On their way out, the nerve fibres
from the motor areas of any side cross over to the opposite side of the
region of the medulla oblongata. This means that the motor areas of the left
hemisphere control the voluntary movements of the right side and vice versa.
The motor neurons of the cerebrum are termed upper motor neurons, and those
of the spinal cord, are called lower motor neurons. Damage to motor areas
results in paralysis.
The motor speech area controls the movements needed for speech.
It is called Broca's area. It is peculiar because
it develops in the left hemisphere in right-handed persons and in the right
hemisphere in the left-handed persons.
Unlike in other vertebrates, the boundary between the cerebrum
and diencephalon is not very distinct in mammals.
The spinal cord is protected within the neural arches of the
vertebrae and also by the meninges and
cerebrospinal fluid. The cord is suspended loosely in the spinal canal. Since
its diameter is much less than that of the canal, there is no danger of it
being injured by the ordinary movements of the vertebral column.
Structure of Neuron
The nervous system of man is made up of millions of nerve cells
Each nerve cell consists of a cell body called cyton.
The cyton contains granular cytoplasm,
called neuroplasm, surrounding a large nucleus.
Scattered in the cytoplasm are found a number of long, delicate
fibrils called neurofibrils.
The neurofibril conducts nerve
From the cyton arise two types of
processes, viz. axon and dendron.
The axon is a single, long process ending in many branches.
The dendrons or dendrites are small
processes. They are many in number and short or long in length. Each dendron is profusely branched at its tip.
The dendrons conduct impulses towards
the cyton, while the axons conduct impulses away
from the cyton.
According to the structure of the axon, two types of nerve fibres have been recognised.
These are medullated and non-medullated
The medullated nerve fibres or myelinated nerve fibres are covered by a fatty sheath called medullary or myelin sheath. The non-medullated
nerve fibres are not covered by myelin sheath.
The myelin sheath of a nerve fibre is
not continuous. Its continuity is broken at regular intervals, forming
constrictions called nodes of Ranvier.
The nodes of Ranvier are the points at
which one Schwann's cell ends and the other Schwann's cell begins.
Types of Neuron
Depending on the number of processes they possess, neurons have
been divided into three types, viz. unipolar,
bipolar and multipolar.
Unipolar Neuron: Neurons which have a
single process dividing into a dendron and an axon
are called unipolar. Such neurons are found in the
dorsal ganglia of the spinal cord.
Bipolar Neuron: Neurons having two processes arising from
the opposite poles of the cyton are called bipolar.
These neurons are found in the retina of the eye.
Multipolar Neuron: Neurons having many
processes arising from the cyton are called multipolar. Such neurons have a single axon and many dendrons. The great majority of neurons are of this type.