Nervous System - Mans
Mansoura UniversityFaculty of ScienceZoology Department(2006 – 2007)
INTRODUCTIONAnimals are characterized by irritability or the ability to detect andrespond to environmental stimuli. This involves a sensory system(detection), a nervous system (interpretation) and a motor system(response). The nervous system is thus a connection between sensoryinputs and motor outputs.As evolution proceeded the nervous system becomes morecomplex. The radically symmetrical animals have simple nervous systemconsisting of nerve net work, conducts signals from sensory cells tomuscle cells. There is no centralization of nervous system. The bilaterallysymmetrical animals have centralized nervous system with enlargedanterior end called brain. The rest of which is the spinal cord. Generally,invertebrate animals tend to be small and have simple nervous system,whereas vertebrates have highly or well developed nervous system.Especially important is that all vertebrates have a similar basicstructure of their nervous system that is divided into: (1) central nervoussystem (CNS) which comprises the brain and spinal cord. (2) Peripheralnervous system (PNS) which comprises cranial nerves that join the brainand nerves of the spinal cord (spinal nerves). Nervous system in fish andamphibians is poorly developed compared to higher vertebrates and thenumber of cranial nerves is ten pairs only. In reptiles and birds, thenervous system is more developed, as the brain increases in size and thenumber of cranial nerves increased by two pairs (spinal accessory and thehypoglossal). In mammals, the nervous system is more complicated thanother vertebrates and is characterized by: (1) increased brain size relativeto body size. (2) increased subdivisions and growth of forebrain,especially the cerebrum, which is associated with the increasinglycomplex behaviour of mammals. (3) cerebral cortex is present that isconcerned with the muscular activity and higher brain functions.١
NERVOUS SYSTEMLife is maintained by coordination of the functions of variousbody systems. Coordination is controlled by two main systems:1- Endocrine system (chemical regulation): is a collection of bloodcarrying chemical messengers (hormones) with slow and long ncludestrillionsofinterconnected neurons with rapid and short standing action.FUNCTIONS OF NERVOUS SYSTEM:Nervous system acts to regulate and coordinate various bodyactivities necessary to life by allowing us to receive stimuli (sensoryinformation) from various sensory receptors and then processing theminto appropriate responses made by body organs (effectors).NERVOUS TISSUENervous tissue is composed of two types of cells: (1) neurons or nervecells (figure 1). (2) neuroglia (also called glia or glial cells "means glue").Figure 1: The general structure of the neuron.٢
GLIAL CELLS:The glial cells (figure 2) represent about 10 to 50 times the numberof neurons and are in direct contact with neurons and often surroundthem. They act to support, nourish and protect the neurons, thus aid intheir ability to do their functions.TYPES OF GLIAL CELLS:1-Oligodendroglia:They are few macroglial cells that form myelin sheath around theaxons in the CNS (like the Shwann cells in the PNS). A single cell canextend and surround large number of neurons for myelination of theiraxons.2-Astroglia:They are the most abundant of glial cells that have star likeappearance. Astroglia fill the spaces between neurons, have numerousprojections that hold neurons to their blood supply ,and help to regulatethe external chemical environment of neurons by removing excess ionsand taking up neurotransmitters released by neurons and recycling them.3-Microglia:They are of various forms that have branched processes. They aremigratory and act as phagocytes to waste products of nerve tissues.Microglia originate outside the brain, mostly in bone marrow, unlikeother glial cells.Figure 2: Types of glial cells.Figure 2: Types of glial cells.٣
