Organization of the nervous system

• Our body is constantly reacting and adjusting to the changes in the outside environment and within the body itself (internal environment), which are called stimuli (sing. Stimulus).
• In other words, any external or internal factor which brings about a response in an organism is called stimulus and the response shown by an organism towards or against that stimulus is called a reaction.
• Stimuli (messages) are detected and conveyed via nerves to the spinal cord and the brain where they are analyzed, combined, compared and coordinated by a process called integration.
• After being sorted out, messages are conveyed by nerves to the muscles and glands of the body.
• The nervous system is a network of organs and tissues that controls and coordinates all the activities of an organism inside and outside its body with the help of nerves.
• The nervous system and the endocrine system are the two major regulatory systems of the body, and both are specialized to make the proper responses to the stimuli.
• The nervous system is faster than the endocrine system as the stimuli received are processed rapidly through a combination of electrical impulses and chemical substances called neurotransmitters.
• Neurotransmitters help in communication between two nerve cells, between a nerve cell and a muscle cell, or between a nerve cell and gland cells.

Divisions of nervous system:

• Although the nervous system is a single, unified communications network, it can be divided on a gross anatomical basis into the central and the peripheral nervous system.

1. Central nervous system (CNS):

• It is the body’s central controlling system and consists of the brain and the spinal cord.
• They are surrounded and protected by the skull and vertebral column respectively.
• It receives and interprets or integrates all stimuli and relays nerve impulses to muscles and glands, where the designated actions usually occur.

2. Peripheral nervous system (PNS):

• It consists of nerves and their fibers emerging from and going to the brain (cranial nerves) and spinal cord (spinal nerves).
• This system allows the brain and the spinal cord to communicate with the rest of the body.
• In terms of function, two types of nerve cells are present in the peripheral nervous system:
  • Afferent or sensory nerves: carry nerve impulses from sensory receptors in the body to the CNS.
  • Efferent or motor nerves: convey information away from the CNS to the effector organs (muscles and glands).
• PNS is divided on a purely functional basis into the somatic nervous system and the visceral nervous system.
  • Somatic nervous system is further composed of afferent and efferent divisions.
    • Somatic afferent (sensory) division: consists of nerve cells that receive and process sensory input from the superficial organs like skin, skeletal muscles, tendons, joints, eyes, tongue, nose and ears and the input is conveyed to the spinal cord and
    • Somatic efferent (motor) division: consists of neuronal pathways that descend from the brain through the brain stem and spinal cord to influence the lower motor neurons of some cranial and spinal nerves. They always excite (never inhibit) the skeletal muscles to contract, i.e. regulates the voluntary contraction of skeletal muscles.
  • Visceral nervous system is also composed of afferent and efferent divisions.
    • Visceral afferent (sensory) division: includes the neural structures involved in conveying sensory information from sensory receptors in the visceral organs (internal large body organs lying in the great body cavities) of the cardiovascular, respiratory, digestive, urinary and reproductive systems.
    • Visceral efferent (motor) division: It is commonly known as autonomous or autonomic nervous system (ANS) and includes the neural structures involved in the motor activities that influence smooth muscles, cardiac muscles, and glands of the skin and viscera.
    • Most regulatory actions of the autonomic nervous system are not under conscious control.
      • Autonomic nervous system is further divided into two sub-divisions:
        1. Sympathetic nervous system
        2. Parasympathetic nervous system

S.N.	Parts of the body	Sympathetic nervous system	Parasympathetic nervous system
1.	Blood vessels External genitalia Salivary glands Skeletal muscles Skin Viscera	Generally constricted Constricted Constricted Dilated Constricted Constricted	Generally dilated slightly Dilated Constricted No innervation Dilated slightly Dilated
2.	Eye Muscle of iris Ciliary muscle	Pupil dilated Relaxed for far vision	Pupil contracted Contracted for near vision
3.	Heart Rate Strength of contraction	Increased Increased	Decreased Decreased
4.	Gastrointestinal tract Motility and tone Sphincters Secretion Gall bladder Liver	Inhibited Stimulated (contracted) Probably inhibited Inhibited Increased glycogenolysis (resulting in increased blood glucose)	Stimulated Inhibited (relaxed) Stimulated Stimulated No innervation
5.	Lungs Bronchial tubes Bronchial glands Bronchial muscles	Lumen dilated No innervation relaxed	Lumen constricted Secretion stimulated Contracted
6.	Glands of head Lacrimal glands Salivary glands	No innervation Increased viscous secretion	Secretion stimulated Profuse water secretion
7.	Urinary bladder	Relaxed	Contracted
8.	Skin Sweat glands Blood vessels Piloerector muscles	Stimulated to secrete Constricted Contracted (hair stands erect)	No innervation No innervation No innervation
9.	Sex organs	Vasoconstricted (orgasm)	Vasodilated (erection in male, lubrication in female)
10.	Adrenal medulla	Epinephrine, norepinephrine secreted	No innervation

 

Some effects of the autonomic nervous system

Organization of the nervous system