levels are o o Cervical or ncx.k levels (C) Thoracic or chest level (T) Lumbar level (L) Sacral level (S) Coccygea or tail level (Co) REFLEXES A reflex is a stereotyped very rapid response to a specific stimulus that occurs at sub-conscious level. Ihe basic unit of integrated neural activity leading to a reflex response is the REFLEX ARC. A reflex arc 1) 2) 3) 4) 5) A SENSORY RECEPTOR that upon application of a stimulus for-ns the GENERATOR potential which v.hen it rerches a threshold value results into fully fledged and self- propagating ACTION POTENTIAL (impulses) at the first node of Ranvier in case of modified nerve endings or in the afferent neuron that with the receptor cell. An AFFERENT NEURON that carry/forms the action potential the generator potential reaches the threshold value. Ille impulses from the receptor are then carried to the integratory areas in the CNS. All afferent neurons enter the spinal cord through the Dorsal rcx)t or through the cramal nerve that contain sensory axons. The IATEGRATORY AREA that is either the spinal cord (spinal reflexes that are simple) or the brain (for complex reflexes). In the integratory area, there can be one synapse (monosynaptic reflexes) or several synapses due to presence of intemeuron(s) (polysynaptic reflexes). "Ihe EFFERENT NEURON that carries the impulses from the integratory area to the Gffector organ. 'Ihe efferent (motor) neurons leave the Spinal cord via the ventral route or cranial nerve that contain efferent neurons. •Ihe EFFECTOR organ that can be a muscle or gland. These respond by contraction or secreting chemical substances. A simple reflex circuit, the knee-jerk response (more formally, the myotatic reflex), illustrates several points about the functional organization of neural circuits. Stimulation of peripheral sensors (a muscle stretch in this case) initiates receptor potentials that trigger action.

. . centrally along the axons of the sensory stimulates sp.nal rmttor neurons by means of »napuc contacts- Ihe action potentials triggered the synapuc potential In motor neurons travel peripherally In efferent axons. gt•.tng nse to muscle contraction and a behmioral One of the purposes of this particular reflex is to help maintain an upnght posture in the face of unexpected changes nere are many types of reflexes that occur in the b«xiy ranging from the simplest ones to very complex ones that involve many neurons, Ille integration of reflexes follows a hierarchy order Simple reflexes are mainly integrated at cord level e.g. stretch and withdrawal reflexes. rx»sitive supportive reaction, limb movement, emptying the unnary bladder (Micturition) and defecatiom which are known SPINAL REFLEXES Iliese reflexes can be exhibited in an animal whose spinal cord is transacted at a point where the medulla begins so to separate the sptnal cord and the rest of the brain. Such animal preparation is called a SPINAL ANIMAL: Immediately follouing the transaction of the cord from the rest of the brain, there is a period when the spinal reflexes are very much depressed and this is called SPINAL SHOCK duratiori of spinal shcEk varies depending on the extent of ENCEPHALISATION betng nunutes to hours in lower animals and days in higher animals like primates higher brain centres may modify these reflexes but the basic pattern is essentially set rn the spinal cord, It 15 thought that the sudden Interrupuon of facilitatory dnses from the bram lead the spinal shock More complex reflexes are Integrated tn the brarnstem and midbram Such reflexes are those for the control the respiratory rhythms, heart fimcuons, blCX)d pressure, balance and.

. . 04 equtltbnurn, feeding and ernouonal refle•ses anger, eroternent. •.exual pleasure BRAIN Ilie bram contams four prtnctpal part'. From the sptnal cord the pan called medulla oblongata and pons. then mid-brain (mecencepha]on). follov•.ed by interbrain (ds.z-ep•ha.iora) finally higher brain (Telencephalon) The medulla oblongata and pom rmdbrzn ze collectively called the brain stem •Itie brajn stem contans rer•.e fiber tract' certre•,. fot regulating autonomic activities such as resptrauon, GIT rmullty. and blood Sid functions. *Ihe cerebellum is considered as part of the rmd-braan. t}Zugh also Included •n tie br:urustem due to its JX)sition. -Ille medulla oblongata, pons and cerebellum ze sorrzurnes referred to as the hindbrain though the term also refers to the cerebellum itself Divisions of the Brain Maior Division Prosencephalon (Forebrain) Mesencephalon (Midbrain) Rhombencephalon (Ilindb rain) Subdivision Telenceph •Jon Diencephalon Mese on Metencephalon Myelencephalon tructu Ne«ortex; Basal Amygdala; Lateral Ventricles Thalamus; Hypothalamus; Epithalamus-. Third Ventridc Tectum: Tegmentum-. A quedgct Cerebellum: Pons; Fourth Ventricle Medulla Chlong*ta; Fourth Ventricle All b,cdy parts are reprsented both on the thalamus md tn the sensory md reas of tie cerebral corte.v The map of bcÆy• parts rqræentauon on the cerebral cortex is called HOMUNCULUS. "Ihe area of the cerebral cortex to which sensory Signals ze projected is caned SOMESTHETIC cortex (Somatic sensory areas I located z: the Ponca-itra.l servory area II located at the wall of Suluan fissure) The proportional representation of ach vem.

