CHAPTER

21

NEURAL CONTROL
AND COORDINATION
 - Neural (Nervous) system is a system that controls and coordinates the
body activities, conducts and integrates the information and responds to
stimuli.
- It includes brain, spinal
cord and nerves.
- It is made up of specialized
cells known as neurons.

Neuron is the structural and functional unit of neural system. It is com-

posed of Cell body (cyton): Contains cytoplasm, cell organelles and Nissl’s
granules (granular bodies).
Dendron: Short fibres projecting from the cyton. Their sub branches (den-
drites) transmit impulses towards the cyton.
Axon: A long fibre which transmit impulses away from the cell body. The
branching of axon is called axonite. Each axonite ends as a bulb-like
structure called synaptic knob.

Types of Neurons
Unipolar: One axon. No Dendron. Found in embryo.
Bipolar: One axon and one dendron. Found in the retina.
Multipolar: One axon and 2 or more dendrons. Most common type. Found
in the cerebral cortex.

Types of axon
Myelinated axon: It is enveloped with Schwann cells that form a myelin
sheath around the axon. Found in spinal & cranial nerves.The white co-
loured area, formed of myelinated nerve fibres is called white
matter. Gaps b/w 2 adjacent myelin sheaths are called nodes of Ranvier.

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 Non-myelinated axon: Schwann cells present but no myelin sheath. The
gray coloured area without myelin sheath is called gray matter. Found in
autonomous & somatic neural systems.

GENERATION & CONDUCTION

OF NERVE IMPULSES
Impulse transmission is electrochemical. It has 3 steps:
1. Maintenance of resting membrane potential
- Neural membrane contains various selectively permeable ion channels.
- In a resting neuron (neuron not conducting impulse), the axonal mem-
brane is more permeable to K+ ions and nearly impermeable to Na+ ions.
Also, the membrane is impermeable to negatively charged proteins in axo-
plasm.
- Therefore, concentration of K+ and –vely charged proteins in axoplasm is
high and concentration of Na+ is low.
- The fluid outside the axon contains low concentration of K+ and high con-
centration of Na+. This forms an ionic or concentration gradient across rest-
ing membrane.
- The ionic gradients are maintained by the active transport of ions by the
Na-K pump. It transports 3 Na+ outwards for 2 K+ into the cell. As a result,
the outer surface becomes positively charged and inner surface becomes
negatively charged (i.e, polarized).
- The electrical potential difference across the resting plasma membrane is
called as the resting potential.

2. Action Potential
- When a stimulus is applied, the membrane at the site A becomes per-
meable to Na+. This causes rapid influx of Na+ and reversal of the polarity
at that site (outer negative and inner positive). It is called depolarization.

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- The electrical potential difference during depolarisation across the plasma
membrane is called action potential (a nerve impulse).

3. PROPAGATION OF ACTION POTENTIAL

- At sites ahead (site B), outer surface is positive and inner surface is
negative. As a result, a current flows on the inner surface from site A to
site B.
- On the outer surface, current flows from site B to site A to complete
the circuit. Hence, the polarity is reversed and action potential is generated
at site B. i.e., action potential at site A arrives at site B.
- The sequence is repeated along the axon and the impulse is conducted.
- The rise in permeability to Na+ is extremely short lived. It is quickly fol-
lowed by a rise in permeability to K+.
- Immediately, diffuses outside the membrane and restores the resting
membrane. Thus the fibre becomes ready for further stimulation.

SYNAPTIC TRANSMISSION
OF IMPULSES
- Synapse is a functional junction between two neurons.
- It is 2 types: Electrical & Chemical.

1. ELECTRICAL SYNAPSES
- In this, the membranes of pre- and post-synaptic neurons are in close
proximity. So impulse transmission is similar to the transmission along
an axon.
- Impulse transmission is faster than in chemical synapse.
- Electrical synapses are very rare in human system.

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2. CHEMICAL SYNAPSES
- In this, there is a fluid filled space (synaptic cleft) between the presyn-
aptic neuron and postsynaptic neuron.
- The presynaptic regions have swellings called Synaptic knob (buttons).
They contain synaptic vesicles filled with neurotransmitters (acetylcho-
line or adrenaline).