THE NEURON:The neuron is the structural or anatomical unit that is responsiblefor the functions normally associated with thenervous system. Itoperates by generating electrical signals that pass from one part of thecell to another part of the same cell and by releasing chemicalmessengers (neurotransmitters) to communicate with other cells.STRUCTURE OF THE NEURON:Neurons vary considerably in size and shape according to theirsites and functions. In general, they are formed of the cell body and cellprocesses (figure 1).1) The cell body (Soma):The cell body is the enlarged part of the neuron. It is a metaboliccenter that provides nutrition for the whole neuron. The cell bodies insidethe CNS are usually collected into groups called (nuclei or centers), butin PNS usually collect to form (ganglia). The cell body is surrounded bythe cell membrane which continues to cover its processes. The cell bodycontains nucleus and surrounding cytoplasm, besides mitochondria andother organelles typical in eukaryotic cell, but no centrosomes. Theabsence of centrosomes indicates that the neurons have lost its power ofdivision.In addition, the neuron contains specialized structuresincluding: Nissl bodies: these are granular materials (free and attached ribosomes)that present in the cell body and not present in dendrites or axon. Nisslbodies are responsible for synthesis of protein in the nerve cell. Neurofibrils: these are thin fibers that present in the cell body andextend into the processes of the cell. They serve as a support of theneuron to maintain its shape.٤
Microtubules: these are distributed through out the cytoplasm of thecell body and extend into the dendrites and axon. They serve to supportthe neuron (as neurofibrils) and to transport materials and organellesdown from the cell body to the axon (axon transport), for regeneration ofdamaged axons. Axon transport of certain materials also occurs in theopposite direction from the axon terminals to the cell body. By this route,growth factors and other chemical signals picked up at the terminal canaffect the neuron. This is also the route by which certain viruses taken upby the peripheral terminals can enter the CNS.2) The Cell Processes:a) The dendrites: are multiple short processes which extend from thecell body. They extend to the surrounding area to act as receptive surface.So, the dendrites increase the surface area of the cell body. The surface ofdendrites collect impulses and transmit them to the cell body.b) The axon (nerve fiber): is a single long process that conductsimpulses away from the cell body to its target cell. Axons vary in lengthfrom only a millimeter to a meter or more (axons extending from spinalcord to foot). The portion of the axon closest to the cell body plus the partof the cell body where the axon is joined known as the initial segment or(axon hillock). It is important in generating the electric signal thatpropagates away from the cell body along the axon. The axon may havebranches called collateral branches along its length (figure 1). Near theends, both the axon and its collaterals undergo further branching. Eachbranch ends in a terminal, which makes junction with one of thefollowing (figure 3): Dendrites or cell body of another neuron, forming a (neuro-neuraljunction) or synapse. Muscle fiber to form a (neuro-muscular junction). Secretory gland to form a (neuro-epithelial junction).٥
Figure 3: Termination of the axonMYELINE (MEDULLARY) SHEATH:All vertebrates have two types of nerve fibers, the large axonsmore than 1um in diameter being myelinated and those of smallerdiameter are generally unmyelinated. Almost all invertebrates areequipped only with non-myelinated fibers, but some differ from those inthe vertebrates in being much larger.Myelin is a white lipoprotein material composed of severalcompressed layers (20-200) which make successive wrappings aroundthe axon. Myelin sheath is not a continuous layer, but is interrupted atregular internods by nodes of Ranvier (figure 4). Through these nodes,٦
ions and water inside the neurons can undergo exchange withsurrounding tissues.In PNS, myelin sheath is usually covered by a thin basementmembrane (the neurilemma). Just beneath it, the Schwann cells are to befound at the mid point of each internode. In contrast, the nerve fiberswithin the central nervous system usually lack the neurilemma.In the CNS, myelin sheaths are formed by oligodendroglia (a typeof glial cells), while in the PNS , Schwann cells are responsible for this .Each Schwann cell only warps an internode ( i.e many Schwann cellswill myelinate one axon ), although one oligodendroglia will myelinatemany axons in the CNS. This difference allows Schwann cells to act forregenerating damaged axons in PNS , but oligodendroglia can not guideregeneration and damage in CNS . It is therefore irreparable.Figure 4 : Part of vertebrate axon (ax.) showing myelinsheath (myel.), nodes of Ranvier (n.) and Schwann cell (S.c.) justbeneath the neurilemma (N.) .TYPES OF NEURONS:I) STRUCTURAL CLASSIFICATION OF NEURONS:There are four types of neurons (figure 5) according to the numberof processes extending from the cell body (polarity):1) Unipolar neuron:Has only one process (exists in the nuclei of cranial nerves).٧
