When we look at the entire nervous system in our body, we can clearly make out its two big divisions. One part consists of nerves ramifying all the parts of the body. This constitutes the peripheral nervous system. The other part consists of brain and spinal cord which from the central nervous system.
The peripheral nervous system consists of three systems of nerves,
- Cranial and spinal nerves
- The sympathetic nerves
- parasympathetic nerves.
The latter two systems together constitute the autonomic nervous system. The former is known as the cranial ans spinal system of nerves. The cranial nerves travel between the brain and the various organs without passing through the spinal cord. The cranial nerves are sensory, motor or mixed nerves. These nerves are associated with responses involving involuntary action.
The autonomic nervous system comprises the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). The former originates from cervical, thoracic and lumber region of the spinal cord while the latter extends its nerves fibers from the cranial and sacral ends of the central nervous system.
The sympathetic nervous system prepares the body for emergency situation and is responsible for responses associated with flight or fight. The heart beat is strengthened and accelerated, blood pressure and blood sugar level are increased and blood moves from vessels within trunk to those of arms and legs to support fighting or running.
The parasympathetic nervous system on the other hand is involved in eating, sexual activity and urination etc. autonomic nervous system regulates involuntary system of the body automatically.
Reflex action and Reflex arc
The neuron or nerve cell is considered the anatomical unit of the nervous system. The reflex arc is regarded as the functional unit of the system. A reflex is an unvarying and automatic response to a specific stimuli.
The most common example of reflex action is of knee-jerk reflex. If the tendon on the muscle lying just below the knee is given a sharp below the leg will extend sharply. This response is performed by creation of an impulse in sensory neurons that is carried to the spinal cord. A synapse with a motor neuron in the cord results in the impulses moving along an axon of the motor neuron to the exterior muscle of the leg. The contraction of the exterior muscle produces knee jerk (another example of reflex action is the touching a hot object by chance and hand is suddenly withdrawn).
Reflexes are mechanisms for maintaining appropriate posture regulation blood pressure and orienting the body to environmental condition that threaten the organisms.
Involuntary action
A spinal nerve arises from spinal cord by two roots, a dorsal root consisting of sensory neurons and a ventral root consisting of motor neurons. The two roots combine to form a spinal nerve. The nerve thus formed is a mixed nerve. In an organ which is supplied by it i.e. fore-arm of man the sensory fibers supply the skin (a sense organ) and the motor fiber supply muscles. When a person accidentally touches something very hot the hand is abruptly withdrawn.
What happens is that the sensory fibers carry the impulse to the spinal cord and from it message is transmitted to the muscles of arm to contract and the hand is withdrawn without the involvement of brain. In the spinal cord (grey matte) the associative neurons transfer the nerve impulse from sensory to the motor neurons.
An involuntary action is an action which is elicited without any conscious recognition are effort by an organism. These actions have adaptive values because these actions required being to be produced at once for the survival of the individual and safe existence.
A nervous pathway which operates automatically without having to wait for directions from brain is a reflex arc. Many such reflexes operate in the body and increase the efficiency of nervous system.
Voluntary action
An action which takes place willingly under the directions from brain is a voluntary action e.g. lifting of book, turning off an electric switch, riding a bicycle so on and so forth.
In voluntary action the sensory stimuli may be conveyed directly to the brain through sensory neurons or may first go to the spinal cord from where the information may be passed on to the brain by associative neurons. The brain then comprehends the information and send motor impulse to the specific effectors for relevant controlled response.
Further Reading: Nerves System