Biology 2121 – Independent Notes

Introduction to the Nervous System – Chapter 11

I. Know the following information concerning the classification of neurons – Table 11.1

There are three different neurons based on structural differences:

  • Multipolar – three or more processes; major neuron type in the CNS; most are interneurons connecting motor and sensory neurons; some serve as motor neurons.
  • Unipolar – one process; found mostly in the PNS; most are sensory neurons (move impulses from sensory receptors in the peripheral parts of the body to the CNS-spinal cord).
  • Bipolar – two processes; very rare; most are sensory neurons; eye-retina for transmission of visual impulses.

II. Refractory Periods

When a section of membrane is producing an AP, the sodium voltage gated channels are open. At this time no other AP can take place at that time. This is called the absolute refractory period.

During the time period in which the sodium gates close and the potassium gates are open, if a very strong stimulus occurs, another AP may take place. This is called the relative refractory period.

When the potassium and sodium gates are both closed, the membrane is ready for another AP to be produced under normal circumstances.

III. Multiple Sclerosis

Condition that occurs due to the myelin sheaths being destroyed. We refer to this as an auto-immune disease. The immune system cells fail to recognize it’s own cells as itself. They begin to attack the myelin on the neuron cells.

As the myelin degrades, problems controlling muscle movement, speech problems, weakness, blindness may occur. Nerve transmissions become erratic, thus messages sent to the muscles and organs are also erratic.

IV. Synapses and Neurotransmitters

You learned previously about neuromuscular junctions and synapses. For nerves to communicate with one another with in the nervous system, they must rely on neurotransmitters. Remember nerves are not linked together structurally. There is a gap in between them.

When a nerve transmission moves through the first neuron, when it reaches the end of the nerve (axon terminal), a neurotransmitter will be released. For muscles the NT is called acetylcholine. For other nerves there are many different neurotransmitters that are released.

At this point it is important to know the following:

  • All neurotransmitters are released across the synapse.
  • When they reach the second nerve (post-synaptic membrane), they bind to a receptor.
  • After binding to a receptor it may do one of the following:
  • Excite the membrane or inhibit the membrane.
  • Yes, some inhibit the membrane. This inhibits the transmission between the neurons at this point.
  • One more thing:
  • AP cannot travel from one neuron to another. After it reaches the second neuron membrane, it stimulates a graded potential only. If the stimulus is strong enough the graded potential may be converted into an action potential at the axon hillock of the second neuron.
  • Remember, APs may only be produced by muscle and nerve cells.
  • Neurotransmitters you need to know:
  • Acetylcholine – an excitatory NT; excites skeletal muscles; Botulism disease blocks the release of ACh causing paralysis.
  • Norepinephrine – main NT of neurons of the sympathetic nervous system (fight or flight reaction) of the PNS; release enhanced by amphetamines or uppers.