2) Pseudounipolar neuron:Has one short process that gives two branches . One is the longperipheral process ends at the receptor . The other branch is the shortcentral process enters the central nervous system (exists in the spinalganglia).3) Bipolar neuron:Has one long axon and one dendrite on the opposite sides of thecell body (less common, exists in certain tissues as retina of eye,olfactory epithelium of the nose & in the ear).4) Multipolar neuron:Has several short dendrites and one long axon (exists widely inbrain & spinal cord).Figure 5: Types of neurons. (a) unipolar neuron, (b)pseudounipolar neuron (c) bipolar neuron and (d) multipolarneuron.II) FUNCTIONAL CLASSIFICATION OF NEURONS:1-Sensory or afferent neurons:Conduct impulses from the sensory receptors to CNS.2-Motor or efferent neurons:Conduct impulses from the CNS to effector organs. There are twotypes of motor neurons (Figure 6):٨
a) Somatic motor neurons: which innervate external (voluntary) organs(skeletal muscles).b)Autonomic motor neurons: which innervate the internal (involuntary)organs (smooth and cardiac muscles and glands).3- Interneurons (about 99% of all neurons):They connect sensory and motor neurons and serve to integratefunctions of nervous system.Figure 6: The sensory- motor neurons.NOTE: Motor and interneurons are multipolar, whereas sensory neuronsare often unipolar. For afferent neurons, both cell body and the long peripheralprocess of the axon are outside the CNS and only a part of thecentral process enters the brain or spinal cord. The interneurons are located inside CNS. For efferent neurons, the cell bodies are within the CNS, but theaxons extend out into periphery. The axons of both afferent and efferent neurons, except for thesmall parts in the brain or spinal cord, form the nerves (nervetrunks) of the PNS.٩
THE NERVE:The nerve (or nerve trunk) is composed of a large number of nervefibers. Each nerve fiber is an axon covered by a myelin sheath. The fibersare bound together in bundles by connective tissue rich in blood vesselsknown as perineurium. Bundles of individual nerve fibers form thenerve trunk which is enclosed in a relatively strong sheath of connectivetissue known as epineurium.REFLEX ACTIONThe activities controlled by the nervous system are called reflexactions or simply reflexes. The reflex is a reaction started by a change inthe surrounding environment which acts as a stimulus, stimulating one ofthe receptors. This leads to initiation of nerve impulse which passesthrough chain of sensory neurons to CNS. From the CNS, impulses passoutwards (reflected) as a response through the motor neurons and reachthe effector organ.REFLEX ARC:The reflex action is carried out through a pathway called reflex arc,which is considered the functional or (plysiological) unit of the nervoussystem. The reflex arc consists of: A receptor organ: It is a sensory cell where stimulus is received. A sensory (afferent) neuron: Its function is to conduct impulse fromthe receptor neuron to CNS. An interneuron: The afferent neurons commonly synapticallycommunicate with interneurons, which in turn send impulses to motorneurons or to other interneurons, which finally influence the activity ofthe motor neurons concerned in the reflex action. A motor (efferent) neuron: It serves to transmit the impulse to theeffector organ.١٠
An effector organ: It is the organ which responds to the transmittedimpulse (e.g. the muscle or gland).The reflex arc in which the afferent sensory neurons directlycommunicate with the efferent motor neurons is a simple reflex arch witha single synapse (monosynaptic arc) and the reflex is monosynapticreflex, while the reflex arc in which one or more interneurons areimposed between the afferent and efferent neurons are polysynaptic (No.of synapses in the arc varies from two to many hundreds).Figure 7: Reflex arcTYPES OF REFLEXES:The reflexes are functionally classified into:(1) The somatic reflexes.(2) The autonomic reflexes.In the two reflexes, the sensory pathway is similar, but the motorpathway differs. In the somatic reflex the motor (efferent) branch consistsonly of one motor neuron (its cell body located in CNS), while it consistsof two neurons in the autonomic reflex. They are pre-ganglionic neuron١١
