Describe and Discuss the Organisation of The

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Describe and Discuss the

Organisation of the Nervous

System.

Lecturer: Dr. Francesco Amico

Course: PSY382

Student: Alan Cummins

Student No: 1165236


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PSY382 - Alan Cummins - 1165236 - 3 of 27

Table of ContentsIntroduction ..................................................................................................................................... 3

Overview of the Nervous System ................................................................................................... 5

The Central Nervous System .......................................................................................................... 7

The Peripheral Nervous system .................................................................................................... 21

How the Nervous System Interacts ............................................................................................... 25

Bibliography ................................................................................................................................. 27

Table of Figures

Figure 1 - General Structure of the Nervous System ...................................................................... 6

Figure 2 - Meninges ........................................................................................................................ 7

Figure 3 - Major Brain Ventricles ................................................................................................... 8

Figure 4 - Central Nervous System Overview ................................................................................ 9Figure 5 - The Brain: Fore, Mid and Hindbrain............................................................................ 10

Figure 6 - Brain Lobes .................................................................................................................. 12

Figure 7 - Basal Ganglia ............................................................................................................... 13

Figure 8 - Thalamus and Hypothalamus ....................................................................................... 14

Figure 9 - Midbrain Structures ...................................................................................................... 15

Figure 10 - Cerebellum and Pons.................................................................................................. 16

Figure 11 - Spinal Cord................................................................................................................. 18

Figure 12 - Spinal Cord Detailed Structures ................................................................................. 19

Figure 13 - Autonomic Versus Somatic Nervous System ............................................................ 21

Figure 14 - Autonomic Nervous System ...................................................................................... 23Figure 15 - Brain, Spinal Cord and Link to Peripheral Nervous System ..................................... 26


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Introduction

This essay will give a brief overview of the structure of the nervous system. It will make

extensive use of illustrations taken from various anatomical guides, as referenced in the

bibliography. The physiological components of the nervous system are too wide-reaching and

vast to give an in-depth discussion and description here and as such the structure and function as

regards the cellular level will be left to one side. Equally not every single structure within the

nervous system will be referenced but the major component pieces will be put in place. The

essay will be broken down into the following sections:

Overview of the Nervous System: High-level description of the nervous system. The Central Nervous System: Specifics of the Central Nervous System. The Peripheral Nervous System: Specifics of the Peripheral Nervous System. How the Nervous System Interacts: How the two Systems interact with some brief

examples.


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Nervous System

PeripheralNervous System

Central Nervoussystem

AutonomicNervous System

Somatic Nervoussystem

Brain Spinal Chord

SympatheticNervous system

ParasympatheticNervous System

Forebrain Midbrain Hindbrain

Mylencephalon

MedullaPons

Cerebellum

MetencephalonMesencephalonDiencephalonTelencephalon

ThalamusHypothalamus

Cerebral CortexBasal GangliaLimbic System

Inferior andSuperior Colliculi

Figure 1 - General Structure of the Nervous System


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The Central Nervous System

The Central Nervous System (CNS) is made up of the spinal cord and the brain. (See

Figure 4 - A) Both are protected by bone and meninges. The brain is protected by the skull and

the spinal cord by the vertebral column. The meninges is a protective sheath surrounding both

the brain and spinal cord made up of three layers, an outer thick tough layer called dura matter, a

middle layer called the arachnoid membrane which is soft and spongy and then the pia mater

which is closely attached to the brain and spinal cord. Between the pia mater and arachnoid

membrane is a gap called the subarachnoid space which is filled with cerebrospinal fluid (CSF)

(See Figure 4 B, Figure 2).

Figure 2 - Meninges

The brain is essentially a mass of neurons, glia and other supporting cells, floating in a pool in

this fluid. It receives a large amount of supply of blood, which acts as a communication pathway

and it is chemically guard by the blood-brain barrier. The brain contains a series of hollow

interconnected chambers called ventricles which are filled with CSF.


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Figure 3 - Major Brain Ventricles

The largest chambers are the lateral ventricles which are connected to the third ventricle (See

Figure 3). The third ventricle is located at the midline of the brain, dividing the brain into

symmetrical halves. A bridge of tissue called the massa intermedia crosses through the middle of

the third ventricle. The cerebral aqueduct connects the third ventricle to the fourth ventricle.


