The Nervous System

It's overall functions are to collect information about the body's external/internal states and transfer this information to the brain to analyse this information and to send impulses out to initiate appropriate motor responses to meet the body's needs

The system is composed of specialized cells, termed nerve cells or neurons, that communicate with each other and with other cells in the body

The cell body, containing the nucleus

The dendrites, hair-like structures surrounding the cell body, which conduct incoming signals

The axon (or nerve fiber), which conduct outgoing signals emitted by the neuron. Axons are encased in a fat-like sheath, called myelin sheath, which acts like an insulator and, along with the Nodes of Ranvier, speeds impulse transmission.

The nerves of the body are organized into two major systems:

The central nervous system (CNS), consisting of of the brain and spinal cord

The peripheral nervous system (PNS), the vast network of spinal and cranial nerves linking the body to the brain and spinal cord

• the autonomic nervous system (involuntary control of internal organs, blood vessels, smooth and cardiac muscles), consisting of the sympathetic NS and parasympathetic NS
• the somatic nervous system (voluntary control of skin, bones, joints, and skeletal muscle)

The central nervous system is divided into two parts:

• the brain
• the spinal cord

The spinal cord is about 43 cm long in adult women and 45 cm long in adult men and weighs about 35-40 grams

The vertebral column, the collection of bones (back bone) that houses the spinal cord, is about 70 cm long

Therefore, the spinal cord is much shorter than the vertebral column.

The somatic nervous system consists of peripheral nerve fibres that send sensory information to the central nervous system AND motor nerve fibres that project to skeletal muscle

The autonomic nervous system is divided into three parts:

• the sympathetic nervous system
• the parasympathetic nervous system
• the enteric nervous system

The autonomic nervous system controls smooth muscle of the viscera (internal organs) and glands

In the peripheral nervous system, neurons can be functionally divided in three ways: 

• Sensory (afferent) - carry information INTO the central nervous system from sense organs or motor (efferent) - carry information away from the central nervous system (for muscle control)
• Cranial - connects the brain with the periphery or spinal - connects the spinal cord with the periphery
• Somatic - connects the skin or muscle with the central nervous system or visceral - connects the internal organs with the central nervous system