with the cell body located in the CNS and post-ganglionic neuron withthe cell body located in a ganglion (called autonomic ganglion) outsideCNS (figure 8).Anatomically, reflexes are either:(1) Spinal reflex, which concerned with immediate withdrawalfrom harmful stimuli (without involvement of brain) .(2) Cranial reflex, which is more complicated reflex action(involves association of specific brain areas).Fiure 8: Somatic and autonomic reflex arcs.Examples Of Reflexes:1) The sudden withdrawal of one’s hand when it comes in contactwith hot surface.2) The act of sneezing and coughing (coughing is mainly due toirritation of the larynx by some materials, while sneezing is due toirritation of the mucous membrane of the nose).3) The secretion of sweat in an increased quantity (warmth acts asthe stimulus for increased secretion).4) The secretion of saliva and gastric juice and the movements of thestomach (food acts as a stimulus).١٢
RECEPTORS:Are specialized endings of afferent (sensory) neurons or separate cells.Generally receptors are classified into two groups: Exteroceptors andInteroceptors.1) Exteroceptors (for detection of external stimuli), they include: Touch and pressure (cutaneous) receptors. Thermal receptors. Photoreceptors. Auditory receptors. Smell and taste receptors (chemoreceptors).2) Interoceptors (for detection of internal stimuli), they inclued: Proprioceptors (respond to change in position of body). Baroreceptors (respond to change in blood pressure). Chemoreceptors (respond to change in circulating, CO2, O2 and H ).NERVE IMPULSE:The neurons (like all other cells) have difference in the concentration ofions on the two sides of their plasma membrane. Under resting condition,the major ions in the extra cellular fluid are sodium and chloride ions,whereas the intercellular fluid contains high concentration of anicmolecules,particularly proteins and phosphates. This unequal distribution of ionsand charges around the membrane results in a net negative charge insideand a positive charge outside the membrane (i.e. the membrane ispolarized).This charge difference at rest, develops an electrical potential(resting membrane potential). In many nerve cells the resting membrane١٣
potential is approximately (– 60 mv). Minus sign indicates that inside ofthe cell is more negative than the extra cellular fluid.Sodium - Potassium Pump :The concentration difference for sodium and potassium on the twosides of the cell is due to the action of plasma membrane active transportsystem (sodium – potassium pump) that maintains this unequalconcentration by actively transporting ions against their concentrationgradient using ATP. Sodium–potassium pump actually pumpsthreesodium ions out of the cell for every two potassium ions that they bring.This unequal transport of positive ions make the inside of the cell morenegative than it would be from simple ions diffusion .The Action Potential (temporary change in the membranepotential):The nerve cell is specialized to respond to various stimuli and toconvert this stimuli into electrical signal (nerve impulse) that isconducted along the nerve fiber to its terminals.This occurs by the rapid changes in the voltage of the membrane froma negative state to a positive value for a brief time. A process that iscalled depolarization and is accompanied by transmission of electricalimpulses by the nerve cell.This is a result of activation of the membrane Na , k pump inresponse to stimuli, causing much more sodium influx than potassium outflux. Consequently, producing a gradual depolarization of the membrane(generation of action potential), that lasts for few milliseconds, thenreturns so rapidly to its resting membrane potential (where sodium pumpundergoes inactivation). As consequence, the membrane potential isrestored to its resting level.The action potential (nerve impulse) propagates in direction fromthe soma to the axon terminal at constant speed. When the actionpotential reaches the end of the axon, it invades the synaptic terminal,١٤
causing the release of a chemical transmitter (neurotransmitter) used inthe communication between neurons in a process called synaptictransmission.NOTE:After passage of the action potential, there is a brief period (therefractory period) during which the membrane can not be stimulated.This prevents the massage from being transmitted backward along themembrane.The Velocity of Action Potential :In unmyelinated nerve fiberthe conduction velocity isproportional to the diameter of the axon. The larger diameter of the axon,the greater speed of propagation. This is because the axons with largediameter do not offer as much resistance to the flow of ions along thelength of axon. In myelinated axon the velocity is greater and isdetermined not only by the diameter of the axon, but also by distancebetween nodes of Ranvier.The formation of myelin around an axon prevents the penetrationof ions needed for the conduction of the action potential, however thereare nodes of Ranvier, where the membrane of the nerve axon is exposedand contains large number of sodium and potassium channels. In thenodes of Ranvier the membrane is depolarized and action potentials aregenerated. The generation of an action potential in a node of Ranviercauses the membrane in the adjacent node of Ranvier to depolarize andalso to generate an action potential.In this way, the propagation of an action potential along amyelinated nerve axon appears to jump (saltus in latin) from one node ofRanvier to the next in the process of saltatory conduction. Thus, thegreater distance between nodes of Ranvier, the greater the velocity ofaction potential propagation (figure 9).١٥