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Figure 4 - Central Nervous System Overview


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The brain has three major anatomical regions and is broken into: the fore, mid and

hindbrain. The forebrain made up of the Telencephalon and Diencephalons, the midbrain made

up of the Mesencephalon and the hindbrain made up of the Myelencephalon containing the

cerebellum, pons and medulla. (See Figure 5)

Figure 5 - The Brain: Fore, Mid and Hindbrain

The forebrain contains the Telencephalon and the Diencephalon. The forebrain is

located above the midbrain. The Telencephalon includes most of the two symmetrical cerebral

hemispheres that make up the cerebrum. Most areas that lie in one cerebral hemisphere have

corresponding areas in the other. The two hemispheres are connected by several large tracts, one

of which is the corpus callosum. These function to keep each hemisphere informed about what

the other is doing. These hemispheres are covered by cerebral cortex and contain the limbic

system and the basal ganglia. The cerebral cortex functions to control thought voluntary

movement, language, reasoning and perception. It is the outer covering of the cerebral


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hemispheres and represents the most recent development in the evolution of the vertebrate

nervous system. The cerebral cortex consists of sulci, fissures and gyri which greatly enhance

the surface area of the cortex. The cerebral cortex consists mostly of glia and cell bodies,

dendrites and interconnecting axons of neurons. The cerebral cortex has what is called gray and

white matter. Gray matter is so called because of the number of cell bodies in that area which

give it a grey colour. Beneath the cerebral cortex are millions of axons that connect neurons of

the cerebral cortex with those located elsewhere in the brain. The large concentration of myelin

within this area gives this tissue an opaque white appearance and is termed white matter. The

cerebral cortex receives information from sensory organs via the primary visual cortex, the

primary auditory cortex and the primary somatosensory cortex. (See Figure 4 - E) With the

exception of olfaction and gestation sensory information from the body or environment is sent to

the primary sensory cortex of the contralateral hemisphere. Control of movement is within the

primary motor cortex, located just in front of the primary somatosensory cortex. Neurons here

are connected to muscles in different parts of the body. These connections are contralateral.

Perceiving, learning, remembering, planning and action take place in the association areas of the

cerebral cortex. The cerebral cortex is split into four areas, the frontal lobe, everything in front of

the central sulcus, the parietal lobe, to the side of the cerebral hemisphere just behind the central

sulcus, caudal to the frontal lobe, the temporal lobe, just forward from base of brain, ventral to

frontal and parietal lobes and the occipital lobe which lies at the back of the brain, caudal to the

parietal and temporal lobes (See Figure 6).


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Figure 6 - Brain Lobes

The brain contains two of each lobe, one in each hemisphere. Each primary sensory area of the

cerebral cortex sends information to adjacent regions called the sensory association cortex.

Similarly there is a motor association cortex located just rostral to the primary motor cortex. The

rest of the frontal lobe, rostral to the motor association cortex is known as the prefrontal cortex.

Each hemisphere cooperate but do not perform identical functions, the unity of these

hemispheres is maintained by the corpus callosum. These lobes have distinct functional areas:

Frontal, which is responsible for motor activity, speech, thought processes, Parietal, which lies at

top of brain and processes information about touch, taste, pressure, pain, heat and cold,

Temporal, which is responsible for processing auditory signals and Occipital, which is located at

rear of the head and is responsible for receiving and processing visual information. Sensory

cortex has areas involved in the analysis of sensory information. Motor cortex has areas that

organise muscle activity. Association cortex has areas associated with complex cognitive

functions such as language and thought and attributes such as personality. The Limbic system

containing the cingulated gyrus, hippocampus and amygdale surround the core of the forebrain.

The fornix is a bundle of axons that connects the hippocampus with other regions of the brain


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including the mammillary bodies. The Limbic system is involved in learning, memory, feelings,

expressing emotion and recognition.

Figure 7 - Basal Ganglia

The Basal Ganglia are a collection of subcortical nuclei in the forebrain which lie

beneath the anterior portion of the lateral ventricles (See Figure 7). It contains the global

pallidus, caudate nucleus, subthalamic nucleus, putamen and the substantia nigra. It is involved

in the control of muscle movement. The Diencephalon is situated between the Telencephalon

and Mesencephalon. It surrounds the third ventricle.