. . part on the degree of its use With pmts that are more used being represented o: bigger area than those not so much used All sensory and motor Signals from one Side of the b cross mer to the opposite Side either at the sprnal or the medulla level and therefore the 1 cerebral cortex control the nght side of the body uhile the right cerebral cortex control the Side of the body MOTOR DIVISION AND THE ROLE OF BRAIN IN THE CONTROL OF MOTOR ACTIVITIES. While the sensory system transduce valious forms of energy into electrical impulses. the motor system transduce electrical energy into mechanical energy resulting into muscle contraction The parts of the CNS involved in the planning, control and regulation of motor activities of muscles and glands is called the MOTOR division. Various inputs play a role in the planning voluntary activity, in adjusting body posture to allow stability dunng movement as well co-ordination of different muscle activities so as to ensure smooth and precise movements. Motor planning takB place in the cerebral cortex in an area called MOTOR CORTEX I, located at the pre-central gyrus though there are other areas with motor actÅities. Like the somatosensory area, all body parts are represented on the motor cortex forming what is known as MOTOR HOMUNCULUS with the degree of representation differing depending on the extent of use of a given group of muscles in a given orgam The motor system can be broadly classified as the UPPER MOTOR NEURONS and LOWER MOTOR NEURONS. LOWER MOTOR NEURONS are the a (alpha) and 7 (gamma) motor neurors that have their cell bodies in the CNS but their axons innervate the extrafusal and intrafusal skeletal muscle fibers respectively. These are the final pathways that bring commands from the CNS leading to muscle contraction Malfunctions of the lower motor neurons manifest themselves as muscle paralysis, muscle wasting (atrophy) and loss of segmental reflexes such as the withdrawal reflexes UPPER MOTOR NEURONS influence the action of the lower motor neurons to muscles. The upper motor neurons are divided into the Pyramidal, extrapyramidal systems, cerebellum and the basal ganglia Defects in the upper motor neurons can differentiated from those in the lower motor neurons by the presence of uncoordinated movements (ataxia), no muscle atrophy and segmental reflexes are present The PYRAMIDAL system is responsible for the initiation of voluntary leamed, skilled movements that are normally performed by flexor muscles in the body. *Ille F„XTRAPYRAMIDAL system is for the maintenance of the correct muscle tone in the antigravity extensor muscles,.

. . v, The neurons in the Pyramidal system are re;ponsible for the initiation of skilled, learned voluntary movements through their influence on the lower motor neurons to the flexor muscles that are distal to the sptne Lesions an the pyramidal system cause weaknesses in muscles •n one side of the body and loss of propnoception. •ne extrapyramidal system is the one whose neurons origmate mamly in the brain stem and do not pass through the pyramid like structures seen tn a cross secuon of the ventral medulla 'Ihe main function of the extrapyramidal system is to maintain the necessary muscle tone In the proximal antigravity extensor muscles. CEREBELLUM ne cer&llum is one of the structures containing the UPPER MOTOR neurons. •the cerebellum ("little brain") is a structure that is located at the back of the brain, underlying the occipital and temporal lobes of the cerebral cortex_ Although the cerebellum accounts for approximately 10% of the brain's volume, it contains over of the total number of neurons in the bram, Historically, the cerebellum has been considertxl a motor structure, because cerebellar damage leads to impairments in motor control and posture and the majority of the cerebellum's outputs are to parts of the motor system. Motor commands are not initiated in the cerebellum; rather, the cerebellum modifies the motor commands of the descending pathways to make movements more (daptive and accurate. In summary the is involved in the following functions: Maintenance of balance and posture. "Ihe cerebellum is important for making postural adjustments in order to maintain balance. Through its input from vestibular receptors and propriou:ptors, it modulates commands to motor neurons to compensate for shifts in body position or in load muscles. Patients with cerebellar damage suffer from balance disorders. and they develop stereotyped postural strategies to compensate for this problem (e.g. a wide-based stance) Coordipation of voluntary movements. Most movernents are composed of a number of different muscle groups acting together in a temporally cmrdinated fashion One major function Of the cerebellum is to cmrdinate the timing and force of these different muscle groups to produce movements. Motor learning. cerebellum is important for motor leaming. "Ihe cerebellum plays a major role in adapting and fine-tuning motor prograns to make accurate movanents through a trial-and- error process. In the early stages of acqurring a new motor skill (riding a bicycle or learmng to hit.

