Nervous system
The nervous system.
Neuron types.
Glial cells.
Spine.
Spinal cord.
Peripheral nervous system.
Somatic nervous system.
Autonomic nervous system.
Sympathetic nervous system.
Parasympathetic nervous system.
The nervous system.
The Brain is part of the nervous system.
It is the most important organ, in the nervous system.
The Brain controls, the nervous system.
The nervous system, plays a central role,
in regulating the functions of the human body.
For example, breathing, heart beat and digestion, are regulated by the nervous system.
The nervous system also coordinates, the activities,
what we know collectively, as human behaviour.
It is the nervous system, which coordinates activities like,
walking, dancing, and smiling.
Even a state of mind, originates in the brain.
An emotion, such as fear originates in the brain.
The state of mind, being alert, motivated, or sleepy comes from the brain.
The Brain depends on a network of nerves, to communicate with the body.
This network of nerves, acts as a communication network.
The nerves reach out, to all parts of the body.
The nerves transmit information to the brain,
and communicate information from the brain.
The nervous system comprises of:
The central nervous system.
and the peripheral nervous system.
The central nervous system is composed of:
The Brain.
and the spinal cord.
The central nervous system, coordinates and controls the nervous system.
The central nervous system, can be called as the CNS, in brief.
There are nerves going from the Brain, and the spinal cord,
to all the parts of the body.
There are nerves going to the cheeks, to the tongue, to the heart, to the hands,
to the stomach, and to the tips of the toes.
Just like blood vessels, nerves also reach out, to all parts of the body.
All these nerves, are collectively called as the peripheral nervous system.
The nerves in the peripheral nervous system, perform two basic functions.
They carry signals, from different parts of the body, to the brain.
They also communicate, command signals from the brain,
to different parts of the body.
The basic unit of the nervous system, is the neuron.
The nervous system, specially the brain,
is a set of highly interconnected neurons.
We can say that the neuron is the building block, of the nervous system.
Neuron types.
From a functional view point, there are three basic types of neurons.
Afferent neurons.
Efferent neurons.
Interneurons.
Afferent neurons.
Afferent neurons convey information, from the tissues and organs, of the body,
to the central nervous system.
Afferent neurons have receptors, that respond to various,
physical and chemical changes in the environment.
The afferent neurons propagate, these electrical signals,
from the receptors, into the spinal chord or brain.
The cell body of afferent neurons, is outside the central nervous system.
Afferent neurons do not have dendrites.
They have an axon.
The axon carries the signal, from the receptor,
to neurons in the central nervous system.
Efferent neurons.
Efferent neurons transmit electrical signals, from the central nervous system,
to the gland or muscle cells.
The cell bodies, and dendrites, of efferent neurons, are within the central nervous system.
The axons, of efferent neurons, extend out into the periphery.
Neuron clusters.
The cell bodies of neurons, having similar function,
are often clustered together.
Groups of neuron cell bodies, in the peripheral nervous system,
are called ganglia.
Groups of neurons in the CNS, are usually called nuclei.
Nerves.
The axons of the afferent neurons,
and the axons of the efferent neurons,
form the nerves of the peripheral nervous system.
The long extension from a single neuron, is called as a nerve fibre.
Nerves refer to a group of many nerve fibres, that are travelling together,
to the same general location in the peripheral nervous system.
Interneurons.
Interneurons connect the afferent and efferent neurons.
Interneurons lie entirely within the central nervous system.
Most of the neurons, about 99%, of all neurons are interneurons.
For every afferent neuron, entering the central nervous system,
there are about 200000 interneurons, in the CNS.
For the simplest reflex action, there might be no interneurons.
For a more complex stimuli, invoking memory or knowledge,
it might involve millions of interneurons.
Glial cells.
Glial cells perform other functions in the CNS.
One type of glial cells, give a coating to the axons.
This is analogous to the insulating plastic coating, of an electrical wire.
These glial cells are called as myelin.
There are other types of glial cells.
One type helps in sustaining the neuron metabolically.
For example, they remove neurotransmitters, from the extracellular fluid,
around the synapse.
Glial cells also perform, other useful functions, in the CNS.
Spine.
The spine is at the centre of the body plan.
We share the spine, with many ancient species of life.
Many types of fish have a spine.
So do many animals, like horses, cows, lions, deers, cats and dogs.
In all forms of life, apart from its structural role,
this spine has played a central role, in the nervous system.
In human beings, the spine houses the spinal cord.
Spinal cord.
The spinal cord, plays an important role in the nervous system.
The spinal cord, act like an information highway,
between the brain, and the peripheral nervous system.
Sensory signals are transmitted from all parts of the body,
to the brain, via the spinal cord.
Motor instructions from the brain, are transmitted to all parts of the body,
from the brain, via the spinal cord.
The spinal cord is part of the central nervous system.
All the nerves, conveying information to and from the brain, and the spinal cord,
constitute the peripheral nervous system.
The spinal cord, is a slender cylinder, with a diameter, comparable to a little finger.
It extends from the base of the spine, to almost the end of the spine.
It comprises of soft tissue.
The central butterfly shaped area of grey matter, is composed of interneurons.
It also contains the cell bodies of efferent neurons,
and the entering fibres of the afferent neurons.
It is called grey matter, because it lacks the whitish myelin coating.
The grey matter is surrounded by white matter.
The white matter mostly comprises of myelinated axons of interneurons.
The myelin coating gives the region, a white appearance.
The group of axons is the path way, that runs longitudinally through the cord.
Some of the axons are descending, conveying information from the brain.
Some of the axons are ascending, conveying information to the brain.
Nerves emanate from both the sides, of the spinal cord.
Different groups of nerves leave, from specific parts of the spinal cord.
They leave as nerve pairs from each side of the spinal cord.
