The peripheral nervous system (PNS) consists of
all the nerves branching out of the brain and spinal
cord. If you imagine the Central Nervous System
as the main highway, then the Peripheral Nervous System forms all the
connecting secondary roads. These allow electrical impulses to travel to and
from the furthest regions, or periphery, of the human body. The PNS is
built almost entirely from nerves. There are two main types; spinal nerves and cranial nerves. Functionally,
the PNS can be divided into the autonomic and somatic nervous systems. Both of these can be further subdivided;
the former into sympathetic and parasympathetic arms and the latter into sensory and motor divisions.
Cranial nerve tissue: The nerve tissues starting from brain and
again ending to brain are called cranial nerve tissues. There are 12 pairs of
cranial nerves emerging from brain which collect information from the organs of
head like eye, ear, tongue, nose etc.
Spinal nerve tissue: The nerve tissues
emerging from spinal cord and spread to different parts of body are called
spinal nerves. There are 31 pairs of spinal nerves emerging from spinal cord
and spread to different parts of body. They carry information and emotions from
different parts of body to brain and brain to different parts of body.
Autonomic nervous system: The autonomic nervous system is a control system that acts
largely unconsciously and regulates bodily functions, such as the heart rate,
digestion, respiratory rate, pupillary response, urination, and sexual arousal.
Autonomic nervous system is divided to two types
1. Sympathetic: this increases the rate of heartbeat, blood
pressure, breathing and function of stomach, intestine and urinary system.
2. Parasympathetic: It is opposite of sympathetic which
normalizes the events mentioned above in general conditions such as heart beat, blood
pressure, urination rate etc.
Reflex Action and Reflex Arc
When you
touch a hot object or when a pin pricks your finger, what is your
immediate reaction? Of course, you remove your hand away from the source
of pain, either the hot object or the pin. In situations like these, your
reactions are always immediate, involuntary and sudden. They happen without
much of a thinking process. In scientific terms, this action is called the
reflex action. Here the spinal cord has a major role to play. The reflex
arc shows the pathway through which the reflex action occurs.
The whole mechanism of reflex action occurs in such a
fashion that there is no conscious control of the brain. Stimulation occurs through the peripheral nervous system and the response to this peripheral
nerve stimulation is involuntary. In a reflex action, the spinal cord along
with the brain stem is responsible for the reflex movements.
A
few examples of reflex action are:
·
When light acts as
a stimulus, the pupil of the eye changes in size.
·
Sudden jerky withdrawal of
hand or leg when pricked by a pin.
·
Coughing or sneezing, because
of irritants in the nasal passages.
·
Knees jerk in response to a
blow or someone stamping the leg.
·
The sudden removal of the
hand from a sharp object.
·
Sudden blinking when an
insect comes very near to the eyes.
The
whole process of reflex action involves some important components. They are
receptor organs, sensory neurons, nerve center, associated neurons, motor
neurons and effector neurons.
The
receptor organs perceive the stimuli. They are situated on the sense organs.
The afferent neurons or the sensory neurons carry the stimuli from receptors to
the spinal cord. The ganglion of the spinal cord has the sensory neurons.
The
spinal cord is the nerve center, where synaptic connections are formed. The
associated neurons are present in the spinal cord. The ventral horn of spinal
cord has the motor neurons. Effector organs are the glands and muscles that
behave in response to the stimuli.
Neuron: The smallest unit of nervous system is called
neuron. It is also called nerve cell. Neurons
are cells within the nervous system that transmit information to other nerve
cells, muscle, or gland cells. Most neurons have a cell body, an axon, and
dendrites. The cell body contains the nucleus and cytoplasm. The axon extends
from the cell body and often gives rise to many smaller branches before ending
at nerve terminals. Dendrites extend from the neuron cell body and receive
messages from other neurons. Synapses are the contact points where one neuron
communicates with another. The dendrites are covered with synapses formed by
the ends of axons from other neurons.
The cell body contains the nucleus
and cytoplasm. The axon extends from the cell body and often gives rise to many
smaller branches before ending at nerve terminals.
Dendrites extend
from the neuron cell body and receive messages from other neurons. Synapses are
the contact points where one neuron communicates with another. The
dendrites are covered with synapses formed by the ends of axons from
other neurons.
When
neurons receive or send messages, they transmit electrical impulses along their
axons, which can range in length from a tiny fraction of an inch (or
centimeter) to three feet (about one meter) or more. Many axons are covered
with a layered myelin sheath, which accelerates the transmission of
electrical signals along the axon. This sheath is made by specialized cells
called glia. In the brain, the glia that make the sheath are called
oligodendrocytes, and in the peripheral nervous system, they are known as Schwann
cells. The
brain contains at least ten times more glia than neurons. Glia perform many
jobs. Researchers have known for a while that glia transport nutrients to
neurons, clean up brain debris, digest parts of dead neurons, and help hold
neurons in place. Current research is uncovering important new roles for
glia in brain function.
