Nervous System and Sensory Organs

This section is divided into three parts covering the nervous system, the
eyes and the ears.

 

 General description

 The nervous system performs three related functions:

1. Senses certain changes within the body and in the outside

 environment;

2. Interprets the changes;

3. Responds to the interpretation by initiating action in the form of

 muscular contractions or glandular secretions. For descriptive purposes,

 the parts of the nervous system are grouped as follows:

 

 For descriptive purposes, the parts of the nervous system are grouped

 as follows:

1. The central nervous system, consisting of the brain and spinal cord.

2. The peripheral nervous system, consisting of:

 31 pairs of spinal nerves.

 12 pairs of cranial nerves

 The autonomic part of the nervous system.

 

Neurones

 The nervous system comprises vast numbers of units called neurones.

 Each neurone consists of a nerve cell and its processes, axons and dendrites.

 Neurones are commonly referred to as nerves. Some initiate nerve

 impulses, others act as “relay stations” where impulses are passed on or

 re-directed.

 

Nerve cells

 Nerve cells vary considerably in size and shape but are all too small to be

 seen with the naked eye. They form the grey matter of the nervous system

 and are found at the periphery of the brain, in the centre of the spinal cord,

 in ganglia, and as single cells in organ walls.

 

Axons and dendrites

Axons and dendrites are extensions of the nerve cells which form the white matter of the nervous system.They are referred to as nerve fibres outside the brain and spinal cord.

 

Central Nervous System

The central nervous system consists of the brain and spinal cord.

 

Brain

Situated in the cranial cavity, the brain is the main area where nerves come together. It consists of the cerebrum, the brain stem (mid brain, pons varolii and medulla oblongata) and the cerebellum or hindbrain.

 

Cerebrum

The cerebrum is the largest part of the brain. It comprises two hemispheres,consisting mainly of nerve cell bodies. Deep within the brain, the hemispheres are connected by a mass of nerve fibres (white matter)

 The main functions of the cerebrum are:

• Mental activities involving memory, intelligence, reasoning, and

 learning attributed to the higher centres;

• Sensory perceptions, including those of pain, temperature, touch,

 sight, hearing, taste and smell;

• Initiation and control of voluntary muscle contraction.

 

Lobes

For descriptive purposes each hemisphere of the cerebrum is divided into lobes. There are four areas, separated by furrows, or fissures, called sulci. They are the frontal, occipital, parietal and temporal lobes.

Hypothalamus

 The hypothalamus is a group of nerve cells situated in the cerebrum,

 immediately above the pituitary gland. It is linked to the pituitary gland

 by nerve fibres and blood vessels. The main function of the hypothalamus

 is to control the release of pituitary hormones.

 

Cerebellum

 Situated immediately below the posterior portion of the cerebrum, is the

 cerebellum. It is ovoid in shape, and has two hemispheres. Associated with

 co-ordination of voluntary muscular movements, posture and balance,

 allows people to carry out very precise actions.

 

Spinal Cord

The spinal cord extends down the vertebral canal as far as the first lumbar vertebra. It is surrounded by the meninges and cerebrospinal fluid. The spinal cord is the main link between the brain and other parts of the body.

 

Meninges

 The brain and spinal cord are completely surrounded by three meninges:

 the dura mater, the arachnoid mater and the pia mater. The outer layer,

 the dura mater, lies next to the skull. The arachnoid mater is in the middle,

 and the pia mater on the inside, closely covering the brain.

 The space between the dura mater and the arachnoid mater is called the

 “subdural” space. The space between the arachnoid mater and the pia mater

 is the “subarachnoid space” Cerebrospinal fluid circulates through the

 subarachnoid space.

 

Cerebrospinal fluid (CSF)

 The brain and spinal cord are surrounded by a watery fluid called the

 cerebrospinal fluid which cushions and protects these vital organs. It is

 secreted continuously into each ventricle of the brain, but the amount around

 the spinal cord remains constant, as absorption keeps pace with secretion.

