The Human’s Nervous System
Human’s nervous system is related to one of the characteristics of living things, which is the sensitivity. Any change in the environment that causes an organism to react is called as a stimulus. Then, organism’s reaction is what we called as the response. The ability to respond to a stimulus is what we called by the term of sensitivity or irritability.
The nervous system helps all the parts of the body to communicate with each other. It also reacts to changes both outside and inside the body. The nervous system uses both electrical and chemical means to send and receive messages.
If you think of the brain as a central computer that controls all bodily functions, then the nervous system is like a network that relays messages back and forth from the brain to different parts of the body. It does this via the spinal cord, which runs from the brain down through the back and contains threadlike nerves that branch out to every organ and body part.
There are two main parts of the human’s nervous system, the central nervous system (CNS) and the peripheral nervous system (PNS). The difference between those two is that the CNS consists of the brain and the spinal cord while the PNS consists of the cranial nerves branches from the brain which is 12 pairs in total and the spinal nerves branches from the spinal cord which is 31 pairs in total and the sense organs or receptors.
How does your nervous system process information? The information is first gathered by the receptors. The sense organs receive stimuli, so they are called as receptors. Then, the information gathered by the receptors is converted into electrical signals called nerve impulses. The nerve impulses are then transmitted by our peripheral nerves to our CNS which is our brain and our spinal cords. Our brain then interpret the nerve impulse and then decide what actions to take. The brain initiated nerve impulses then transmit it to the effectors. The effectors will then carry out the intended actions. Here we have a video to show how the nerve impulse travels when you click the computer mouse :
http://www.youtube.com/watch?v=xRkPNwqm0mM
The nervous system is made up of nervous tissue. Nervous tissue consists of nerve cells called neurones. Unlike other cells in the body, neurones aren’t easily replaced if they die or are damaged by infection or injury. There are three types of neurons, which are sensory neurone, motor neurone, and the relay neurone.
Sensory neurones transmit nerve impulses from the sense organs or receptors to the central nervous system (CNS). It has a circular shape of the cell body and usually found in the ventral root ganglion. Sensory neurone has only one long dendron and a short axon which differ it with the other neurone types.
Relay neurones transmit nerve impulses from the sensory neurones to the motor neurones. Relay neurones can be found within the Central Nervous System. Relay neurones are also called as intermediate neurones.
Motor neurones transmit nerve impulses from the Central Nervous System to the effectors. It has irregular shape of the cell body due to the many presence of the dendrons. It has irregular shape in order to increase the surface area so that more area can receive the impulse. Motor neurones usually found in the dorsal root ganglion.Structure of a neurone
• Dendron
The nerve fibres that transmit nerve impulses towards the cell body in a motor neurone are called the dendrons. Motor neurone has a lot of dendrons. In the other hand, there is only one long dendron in a sensory neurone. There are dendrites at the end branches of the dendrons which receive and allow nerve impulses from one neurone to the other. The nerve impulses transmitted electrically through the neurone.
• Cell Body
Inside the cell body, we can find the nucleus and the cytoplasm. In a motor neurone, the cell body is irregular in shape while in a sensory neurone, the shape of the cell body is circular.
• Axon
There are also axons which is the nerve fibres that transmit nerve impulses away from the cell body. In the other words, what the axon does is vice versa with what the dendron does. The axon in a motor neurone is longer than the dendron. Differ from motor neurone which has long axon, sensory neurone has short axon.
• Myelin Sheath & Neurilemma
The nerve fibres are surrounded by layer of fatty substances called the myelin sheath. It protects and insulates the axon. Not only the axon is surrounded, the myelin sheath itself is also surrounded by a thin membrane or the neurilemma.
• Node of Ranvier
In both sensory and motor neurone, not all regions of the axon is enclosed by the myelin sheath. The regions where the axon is not covered by the myelin sheath are what we called as the nodes of Ranvier. The presence of node of Ranvier allows the nerve impulses to jump from one node to another which help in speeding up the transmission of nerve impulses between nerve fibres.