. . a baseball). a high concentration needed At th'S stage the cerebral cortex cerebellum to the varrous muscles tnvohed are h:ghl:. actne Houever. once the skill ts can be earned out easily mthout much concentration and it IS thought that the cerebellum btg role tn the execuuon of such learned skil!s functions Although lhe cerebellum ts nwst understood terms of Its contnbuuons motor control. It IS also tmoived In certam cognitive functions. such as language Ihus. like be-Bal ganghx the cerebellum is lustoncally considered as part of the motor system. but its funcuors extend beyond motor control in that are not Vt v,ell understood If there is any traumatic brain injury or brain cancer. the function of cerebellum may go hayqtre It causes slow and uncoordinated movements tn the body. Iherefore, people "lth cerebellum lestons sway and stagger while they walk. damage to cerebellum may lead to may problems in an individual. Thße problems affect the brain as follows: Asynergia: is loss of coordmauon of motor movement Dysmetrix The person finds it difficult to judge distance and to stop Adiadochokinesia: Ihis is a condition v.here the person ts unable to perform rapid alternating movements Intention tremor -nte patient may tremor carrying out certam movements (tremors v,hen an animal intends to move). Ataxic gait; Staggering and suaying while ssalking Hypotonia: A person develops weak muscles. THE BASAL GANGLIA Ihe basal ganglia are a collection of nuclei found on both sides of the thalamus, outside and the limbic system. but below the cingulate gyrus and wtthin the temporal lobs Although glutamate is the most common neurotransmitter here as everywhere in the bratn, the Inhibitory neurotransmitter GABA plays the most important role an the basal ganglia Ihe largest group of these nuclei are called the corpus striatum Cstriped body"). made up of the caudate nucleus ("tail"). the putanrn Cshell"). the globus pallidus ("pale globe"). and the nucleus ("leamn•g"). All of these structurs a double ones, one set on each side of the central septum Its functions in animals are poorly understocxi. although in man, lesions tn the bxal gangha been linked with anomalies in movements such as mperkinetic (excessive rnovements and l»pok.rnesis (reduced.

. . head of QILIdate nuc leus globus pallidus (unclernea putatnen optic tract amygdaloid nucleus thalarnus lateral geniculate body tail of caudate nucl eus 'Ihe caudate begins just behind the frontal lobe and curves back towards the occipital lobe. It sends its messages to the frontal lobe (especially the orbital cortex, just above the eyes), and appears to be responsible for informing the body that something is not right and the brain should do something about it: Wash your hands or Lock your door! an underactive caudate may be involved in various disorders, such as depression and schizophrenia. The putarnen lies just under and behind the front of the caudate It appears to be involved in coordinating automatic behaviors such as riding a bike, driving a car, or working on an assembly line. "Ihe globus pallidus is located just inside the putamen, with an outer part and an inner part. It receives inputs from the caudate and putamen and provides outputs to the substantia nigra 'Ihe nucleus accumbens is a nucleus just below the previous nuclei. It receives signals from the prefrontal cortex (via the ventral tegmental area) and sends other signals back there via the globus pallidus inputs use dopamine, and many drugs are known to greatly increase these messages to the nucleus accumbens. Another nucleus of the basal ganglia is the substantia nigra ("black substance"). Located in the upper portions of the midbrain, below the thalamus, it gets its color from neuromelanin, a close relative of the skin pigment. One part (the pars compacta) uses dopamine neurons to send signals up to the striatum exact function isn't known, but is to involve reward circuits and Parkinson's disease due to the death of dopamine neurons . Ille other part of the substantia nigra.