 

Ventricles

Within the brain there are four irregular shaped cavities, containing cerebro- spinal fluid.They are: right and left lateral ventricles, and the third and fourth ventricles.

 

Peripheral Nervous System

 The peripheral nervous system comprises:

 31 pairs of spinal nerves

 12 pairs of cranial nerves

 The autonomic part of the nervous system.

 

Spinal nerves

 There are 31 pairs of spinal nerves that leave the vertebral column and are

 named according to the associated vertebrae i.e. 8 cervical, 12 thoracic,

 5 lumbar, 5 sacral, 1 coccygeal. The spinal nerves arise from both sides of

 the spinal cord and emerge through the intervertebral foramina. Each nerve

 is formed by the union of a motor and sensory nerve root and is,

 therefore, a mixed nerve.

 

Cranial nerves

There are 12 pairs of cranial nerves originating from the brain. A motor nerve
conducts away from a centre and causes contraction of muscle or secretion of
glands. A sensory nerve perceives sensation such as pain, touch, or pressure,
and conducts them toward a centre.

The names and numbers are:  

1. Olfactory (sensory)
2. Optic (sensory)
3. Oculomotor (motor)
4. Trochlear (motor)
5. Trigeminal (mixed)
6. Abducent (motor)
7. Facial (mixed)
8. Vestibulocochlear (auditory) (sensory)
9. Glossopharyngeal (mixed)
10. Vagus (mixed)
11. Accessory (motor)
12. Hypoglossal (motor)

 

Autonomic nervous system

 The autonomic, or involuntary part of the nervous system, controls the

 automatic functions of the body. Although stimulation does not occur

 voluntarily, the individual may be conscious of its effect e.g. an increase

 in the heart rate.

 The autonomic nervous system is divided into two parts, sympathetic,

 and parasympathetic. Both systems are distributed to organs of the

 body, where they regulate function by a kind of balanced opposition.

 

Sympathetic

 The sympathetic portion of the autonomic system leaves the central

 nervous system at the thoracolumbar region. Its effect is similar to those

 of hormones secreted by the adrenal glands, preparing the body for

 “fight and flight” in exciting or stressful situations. It stimulates heart

 action, constricts blood vessels in the head, dilates pupils, dilates skeletal

 blood vessels, and increases the secretion of adrenaline and

 noradrenaline. Functions in the gastrointestinal and urinary tracts are

 decreased.

 

Parasympathetic

 The parasympathetic portion of the autonomic system leaves the central

 nervous system at the cranial and sacral regions of the spine. It tends to

 slow down body processes, except digestion and absorption of food and

 the functions of the genitourinary systems. It has a general calming

 effect — the opposite of the sympathetic nervous system.

 

Nervous System

 

Glossary of Terms Nervous System

Autonomic nervous system

 (aw-to-nom’ik) Involuntary nervous system not subject to control by human

 will. Parasympathetic: Portion of the autonomic system which leaves the

 central nervous system in the cranial and sacral regions of the spine. The

 parasympathetic nervous system has a general calming effect on the body.

 Sympathetic: Portion of the autonomic system which leaves the central

 nervous system at the thoracolumbar region. The sympathetic nervous

 system prepares the body for stressful situations.

 

Brain

 Extremely complex mass of nervous tissue organised in five distinct

 sections: 1) cerebrum (ser’e-brum) 2) midbrain, 3) pons varolii (ponz

 vah-ro’- le-i), 4) medulla oblongata (me-dul’lah ob-long-ga’ta), and

 5) cerebellum (ser-e- bel’um).

 

Central nervous system

Brain and spinal cord.

 

Cerebrospinal fluid

(ser-e-bro-spi’nal) Watery fluid which surrounds the brain and spinal
cord to cushion and protect these vital organs.

 

Cranium

(kra’ne-um) Skull bones which protect the brain.

 

Ganglion

A group of nerve cells located outside the central nervous system.