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Figure 8 - Thalamus and Hypothalamus

The thalamus makes up the dorsal part of the Diencephalon, near the middle of the cerebral

hemispheres, immediately medial and caudal to the basal ganglia (See Figure 8 ). It has two

lobes connected by a bridge of gray matter called the massa intermedia. Most neural input to the

cerebral cortex is received from the thalamus. It is divided into several nuclei that relay sensory

information to specific sensory projection areas of the cerebral cortex. E.g. the lateral geniculate

nucleus, the medial geniculate nucleus and the ventrolateral nucleus. The Hypothalamus lies at

the base of the brain under the thalamus. (See Figure 8) It controls the autonomic nervous system

and endocrine system. It is situated on both sides of the ventral portion of the third ventricle. The

thalamus functions to integrate sensory information. It receives information and relays this

information to the cerebral cortex. The cerebral cortex also sends information to the thalamus

which then transmits this information to other areas of the brain and spinal cord. The

hypothalamus controls body temperature, emotions, hunger, thirst and the circadian rhythm.


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The midbrain lies between the hindbrain and the forebrain (See Figure 9). It controls

vision, hearing, eye movement and body movement. It comprises of the Mesencephalon

consisting of structures such as the substantia nigra, the superior and inferior colliculli and the

red nucleus. The midbrain surrounds the cerebral aqueduct and consists of two major parts, the

tectum and the tegmentum. The tectum is located in the dorsal portion of the mesencephalon.

Figure 9 - Midbrain Structures

Its principle structures are the superior colliculli and the inferior colliculli. The brain stem

contains the diencephalons, midbrain and hindbrain. It functions to control visual and auditory

sensory information respectively. The Tegmentum consist s of the portion of the mesencephalon

beneath the tectum. It includes the rostral end of the reticular formation, periaqueductral gray

matter, red nucleus, substantia niagra and the ventral tegmental area.


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The hindbrain connects the spinal cord with the rest of the brain. The hindbrain

surrounds the fourth ventricle and consists of the metencephalon and the myelencephalon. The

metencephalon consists of the Pons and Cerebellum.

Figure 10 - Cerebellum and Pons

The cerebellum consists of two hemispheres, covered by cerebellar cortex and has a set of deep

cerebellar nuclei. (See Figure 10) The Cerebellum functions to control movement, balance and

posture. It is divided into two and has a cortex that surrounds these hemispheres. Each

hemisphere is connected to the dorsal portion of the pons by bundles of axons called superior,

middle and inferior cerebellar peduncles. The pons is a large bulge in the brain stem that lies

between the Mesencephalon and the medulla oblongata immediately ventral to the cerebellum

(See Figure 10). The Pons functions to control functions of internal organs. It consists mainly of

sensory and motor tracts travelling to and from the cortex. It also contains a section of the

reticular formation involved in producing cortical arousal during REM sleep. The

Myelencephalon contains the medulla oblongata. Its lower border is the rostral end of the spinal


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Figure 11 - Spinal Cord

The spinal cord is a long conical structure. (See Figure 11) The major function of the

spinal chord is to receive and process sensory information from the skin, joints and muscles of

the limbs and trunk and to control the activity of these muscles. It distributes motor fibres to the

effector organs of the body and collects somatosensory information and passes it on to the brain.

It is protected by the vertebral column which is composed of twenty-four individual vertebrae of

cervical, thoracic, lumbar and sacral and cocygeal portions. The spinal cord passes through

spinal foramens in each vertebra.


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Figure 12 - Spinal Cord Detailed Structures

The spinal cord is rapped by meninges and small bundles of fibres emerge from each side of the

spinal cord in two straight lines along its dorsolateral and ventrolateral surfaces. Groups of these

bundles fuse together and become thirty-one paired sets of dorsal and ventral roots. These join


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The Peripheral Nervous system

The brain and spinal cord communicate with the rest of the body via the cranial nerves

and spinal nerves. These nerves are part of the peripheral nervous system (PNS). The PNS is

subdivided into autonomic and somatic divisions.

Figure 13 - Autonomic Versus Somatic Nervous System

The autonomic nervous system (ANS) is concerned with regulation of smooth muscle,

cardiac muscle and glands. (See Figure 13) Essentially it is responsible for maintaining the

activity of systems that control normal body functions such as the cardiovascular system and the

digestive system (See Figure 14). The ANS is involved in life-sustaining internal processes. It is

concerned with behaviour that is beyond voluntary control. It makes sure that internal activity is

matched to the demands of external behaviour and the external environment. The ANS

modulates behaviour rather than controls it. The Autonomic system is further subdivided into


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Parasympathetic and Sympathetic systems. This distinction is based on the distribution of nerves

and ganglia.


