Neurons: An introduction

Nerve cells are called neurons. Neurons have the ability to gather and transmit electrochemical signals. Neurons share the same characteristics and have the same parts as other cells, but the electrochemical aspect lets them transmit signals over long distances and pass messages to each other.

Neurons have three basic parts:
1) Cell body - This main part has all of the necessary components of the cell, such as the nucleus (contains DNA), endoplasmic reticulum and ribosomes (for building proteins) and mitochondria (for making energy). If the cell body dies, the neuron dies.

2) Axon - This long, cable-like projection of the cell carries the electrochemical message (nerve impulse or action potential) along the length of the cell.
Note: Depending upon the type of neuron, axons can be covered with a thin layer of myelin, like an insulated electrical wire. Myelin is made of fat, and it helps to speed transmission of a nerve impulse down a long axon. Myelinated neurons are typically found in the peripheral nerves (sensory and motor neurons), while non-myelinated neurons are found in the brain and spinal cord.

3) Dendrites or nerve endings - These small, branch-like projections of the cell make connections to other cells and allow the neuron to talk with other cells or perceive the environment. Dendrites can be located on one or both ends of the cell.

Neurons and Synapses

The basic computational unit in the nervous system is the nerve cell, or neuron. A neuron has:
* Dendrites (inputs) a neuron
* Cell body
* Axon (output)

A neuron receives input from other neurons (typically many thousands). Inputs sum (approximately). Once input exceeds a critical level, the neuron discharges a spike - an electrical pulse that travels from the body, down the axon, to the next neuron(s) (or other receptors). This spiking event is also called depolarization, and is followed by a refractory period, during which the neuron is unable to fire.

The axon endings (Output Zone) almost touch the dendrites or cell body of the next neuron. Transmission of an electrical signal from one neuron to the next is effected by neurotransmittors, chemicals which are released from the first neuron and which bind to receptors in the second. This link is called a synapse. The extent to which the signal from one neuron is passed on to the next depends on many factors, e.g. the amount of neurotransmittor available, the number and arrangement of receptors, amount of neurotransmittor reabsorbed, etc.

The human brain contains about 10 billion nerve cells, or neurons. On average, each neuron is connected to other neurons through about 10,000 synapses. (The actual figures vary greatly, depending on the local neuroanatomy)
Neurons typically operate at a maximum rate of about 100 Hz

Basic Neuron Types

Neurons come in many sizes. For example, a single sensory neuron from your fingertip has an axon that extends the length of your arm, while neurons within the brain may extend only a few millimeters. Neurons have different shapes depending on what they do. Motor neurons that control muscle contractions have a cell body on one end, a long axon in the middle and dendrites on the other end; sensory neurons have dendrites on both ends, connected by a long axon with a cell body in the middle.

Some types of neurons:
- Bipolar (Interneuron)
- Unipolar (Sensory Neuron)
- Multipolar (Motoneuron)
- Cortical Pyramidal Cell

Neurons also vary with respect to their functions:
* Sensory neurons carry signals from the outer parts of your body (periphery) into the central nervous system.
* Motor neurons (motoneurons) carry signals from the central nervous system to the outer parts (muscles, skin, glands) of your body.
* Receptors sense the environment (chemicals, light, sound, touch) and encode this information into electrochemical messages that are transmitted by sensory neurons.
* Interneurons connect various neurons within the brain and spinal cord.

Neuronal types

Motor neurons
These lower motor neurons are located on the ventral aspect of the cord. They are either alpha or gamma cells.
- Alpha cells are the principle lower motor neurons of the spinal cord and form the main portion of the final common pathway. They conduct rapid motor impulses, with each alpha cell innervating approximately 200 muscle fibers.
- Gamma neurons are also part of the final common pathway according to some sources but they are only half as numerous as alpha cells. Gamma cells conduct slow motor impulses. Their major function is to stretch muscle spindles.

Association neurons
Interneurons connect the anterior and posterior horns of the gray matter and are involved in the reflex arc. They work within the same segment of the spinal cord, with a segment being defined as the horizontal section of the cord that gives rise to one pair of spinal nerves.
Internuncial Neurons travel between segments, sending projections up to the brain stem and cerebellum. They project in an ascending, not descending manner.
These association neurons are found throughout the central nervous system. They are much more numerous than motor neurons; the ratio between the two types of cells is 30:1.

The main function of the association neurons in the spinal cord is that of inhibitory control. They also interconnect other cells with one another.