 

Meninges

(me-nin’jes) Three membranes which cover and protect the brain and spinal cord. The innermost membrane is pia mater (pi-ah ma’ter); the middle membrane is arachnoid mater (ah-rak’noid ma’ter); the outermost membrane is dura mater (du’rah ma’ter).

 

Nerve

Bundle of nerve fibres which convey impulses between a part of the
central nervous system and another part of the body.

 

Nerve impulse

The electrochemical force conveyed along nerve fibres.

 

Nerve plexus

(plek’sus) Network of nerves.

 

Peripheral nerve

(pe-rif’er-al) An outlying nerve, distal from its origin at the spinal cord. Motor

 nerve: A nerve which conducts away from a centre and causes contraction

 of muscle or secretion of glands; an efferent nerve. Sensory nerve: A nerve

 which perceives sensations such as pain, touch, and pressure, and conducts

 them toward a centre; an afferent nerve.

 

Sacrum

(sa’krum) Broad, flat, triangular-shaped plate of bone formed by five fused vertebrae.The sacrum is located between the pelvic bones with the lumbar vertebrae above and the coccyx below.

Spinal cord

The major pathway for transmission of nerve impulses between the brain and
all parts of the body. The spinal cord extends about 45 cm from the base of
the brain down the spinal canal.

 

Thoracolumbar region

(tho’rah-ko lum’bar) Thoracic and lumbar regions of the spine.

 

Vertebral column

(ver’te-bral) Series of bones or vertebrae which make up the bony spine.

 

The Eyes

 The eyes are sensory organs which function to give sight. Situated in the

 orbital cavity for protection, they are coordinated to function as a pair,

 giving three dimensional vision. Structures which are part of the normal

 eye function are the globe, (eyeball) and its contents, the bony orbit

 (socket), muscles and tendons, conjunctiva, eyelids, tear ducts and glands,

 glands, the optic nerve and that portion of the occipital lobe of the brain

 which is concerned with vision.

 

Sclera

 The sclera, or white of the eye is a firm, fibrous membrane forming the

 outermost layer of tissue of the posterior and lateral aspects of the eyeball

 and is continuous with the transparent cornea. It maintains the shape of

 the eye and gives attachment to muscles of the eye.

 

Cornea

The cornea is a clear, transparent epithelial membrane, shaped to bend and focus
the light rays which pass through it onto the retina.

 

Choroid

Lining the majority of the inner surface of the sclera, is the choroid. It is a chocolate
brown in colour and rich in blood vessels. The choroid absorbs light after it has
passed through the retina.

 

Retina

 The innermost layer of the wall of the eye is the retina where sensory

 receptors (rods and cones), highly sensitive to light, are located. The

 retina covers about three quarters of the eyeball, thickest at the back,

 and thinning out to end just behind the ciliary body. The small area of

 the retina where the optic nerve leaves the eye is the optic disc or

 blind spot. It has no light sensitive cells.

 

Optic nerve

Signals picked up by the retina are transmitted by the optic nerve to the brain where they are interpreted. Fibres originate in the retina and converge to form the optic nerve.

 

Ciliary body and muscle

 Ciliary body and muscle are an extension of the choroid, consisting of

 ciliary muscle and secretory cells that secrete aqueous fluid into the

 posterior chamber of the anterior segment of the eye.

 Contraction and relaxation of the ciliary muscles changes the thickness

 of the lens to focus light rays on the retina. The ciliary body also gives

 attachment to the suspensory ligament.

 

Iris

 Extending from the ciliary body and lying behind the cornea in front of the

 lens is the iris. The iris divides the anterior segment of the eye into anterior

 and posterior chambers. It is a circular body composed of pigment cells

 (which determine eye colour), and two layers of muscle fibres, one circular,

 the other radiating that cause it to contract or expand depending on the

 light. In bright light the circular muscles contract which constricts the

 pupil to restrict the amount of light entering. In dim light the radiating

 muscles contract which dilates the pupil to let in more light.

