Nerves

Nerves

The structural units of the vertebrate nervous system are neurons. These are specialized cells of different shapes and sizes in which the properties of irritability and conductivity have reached their maximum expression.

These respond to stimuli like light, electricity, chemical and mechanical agents, and transmit the excitations aroused by these stimuli to other neurons and to other cells of the body. Messages are conducted by nerves in the form of electrical impulses.

Types of Nerves

There are three types of nerve fibres depending upon their function that in which direction they transmit the nerve impulse.

Sensory Nerves :

These receive sensory impulses from the receptor neurons and lie next to them. These are also called afferent nerves (Latin :afferent means carry toward). There nerves contain neurons which carry message (impulse) from sensory areas toward the central nervous system.

Motor Nerves :

These are final nerve cells that carry out the appropriate action in transmitting impulses to the effectors. These are also called efferent nerves (Latin :efferent means carry away). These nerves contain neurons which take messages away from the central nervous system towards the effector organ, such as muscles and glands.

Mixed Nerves or Association neuron :

There are many nerves which are comprised of both afferent (sensory) and efferent (motor) nerve fibres. It interlink axon of sensory neuron and dendron of motor neuron. These are called mixed nerves. For example, most cranial and spinal nerves are called mixed nerves.

Structure of Neuron

A nerve cell is an elongated cell which can be divided into 3 parts:

Dendrite

It is a hair-like process which is hollow. It is connected to the cyton. The number of dendrites may be more than one. Dendrites may also be branched. Dendrites receive sensation or stimulus which may be physical, chemical, mechanical or electrical. It passes on the stimulus to the cyton.

Cyton

This part of the neuron has a central nucleus and surrounding cytoplasm. Around the nucleus there are granules called Nissl granules. Stimulus is changed in cyton to another form called impulse. From one side of cyton arises a cylindrical process filled with cytoplasm. This process is called axon.

Axon

It is the longest part of the neuron. It transmits the impulse from cyton to the tip of the axon called axon bulb. The axon is generally covered by a sheath of lipoprotein called myelin sheath. This sheath is formed by a type of cell called Schwann cell. At one point the axon is slightly depressed (a notch, an indentation). This is called the node of ranvier. When an impulse travels along the axon, an electromechanical change can be seen.

A Neuron (Nerve cell)

Receptor and Effector:

There are five sense organs in our body: eyes, ears, nose, tongue and skin. In a sense organ a receptor is present, which is a cell or group of cells sensitive to a particular type of stimulus (change in environment) such as light, heat, sound etc.

Types of receptors

The part of a body which can respond to stimulus according to the instruction sent from the nervous system is called effector. Effectors are mainly muscles and glands.

Functioning of the nervous system

• Excitability is an inherent properly of nervous tissue and excitation does not remain at the site of its origin but is transmitted along nerve fibres. This properly of nervous tissue to transmit excitation is called conductivity.
• All the nerve fibres carry messages in the form of nerve impulses.

• The nervous system receives a stimulus, through a receptor organ integrates or coordinates it, and effects a response through the effector organ. Thus, a coordinated behaviour has five main components – stimulus, receptor, coordinator, effector, and response.

• In such a coordinated behaviour, any stimulus of sound, sight, smell, etc. is perceived by receptor organs like eyes, ears, skin, etc.

• The brain and spinal cord are the coordinators which receives information in the form of messages called nerve impulses, from receptor organs via neurons. The information flows to the effector organs, i.e. muscles, and the response occurs.

• The passage of nerve impulse along a fibre involves a number of physical and chemical changes, non of which is directly visible.

Central Nervous System

In vertebrates, the central nervous system consists of brain, and the spinal cord.

Brain:

It is the most important organ which is lodged in the brain box, calledcranium. Brain is covered by membranes calledmeninges.Between the membranes and the brain and also inside the brain, there is a characteristics fluid, calledcerebrospinal fluid.The brain can be divided into three main parts:

(i) Fore-brain:

This is the anterior part which includes (a) olfactory lobes, the centers of smell; (b) cerebral hemispheres, the seat of intelligence & voluntary action; (c) diencephalons, the centre of hunger, thirst, etc.

(ii) Midbrain:

This part includesoptic lobeswhich are centre of vision.

(iii) Hind-brain:

This is the posterior part which includes the (a)cerebellum,the co-ordination centre of involuntary actions. Medulla oblongata is continued behind into the spinal cord. The brain is hollow. It has four longitudinal cavities called ventricles. In land vertebrates, 12 different cranial nerves are connected with the brain. Spinal cordIt is a long cord which arises from medulla oblongata and runs all along the vertebral column. It pass through the neural canal which is a canal of vertebra.

In the transverse section of a spinal cord, a central canal can be seen. This canal remains filled with cerebrospinal fluid. Immediately surrounding the canal, there are clusters of cytons which form thegrey matter.In the peripheral part, axons are concentrated and, therefore, this area is calledwhite matter.From each side of spinal cord, there are two horns, the dorsal horn and the ventral horn. To the dorsal horn joins a nerve which picks up sensation from the organ. It is called sensory organs. From the ventral horn or root arises the motor nerve which takes the message from the spinal cord to organ concerned. These two nerves constitute thereflex action. This action is very quick, for example, movements of eyelids, sneezing, coughing, yawning, hiccupping, shivering etc. In man 31 pairs of spinal nerves can be seen; eight in the neck region, 12 in chest region, five in abdominal region, five in hip region, and one in coccyx region. Coccyx is the last bone of the vertebral column.

Autonomic nervous system:

Nerves from the brain and the spinal cord connect the skeletal muscles and control their activities according to the direction and demand of the body. These nervous are, therefore related to the voluntary acts, i.e., acts according to the desire. But the internal organs are not under the control of our will. We cannot rotate the stomach or accelerate the heart beat by our conscious effort. For the control of the activities of the internal organs there is another type of nervous system called autonomous nervous system.You have seen the case of reflex action that motor neuron arises from the spinal cord and pass along the ventral root uninterruptedly all the way to the skeletal muscle. The cyton of this neuron is located in the spinal cord. But in case of an autonomous nervous system, i.e., spinal cord. This neuron runs for some distance only. It terminates in the sympathetic ganglion where it passes through the massage to the second nerve or neuron through asynapsewhich carries the impulse to the muscle or gland.

The autonomous nervous system has two subdivisions:sympatheticandparasympathetic.The sympathetic nervous system originates from the thoracic (chest) and lumber (abdominal) areas of spinal cord. Parasympathetic nerve which arises from spinal cord runs for a considerable distance and it forms asynapse(the point of exchange of impulse from one nerve to another) near the target organ where it has to send the message. The sympathetic nerves which arise from the spinal cord form a synapse in regular chain of ganglia running parallel to the spinal cord.

Sympathetic and parasympathetic nervous systems function in just the opposite manner. For example, if a person become angry, it may be due to the discharge of a chemical to different organs by the sympathetic nerves leading to increased heartbeat, etc. the parasympathetic nerve may discharge a different chemical, and thereby slow down the heartbeat and bring the person to normal state.

Nervous system in grasshopper (an Insect):

In insects, the nervous system consists of a brain, ganglia (singular ganglion) & nerve cord. A mass of nerve cells is called ganglion. The nerve cord runs along the entire length of the body. At intervals, it has ganglia. Small nerves are given out from each ganglion. Near the anterior end of the insect body, a large bi-lobed ganglion, called the brain, is present. Thus the nervous system of grass hopper consists of a brain, a long nerve cord, the ganglia and nerves spreading from the nerve cord.