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.
A motor neuron
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, which have numerous
cell processes (an axon and many
dendrites) are often referred
to as multipolar neurons.
Interneurons are also multipolar.
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
A diagram showing the different neurons
- 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.