Figure 9: In a myelinated axon, the impulse jumps from onenode of Ranvier to the next (saltatory conduction).SYNAPSES:The communication between nerve cells occurs at junctions calledsynapses. In the nervous system, there are two types of synapses(according to presence of chemical transmitter), (1)chemical synapsesand (2)electrical weendistant cells:In the nervous system, the predominant type of synapse is thechemical synapse (figure 10). In chemical synapses, at least two cellsparticipate–the cell producing the nerve impulse, called the presynapticcell, and the target cell receiving the impulse, called the postsynaptic cell.The presynaptic component of the synapse consists of the terminalending which contains vesicles called synaptic vesicles. These vesiclesfilled with chemicals referred to as neurotransmitters. When an actionpotential in the presynaptic neuron reaches the end of the axon, it causesthe release of theneurotransmitter into the synaptic cleft. Thetransmitters bind to receptors located in postsynaptic cell membranes.This results in depolarization of its plasma membrane, causingtransmission of action potential along the postsynaptic neuron.١٦
Figure 10: The structure of the chemical weenadjacent cells:Is a gap junction (with no synaptic vesicles) in which impulses areconducted directly from one cell to another adjacent cell. Its conductionis faster than the chemical synapse.Electrical synapses are also found in cardiac cells of the heart,smooth muscle cells, and other cells that display a synchronization ofactivity.١٧
I- BASIC ORGANIZATION AND FUNCTIONS OFTHE CENTRAL NERVOUS SYSTEMThe central nervous system (CNS) is composed ofbrain andspinal cord . The CNS is surrounded by bony skull and vertebrae. Boththe spinal cord and brain consist of a white matter (bundles of axons eachwith a sheath of myelin) and grey matter (masses of cell bodies anddendrites). In the spinal cord, the white matter is at the surface and thegrey matter inside. In lower vertebrates (like fish & amphibians), theyhave their white matter on the outside of their brain as well as their spinalcord. However, in the brain of mammals this pattern is reversed. Both thebrain and spinal cord are enveloped by three membranes called meninges(figure 11) which support and protect the CNS . They are :1- Dura mater: The outer membrane, next to the interior surface of theskull and bony vertebrae.2- Arachnoid : The middle membrane.3- Pia mater: The inner membrane, covering the entire surface of thebrain and spinal cord.Figure 11: Diagram showing the meninges surrounding the brain andspinal cord.١٨
The space between the arachnoid and pia mater (subarachnoidspace) is filled with fluid called cerebrospinal fluid (CSF). The rest of theCSF lies in the ventricles of the brain.CEREBROSPINAL FLUID (CSF):The CSF is a clear colorless fluid secreted from the choroid plexus(network of blood capillaries) which is found in the lining of theventricles of the brain.The CSF has a composition identical to that of the nervous systemextra cellular fluid (ECF) but differs from that serving as the ECF of thecells in the rest of the body. This compositional difference of CSF ismaintained by the blood brain barrier (BBB), which is a system of tightjunctions between endothelial cells of the nervous system bloodcapillaries.The CSF circulates from the interconnected ventricular system into thecentral canal of the spinal cord (figure 12). It flows through threeforamena (apertures) at the fourth ventricle into the subarachnoid spacealong the brain and spinal cord, where it is directly absorbed into thecerebral veins to enter the blood stream.Normally, the total volume of CSF is about 150 ml and its dailyrate of secretion equals its rate of absorption. If the flow of CSF isobstructed, CSF accumulates, causing hydrocephalus. In sever cases, theelevation of CSF in ventricles leads to compression of the brain bloodvessels, which may lead to inadequate blood flow to the neurons,neuronal damage and mental retardation.Functions:(1) It acts as bath around brain and spinal cord to protect them frominjury with position or movement.(2) It serves to transport nutrients into the nervous system and to removeharmful metabolites from nervous system into blood.(3) It keeps constant intracranial pressure, if the volume of the brainincreases, CSF drains away and if the brain shrinks, more fluid isretained.١٩