. . (the pars tettculata) mostly CiABA neurons. II's mam known function is controlling mmements It also Imohed Parkinson's. as well epilepsy. Parkinson's disease Parkituson's is characterized by tremor (shaking), rigid tnusc/es, difficulty making quick. smooth mm ements. and difficulty standing and walking. Many people also develop depression and anxiety and. later in life, problems with memory loss and dementia. Parkinson's is originates in the death of cells in the substantia nigra and the loss of dopamine and melanin produced by those cells. It progresses to other parts of the basal ganglia and to the nerves that control the muscles, involving other neurotransmitters. Possible causes or contributing factors include environmental toxims, head trauma, and genetics. Huntington's disease Huntington's is characterized by loss of memory and Odd jerking movements called chorea ("dance"). It is a hereditary disease (with a dominant gene) involving cell death in the caudate nucleus. It usually starts in a person's 30s, but may start at any age. There is no cure, but there are treatments that can reduce the symptoms. It is fatal, although it is complications of the disease that usually cause death, rather than the disease itself Many Huntington's sufferers commit suicide,.

. . Main motor area Complex movement motor area H@her functions association area Spe ech association area Sme sensory area Man sensory area Visual and auditory association area Proprioception sensory area Visual assoaation area Visual sensory area omprehension association 39 Sensory areas Associatnn areas Motor areas And dory sensory area.

. . CONTROL OF BEHAVIOUR AND EMOTIONS patterns (sexual. fear. rage) and emotions are controlled by the Limbic system limbic system conststs of a nm of ussue around the hillus of the cerebral hemispheres allocortex and juxtallocortex) and a group of associated deep structures that include hippocampus. amygdala and septal nuclei. 'Ihe Iltnbtc system is a convenient way of describing several functionally and anatomicallv Interconnected nuclei and cortical structures that are located in the telencephalon and diencephalon. 'Ihese nuclei serve several functions; however most have to do with control of functiorus necessary for self-preservation and species preservation. They regulate autonomic and endocrine function, particularly in response to emotional stimuli. They set the level of arousal and are involved in motivation and reinforcing behaviours Additionally, many of these areas are critical to particular types of memory. Some of these regions are closely to the olfactory system, since this system is critical to survival of many species. Areas that are typically included in the limbic system fall into two categori(h. Some of these are subcortical structures, while many are portions of the cerebral cortex. Cortical regions that are involved in the limbic system include the hippocampus as well as areas of neocortex including the insular cortex, orbital frontal cortex, subcallosal gyrus, cingulate gyrus and parahippocampal gyrus. This cortex has been termed the "limbic lobe" because it makes a rim surrounding the corpus callosum, following the lateral ventricle. Subcortical portions of the limbic system include the olfactory bulb, hypothalamus, septal nuclei and some thalamic nuclei, including the anterior nucleus and possibly the dorsomedial nucleus. One way in which the limbic system has been conceptualized is as the "feeling and reacting brain" that is interposed between the "thinking brain" and the output mechanisms of the nervous system In this construct, the limbic system is usually under control of the "thinking brain" but obviously can react on its own. Additionally, the limbic system has its input and processing side (the limbic cortex, amygdala and hippocampus) and an output side (the septal nuclei and hypothalamus). Most of these regions are connected by pathways that are shown below.

. . HYPOTHALMUS The hypothalamus, the primary output node for the limbic system, has many important connections. It is connected with the frontal lobes, septal nuclei and the brain stem reticular formation via the forebrain bundle. It also receives inputs from the hippocampus via the fomix and the amygdala via two pathways (ventral amygdalofugal pathway and stria terminalis). Ihe hypothalamus has centres involved in sexual function, endocrine functiom behavioral function and autonomic control. In order to perform its essential functions, the hypothalamus requires several type; of inputs. *lhere are inputs from most of the body as well as from olfaction, the viscera and the retina It also has internal sensors for temperature, osmolarity, glucose and sodium concentration. In addition, there are receptors for various internal signals, particularly hormones. Iliese include steroid hormones, and other hormones as well as intemal signals (such as hormones involved in appetite control such a.s leptin and orexin). Ihe hypothalamus strongly influences many functions including autonomics, endocrine functions and behaviors. Autonomic functions are controlled via projections to the brain stem and spinal cord. lhere are localized areas in the hypothalamus that will activate the sympathetic nervous system and some that will increase parasympathetic activity. Endocrine functions are controlled either by direct axonal connections to the posterior pituitary gland (vasopressin and oxytocin control) or via release of releasing factors into the hypothalamic-I»pophyseal portal system (to.

