NEURAL CONTROL AND CO-ORDINATION
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 (nerve cell)-
Neuron is the structural and functional
unit of neural system. It is composed of
Cell
body (cyton): Contains cytoplasm,
cell organelles and Nissl’s granules (granular bodies).
Dendron: Short fibres projecting from
the cyton. Their sub branches (dendrites) 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
1.Unipolar: One axon. No Dendron. Found
in embryo.
2.Bipolar:
One axon and one dendron. Found in
the retina.
3.Multipolar: One axon and 2 or more dendrons. Most common type.
Found in the cerebral cortex.
Types
of axon
a) Myelinated
axon: It is enveloped with Schwann cells that form a myelin
sheath around the axon. Found in spinal & cranial nerves.
The white coloured area, formed of myelinated nerve fibres is called white
matter. Gaps b/w 2 adjacent myelin sheaths are called nodes of Ranvier.
b) 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 membrane is more permeable to K+ ions and
nearly impermeable to Na+ ions. Also, the membrane is impermeable to negatively
charged proteins in axoplasm.
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 concentration of Na+. This forms an
ionic or concentration gradient across resting 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 permeable 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.
The electrical potential
difference during depolarization across the plasma membrane is called action
potential (a nerve impulse).
3.
Propagation of action potential
1.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.
2.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.
3.
The sequence is repeated along the axon and the impulse is conducted.
4 .The
rise in permeability to Na+ is extremely short lived. It is quickly followed by
a rise in permeability to K+.
5. Immediately, K+ 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.
2.
Chemical synapses
In
this, there is a fluid filled space (synaptic cleft) between the
presynaptic neuron and postsynaptic neuron.
The presynaptic regions have swellings called Synaptic knob (buttons). They contain synaptic vesicles filled with neurotransmitters (acetylcholine or adrenaline).
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.
HUMAN NERVOUS (NEURAL)
SYSTEM
It has 2 parts:
1.Central neural system (CNS): Brain & spinal cord.
2.Peripheral
nervous system (PNS): All nerves.
CENTRAL NEURAL SYSTEM
(CNS)
A. BRAIN -
It
is protected in cranial cavity.
It has 3-layered connective tissue membranes called cranial
meninges.
Meninges consist of outer dura mater, middle arachnoid
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.
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
a. Forebrain (Prosencephalon)
It is the anterior
part. Consists of cerebrum & diencephalon.
Cerebrum
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. Responsible
for intersensory associations, memory and communication.
Integrated activities of different
centres of cerebral cortex control intelligence, memory, judgment, learning,
thinking and articulate speech.
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:
a. Regulates temperature,
thirst, hunger and emotions.
b. Secretes hypothalamic hormones.
c. Controls pituitary gland.
d. Controls sleep, wakefulness, blood pressure, heart rate.
b. Midbrain (Mesencephalon)
It is located between thalamus/hypothalamus and Pons.
A canal (cerebral aqueduct) passes through the mid
brain.
Mid brain consists of 4 round lobes called Corpora
quadrigemina. Their anterior pair is the centre of visual reflexes and
the posterior pair is a centre of auditory reflex.
Cerebellum (“little cerebrum”): It
has very convoluted surface to accommodate more neurons. It co-ordinates
muscular activities and body equilibrium.
Pons varoli: It
consists of fibre tracts that interconnect different regions of the brain. It
co-ordinates 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.
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 co-ordinates muscular activities and body
equilibrium.
Pons varoli: It
consists of fibre tracts that interconnect different regions of the brain. It
co-ordinates 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.
Functions:
a. Conduction of impulses to and from the brain.
b. Centre of spinal reflexes.
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 &
parasympathetic nerves. Sympathetic system prepares body to cope with
emergencies, stresses & dangers. It increases heartbeat, breathing rate,
constricts 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.Withdrawal of the hand when it touches a hot object. Touching lips of a nursing baby evokes sucking reflex.