Simple Reflex: Reflex Arc
Reflex arcs involve five essential components: the receptor, the sensory neuron, the interneuron in the spinal cord, the motor neuron, and the effector (muscles). Most reflexes occur without brain coordination.
For example, if you burn your hand on a hot iron, the sensation of heat is detected by receptors in your skin, and an impulse is initiated in a sensory neuron. The sensory neuron carries the impulse to the spinal cord and passes it to a interneuron. Then, the interneuron transmits the impulse to the motor neuron. The motor neuron causes the muscles in your hand to contract and pull away. This process occurs in less than a second, before even the brain is informed. Reflexes are involuntary and often unconscious.
The brain becomes aware of an automatic refelx action when the sensory neurons pass impulses to various interneurons, some of which send impulses to the brain.
Nerve Impulse Transmission
A nerve impulse is an electrochemical event involving the movement of unequally distributed ions across the nerve cell membrane.
The following table explains what occurs in three different stages in a neuron:
A Neuron at Rest: |
· Sodium ions (Na+) remain in a greater concentration outside the neuron
· Potassium ions (K+) are in a greater concentration inside the neuron
· The cell membrane is impermeable to Na+
· The unequal distribution is maintained by a sodium-potassium pump
· The membrane potential inside the nerve cell is negative (approx. -60 millivolts), caused by the large negatively charged proteins
· This is called resting potential
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A Stimulated Neuron: |
· The stimuli could be electrical, mechanical, or chemical
· Membrane becomes permeable to Na+
· The Na+ rush into the nerve cell through protein channels, known as ion gates
· The membrane potential inside the cell becomes positive (approx. +40mv) relative to the outside of the cell
· This is called depolarization, or charge reversal, and it is the nerve impulse
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Restoring a Neuron: |
· As Na+ diffuses into the cell, K+ begins to diffuse out of the cell, restoring the membrane potential to -60mv
· This is called repolarization
· The sodium-potassium pump moves Na+ out of the cell and K+ into the cell through the same type of protein channels
· ATP is used as the energy to maintain the pump
· When the sodium-potassium distribution has returned to normal, the membrane is said to have recovered
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Movement of the Action Potential
The movement of sodium ions into the nerve cell causes a depolarization of the membrane and this signals an action potential. The areas adjacent to the depolarized membrane are in turn affected, resulting in the movement of the impulse along the axon. |