Where Are the Nerves?
Your nerves are spread all over your body. The organs of the body are composed of tissues. These in turn consist of microscopic units called cells, specialized to perform particular functions such as secretion (glands), contraction (muscles), or conduction (nerves).
Likewise the tissues that make up our nervous system are composed of billions of individual cells located all over the human body. No part of our body is completely insensitive to pain or some other sensation, because nerves are to be found in every part of the human anatomy.
The nervous system is usually considered to have two parts, the peripheral or outside system, which is in direct contact with the things that cause pain or pleasure, and the central system, consisting of the brain and the spinal cord. It is in our central nervous system that all our sensations and reactions are finally registered.
Nerves can be categorized into two groups based on function: Sensory nerves conduct sensory information from their receptors to the central nervous system, where the information is then processed. Thus they are synonymous with afferent nerves. Motor nerves conduct signals from the central nervous system to muscles. Thus they are synonymous with efferent nerves.
In the central nervous system, the analogous structures are known as tracts. Neurons are sometimes called nerve cells, though this term is potentially misleading since many neurons do not form nerves, and nerves also include non-neuronal Schwann cells that coat the axons in myelin.
Each nerve is a cordlike structure containing bundles of axons. Within a nerve, each axon is surrounded by a layer of connective tissue called the endoneurium. The axons are bundled together into groups called fascicles, and each fascicle is wrapped in a layer of connective tissue called the perineurium. Finally, the entire nerve is wrapped in a layer of connective tissue called the epineurium.
A nerve conveys information in the form of electrochemical impulses (known as nerve impulses or action potentials) carried by the individual neurons that make up the nerve. These impulses are extremely fast, with some myelinated neurons conducting at speeds up to 120 m/s. The impulses travel from one neuron to another by crossing a synapse, the message is converted from electrical to chemical and then back to electrical.