SYNAPSE STRUCTURE

PRESYNAPTIC AXON

MICROTUBULE

LOTS OF MITOCHONDRIA

NEUROFILAMENT

SYNAPTIC VESICLES

VESICLE RELEASING NEUROTRANSMITTER

NEUROTRANSMITTER

DENDRITES

Impulse transmission through chemical synapse: Impulse reaches at

axon terminal --- synaptic vesicles bind on
plasma membrane ---- release of neurotransmitter ---- It
diffuses across synaptic cleft ---- combine with receptors
on the post synaptic membrane ---- opening of ion
channels allowing entry of ions ---- generates action
potential.
- This action potential may be excitatory or inhibitory.

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HUMAN NERVOUS (NEURAL)
SYSTEM
It has 2 parts:
Central neural system (CNS): Brain & spinal cord.
Peripheral nervous system (PNS): All nerves.

A. BRAIN
- It is protected in cranial cavity .
- It has 3-layered connective tissue membranes called cranial meninges .
- Meninges consist of outer dura mater , middlearachnoid mater and inner
pia mater .
- The subarachnoid space (space between pia mater and arachnoid mater)
is filled with cerebrospinal fluid (CSF).
The ventricles of brain are also filled with CSF.
- Brain has 3 divisions: Forebrain, Midbrain & Hindbrain

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A. FOREBRAIN (PROSENCEPHALON)
It is the anterior part. Consists of cerebrum & diencephalon. Cerebrum
- Largest part. It has 2 cerebral hemispheres held together by a tract of
nerve fibres (Corpus callosum).
- Outer part of cerebrum is called cerebral cortex. It has convulsions &
depressions and is formed of gray matter.
- Inner part of cerebrum is formed of white matter.
- Cerebral cortex consists of
Motor area: Controls voluntary movements of muscles.
Sensory (Somaesthetic) area: Controls the
functioning of sense organs.
Association area: It is neither clearly sensory nor motor in function. Re-
sponsible for intersensory associations, memory and communication.

DIENCEPHALON (THALAMUS & HYPOTHALAMUS)

Thalamus: It is the structure around which the cerebrum wraps. It is a

coordinating centre (relay station) for sensory and motor impulses.
Hypothalamus: It
a. Regulates temperature, thirst, hunger and emotions.
b. Secretes hypothalamic hormones.
c. Controls pituitary gland.
d. Controls sleep, wakefulness, blood pressure, heart rate.
- The inner parts of cerebral hemispheres and a group of associated deep
structures like am ygdala , hippocam pus, hypothalam us, etc. together
constitute Limbic system (Limbic lobe) . It regulates sexual behavior, mo-
tivations, emotions (excitement, pleasure, rage, fear etc).

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B. MIDBRAIN (MESENCEPHALON)

It is located between thalamus/hypothalamus and Pons .

A canal (cerebral aqueduct) passes through the midbrain.
Mid brain consists of 4 round lobes called Corporaquadrigem ina . Their
anterior pair is the centre of visual reflexes and the posterior pair is a
centre of auditory reflex.

C. HINDBRAIN (RHOMBENCEPHALON)

It consists of cerebellum, Pons & Medulla oblongata. Midbrain &

hindbrain form the Brain stem.
Cerebellum (“little cerebrum”): It has very convoluted surface to
accommodate more neurons. It coordinates muscular activities
and body equilibrium.
Pons varoli: It consists of fibre tracts that interconnect different
regions of the brain. It coordinates the activities of eye and ear
and regulates respiration.
Medulla oblongata: It is connected to spinal cord.
It controls respiration, cardiovascular reflexes,
gastric secretions, peristalsis etc. It also controls
salivation, vomiting, sneezing & coughing.

B. SPINAL CORD
- It is enclosed within the spinal canal of vertebral column.
- It is also protected by meninges.
- Spinal cord has a central canal containing CSF.
- Outer white matter and inner gray matter.
a. Conduction of impulses to and from the brain.
b. Centre of spinal reflexes.