. . "ifluence antenor pnultan• funcuon) are also projections to the reticular formation that im Olved certam behavours. particularly emotional reactions Some funcnons are jntnnstc to the mvothalamus Iliese are functions that require a direct input to the hypothalamus and uhere the response generated directly via hypothala:nic outputs, Included are such things as temperature and osmolarity regulation. There are many functions where the monitors the Internal milieu and produces a regulatory response. These Include the regulation of endocnne functions and appeute For example, the ventromedial nucleus of the hypothalamus is considered a satiety area, while the lateral hypothalamic area is a feeding centre. Additionally, there are many complex behaviours that are patterned by the hypothalamus, including sexual responses. The preoptic area is one Of the areas Of greatest sexual dimorphism (i e, difference in structure between the sexes) and, along with the septal nuclei, is an area of gonadotropin releasmg hormone projections to the median eminence region of the hypothalamus, sexual responses involve autonomic, endocrine and behavioural responses. Finally, the suprachiasmauc nucleus receives direct retinal input enlis nucleus is responsible for entraining circadian rhythms to the day-night cycle. Amygdala ne amygdala is an important structure located in the anterior temporal lobe within the uncus. Ihe amygdala makes reciprocal connections with many brain regtons including the thalamus, hypothalamus, septal nuclei, orbital frontal cortex, cingulate gyrus, hippocampus, parahippocampal gyrus, and brain stem. The olfactory bulb is the only area that makes input to the amygdala and dos not receive reciprocal projections from the amygdala amygdala is a critical centre for coordinating behavioural, autonomic and endocrine responses to environmental stimuli, especially those with emotional content. It is important to the coordinated responses to stress and integrates many behavioural reactions involved in the survival of the individual or of the species, particularly to stress and anxiety. Lesions of the amygdala reduce responscs to stress, particularly conditioned emotional ftxponses. Sumulation of the amygdala producB behavioural arousal and can produce directed rage reactions. Various stimuli produce respomes mediated by the amygdala The convergence of inputs is important since it allows the generation of learned emotional responses to a variety of situations The amygdala responds to a variety of emouonal stimuli, but mostly those related to fear and anxiety/rage..

. . HIPPOCAMPUS "Ihe hippocampus is an ancient area of cerebral cortex that has three layers. Iliis is in the medial aspect of the temporal lobe, forming the medial wall of the lateral ventricle in this area There are several sources of himxycampal afferents. *Iliese are primarily from the septum and hypothalamus via the fornix and from the adJacent entorhinal cortex. "Ihis cortical region receives input from diffuse areas of Lhe neocortex, especially the limbic cortex, and from the amygdala The entorhinal cortex projects to the dentate gyrus of the hippocampus via the perforant pathway, synapsing on granule cells. The physiology of these pathways has been studied extensively, particularly in terms of long-term physiological changes associated with memory. Himxjcampal neurons have been studied extensively in terms of long-term potentiation lhis requires activation of glutamate receptors and rsults in long-term changes in neuronal excitability by way of calcium mediated physiologic effects. Outputs from the hippocampus pass primarily via two pathways. The first of these outputs is through the fornix These fibers project to the mamillary bodies via the post-commissural fornix, to the septal nuclei, to the preoptic nucleus of the hypothalamus, to the ventral striatum and to portions of the frontal lobe through the precommisural fomix There are large numbers of projections from the hippocampus back to the entorhinal cortex Note the hippocampus has rw•iprocal connections with the cortex as well as outputs along the fomix. Historically, the loop starting with the hippocampus projecting to the mamillary bodies, Vsith relay to the anterior thalamic nucleus, then the cingulate gyrus, entorhinal cortex and back to the hippocampus was thought to be an important circuit. This received the name "Papez circuit". The circular nature of this connection, however, does not appear to be of functional significance. Ihe hippocampus has several functions. It helps control corticosteroid production. It also has significant contribution to understanding spatial relations within the environment. Additionally the hippocampus is critically involved in many declarative memory fimctions.