Nerves are part of the peripheral nervous system.
Peripheral nervous system.
Nerve fibres, in the peripheral nervous system, transmit signals,
from the CNS to all other parts of the body.
The nerve fibres are grouped into bundles, called nerves.
The peripheral nervous system consists of 43 pairs of nerves.
12 of these pairs connect directly to the brain.
They are called cranial nerves.
31 of these pairs connect with the spinal cord.
Nerve fibres can be classified as afferent fibres,
and efferent fibres.
All the spinal nerves contain both afferent and efferent fibres.
Some cranial nerves contain only afferent fibres.
The peripheral nervous system,
has an afferent division,
and an efferent division.
Afferent division.
Afferent neurons have receptors at their peripheral endings.
They convey information from the peripheral endings, to the brain.
For example, one peripheral ending, could be at the tip of a little finger.
The long part of the axon, of the afferent neuron, is outside the CNS,
and is part of the peripheral nervous system.
Efferent division.
Efferent neurons carry signals from the CNS to muscles, or glands.
The efferent division of the peripheral nervous system,
has 2 main subsystems.
1. The somatic nervous system.
2. The autonomic nervous system.
Neurons of the somatic nervous system, innervate skeletal muscles.
Skeletal muscles, are attached to bones.
We use skeletal muscles, for movement.
Neurons of the autonomic nervous system, innervate smooth,
and cardiac muscles, glands, etc.
Somatic nervous system.
The somatic nervous system, comprises of all the nerve fibres,
going from the CNS, to the skeletal muscles cells.
The cell bodies of these neurons, are located in groups,
in the brain stem, or spinal cord.
Myelinated axons, leave the CNS, and go directly to the muscle cells.
We note, that there is only a single neuron,
between the CNS, and the effector organ.
The end of the axons, are called axon terminals.
Acetylcholine are the neurotransmitters, that are released from these neurons.
These neurotransmitters cause the contraction of skeletal muscles.
They are called motor neurons.
Excitation of motor neurons, leads to muscle contraction, and movement.
The somatic nervous system, involves conscious control.
If we wish to lift a cup, and bring it to our lips,
we use the somatic nervous system.
Autonomic nervous system.
The autonomic system, innervates all the tissues,
other than the skeletal muscles.
The heart, many glands and smooth muscles, are innervated,
by the autonomic nervous system.
Autonomic nervous system, usually work without conscious control, or awareness.
For this reason, they are sometimes referred to as the involuntary nervous system.
For example, we do not make a conscious effort to make our heart beat.
But, we do make a conscious effort, to lift our hand, using the somatic nervous system.
In this system, the group of axons, between the CNS,
and the effector cells consists of two neurons.
There is a synapse between these neurons.
This synapse is located outside the CNS.
The neurons outside the CNS form clusters.
A cluster of such nerve cells, is called ganglia.
The nerve fibres between the CNS, and the ganglia,
are called pre-ganglionic autonomic fibres.
The nerve fibres between the ganglia, and the effector cells,
are called post-ganglionic fibres.
The autonomic nervous system, is comprised of two sub systems.
1. The autonomic sympathetic nervous system.
2. The autonomic parasympathetic nervous system.
The nerves of the sympathetic nervous system,
leave from the thoracic and lumbar, regions of the spinal cord.
The parasympathetic nerve fibres,
leave from the brain and cervical regions, of the spinal cord.
Sympathetic nervous system.
Most of the sympathetic ganglia, lie close to the spinal cord.
There are two chains of ganglia, on either side of the cord.
These are called sympathetic trunks.
The sympathetic trunks extends, from the cervical levels,
high in the neck, down to the sacral level.
The ganglia can act as automatic relay stations.
The ganglia can also act as integrating centres.
The anatomical arrangement of the ganglia, in the sympathetic nervous system,
help to tie the entire system together, so it can act as a single unit.
The neurotransmitter between the pre-ganglionic,
and the post-ganglionic fibres, is acetylcholine.
The major transmitter between the post-ganglionic fibre,
and the effector cell, is norepinephrine.
Parasympathetic nervous system.
The parasympathetic ganglia, lie within the effector organ.
The parasympathetic system, is made of relatively independent components.
The major neurotransmitter between the pre-ganglionic,
and the post-ganglionic fibres is acetylcholine.
Acetylcholine is also the neurotransmitter between the post-ganglionic fibre,
and the effector cells.
Many of the drugs, that stimulate or inhibit various components,
of the autonomic nervous system affect receptors,
for acetylcholine, or norepinephrine.
Dual innervation.
The heart, many glands and smooth muscles, are innervated,
by both sympathetic and parasympathetic fibres.
This is called dual innervation.
Typically, when the sympathetic system has one kind of effect, in the target gland,
the parasympathetic system, has the opposite effect.
This is analogous, to having both an accelerator and a brake, in a car.
We have a choice of applying only the accelerator.
We can apply only the brakes.
For better results, we simultaneously release the accelerator,
and apply the brake.
A fine degree of control, can be achieved by appropriately applying,
a combination of both.
The sympathetic and parasympathetic systems, work in a similar fashion.
They work reciprocally.
For example, one system might increase the heart rate,
while the other system, will decrease the heart rate.
Adrenal medulla.
The ganglion is not a ganglion in the real sense.
This releases transmitters into the blood stream.
It is an endocrine gland, whose secretion,
is controlled by the sympathetic pre-ganglionic nerve fibres.
It releases a mixture of epinephrine and norepinephrine.
These substances in this context act more like hormones.
They are transmitted via the blood stream, to the effector cells,
which are sensitive to them.
In this module, we discussed the relationship between the brain,
and the network of nerves connecting it to the body.
In other modules, we will discuss, other parts and functions, of the nervous system.