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PERIPHERAL NEURAL
SYSTEM (PNS)
It includes cranial nerves and spinal nerves.
Nerve fibres of PNS are 2 types:
Afferent (sensory) fibres: Carry impulses from sense
organs to CNS.
Efferent (motor) fibres: Carry impulses from CNS to muscles and glands.
PNS has 2 divisions. They are
Somatic neural system: Relays impulses from the
CNS to skeletal muscles.
Autonomic neural system: Transmits impulses from
CNS
to involuntary organs & smooth muscles. It includes sympathetic & para-
sympathetic nerves. Sympathetic system prepares body to cope with emer-
gencies, stresses & dangers. It increases heartbeat, breathing rate, con-
stricts arteries and elevates BP. Parasympathetic system returns the body
to a resting state after stressful situations and slows down
heartbeat, dilates arteries, lowers BP etc. Visceral nervous system is the
part of PNS. It includes nerves, fibres, ganglia & plexus by which
impulses travel from CNS to the viscera and from viscera to CNS.

REFLEX ACTION
It is the rapid, involuntary and unconscious actions
of body in response to a stimulus. E.g.
- Withdrawal of the hand when it touches a hot object.
- Touching lips of a nursing baby evokes sucking reflex.

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- Closing of the eyelids when light falls on them.
- Knee jerk phenomenon.
- If a child sees or smells a food unknown to him, he does not salivate.
But if he sees or smells that food every time before tasting it, he sali-
vates (conditioned reflex).
The pathway of impulses in a reflex action is called Reflex arc. It con-
sists of
- A receptor organ: It receives the stimulus.
- Sensory (afferent) neuron: It transmits impulses from sense organ
to CNS.
- Interm ediate (connector) neuron : It connects sensory and motor
neurons.
- Motor (efferent/effector/excitor) neuron: It conducts impulse from the
CNS to effector organ.
- An effector organ (muscle/gland): It responds to impulse.

SENSE ORGANS
- These are the organs that detect the changes in the environment and
convey the information to the CNS.
- It includes eye, ear, nose, tongue & skin.

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- Two eyes are located in sockets of the skull called orbits.
- The adult human eyeball is nearly a spherical structure.
- Eyeball has three layers: Sclera, Choroid & Retina.

a. Sclera
- The external layer formed of a dense connective tissue.
- Anterior transparent portion of sclera is called cornea.

b. Choroid
- Bluish middle layer. Contains many blood vessels.
- Choroid is thin over posterior two-thirds of the eyeball, but it is
thick in the anterior part to form ciliary body.
- Ciliary body continues forward to form a pigmented and opaque
structure called the iris. It is the visible coloured portion of the eye.
- Iris has a central opening called pupil. The diameter of the pupil is
regulated by the muscle fibres of iris.
- The eyeball contains a transparent crystalline lens. It is held in place
by ligaments attached to the ciliary body.

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 c. Retina
- Inner layer. It contains 3 layers of cells – from inner to outer
– ganglion cells, bipolar cells & photoreceptor cells.
- Photoreceptor cells are 2 types: rods and cones. They contain photosen-
sitive proteins (photopigments).
- Photopigments are formed of opsin (a protein) and retinal (an aldehyde
of vitamin A).
Cone cells:
Function: Daylight (photopic) vision & colour vision. There are 3 types of
cones containing photopigments that respond to red, green and blue
lights. The sensations of different colours are produced by combinations
of these cones and their photopigments. When the cones are stimulated
equally, a sensation of white light is produced.

Rod cells:
Function: Twilight (scotopic) vision.
They contain a purplish-red protein called rhodopsin
(visual purple). It contains a derivative of Vitamin A.
- Slightly above the posterior pole of the eyeball, there is
a region where optic nerves leave the eye and retinal
blood vessels enter it. Here, photoreceptor cells are
absent. It is called blind spot.
- Lateral to the blind spot, there is a yellowish
pigmented spot called macula lutea with a central pit
(fovea).

- The fovea is a thinned-out portion of the retina where only the cones
are densely packed. It is the point of greatest visual acuity (resolution).

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- The space between the cornea and lens is called aqueous chamber. It
contains aqueous humor (thin watery fluid).
- The space between the lens and retina is called vitreous chamber. It con-
tains vitreous humor (a transparent gel).