. . MEMORY Memory ts the abilitv of the bram to store. retauu and subsequently recall Infotmauon sewral of clxsstfyng memones. based on duration. nature and retncnal of From Information processing perspectne there are thnx mam stages m the formation and of memory. • Encoding (processing and combining of received information) Storage (creation of a permanent record of the encoded Information) • RetrievaL/RecaIl (calling back the stored information in response to cue for use in some process or activity) CLASSIFICATION BY DURATION A basic and generally accepted classification of memory is based on the duration of memory retention, and identifies three distinct types of memory: sensory memory, short-term memory, and long-term memory. The sensory memory corresponds approximately to the initial moment that an item is perceived. Some of this information in the sensory area proceeds to the sensory store, which is referred to as short-term memory. Sensory memory is characteri7J2d by the duration of memory retention from milliseconds to seconds and short-term memory from seconds to minutes and hours. These storß are generally characterind as of strictly limited capacity and duration. whereas in general stored information can be retrieved in a period of time which ranges from days to years; this type of memory is called long-term memory. It may be that short-term memory is supported by transient changes in neuronal communicauom uhereas long-term memories are maintained by more stable and permanent changes in neural structure that are dependent on protein synthesis. Some psychologists, however, argue that the distinction benveen long- and short-term memories is arbitrary, and is merely a reflection of differirig levels of activation within a single store. If we are given a random seven-digit we may remember it only for a few seconds and then forget (short-term memory). On the other hand, we can remember telephone numbers for many years (assuming we use them often enough). Those long-lasting memories are said to be stored in long-term memory..

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. . from septal nuclei and has reciprocal connections With each of tjk9.e areas and •mth the dorsomedial nucleus of the thalamus Damage to the prefrontal area produces difficultje% With abstract reazontng. Judvnent moods and solung •Ihe effect of frontal lobe damage on mood depend'. on the •upeofic part of the prefrontal cortex damaged The patient's behavjour often dce..cnbed as tactless Also. thr• pan of the cortex can also be strongly affected by alcohol Prefrontal cortex function abnormal jn rn€xjd disorders Depression most often associated Bith Increased activity rn 'X'ruons of the frontal lobe. especially the medial regions including the subgaaual portion of the amenor ctngulate cortex, and decreased activity in the pstenor cingulate gyrus Olfacucm makes strong connections With the rxjrtions of the temporal lobe and amygdala The olfactory cortex is structurally Simpler than other portions of the cerebral cortex and is terrned allocortex (see section XI). It includes the prepjriform md periamygdaJ01d cortex that the anterior part of the parahiw»campal gyrus covenng the uncus. In sorne species, of course, olfaction IS rmre trnjx)rtant than in others Olfactory filaments CtOSS the cnbiform plate and synapse with mitral cells in the olfactory bulbs. Axons from these cells make up the olfactory tract which exteruls to antenor terrpral structurß bilaterally as well as bcaJ forebrain Ihe steps in emotional responses include (l) COGNITION which is the auareness of the sensation and its cause (2) AFFECT which is the feeling of sensation (3) CONATION which is the urge to take action. (4) PHYSICAL changes that are the notable changes like sweating. elevated heart rate and pressure neuronal Circuits in the limbic system are characterised with long after-discharge (cornplex reverbéatory Circuits) that explains the duration of the rest»nses long after the initial stimulauom Stimulauon of specific areas in the limbic systan cause certam resporzes related to båavtour Üld'or emotions Fear and rage in animals In genaal, animals rnmntarn that is mid-way between rage placidlty (very docile) In monkeys, fear is controlled by the nuclea The cerebral cortex, ventrorndial.