• Dendrites of an Axon
Near the end of the motor neurone, there are dendrites of axon that send nerve impulses to the effector. It can only be found in the motor neurone.
• Motor End Plate
Last, there is motor end plate which is the junction between the dendrite and the muscle fibres. In this region, the nerve impulses are transmitted not only electrically but also chemically. Motor end plate is formed when the end branch of an axon is in contact with the muscle fibre. Therefore, motor end plate can only be found in the motor neurone.
To know more about the structure of a neurone let us see this video :
http://www.youtube.com/watch?v=i-NgGKSNiNw
Synapse
Synapse is a junction or gap that connects or links two neurones. It can also be said that the synapse is a junction of a neurone and an effector such as muscle. The nerve impulses travelled through the synapse are transmitted from the axon of a neurone to the dendron of another neurone. The transmission of nerve impulses across the synapse is done by several chemicals in the body which are called as the neurotransmitter.
Grey Matter & White Matter
The grey matter and white matter both made up the brain and the spinal cord. The grey matter which is closely packed cell bodies of the neurones forms the outer part of the brain and the central or the middle part of the spinal cord. On the other hand, the white matter which mainly made up of the nerve fibres forms the central parts of the brain and the outer part of the spinal cord. The white matter is a neuronal tissue containing mainly long, myelinated axons.
The spinal cord consists both of the grey and white matter. The grey matter that covers the mid part of the spinal cord has a letter ‘H’ shape. There is a narrow central canal that runs through the mid part of the spinal cord. The canal has fluid that transports nutrients to the spinal cord. The fluid is acknowledged as the cerebrospinal fluid.The nerves in the spinal cord are known as the spinal nerves. Before they join the spinal cord, each of the spinal nerves divides into two roots, the dorsal and the ventral root.
Structure of the spinal nerves
• Dorsal Root
It joins the dorsal part or towards the back of the spinal cord. As has been discussed before, the dorsal part contains only sensory neurones.
• Dorsal Root Ganglion
It is a small swelling that clustered together the cell bodies of the sensory neurones.
• Grey Matter
It is where the axons of the sensory neurones of the spinal cord end while on the other hand, the dendrons are located in the dorsal root and spinal nerve.
• Ventral Root
It joins the ventral part or towards the front (opposite direction of the dorsal root) of the spinal cord. This is where the motor neurones are only located in.
The cell bodies and dendrons of the motor neurones stay in the grey matter of the spinal cord while the axons leave the spinal cord and then enter the ventral root and also the spinal nerve.
• Spinal Nerve
It contains nerve fibres from the dorsal and ventral root. Therefore, it is also called as a mixed nerve since the nerve fibres from the dorsal root are the sensory neurones and from the ventral root are the motor neurones.
After the nerves leave the spinal cord, it subdivides into branches providing the nerve fibres to different parts of the body.
• Neurones Separate
In this part of the spinal cord, the sensory and motor neurone separate. The one who go to the receptors are the sensory neurones whil the motor neurones go to the effectors.
Voluntary & Involuntary Action
Voluntary action is action that is done under the control of the brain and also by a will. It may have to do or do not any receptor or sensory neurone.
Pathway of nerve impulses in a voluntary action:
Sensory neurone (receptor) -Brain-Spinal Cord- Motor neurone (effector)
Involuntary actions or reflex action is action or response that is done without any control. Breathing and peristalsis movements are the examples of an involuntary action. The knee-jerk reflex and withdrawal of hand when touching a hot object are also examples of involuntary action.
Pathway of nerve impulses in an involuntary action:
Receptor- Sensory neurone-Spinal Cord-Motor neurone -Effector
The pathway above, which is the pathway where nerve impulses travel through in a reflex action (from receptor to effector) is known as the reflex arc.
http://www.brainexplorer.org/brain_atlas/Brainatlas_index.shtml#image
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookNERV.html
