Human nervous
system
NERVE CELL OR
NEURONS.
Nerve cell
or neuron is the unit of 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 the cell body. These are called dendrites and axons. Dendrites
conduct impulses toward cell body and axons conduct impulses away from cell
body.
Schwann's cells are special neuralgia 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-militated points;
called the nodes of rangier. Myelin sheath is an insulator, so the membrane
coated with this sheath does not conduct nerve impulses. In such a neuron,
impulses jump over the area of myelin going from node to node. Such impulses
are called salutatory (jumping) impulses. This increase the speed of nerve
impulses.
The basis
of their functions, neurons are of three types:
TYPES OF
NEURONS
1.
Sensory neurons: sensory neurons conduct sensory information (nerve impulses) from
receptors towards the CNS. Sensory nerves have one dendrite and have an axon.
2.
Interneurons: interneuron’s from brain and spinal cord. They receive information,
interpret them and stimulate motor neurons. They have many dendrites and axons.
3.
Motor nerves: motor nerves carry information from interneuron’s to muscles or glands
(effectors).they have many dendrites but have only one axon.
TYPES OF NERVES
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.
1: sensory nerves: sensory nerves contain the axons of the sensory neuron only.
2: motor nerves:
motor nerves contain the axons of motor neurons only.
3: mixed nerves:
mixed nerves contain the axons of both. I.e. sensory and motor nerves
Divisions of nervous system.
The details of the central nervous system are given below.
Central nervous system consists of the brain and spinal cord.
A: The Brain
Brain (its structure)
In animals
and human being, all life activities are under control of the brain. The structure
of the brain is suitable to perform this function .the brain is situated inside a bony
cranium (part of skull).inside the cranium, the brain is covered by three layers called
meninges.meninges protect the brain and also provide nutrients and oxygen to brain
tissue through their capillaries.
The brain
contains fluid – filled ventricles that are continues with the central canal of
spinal cord. Fluid within ventricles and central canal is called cerebrospinal
fluid (CSF).
THE DIVISION OF BRAIN
There are
three major parts of brain. It is most highly developed in humans. The following are important parts of the brain that are described below.
Ø Forebrain
Forebrain
are the largest part/ area of brain. It is most highly developed in humans. The following are important parts of forebrain.
§ Thalamus lies just below the cerebrum. It serves as a relay
centre between various parts of the brain and spinal cord. It also receives and
modifies sensory impulses (except from the nose) before they travel to cerebrum.
Thalamus also involves in pain perception band and consciousness (sleep and
awakening.
§ Hypothalamus lies above midbrain and just below the thalamus. In
humans, it is roughly the size of an almond. One of the most important
functions of the hypothalamus is to link the nervous system and endocrine system.
It controls the secretion of
pituitary gland. It also controls feeling such as rage, pain, pleasure and
sorrow. Likewise, it is the structure that deeps in the cerebrum. Furthermore, it functions for the
formation of new mummeries. Peoples with damage to it cannot remember the
things that occurred after the damage but remembers the things that happened
before the damage to the hypothalamus.
§ Cerebrum is 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 hemisphere are called
olfactory bulbs which receive impulses from nerves and create the sensation of
smell.
|
lobe |
function |
|
frontal |
Controls motor functions, permits conscious control of skeletal
muscles. |
|
parietal |
Contains sensory areas that receive impulses from skin. |
|
occipital |
Receive and analyzed visual information. |
|
temporal |
Concerned with hearing and smell. |
The upper layer of cerebral hemisphere 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 the nervous system consist 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.
Ø Midbrain
Midbrain is
lies between hind brain and forebrain and connects the two. It receives
sensory information and sends it to the appropriative part of forebrain.mid-brain also control some auditory reflexes and posture.
Hindbrain
It consists of two of the major parts
1) Medulla oblongata
lies on the top of the spinal cord. It controls breathing, heart rate and blood
pressure. It also controls many reflexes such as vomiting, coughing, sneezing
etc Information that pass through
medulla.
2) Cerebellum
is behind medulla. It coordinates muscles movements.
3) Pons is
present on top of medulla. It assists the medulla in controlling breathing. It also
serves as a connection between cerebellum and spinal cord. The medulla
oblongata, Pons and midbrain connect the rest of the brain to the spinal cord. They are
collectively referred to as brain stem (supporters of brain).
The spinal cord is being discussed in another post.
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