Nervous tissues

General

All living cells have the ability to react to stimuli. Nervous tissue is specialised to react to stimuli and to conduct impulses to various organs in the body which bring about a response to the stimulus. Nerve tissue (as in the brain, spinal cord and peripheral nerves that branch throughout the body) are all made up of specialised nerve cells called neurons. Neurons are easily stimulated and transmit impulses very rapidly. A nerve is made up of many nerve cell fibres (neurons) bound together by connective tissue. A sheath of dense connective tissue, the epineurium surrounds the nerve. This sheath penetrates the nerve to form the perineurium which surrounds bundles of nerve fibres. blood vessels of various sizes can be seen in the epineurium. The endoneurium, which consists of a thin layer of loose connective tissue, surrounds the individual nerve fibres.
Although the system forms a unit it can be divided into the following parts: the central nervous system (CNS) which consists of the brain and spinal cord, the nervous system consists of the nerves outside the CNS which connect the brain and spinal cord to the organs and muscles of the body and the automatic or involuntary nervous system consists of nerve centres and fibres inside as well as outside the central nervous system.
There are three main types of neurons, which are classified according their function: Those that conduct impulses from the sensory organs to the central nervous system (brain and spinal cord) are called sensory (or afferent) neurons; those that conduct impulses from the central nervous system to the effector organs (such as muscles and glands) are called motor (or efferent) neurons. Interneurons (also known as connector neurons or association neurons) are those that connect sensory neurons to motor neurons.

Structure of a Motor Neuron

A motor neuron has many processes (cytoplasmic extensions), called dendtrites, which enter a large, grey cell body at one end. A single process, the axon, leaves at the other end, extending towards the dendrites of the next neuron or to form a motor endplate in a muscle. Dendrites are usually short and divided while the axons are very long and does not branched freely. The impulses are transmitted through the motor neuron in one direction, i.e. into the cell body by the dendrites and away from the cell body by the axon . The cell body is enclosed by a cell (plasma) membrane and has a central nucleus. Granules, called Nissl, bodies are found in the cytoplasm of the cell body. Within the cell body, extremely fine neurofibrils extend from the dendrites into the axon. The axon is surrounded by the myelin sheath, which forms a whitish, non-cellular, fatty layer around the axon. Outside the myelin sheath is a cellular layer called the neurilemma or sheath of Schwann cells. The myelin sheath together with the neurilemma is also known as the medullary sheath. This medullary sheath is interrupted at intervals by the nodes of Ranvier.

A motor neuron

Nerve cells are functionally connected to each other at a junction known as a synapse, where the terminal branches of an axon and the dendrites of another neuron lie in close proximity to each other but never make direct contact.

A Synapse

Classification of Neurons

On the basis of their structure, neurons can also be classified into three main types:

• Unipolar Neurons. Sensory neurons have only a single process or fibre which divides close to the cell body into two main branches (axon and dendrite). Because of their structure they are often referred to as unipolar neurons.
  
• Multipolar Neurons. Motor neurons, which have numerous cell processes (an axon and many dendrites) are often referred to as multipolar neurons. Interneurons are also multipolar.
  
• Bipolar Neurons. Bipolar neurons are spindle-shaped, with a dendrite at one end and an axon at the other . An example can be found in the light-sensitive retina of the eye.
  

A diagram showing the different neurons

Functions of Nerve Tissue

• Nervous tissue allows an organism to sense stimuli in both the internal and external environment.
• The stimuli are analysed and integrated to provide appropriate, co-ordinated responses in various organs.
• The afferent or sensory neurons conduct nerve impulses from the sense organs and receptors to the central nervous system.
• Internuncial or connector neurons supply the connection between the afferent and efferent neurons as well as different parts of the central nervous system.
• Efferent or somatic motor neurons transmit the impulse from the central nervous system to a muscle (the effector organ) which then react to the initial stimulus.
• Autonomic motor or efferent neurons transmit impulses to the involuntary muscles and glands.