The nervous system in man and in other higher animals is composed of two major components:
Central nervous system
Peripheral nervous system
Central nervous system comprises of coordinators i.e. brain and spinal cord while peripheral nervous system consists of nerves that arise from central nervous system and spread in different parts of body. All these components are made of neurons.
Nerve cell or Neuron
Nerve cell or neuron is the unit of the nervous system. The human
nervous system consists of billions of neurons plus supporting (neuroglial)
cells. Neurons are specialized cells that are able to conduct nerve impulses
from receptors to coordinators and from coordinators to effectors. In this way
they communicate with each other and with other types of body cells.
The nucleus and most of the cytoplasm of a neuron is located in its
cell body. Different processes extend out from cell body. These are called dendrites
and axons.
Dendrites conduct impulses toward cell body and axons conduct
impulses away from cell body.
Unlike ordinary cells, mature neurons never divide. But a protein called nerve-growth-factor promotes the regeneration of broken nerve cells. The degenerating brain cells could be repaired, by using embryonic stem cells.
Schwann cells are
special neuroglial cells located at regular intervals along axons. In some
neurons, Schwann cells secrete a fatty layer called myelin sheath, over axons.
Between the areas of myelin on an axon, there are non-myelinated points, called
the nodes of Ranvier. Myelin sheath is an insulator so the membrane coated with
this sheath does not conduct nerve impulse. In such impulses are called
saltatory (‘jumping’) impulses. This increases the speed of nerve impulse.
On the basis of their functions, neurons are of three types;
Sensory neurons conduct
sensory information (nerve impulse) from receptors towards the CNS. Sensory
neurons have one dendrite and one axon.
Interneurons form
brain and spinal cord. They receive information, interpret them and stimulate
motor neurons. They have many dendrites and axons.
Motor neurons carry
information from interneurons to muscle or glands (effectors). They have many
dendrites but only one axon.
Nerve
A nerve means the union of several axons that are enveloped by a
covering made of lipid. Based on the property of axons, the nerves are
classified into three types.
Sensory nerves
contain the axons of sensory neurons only.
Motor nerves contain
the axons of motor neurons only.
Mixed nerves contain
the axons of both i.e sensory and motor neurons.
Divisions of the Nervous System. Nervous System Consist of:
Central Nervous System
Peripheral Nervous System
Central nervous system. It consist of;
·
Brain
·
Spinal Cord
Brain
In animals, all life activities are under the control of brain. The
structure of brain is suitable to perform this function. Brain is situated
inside a bony cranium (part of skull).
Inside cranium, brain is covered by three layers called meninges.
Meninges protect brain and also provide nutrients and oxygen to brain tissue
through their capillaries.
The brain contains fluid-filled ventricles that are continuous with
the central canal of spinal cord. Fluid within ventricles and central canal is
called cerebrospinal fluid (CSF).
The Divisions of Brain
There are three major regions in the brain of human and other
vertebrates. These are forebrain, midbrain and hindbrain.
Forebrain is the
largest area of brain. It is most highly developed in humans. Important parts
of this region are:
Thalamus: Lies just below cerebrum. It serves as a
relay centre between various parts of brain and spinal cord. It also receives
and modifies sensory impulses (except from nose) before they travel to
cerebrum. Thalamus is also involved in pain perception and consciousness (sleep
and awakening).
Hypothalamus: Lies above midbrain and just below
thalamus. In humans, it is roughly the size of an almond. One of the most
important functions of hypothalamus is to link nervous system and endocrine
system. It controls the secretions of pituitary gland. It also controls
feelings such as rage, pain, pleasure and sorrow.
Cerebrum: Its the largest part of forebrain. It
controls skeletal muscles, thinking, intelligence and emotions. It is divided
into two cerebral hemispheres. The anterior parts of cerebral hemispheres are
called olfactory bulbs which receive impulses from olfactory nerves and create
the sensation of smell. The upper layer of cerebral hemispheres i.e. cerebral
cortex consists of grey matter. The grey matter of nervous system consists of
cell bodies and non-myelinated axons. Beneath this layer is present the white matter.
The white matter of nervous system consists of myelinated axons. Cerebral
cortex has a large surface area and is folded in order to fit in skull. It is
divided into four lobes.