MECHANISM OF VISION
Light reflected from an object ---- enters the eye through
cornea & lens ---- focus on retina ---- dissociation of
retinal
from opsin ---- changes in membrane permeability ----
generates potential differences (impulse) in photoreceptor
cells (rods & cones) ---- generates action potentials in
ganglion cells through bipolar cells ---- impulses are
transmitted by optic nerves to brain (visual cortex) ----
impulses are analyzed and the image is recognized based
on earlier memory and experience ---- vision.

EAR (STATOACOUSTIC ORGAN)

- It is the organ for hearing & balancing.
- It has 3 divisions: External ear, middle ear & inner ear.

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EXTERNAL EAR
- Consists of pinna (ear lobe) & auditory meatus (ear canal).
- At the opening of ear canal, hairs are seen.
- Ear canal contains ceruminous glands (modified sebaceous glands). They
secrete wax (cerumen).
- Wax and hairs prevent entry of foreign objects like insects.
- Ear canal ends in tympanic membrane (Tympanum or ear drum). It is a
semi-transparent membrane covered by a thin layer of skin on its outer
surface and by mucous membrane on the inside. Middle ear
- Consists of tympanic cavity and ear ossicles.
- Tympanic cavity is an air filled space that separates the external and
inner ear portions.
- An auditory tube (Eustachian canal) connects middle ear to the pharynx.
It maintains an equal pressure on either side of the eardrum.
- Ear ossicles include 3 small bones namely Malleus, Incus and stapes.
Malleus is attached to tympanum.
- Stapes is the smallest bone of the body. It is attached to membrane of
oval window (fenestra ovalis) of inner ear. Inner ear
- It consists of bony labyrinth & membranous labyrinth.
- Bony labyrinth is a cavity filled with perilymph.
- The membranous labyrinth consists of cochlea and Vestibular apparatus.

COCHLE A (ORGAN OF HEARING):

- It is a coiled structure having 3 canals - upper scala vestibula, middle

scala media and lower scala tympani.
- Scala vestibula and scala media are separated by Reissner’s membrane.
Scala media and scala tympani are separated by basilar membrane.

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 - S. vestibula & S. tympani are filled with perilymph and scala media is
filled with endolymph.
- Resting on the basilar membrane and projecting into scala media is
complex receptor organ called Organ of Corti. It consists of row of sen-
sory hair cells. The hairs (stereo cilia) of these cells project upwards
and lie in contact with tectorial membrane, which projects above them.

VESTIBULAR APPARATUS :
- It consists of 3 semicircular canals and otolith organ.
- 2 semicircular canals are vertical and one is horizontal. One end of
each canal has a bulging called ampulla. Inside it is a lump called
crista ampullaris. Long cilia of cells of crista are grouped together in a
bundle (cupula).
- Otolith organ consists of utricle and saccule.
- Utricle & Saccule have a projecting ridge called macula.
- Crista and Macula are specific receptors in vestibular apparatus.
They contain sensory hair cells. They are responsible equilibrium &
posture of body.

Mechanism of hearing
Pinna collects sound waves ---- waves pass through ear
---- canraelach the tympanic membrane ---- tympanic membrane
vibrates ---- vibrations transmit to ear ossicles & oval
window ---- perilymph in the vestibular canal vibrates ---- vibrations
reach the scala tympani and force the basilar
membrane to vibrate ---- hair endings of sensory hair cells
press against tectorial membrane ---- sensory hair cells are
excited ---- auditory nerve carries impulses to
auditory centre of the brain ---- hearing.

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NOSE
Organ of smell (olfaction).
- It contains mucus-coated receptors (olfactory receptors) made up of
olfactory epithelium. They receive sense of smell. It contains 3 kinds of
cells.
The neurons of olfactory epithelium extend from the outside environ-
ment directly
NEURAL CONTROL AND COORDINATION
into apair of broad bean-sized organs, called olfactory bulb.
These are extensions of the brain’s limbic system.

TONGUE
- Organ of taste (gestation).
- 4 primary tastes are sweet, salt, sour and bitter.
- Taste buds (Gustatoreceptors + supporting cells) are seen around the
bases of taste papillae. Nose & tongue are chemoreceptors (detect dis-
solved chemicals). Senses of taste & smell are functionally similar and
interrelated. The brain integrates different input from taste buds and a
complex flavour is perceived.

SKIN (Cutaneous receptors)

- Largest sense organ.
- It contains receptors for heat, cold, touch, pain &
pressure.

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