. . hyothalam,c septal nuclei are thought to inhibit the litubic area responsible for rage. The marufestauon of fear fleeing or avoldance response while for the rage the animal threatemng hissing sounds. and fighting or attacking moves. Most emotional and responses also invohe the autononuc responses eg elevated blood pressure, sweating •.Oldmg reflexes and m some cases. hormones have been kncj'.\T1 to influence the behaviour of an ammal For example. castration of a male animal leads to docility while an Increase in circulating male gonadal hormones causes an increased hostility. Reward (approach) and Punishment (avoidance) systems in the brain Experiments using electrodes implanted in various areas of the brain have shown that stimulation of certain areas caused the animal to repeat the stimulation over and over again (reward areas) while stimulation of other areas led to the animals avoiding the source of stimulation (punishment or avoidance areas). It has been shown that the neurons in the reward system of the bran secrete Dopamine as neurotransmitter substance. Dopamine exert is effects through interaction with Dopaminergic receptors, agonists stimulate the brain reward system. Addiction is a condition where an animal continues to use a chemical/drug or indulge in an activity that has harmful effects but finds it very hard to abandon the habit. For example in humans, drugs like cocaine, ethyl alcohol and nicotine cause addiction. Though the mechanism of action Of these drugs differs, but they all cause an increase in Dopamine secretion, which stimulates the brain reward system ne knowledge of the various neurotransmitter substances secreted by neurons in the brain led to the use Of drugs (e.g agonist or antagonist of the neurotransmitters or their receptors) to manipulate the behaviour of an animal. For example hallucinergic drugs cause hallucination (extreme ecstasy accompanied by imaginary visions). tranquilliser drugs alley anxiety and antidepressant drugs alleviate deprsion Sexual behaviour animals The limbic system together with the hypothalamus is greatly involved sexual behaviour in mature male and female animals. Most animals have inbom instincts to copulate when the female is ready (during estrus) period. •Ihe levels of sex hormones (Estrogen in female and Testosterone in males) influence the sexual behaviour greatly There are specific areas in the limbic system and hmthalamus that influence sexual behaviour, For example. cats and monkeys with lesiotus on the limbic area in the piriform cortex just over the amygdala, become abnormally hypersuxual. Such animals mount other animals (males and females, mature and immature) of their own species and of other species and even inanimate.

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. . AROI SAI, MECHANISM: SLEEP AND WAKEFULN ESS Tile core of the brainstem and nudbrmn contam a collection of two pes of neurons (1) (2) Specific neurons/nuclet to a particular sensation that project to specific areas in cerebral cortex Non-specific neurons/nuclej stimulated vanous stimuli and send Impulses to vade area: in the cerebral cortex The Impulses from the non-specific neuorns are d' ffusely sent to the cerebral neocortex ua the non-specific nuclei at the dorsal thalamus The Impulses are proJected to layers 14 In the neocortex while Impulses from specific thalamic nucle' are projected to layer 4 in the neocortex. The non-specific neurons compnse v.hat IS called RETICULAR FORMATION ACTIVATING SYSTEM v.hose acuvlty bnngs about concrousness. High frequency surnulauon from the reticular formanon promotes wakefulness while low frequency (about 8/s) stimulation promotes sleep.

. . AUTONOMIC NERVOUS SYSTEM (ANS) autonomic nervous sytetn •s the part of the penpheral (motor) nersous system that Innervates visceral organs such as smooth muscles, cardiac muscle•. and some glandular ussues autonomtc nenous system operates at a subconscjous level and the name autonomic demed from Grcx•k vsord mearung self-governmg or Independent autonomic nervous sytem differs from the sornauc rmtor system •n two major aspects (l ) In the tissues Innervated and (2) In the number of nerurons from the CNS to effector organ and location of cell bodies of the neurons In the sornauc motor system the motor neurons hau• their cell bedtes tn the CNS and axons rm»ve uninterruptel to innervate skeletal muscles But for the automyrnic nervous system. the pathuIY to the effector organ Involves two neurons. -Ihe first one called preganglionic neuron has its cell body in the CNS and its axon run Into a ganyon outside the CNS where it synapses with a second neuron called the postganglionic neuron which then innervate the effector organ •ahich can be smooth muscle, cardiac muscle and CLASSIFICATION OF THE AUTONOMIC NERVOUS SYSTEM The ANS is divided into two major divisions Insed on the anatomical origin of the preganglionic neurons (Anatomic drvasion) and also on the neurotramsmitter substance secreted bv the postganglionic neuron (Chemical division). nus we have the Sympathetic and the Parasympathetic divisions. SYMPA DIVISION Anatomically, the preganglionic neurors of the sympathetic division leave the spinal cord by way of ventral roots of the first thoractc to the fourth lumber spinal nerves and is sometimes referred to as THORACOLUMBER division The axons of preganglionic neurons are short (except those going to the adrenal medulla) and enter the paravertebral sympathetic ganglion chain where they synapse with the postganglionic neurons. Ihe preganglionic neurons secrete acetylcholine as a neurotransmitter substance. The postganglionic neurons then travel to the effectors others may re-enter the spinal nerves and travel with the spinal nerves to more distal structures. Ihe preganglionic neurons to the adrenal medulla synapse With vestigial (rudimentary) postganglionic neurons that are transformed to secretory cells that produce catecholamines (Dopamine, Epinephrine and Norepinephrine), The catecholamines are secreted directly to the blood stream PARASYMPA mvmo,v Anatomically, the preganglionic neurons of the leave the CNS from the brainstem through cranial nerves number III, VII, IX and X and also through spinal nerves in the.