Midbrain lies
between hindbrain and forebrain and connects the two. It receives sensory
information and sends it to the appropriate part of forebrain. Midbrain also
controls some auditory reflexes and posture.
Hindbrain consists
of three major parts.
Medulla oblongata lies on the top of spinal cord. It controls
breathing, heart rate and blood pressure. It also controls many reflexes such
as vomiting, coughing, sneezing etc. Information that passes between spinal
cord and the rest of brain pass through medulla.
Cerebellum is behind medulla. It coordinates muscle
movements.
Pons is present on top of medulla. It assists
medulla in controlling breathing. It also serves as a connection between
cerebellum and spinal cord.
Spinal Cord
The spinal cord is in fact a tubular bundle of nerves. It starts
from brain stem and extends to lower back. Like brain, spinal cord is also
covered by meninges. The vertebral column surrounds and protects spinal cord.
The outer region of spinal cord is made of white matter (containing myelinated
axons). The central region is butterfly shaped that surrounds the central
canal. It is made of grey matter (containing neuron cell bodies). 31 pairs of
spinal nerves arise along spinal cord. These are “mixed” nerves because each
contains axons of both sensory and motor neurons.
At the point where a spinal nerve arises from spinal cord, there
are two roots of spinal nerve. Both roots unite and form one mixed spinal nerve.
The dorsal root contains sensory axons
and a ganglion where cell bodies are located.
The ventral root contains axons of motor
neurons.
Spinal cord performs two main functions:
It serves as a link between body parts
and brain. Spinal cord transmits nerve impulses from body parts to brain and
from brain to body parts.
Spinal cord also acts as a coordinator,
responsible for some simple reflexes.
Peripheral Nervous System
The peripheral nervous system (PNS) is composed of nerves and
ganglia. Ganglia are the clusters of neuron cell bodies outside CNS. Nerves
arise or lead to brain and spinal cord. So they are named as cranial and spinal
nerves. Spinal cord is roughly 40cm long and about as wide as our thumb for
most of its length.
Humans have 12 pairs of cranial nerves and 31 pairs of spinal
nerves. Some cranial nerves are sensory, some are motor and some are mixed. On
the other hand, all spinal nerves are mixed nerves. The cranial and spinal
nerves make two pathways i.e. sensory pathway (conducting impulses from
receptors to CNS) and motor pathway (conducting impulses from CNS to
effectors). Motor pathway makes two systems.
Somatic Nervous System
It is responsible for the conscious and voluntary actions. It
includes all of the motor neurons that conduct impulses from CNS to skeletal
muscles.
Autonomic Nervous System
It is responsible for the activities, which are not under conscious
control. It consists of motor neurons that send impulses to cardiac muscles,
smooth muscle and glands. Autonomic nervous system comprises of sympathetic
system and parasympathetic system. Sympathetic nervous system prepares body to
deal with emergency situations. This is often called the “fight or flight”
response. During an emergency situation, this system takes necessary actions.
For example; it dilates pupils, accelerates heartbeat, increases breathing rate
and inhibits digestion. When stress ends, the parasympathetic nervous system takes
action and normalizes all the functions. It causes pupils to contract, promotes
digestion, and slows the rate of heartbeat and breathing rate.
Reflex Action
When central nervous system sends impulses to muscles and glands,
two types of actions (responses) result.
The higher centers of brain control the
conscious action or voluntary actions.
When impulses are not passed to the
higher centers of brain, it results in responses which are not under conscious
control. Such responses are called involuntary actions. Sometimes, the
involuntary response produced by the CNS is very quick. Such a response is
called reflex action. The pathway followed by the nerve impulses for producing
a reflex action, is called reflex arc.
The most common example of reflex action is the withdrawal of hand
after touching a hot object. In this reflex action, spinal cord acts as
coordinator. Heat stimulates temperature and pain receptors in skin. A nerve
impulse is generated which is carried by sensory neurons to the interneurons of
spinal cord.
From interneurons, the impulse is passed to motor neurons, which
carry it to the muscles of arm. As a result, the muscles contract to withdraw
hand. During it, other interneurons transmit nerve impulses up to brain so that
the person becomes aware of pain and what happened.