Figure 12: Diagram illustrating the location of the cerebrospinalfluid in the ventricles and the spinal canal.1- THE BRAINDuring development, the central nervous system is formed from a longtube of ectoderm (neural tube). The anterior end of the tube becomes thebrain. The lumen of which becomes dilated and produces a largeventricular system within the brain (4 ventricles), while the lumen of thecaudal end of the tube (spinal cord) remains very small and is recognizedin the adults as the central canal. In human, the brain weighs 350- 400 gat birth. As a child grows, the number of cells remains stable, but cellsgrow in size and the number of connecting cells increases. So, brainreaches 1300-1400g in adult human and is differentiated into 3 mainsections, they are:٢٠
(1) Prosencephalon (forebrain): subsequently divided into thetelencephalon (cerebrum) and diencephalon (thalamus & hypothalamus).(2)Mesencephalon (midbrain): develops without subdividesintothemetancephalon (pons & cerebellum) and myelencephalon (medullaoblongate).During continuing formation of the brain, four different regionsbecome apparent . These regions are:1-Cerebrum2- Diencephalon3- Cerebellum4- Brainstem(consists of midbrain, pons and medulla oblongata) (figure 13).Figure 13 : The Brain Regions.BRAIN VENTRICLES:The brain contains four interconnected cavities (ventricles), whichare filled with circulating CSF. This fluid is secreted into the ventriclesby the cells of the choroid plexus. These ventricles are, the first andsecond (lateral) ventricles being the largest and are found in each cerebral٢١
hemisphere. The third ventricle is a narrow cleft that lies between theright and left thalami. It connects the lateral ventricles by means ofinterventricular foramena (figure 12). The fourth ventricle is a flattenedpyramidal cavity found between the cerebellum and medulla oblongataand is connected to the third ventricle by the cerebral aqueduct (alsocalled aqueduct of midbrain). The fourth ventricle is connected also withthe central canal of the spinal cord and it has three openings (two lateral& one medial) through which CSF flows into the subarachnoid space.THE MAIN STRUCTURES OF THE BRAINI- CREBRUM :The cerebrum is the largest portion of the brain associated with thehigher brain functions, such as thought and action. It is divided into leftand right hemispheres being largely separated by a longitudinal division .However the two hemispheres are still connected by bundles ofmyelinated nerve fibers (corpus callosum) to permit transfer ofinformation between them.The outer surface of each hemisphere: consists of grey matterthat represents the cerebral cortex. The cortex in each hemisphere isabout 4 mm thick. It consists mostly of cell bodies and processes havingno myelin covering. The neurons in the cortex are arranged in severallayers (six) one above the other. They are mostly pyramidal shaped cellswith dendrites extensively arborized to reach cortical surface. Cerebralcortex is highly convoluted and this makes the brain more efficientbecause it increases the surface area of the brain and the number ofneurons in it.The surface of the cortex in each hemisphere is marked by groovescalled sulci (singular sulcus) including two main lateral sulci and thecentral sulcus. The sulci divide the cortex into four lobes visible fromoutside, frontal lobe in front of the central sulcus, parietal lobe٢٢
(immediately behind the central sulcus), temporal lobe (below the lateralsulcus) and occipital lobe at the back of the brain. Hidden beneath theselobes of the cortex are the olfactory bulbs, which receive inputs from theolfactory nerves (figure 14).The deeper parts of the cerebral hemispheres: consist of whitematter that is made up of myelinated nerve fibers (nerve tracts), which inturn overlies cell clusters (grey matter) collectively termed subcorticalnuclei. The nerve tracts may run as:1) Association fibers from one part to another at the same hemisphere.2) As connecting fibers from one hemisphere to the other.3) As efferent (descending) or afferent (ascending) fibers, which carryimpulses from or to the brain.LOBES OF THE CEREBRAL CORTEX:1- FRONTAL LOBE:Frontal lobe is associated with the primary motor cortex and isimportant in