The human body is made up of trillions of cells. The cells of the nervous system are called nerve cells or neurons. These are specialized to carry messages through an electrochemical process. The human rain has approximately 100 billion neurons.
Structure of a neuron:
- Cell body:
It is also known as soma. It is the factory of the neuron. It produces all the proteins for the dendrites, axons and synaptic terminals and contains specialized organelles such as mitochondria, Golgi bodies, endoplasmic reticulum, ribosomes, polysomes, etc.
The cell body contains watery and salty fluid known as cytosol. The cytosol contains the most important part of the neuron known as the nucleus. The cell organelles float within the boundaries of the cellular membrane in cytoplasm. It also contains nucleus.
These structures branch out in tree like fashion and serve as the main apparatus for receiving signals from other nerve cells. A nerve cell can have many dendrites which branch many times.
The axon hillock gives rise to the axon. It is also known as a nerve fiber. The axon receives outgoing signals from the soma and passes them along to the other cells via the synaptic terminal.
The diameter of the axon ranges from 1-20 micro meters or may be more than a meter long. Axon can be myelinated or unmyelinated. Myelin sheath is protective layer covering the axon that affects the speed of the signal conduction. Nodes of reviver are the gaps between the myelinated segments.
Axon terminal is where the axon ends. These terminals end in relation to other structures such as another neuron, muscle cells or gland cells.
It is the junction between the axon terminal and other neuron, muscle cells or gland cells. At the synapse, there is a small gap between the terminal end of the axon and the target cell. The neuro- transmitters easily diffuse from their synaptic gap.
When an impulse travels down the axon from the soma, it eventually reaches the axon terminal. When it reaches, the terminal the change in electric charge triggers biochemical reactions that lead to release of neuro transmitters into the synaptic cleft. These neurotransmitters can then diffuse across the synaptic cleft and cause either excitation or inhibition on their target tissue.
- If the target tissue is a muscle, it will cause construction.
- If the target tissue is another neuron, it will trigger or inhibit another action potential in that downstream neuron.
All function of the central nervous system occurs as a result of neuronal synapses.
Types of nerves in central nervous system:
There are three types of nerves in the central nervous system:
- Motor nerves:
Motor neurons send impulses or signals from the brain and signal cord to all of the muscles in the body. These nerves control muscle contraction allowing movements and activities such as wiggling your fingers, walking, catching a baseball or kicking a soccer ball.
- Sensory nerves:
Sensory nerves send message from sensory organs such as skin, nose, ear, tongue, and eye. For example; sensory nerves in skin help you identify if an object is sharp, rough or smooth, hot or cold.
- Autonomic nerves:
Controlling involuntary or semi-voluntary function such as heart rate, blood pressure, digestion, temperature regulation and sweating.
Structural classification of nerve cell:
- Unipolar (pseudo-unipolar):
Dendrite and axon are emerging from same process.
Axon and single are dendrite on opposite ends of the soma.
Two or more than two dendrites, separate from the axon.