The neuron, a nerve cell, is the functional unit of the nervous system that carries the impulse (the message) to the appropriate part of the nervous system or interprets the impulse and allows a response. There are three types of neurons and each has different functions.
1) sensory neurons—receive impulses and carry them from the sense organs to the spinal cord or brain.
2) interneurons—connect sensory and motor neurons and interpret the impulse; only in the brain and spinal cord.
3) motor neurons—carry impulses from the brain and spinal cord to muscles or glands
The Response Mechanism
When a stimulus is received by a sensory neuron, the impulse (or message) is carried through fibrous extensions called dendrites to the cell body. The cell body is made up of cytoplasm, cytoplasmic structures, and a nucleus, which controls neuron function. The impulse travels through the cell body and is carried through the axon to the end brush, a collection of fibers that extend off the axon. Here, the impulse triggers a release of chemicals that allow the impulse to travel through the synapse—the space between the axon of one neuron and the dendrites of the next.
An impulse travels along the neuron pathways as electrical charges move across each neural cell membrane. Ions moving across the membrane cause the impulse to move along the nerve cells.
The difference in electrical charge on each side of the cell membrane (caused by differing numbers of positively and negatively charged ions) produces a resting potential. Neurons have a resting potential of approximately 70 millivolts (mV).
Specifically, the cell membrane proteins pump sodium ions (Na+) out of the neuron and pump potassium ions (K+) into the neuron. Active transport mechanisms and leaking back and forth of both the Na+ and K+ ions produce a negative charge on the inside of the neuron’s cell membrane.
An impulse begins when a neuron is stimulated by another neuron or by a stimulus in the environment. The cell membranes begin to change the flow of ions and a reversal of charges, the action potential, results. An impulse that changes one neuron, changes the next. The impulse movement continues along the pathway in this way.
An impulse can travel quickly through the nervous system. Many vertebrates’ neurons, including humans, have several features that allow for maximum reactivity. Vertebrate axons have a myelin sheath that allows for faster travel of the impulse. Nodes on the axons also allow impulses to jump from node to node instead of traveling through the entire axon. This also allows for faster response. The more quickly an organism can respond, the better adapted it is to its environment.
Nervous System Organization
Two main parts of the human nervous system are the:
1) Central nervous system—consists of the brain and spinal cord.
The brain and spinal cord are protected by three layers of tissue called the meninges. Between the layers is a space filled with cerebrospinal fluid. This fluid acts of a shock absorber to protect against injuries.