 

Lens

The lens is a biconvex elastic body, suspended from the ciliary body behind the iris,by the suspensory ligament. The lens changes shape, under control of the ciliary muscles, to focus light on the retina.

 

Anterior segment of the eyeball

 The space between the cornea and lens is completely divided into

 anterior and posterior chambers, by the iris. The chambers contain a

 clear aqueous fluid (humour), that flows from the posterior to anterior

 chamber via the pupil. The fluid returns to the venous circulation

 through the canal of Schlemm in the angle between the iris and the

 cornea. There is continuous production and drainage of aqueous fluid.

 

Vitreous body

Behind the lens and filling the cavity of the eyeball is the vitreous body (humour).It is a soft, colourless, transparent jelly-like substance. The eye keeps its shape because of the pressure exerted by the vitreous body and aqueous fluid.

Eyelids

 Two moveable folds of tissue situated above and below the front of

 each eye form the eyelids. They move together when blinking. On the

 free edges there are short curved hairs, eyelashes. Along the edges of

 the lids there are numerous sebaceous glands. Eyelids and eyelashes

 protect the eye from injury. The eylids join at the inner and outer

 margins of the eye at the canthi (canthus singular).

 

Conjunctiva

The conjunctiva is a fine transparent protective membrane, lining the eyelids, which folds on to the front of the eyeball. The anterior corneal epithelium is continuous with that of the conjunctiva. When the eyelids are closed the conjunctiva becomes a closed sac.

 

Eyebrows

The eyebrows are two arched ridges of hairs projecting obliquely from the surface
of the skin. They protect the anterior aspect of the eyeball from sweat and dust.

 

Lacrimal system

The lacrimal system comprises those parts of the eye that circulate tears over the outside of the eyeballs.

 

Lacrimal gland and ducts

 The lacrimal glands, each the size of an almond, are situated in the recesses

 in the frontal bones on the lateral aspect of each eye. The glands secrete

 tears composed of water, salts and lysozyme (a bacterial enzyme).The inferior

 and superior canaliculi drain tears from the eye into the lacrimal sac.

 

Lacrimal sac

The small reservoir at the top of the nasolacrimal duct into which tears
drain is called the lacrimal sac.

 

Three Views of the Eye

 

Glossary of Terms—The Eyes

Anterior chamber

Frontal space in the eyeball containing aqueous humour and bounded by cornea,
part of the sclera, iris, part of the ciliary body and that part of the lens which
presents through the pupil.

 

Aqueous humour

(a’kwe-us hu’mor) Watery, transparent fluid produced in the eye and found in the anterior and posterior chambers of the eye; helps maintain conical shape of the front of the globe and assists in focusing light rays on the retina.

Orbit

Bony cavity in the cranium which contains the eyeball and associated
structures.

 

Conjunctiva

(kon-junk-ti’vah) Mucous membrane which lines the eyelids and covers
the exposed surface of the sclera.

 

Cornea

(kor’ne-ah) Transparent frontal layer of the eye.

 

Crystalline lens

(kris’tal-lin) Transparent, biconvex body of the eye located between the
posterior chamber and the vitreous body.

 

Extraocular

(eks-trah-ok’u-lar) Outside the eye.

 

Globe

Eyeball.

 

Intraocular

(in-trah-ok’u-lar) Inside the eye.

 

Iris

Coloured membrane of the eye which separates the anterior and posterior chambers; contracts and dilates to regulate entrance of light rays.

 

Lacrimal ducts and glands

(lak’ri-mal) System of ducts and glands which secretes and conducts tears.

 

Occipital lobe

(ok-sip’i-tal) Posterior lobe of the brain.

 

Optic nerve

(op’tik) Second cranial nerve concerned with sight.

 

Posterior chamber

The space containing aqueous humour between the iris and the lens
and suspensory ligaments.

 

Pupil

(vit’re-us) Transparent jelly-like substance which fills the vitreous body
in posterior of the eye.