conducting three functions. (a) speech. (b) thoughts andmake decisions. (c) initiation and control of voluntary movement.2- PARIETAL LOBE :Is a sensory area (primary somatosensory cortex) associated withspecialized area for taste sensation. Somatosensory cortex is responsiblefor receiving impulses from somatic receptors to give sensation abouttouch, pain, pressure, temperature and position of the body in the space.The specialized sense organs of body (eyes, ears and nose) have otherfunctional areas on the cortex .3- THE TEMPORAL LOBE :Is a sensory area that receives signals from auditory nerves andconcerned with hearing, speech and memory.4- OCCIPITAL LOBE :Is a sensory area that receives signals from optic nerves andconcerned with many aspects of vision and reading ability.٢٣
Figure 14: Lobes of the cerebral cortex.THE SUBCORTICAL NUCLEI (BASAL GANGLIA):The name “basal ganglia” is an exception to the generalization thatganglia are neuronal cell clusters that lie outside the central nervoussystem. They are group of large nuclei (grey matter) that lie deep withinthe cerebral hemispheres and connect with motor cortex and other brainareas. They are important for controlling voluntary movement throughactivation by the motor cortex.II- THE DIENCEPHALON:It is the second component of the forebrain which is divided intotwo major parts (figure 15):1) The thalamus.2) The hypothalamus.٢٤
(1) THE THALAMUS:Is a collection of several large nuclei (separated by the thirdventricle) into two thalami lie close together. Functionally, the thalamicregion serves as a relay station for various sensory inputs (exceptolfaction) before arriving to the primary cortical areas responsible forsensation (sensory function).It is also important to relay motor signals coming from basalgangl
whereas vertebrates have highly or well developed nervous system. Especially important is that all vertebrates have a similar basic structure of their nervous system that is divided into: (1) central nervous system (CNS) which comprises the brain and spinal cord. (2) Peripheral nervous system (PNS) which comprises cranial nerves that join the .
I. Central Nervous System vs Peripheral Nervous System II. Peripheral Nervous System A. Somatic Nervous System B. Autonomic Nervous System III. Autonomic Nervous System A. Parasympathetic Nervous System B. Sympathetic Nervous System IV. Reflex Actions V. Central Nervous Sys
peripheral nervous system is subdivided into the a. sensory-somatic nervous system and the autonomic nervous system (ANS). The peripheral nervous system carry information to and from the central nervous system. The central nervous system is composed of the brain and spinal cord. Figure 1 The Human Nervous System
nervous system is separated into two major divisions: the central nervous system and the peripheral nervous system. The central nervous system is the control center of the body. The functions of the central nervous system are similar to those of the central processing unit of a computer. The central nervous system relays messages, processes .
Nervous System: Consists of two main divisions: Central nervous system (CNS) Brain and spinal cord Peripheral nervous system: Somatic nervous system Autonomic nervous system Central Nervous System
Divisions of the nervous system 1. Central Nervous System (CNS) 2. Peripheral Nervous System (PNS) Central Nervous System (CNS) a. Brain b. Spinal cord . Peripheral Nervous System (PNS) a. Sensory b. Motor . Divisions of the Nervous System which consists of is divided into that make up which is divided into The Nervous
The nervous system can be divided into two major regions: the central and peripheral nervous systems. The central nervous system (CNS) is the brain and spinal cord, and the peripheral nervous system . function of the nervous system. oT describe the functional divisions of the nervous system, it is important
The Structure Of The Nervous System – Once destroyed,nerve cells/tissues never grow back! Regions of the Nervous System. A. C.N.S. –Central Nervous System The central nervous system (CNS) is the command and processing center for the nervous system. It receives information from and sends information to the peripheral nervous system. The two main
Literary Theory and Schools of Criticism Introduction A very basic way of thinking about literary theory is that these ideas act as different lenses critics use to view and talk about art, literature, and even culture. These different lenses allow critics to consider works of art based on certain assumptions within that school of theory. The different lenses also allow critics to focus on .
