The Nervous System
This page outlines basic concepts related to the nervous sytem.
Separate pages describe the brain and spinal cord, and
control of skeletal muscle.
- The nervous receives information about conditions both
within and around the body. It processes and integrates this
information on a variety of levels, and directs the body to respond
appropriately. The basic organization of the nervous system follows
this flow of information:
Afferent or sensory neurons collect stimuli received by
receptors throughout the body, including the skin, eyes, ears, nose, tongue
as well as pain and other receptors in the internal organs.
Sensory information is transmitted to the central nervous system,
which includes the brain and spinal cord. The CNS is responsible for
integrating the sensory information and directing any necessary response.
The CNS controls the rest of the body via efferent neurons, of
which there are two subdivisions:
Efferent neurons to the skeletal muscles, which are under voluntary or
conscious control, comprise the somatic motor division.
Efferent neurons which direct contraction and secretion in the internal
organs fall within the autonomic division. The autonomic
division is in turn divided into the sympathetic and
The neuron, or nerve cell, is the basic functional unit of the nervous
system. There are many types of neurons throughout the nervous system, but they share some common features:
The cell body contains the nucleus and other organelles essential for the
survival of the neuron. It is usually small compared to the rest of the
One or more dendrites extend like tendrils from the cell body. The
dendrites serve to receive incoming electrical signals from other neurons.
Most neurons have a single axon to transmit outgoing signals. Axons
vary in length from micrometers to over a meter. Portions of the axon are
insulated by supporting cells with myelin, a phospolipid membrane.
Neurons carry information from one end of the cell to the other by generating
and propagating electrical signals.
The potential difference across the neuron cell membrane is the
basis for generating electrical signals. Much like a battery, this
potential is creating by the uneven distribution of ions on either
side of the membrane.
Two factors influence the membrane potential difference:
The concentration gradient, or difference in concentration, of different types
of ions across the neuron cell membrane. The two major ions that influence
potential difference are sodium, which is abundant outside the cell, and
potassium, which is abundant inside the the cell. Both of these
ions have a charge of +1.
The permeability of the membrane is differenct for different types of ions.
Ions can only
move across the membrane through pores or channels that only allow specific
types of ions to pass through.
At rest, neuron cell
membranes are impermeable to sodium and only slightly permeable to
potassium. Potassium tends to leak out of the neuron, leaving the inside
of the membrane slightly more negative than the outside due to the loss
of positive charge.
The neuron generates electrical signals by sudden changes in permeability
to ions, particularly sodium. The process begins with the opening of
sodium channels. Because sodium is more abundant outside the membrane, and
because the inside of the membrane is slightly more negative than the
outside, sodium ions tend to rush into the cell through the open channels.
This tends to depolarize the cell, reducing the charge difference
across the membrane.
The initial opening of sodium channels may be caused either chemically or
mechanically (deformation of the cell membrane). This signal
is propagated by sodium channels that are sensitive to the initial voltage
change. These sodium channels are opened the sudden influx of positive
charge through neighboring channels, causing the signal to spread from the
site where it began.
Once an area of the neuron cell membrane has depolarized and passed on a
signal, it needs to repolarize before it can transmit another signal.
This is accomplished by the opening of potassium channels in the membrane.
Since potassium is much more abundant inside the cell, it tends to leak
out and carry positive charge with it. This tends to restore the resting
Neurons communicate with neighboring neurons and other types of cells
by secreting minute amounts of different types of small molecules, which
collectively are called neurotransmitters.
The space between cells where this transmission occurs is known as the
synapse. A sequence of steps typically occurs at the synapse whenever
a neuron communicates with another cell:
Electrical signals originating in the body a neuron reach the end
of the cell's axon.
Depolarization of the axon terminal leads to fusion of packets of
neurotransmitter with the cell membrane, releasing the molecules
into the synapse.
The neurotransmitter molecules diffuse across the membrane to reach
a dendrite or cell body of the target cell.
The neurotransmitter molecules bind to specific receptors on the target
cell membrane, leading to the creation of an electrical signal or other