The Neuron

Basic Anatomy of a Neuron

• Soma or cell body
• Dendrites -- relatively short, branch-like structures extending from the soma.
• Axon -- relatively long fiber extending from the soma. It usually branches near the end furthest from the soma.
• Terminal buttons -- endings that terminate the axon branches.

Functions of the neuron and its parts

• You can think of a neuron as a device for transmitting information from one place to another and for doing computations on that information.
• Signals are received by the neuron at specialized sites called synapses, located on the dendrites and cell body.
• Signals are conducted by the axon from the cell body to the terminal buttons.
• Signals are transmitted from the terminal buttons to the receiving cell across a synapse (if the receiving cell is a neuron) or a neuromuscular junction (if the receiving cell is a muscle cell).

The Neural Impulse

• The neuron normally maintains a resting potential of about -70 to -90 millivolts (mV). (By contrast, a single cell in a flashlight has a charge of about 1500 mV or 1.5 volts.) The negative sign indicates that the interior of the cell is negative with respect to the exterior.
• If the resting potential is temporarily reduced to a threshold value of perhaps -50 mV, this sets in motion a chain of events that begins at the base of the axon. Positively charged sodium ions rush in, rapidly causing the local charge at the base of the axon to reverse (to perhaps +30 mV). Positively charged potassium ions in the axon then rush out, bringing the charge back to negative. The ions are then pumped back across the membrane to their starting positions.
• The narrow region of positive charge causes the next segment of axon to trigger, starting the same process there, and so on; the region of positive charge moves rapidly down the axon. This is the action potential or neural impulse.

The Synapse and the Neuromuscular Junction

• Each terminal button joins to either another neuron, a gland cell, or a muscle cell. The connection to another neuron, which takes place at a dendrite or on the soma, is termed a synapse. When the terminal button connects to a muscle cell, the connection is termed a neuromuscular junction.
• The base of the terminal button connects to a portion of the membrane of the receiving cell via tiny fibers, leaving a small, fluid-filled gap between the two membranes.
• The terminal button contains numerous small bags called vesicles. Each vesicle contains about 10,000 molecules of a special chemical called a neurotransmitter substance.
• The surface of the receiving cell's membrane contains molecules called receptor protiens.
• When an action potential arrives at the terminal button, the vesicles move toward the synaptic gap. Those that reach the surface burst open, spilling their contents into the gap where they quickly diffuse to the other side and attach to the receptor protiens of the receiving cell.
• The receptor protiens then change their shape, allowing ions (charged atoms) to enter the receiving cell. An influx of positive ions (e.g., sodium ions) would tend to reduce the negative charge inside the receiving cell (resting potential). If this occurs in enough places, the receiving cell will reach threshold and fire its own action potential. Or, in the case of the neuromuscular junction, the receiving muscle cell will contract.
• Finally, the neurotransmitter is removed from the receptor protiens, terminating the action. One way this happens is when a special enzyme breaks down the neurotransmitter.

How Drugs Affect the Synapse or Neuromuscular Junction

• Blocking -- the drug can occupy the receptor sites for a neurotransmitter but fail to activate the receptor. This is like placing chewing gum in a lock -- even if you have the right key, it can't get into the lock and open it.
• Activating -- the drug can occupy the receptor sites for a neurotransmitter and activate the receptors like the "real thing."
• Prolonging the action -- by interfering with the mechanism that removes the neurotransmitter from the receptors. The natural neurotransmitter then continues to stimulate the receiving cell for a longer-than-normal time. One way this happens is by inhibiting the enzyme that normally breaks down the neurotransmitter.

Excitatory versus Inhibitory Synapses

• Excitatory synapses -- activation of the synapse causes the receiving neuron to become depolarized (the negative charge is reduced toward zero). This makes it more likely that the neuron will fire its own action potential.
• Inhibitory synapses -- activation of the synapse causes the receiving nturon to become hyperpolarized (the negative charge of the receiving neuron is increased). This makes it less likely that the neuron will reach threshold and fire an action potential.
• A given synapse is always either excitatory or inhibitory.
• Whether a synapse is excitatory or inhibitory depends on the type of neurotransmitter it uses and on the nature of the receptor protiens.

Some Principles of Operation

• All or none principle -- When a neuron fires an impulse, it is always the same: you either get a full impulse or none at all.
• Frequency coding -- the summed strength of all inputs to a neuron (excitatory and inhibitory) determine the frequency (number of impulses per second) of firing. Thus the intensity of stimulation is coded (represented) by the frequency of firing.
• "Doctrine of specific nerve energies" -- how a stream of neural impulses is interpreted by the brain depends not on the source of these impulses, but on where in the brain they are analyzed. Thus if impulses coming in from the auditory nerve from the ears were to arrive at the visual cortex rather than the auditory cortex, one would experience patterns of light rather than sounds.