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Figure 14 - Autonomic Nervous System


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The sympathetic division is most involved in activities associated with the expenditure

of energy from reserves that are stored in the body. Sympathetic motor neurons are located in the

gray matter of the thoracic and lumbar regions of the spinal cord. Fibres of these neurons exit via

the ventral roots. After joining the spinal nerves the fibres branch off and pass into the

sympathetic ganglia, these are interconnected leading to sympathetic ganglion chain. Axons that

leave the spinal cord through the ventral root belong to the preganglionic neurons. Most form

synapses with neurons located in one of the ganglia. These are postganglionic neurons.

The parasympathetic division of the autonomic nervous system supports activities that

are involved with increases in the bodys supply of stored energy. It comprises all neural

material located outside the brain and spinal cord, which are all neurons that are not part of the

CNS. The sympathetic and parasympathetic systems have opposing actions. The sympathetic

system promotes the fight or flight response whereas the parasympathetic system maintains the

resting state of the internal organs. The Postganglionic neurons of the sympathetic system release

noradrenaline. The postganglionic neurons of the parasympathetic system release acetylcholine.

Both systems release acetylcholine at the preganglionic level.

The somatic nervous system is made up of sensory neurons that innervate the skin,

skeletal muscles and joints and the axons the project onto effector cells (See Figure 13). It

mediates our interactions with the external world. The motor neurons in the somatic nervous

system control two types of behaviour. Voluntary behaviour is any motion you make

intentionally. Inputs from higher centres of the brain produce voluntary behaviour. Reflex

behaviour occurs without conscious direction. Inputs directly from the sensory neurons produce

reflex behaviour.


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How the Nervous System Interacts

The nervous system has been described in terms of separate systems. They however are

closely interlinked and work as one complete system. The PNS functions as an input-output

system acting as an intermediary between the CNS and the environment outside the body and

connects the CNS to the organs and glands inside the body. It transmits information, in the form

of nerve impulses, from the environment to the CNS and then conveys information from the

CNS to the muscle, organs and glands. Afferent, incoming information is processed in the CNS

and efferent, outgoing signals are sent out from the CNS to inhibit or excite a relevant sensory or

muscle function. The spinal cord contains sensory and motor pathways that connect the brain and

the PNS. The dorsal root ganglion cells are pseudounipolar cells that convey sensory information

to the spinal chord about: Skin touch and pressure. Temperature, Noxious stimuli, tissue damage,

joint position and movement, tension within the muscle (proprioception). Each motor neuron

contacts a small number of muscle fibres within an individual muscle. The motor neuron and the

fibres it contacts form what is collectively known as a motor unit (See Figure 15).

The nervous system, taken as a whole, has several important features. It makes

use of a simple, in terms of theory, in-integrate-out system. Essentially the brain functions by its

connection to what appears to be pretty much everything within the body. These multiple

connections are summed and affect an outcome. This outcome works through excitation and

inhibition. Sensory and motor divisions exist throughout the entire nervous system and these

functions can be both localised and distributed. The systems are organised to run both in a

hierarchical and parallel fashion with the CNS functioning on multiple levels, some functions

working to varying degrees at several parts of the nervous system. This parallel processing is

aided by the concept of symmetry. The brain is both symmetrical and asymmetrical with many of

the brains circuits crossed, vision being a clear example of such a crossing.


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Figure 15 - Brain, Spinal Cord and Link to Peripheral Nervous System


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Bibliography

Carlson N.R., (2004). Physiology of Behaviour. (8th

Ed). Pearson Education.

Despopoulos, A., (2003). Color Atlas of Physiology (5th Ed).Georg Thieme Verlag.

Feneis, H., Dauber, W., (2000). Pocket Atlas of Human Anatomy Based on the International

Nomenclature.Thieme Medical Publishers.

Fox, S.I., (2006).Human Physiology (8th Ed).McGraw-Hill Science/Engineering/Math.

Greenstein, B., Greenstein, A., (2000). Color Atlas of Neuroscience Neuroanatomy and

Neurophysiology. Georg Thieme Verlag.

Haines, D.E., (2007). Neuroanatomy- An Atlas of Structures, Sections, and Systems. Lippincott

Willams & Wilkins.

Kolb, B., & Whishaw, I.Q., (2006). An Introduction to Brain & Behaviour (2nd

Ed). New York:

Worth Publishers.

Netter, F.H., (2005). Atlas of Neuroanatomy and Neurophysiology.CRC.

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