 

Vitreous humour

(vit’re-us) Transparent jelly-like substance which fills the vitreous body
in posterior of the eye.

 

The Ears

 General description

 The ears are concerned with hearing and maintaining physical balance.

 The ear has three distinct parts: the external ear, the middle ear, and

 the inner ear. The external ear (pinna and external auditory canal) is

 divided from the middle ear by the tympanic membrane (eardrum). In

 the middle ear are three bones called the malleus, incus and stapes. At

 the oval window, the internal ear begins; consisting of the vestibule,

 the cochlea in which the sensory receptors of hearing are located, and

 the semi-circular canals which maintain balance.

 

External ear

The external ear, comprises the pinna (auricle), and the external auditory meatus,
concentrates sound waves and directs them along the external auditory canal. The
auricle, which includes the helix and the lobule, is the visible part of the ear.

 

External auditory canal

The tube leading from the external opening to the tympanic membrane is the external auditory canal. Numerous glands lining the canal secrete wax,which, with hairs, protect the tympanic membrane from foreign matter.

 

Tympanic membrane (ear drum)

The tympanic membrane completely separates the external auditory canal from the
middle ear. It is oval in shape and vibrates when hit by sound waves.

 

Middle ear (tympanic cavity)

A cavity located within the temporal bone filled with air from the eustachian tube
and containing ossicles (small bones).

 

Auditory ossicles

 Three very small bones that extend across the cavity from the tympanic

 membrane to the oval window. They form a series of movable joints

 which are held in position by fine ligaments. They transmit vibrations

 of the tympanic membrane to the inner ear through the oval window.

 They are: the malleus, the incus and the stapes, and are commonly

 referred to as the ossicular chain.

 

Oval window

A small hole in the bone separating the middle and inner ears which is partially
covered by the base of the stapes. It transmits sound waves to the inner ear.

 

Inner (internal) ear

Contains the organs of hearing and balance. The inner ear consists of:
the vestibule, the cochlea and three semi-circular canals.

 

Vestibule

The expanded portion nearest the middle ear. It contains the oval
window and the round windows.

 

Cochlea

Resembles a snail’s shell. It has a broad base where it is continuous with the
vestibule and a narrow apex. It spirals round a bony column. Contains
the organ of Corti. See following diagram

 

Semicircular canals

 Three tubes at right angles to each other and continuous with the

 vestibule. The semicircular canals have no connection with hearing

 although closely associated with the cochlea. They provide information

 about the position of the head in space, contributing to maintenance

 of equilibrium and balance..

 

Organ of Corti

Cells in the cochlea that are the true organs of hearing. Nerve fibres in the
organ of Corti transmit signals to the brain along the auditory nerve.

 

Auditory nerve

Transmits sound signals generated in the organ of Corti via the
auditory part of the eighth cranial nerve to the brain.

 

Eustachian tube

An air tube that extends from the middle ear to the nasal part of the
pharynx. It is also called pharyngotympanic tube or canal.

 

Interior View of the Ear

 

Glossary of Terms—The Ears

Cochlear nerve

(kok’le-ar) That part of the eighth cranial nerve which is concerned with hearing

 

Equilibrium

(e-kwi-lib’re-um) A state of balance.

 

Eustachian tube

(u-sta’shan) Conduit 3-4cm long from middle ear to nasopharynx

 

External ear

Pinna and external auditory canal

 

Hair cells

The final sensory receptors of sound.

 

Incus

(in’kus) One of the three auditory ossicles; also called anvil due to its shape.

 

Internal ear

Consists of vestibule, semicircular canals, and cochlea; contains end
organs of sound perception and equilibration.

 

Malleus

(mal’e-us) One of the three auditory ossicles; hammer-shaped.

 

Middle ear

Extends from eardrum to oval window; contains auditory ossicles.

 

Organ of Corti

The true organ of hearing consisting of hair cells and their nerve fibres.