. . sacral regton hence 't IS sometimes called the CRANIOSACRAL dtvr.ton wall of the mnenated structure, Classification of neurons in the ANS based on the neurotransmitter substance released Based on the type of neurotransmitter substances, the neurons the autononuc nervou•. system are cklsslfied as follows (2) (3) (4) All preganglionic neurons for both sympathetic and parasympathetic release acety Ichohne as a neurotransmitter substance that interact With Nicounjc (N) receptors tn the postganglionic side in the ganglion. "Ihey are thus called Cholinergic. All the postgangliomc neurons of the Parasympathetic secrete Acetylcholine which interact with Muscarinic (M) receptors in the rnembrane of the effectors. They are also Cholinergic, All the postganglionic sympathetic neurons except those to sweat glands and blood vessels tn the skeletal muscles secrete Norepinephrine as the neurotransmitter substance and are therefore called Noradrenergic (though it is nu»re common to call them adrenergicy Ille postganglionic sympatheuc neurons to sweat glands and skeletal muscle blood secrete Acetylcholine that cause sweating and vasodilatation •Iiiey are thus called the sympathetic cholinergic neurons, Postsynaptic receptors in the ANS "Ihe postsynaptic membranes in the ANS contain proteins that are receptors to which neurotransmitter substances bind in order to initiate the change in membrane permeability leading to either depolarization (action potential formed when threshcjld level rs reached) or hyperpolari7Ætion (reduced chances of forming an action potential). nere are two types of receptors that interact with acetylcholine. •these are: (2) Nicotinic (N) receptors that were found to interact with nicotine to mimic the action of acetylcholine These are mainly found in skeletal muscles and also the autonorruc ganglia •Ihe drug called curare can block the of N receptors, but atroprne has no effect on such receptors Muscarinic (M) receptors that were found to interact With muscarine (a touc substmce extracted from toadstools) to mimic the action Of acetydcholtne M receptors Me mainly found in visceral organs such as the hairt and intestinß Atropine blocks effæt of acetylcholine on tiE M receptors by compeung for the sxne btndrng sites but curUe hZ no effect on the M receptors.

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. . v, opcratne ot both systems For example 'pluncten In , hu•nan lhe and clitort•, while sympathetic and le OV ON SOME '1 S'ORGAVS harze ot to tn rn1-os1sJ of 02 Response Relaxation ter Increased heatl rate Iru:rease rn the and Heart stem Arteriae* Skin arxå Skeletal . Pulmon.r. muscle Stomach , Motility & Tcr»e Intestine Unnambladder Skin rnn.'clc Adrnal medulla Liver Salivary glands Adipose tissue , C.m1ra.tJcq tot rear yr.art rate (vagal arrest) Ikcteasc tn contract1iiiy hut m the 'selætt A-V biæk and decreased cmductlon velocttv Dilauon D' lauon Cxm.stncuon Incrca•.c Relaxat.lon Stimulate Increase Relaxation StunulatHjn Contractron of Profuy:. "atcrv tix conduction vel'X1t"' trxreascd conductim in conducuon Ctn•.trtction CmstncUm C '*i.stnction Decrease ( Lsually) C cnstnet:on Ikcrexse Constriction Inhibltton -lhiek_ and j -4.