Neuron
Function
The neuron (nerve cell) is the fundamental unit of the nervous system. The basic
purpose of a neuron is to receive incoming information and, based upon that
information, send a signal to other neurons, muscles, or glands. Neurons are
designed to rapidly send signals across long distances in the body. They do this
using electrical signals called nerve impulses. When a nerve impulse reaches the
end of a neuron, it triggers the release of a chemical called a neurotransmitter.
The neurotransmitter travels rapidly across the short gap between cells (the
synapse) and acts to signal the adjacent cell.
Communication by neurons can be divided into four major steps. First, a neuron
receives information from the external environment and/or from other neurons.
For example, one neuron in the human brain may receive input from as many as one
hundred thousand other neurons. Second, the neuron processes the information
from all of its inputs and determines whether or not to send an output signal.
Third, the neuron sends the signal along its length at high speed. The distance may
be up to several meters (in a giraffe or whale), with rates up to 100 meters (328
feet) per second. Finally, the neuron converts this electrical signal to a chemical
one and transmits it to another neuron or to an organ cell such as a muscle or
gland.
Structure
When combined into networks, neurons allow the human body memory, emotion, and
abstract thought as well as basic reflexes. The human brain contains an estimated
one hundred billion neurons which send, process, and store information
The ability of a neuron to carry out its function of receiving and sending nerve
impulses depends both upon its structure. While different neurons have different
shapes, all neurons share the same signaling abilities.
A typical neuron in a vertebrate (such as a human) consists of four major regions: a
cell body, dendrites, an axon, and synaptic terminals. Like all cells, the entire
neuron is surrounded by a cell membrane. The cell body (soma) is the enlarged
portion of a neuron that most closely resembles other cells. It contains the nucleus
and other organelles (for example, numerous mitochondria). The dendrites and
axon are thin extensions of the neuron. The dendrites, which branch out in treelike
fashion from the cell body, are specialized to receive signals and transmit them
toward the cell body. The single long axon carries signals away from the cell body.
In humans, a single axon may be as long as 1 meter (about 3 feet). Some neurons
that have cell bodies in the spinal cord have axons that extend all the way down to
the toes. Axons generally divide and redivide near their ends and each branch gives
rise to a specialized ending called a synaptic knob (synaptic terminal). It is the
synaptic terminals of a neuron that form connections either with the dendrites or
organ cells. Once an electrical signal has arrived at the end of an axon, the synaptic
terminals release a chemical messenger called a neurotransmitter, which relays the
signal across the synapse to the next neuron or to the organ cell.
Structure-Function Connection:
1. Why do you think there are many dendrites branching out like tree
branches?
2. Why might it be beneficial for the axon to be so long?
3. Why might it be beneficial for axons to branch at the ends to have multiple
synaptic terminals?