 

Oval window

Division between middle and inner ear.

 

Pinna (auricle)

(pin’nah) Auricular appendage; portion of external ear visible at sides of the head.

 

Round window

Membrane upon which sound waves are expended after passing hair cells.

 

Semicircular canals

Three bony canals of the inner ear concerned with the maintenance of balance.

 

Stapes

(sta’pez) One of the three auditory ossicles; stirrup-shaped.

 

Tympanic membrane

(tim-pan’ik) Eardrum.

 

Vestibule

(ves’ti-bul) Central cavity of the inner ear.

 

Exercises on the Nervous System and Sensory Organs:
Eyes and Ears

 

A. Write in the table below the medical terms for the numbered items in the diagram.Then check your answers against the fully labelled diagram in this section

 

B. Write in the table below the medical terms for the numbered items in the diagram.Then check your answers against the fully labelled diagram in this section.

 

C. Read the definitions of terms below. Each describes a part or
function of the nervous system and sensory organs. You may refer to
the previous text to fill in the appropriate word or phrase.

 Definition

1. The two components of the central nervous system.

2. Twelve pairs of cranial nerves, 31 pairs of spinal  nerves and the auto- nomic system make up this part  of the nervous system.

3. The largest portion of the brain, divided into two  hemispheres and 4 lobes.

4. Group of cells in the cerebrum whose main function is to  control the release of pituitary hormones. It is connected  to the pituitary gland by nerve fibres and blood vessels.

5. Covers the brain and spinal cord and has three layers.

6. The space between the dura mater and arachnoid mater.

7. The space between the arachnoid mater and the pia mater.

8. The watery fluid which surrounds the brain and spinal cord,  providing a protective cushion.

9. Sympathetic and parasympathetic are components of  this part of the nervous system.

10. Part of the autonomic system which prepares the body for  “fight and flight”.

11. The firm, fibrous membrane forming the outermost  layer of the eye (white of the eye).

12. The innermost light sensitive layer of the eye where  receptors(rods and cones) are located.

13. Nerve which transmits signals picked up by the retina  to the brain where they are interpreted.

14. The segment of the interior of the eyeball which is  further divided into two chambers.

15. A transparent jelly-like substance which fills the  posterior segment of the eyeball.

16. A biconvex elastic body which alters its shape to  focus light on the retina.

17. Part of the external ear. A tube along which sound  waves are transmitted, and cells produce wax.

18. Three very small bones making up the ossicular  chain, located in the middle ear.

19. Located in the inner ear. Groups of cells in the  cochlea that are the true organs of hearing.

20. Closely associated with the cochlea, but has no  connection with hearing. These canals contribute to  maintenance of balance.

 

D. Use a medical dictionary to look up the meaning of these terms
which are commonly used in describing conditions related to the
nervous system and sensory organs.

 

 accommodation

 Alzheimer’s disease

 anaesthesia

 analgesia

 anencephalia

 aphasia

 astigmatism

 ataxia

 aura

 burr hole

 cataract

 cerebral palsy

 cerebrovascular accident

 coma

 concussion

 cones

 conjunctivitis

 convulsion

 dacryocystorhinostomy

 dyskinesia

 ectropion

 encephalitis

 Entropion

 epilepsy

 glaucoma

 glue ear

 hemiparesis

 hemiplegia

 Huntington’s chorea

 hydrocephalus

 hypermetropia

 ischaemia

 keratitis

 Ménière’s syndrome

 meningitis

 meningocele

 multiple sclerosis

 myasthenia gravis

 myelitis

 myopia

 neurologist

 nystagmus

 ophthalmologist

 otitis media

 otosclerosis

 papilloedema

 Parkinson’s disease

 poliomyelitis

 presbycusis

 presbyopia

 rods

 stapedectomy

 shingles

 spina bifida

 strabismus

 subdural haematoma

 syncope

 tic

 tinnitus

 tympanoplasty

 vertigo