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Human  Nervous System &Division of the                                    Nervous system 


               Human  Nervous System 




We have understood the basic model of  the working of the nervous system.The nervous system in man and in other higher animals is composed of two major components i.e. central nervous system and peripheral nervous system. Central nervous system comprises of coordinates i.e brain and spinal cord while peripheral nervous system consists of nerves that arise from central nervous system and spreads in different parts of body. All these components are made of neurons. Now we will first examine the structure and types of neurons and then e will go to  the divisions of nervous system.


Nerve cell or Neuron : 


Nerve cell  or neuron is the unit of  the nervous system.The human nervous system consists of billions of neuron plus supporting 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 neuron  is located in its cell body. Different processes extend out from cell body.These are called dendrites and axons. Dendrites conduct impulses toward cell body and axons conduct impulses away from cell body.Schwann cells are special neuroglial  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-myelinated
points,called the nodes of Ranvier.Myelin  sheath is an insulator so the membrane coated with this sheath does not conduct nerve impulse. In such a neuron impulses, impulses 'jump' over the areas of myelin going from node to node. Such impulses are called saltatory impulses.This increases the speed of nerve impulses.


On the basis of their functions,neurons of are three types;



1.Sensory neurons conduct  sensory information  from receptors to the CNS.Sensory neurons have one dendrite and one axon.



2.Interneurons from brain and spinal cord.They receive  information, interpret them and stimulate motor neurons. They have many dendrites and axons.






3.Motor neurons carry information interneurons  to muscles or glands.They have many dendrites but only one axon.



Nerve:


Human  Nervous System &Division of the nervous system. 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 contain the axons of sensory neurons only.


2.Motor nerves contain the axons of motor neutrons only.


3.Mixed nerves contain the axons of both i.e sensory and motor neurons.


Ganglion:


In certain parts  of body, the cell body of many neurons from a group enveloped by a membrane.this is called ganglion.


Division of nervous system:


The detail of central and peripheral nervous system are given below.


Central Nervous System.



                      

The central nervous system consist of brain and spinal cord.


A. Brain.




Human  Nervous System &Division of the nervous system.In animals, all life activities are under the control of brain.  The structure of brain is suitable to perform this function. Brain is situated inside a bony cranium(part of skull).

Inside cranium, brain is covered by three layers called meninges. Meninges protect brain and also provide nutrients and Oxygen to brain Tissue through capillaries.

The brain contain of fluid-filled ventricles that are continuous with a central canal of spinal cord. Fluid with in ventricles and central canal is called cerebrospinal fluid of (CSF).

The Division of Brain


There are three major regions in the brain of human and other vertebrates. Theseare forebrain, Midbrain, and hindbrain,

Important parts of each of these region are described below.

Fore brain.


Fore brain is the largest area of brain. It is most highly developed in humans. Following are the important part of this region.


1. Thalamus:


Thalamus lies just below cerebrum. It serves as relay center between various part of brain and spinal cord. It also received and modifies sensory impulses(except from nose) before they travel to cerebrum. Thalamus is also involved is pain perception and consciousness(sleep and awakening)


2. Hypothalamus:


Hypothalamus lies above midbrain and just below thalamus. In humans, It is roughly the size of an almond. One of the most important functions of hypothalamus ti link with nervous system and endocrine. It control the secretion of pituitary gland.It also control feelings such as rage, pain, pleasure, and sorrow.

3. Cerebrum:


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 hemispheres are called olfactory bulbs which receive impulses from olfactory nerves and create the sensation of smell.
The upper layer of cerebral hemispheres i.e cerebral cortex consists of grey matter. The grey matter of nervous system consists of cell bodies an non-myelinated axons. Beneath this layers is present the white matter. The white matter of nervous system consist of myelinated axons. Cerebral cortex as a large surface area and is folded in ordered to fit in skull. It is divided four lobes


Midbrain:

Human  Nervous System &Division of the nervous system. Midbrain lies between hindbrain  and forebrain and connects the two.It receives sensory information   and sends it to the appropriate part of forebrain.Midbrain also controls sime auditory reflexes and posture.

Hinbrain:

Human  Nervous System &Division of the nervous system.
Hindbrain consists of three major parts.


(i)Medulla oblongata :


Medulla oblongata lies on the top of spinal cord. It controls breathing and heart rate and blood pressure.It also controls many reflexes such as vomiting,coughing, sneezing, etc.Information that passes between spinal cord and the rest of brain pass through medulla.

(ii)Cerebellum:

Cerebellum is behind medulla.It coordinates muscle movements.

(iii)Pons:

Pons is present of top  medulla.It assists medulla in controlling breathing.It also serves as a connection between cerebellum and spinal cord.


B.Spinal Cord:


Human  Nervous System &Division of the nervous system.The spinal cord is in fact a tubular bundle of  nerves.It starts from brain stems and extends to lower back. Like brain, spinal cord is also covered by meninges. The vertebral column surrounds  and protects spinal cord.

The outer region of spinal cord is made of white matter (containing myelinated axons). The central region is butterfly shaped that surrounds the central  canal. It is made  grey matter(containing neuron cell bodies).

31 pairs of spinal nerves arise along spinal cord.These are mixed nerves because each contains axons of both sensory and motor neurons.At the point where a spinal nerve arises from spinal cord,there are two roots of spinal nerve.Both roots unite and form one mixed spinal nerve.

• The dorsal root contains sensory axons and a ganglion where cell bodies  are located.

• The ventral root contains axons of motor neurons.