Biology 3201 Unit 1 – Maintaining Dynamic Equilibrium II Section 1 – Nervous System “The Neuron”

The Neuron  The basic functional unit of the nervous system.  Function: Send impulses to and from the CNS and PNS and the effectors (muscles/glands)

Neuron Structure

 Dendrite  Fine hair-like extensions on the end of a neuron. Function: receive incoming stimuli.  Cell Body or Soma  The control center of the neuron. Function: Directs impulses from the dendrites to the axon.  Nucleus  Control center of the Soma. Function: Tells the soma what to do.  Axon  Pathway for the nerve impulse (electrical message) from the soma to the opposite end of the neuron.  Myelin Sheath  An insulating layer around an axon. Made up of Schwann cells.  Nodes of Ranvier  Gaps between schwann cells. Function: Saltatory Conduction (Situation where speed of an impulse is greatly increased by the message ‘jumping’ the gaps in an axon).

Types of Neurons  There are 3 types of neurons. 1. Sensory Neurons  Neurons located near receptor organs (skin, eyes, ears).  Function:receive incoming stimuli from the environment. 2. Motor Neurons  Neurons located near effectors (muscles and glands)  Function: Carry impules to effectors to initiate a response. 3. Interneurons  Neurons that relay messages between other neurons such as sensory and motor neurons. (found most often in Brain and Spinal chord).

Types of Neurons

Nerves  Nerves  Collections of neurons that are joined together by connective tissue.  Responsible for transferring impulses from receptors to CNS and back to effectors.

How Do Neurons Operate?  Neuron at Rest  Resting Potential Occurs when the neuron is at rest. A condition where the outside of the membrane is positively(+) charged compared to the inside which is negatively(-) charged. Neuron is said to be polarized. Neuron has a voltage difference of -70 mV

How is resting potential maintained? Ion Distribution

How is resting potential maintained?  At rest, the sodium gates are closed.  Membrane is 50 times more permeable to K + ions causing them to “leak” out.  This causes outside of membrane to have an abundance of + charges compared to inside. The inside of the membrane is negative compared to the outside. This is helped by the (-) proteins etc.  The “sodium-potassium” pump pulls 2 K + ions in for 3 Na + ions sent out. This further creates a charge difference!!

Action Potential The mechanism by which neurons send impulses. They are comprised of electrical signals generated at the soma and moving along the axon toward the end opposite the soma (motor neurons) Action potentials occur in two stages:  Depolarization  Repolarization

Depolarization in an action potential  When the neuron is excited past its “Threshold” the following events occur: Sodium ions (Na + ) rush into the axon. This neutralizes the negative ions inside. The inside of the axon becomes temporarily (+) while the outside becomes temporarily (-). The reversal of charge is known as “depolarization” Nearby Sodium (Na + ) channels open to continue the depolarization.

Let’s Have a look.  urces/toc/unit+4+homeostasis+maintaining+dynamic+equilibriu m/chapter+12+the+nervous+system/cool+stuff+to+see+and+do /movie+channel+behaviour.php

Repolarization  This is the restoring of the (+) charge on the outside of the axon and (-) on the inside. Potassium gates open and potassium floods out.  This generates positive charge on the outside of membrane. Sodium Channels Close (no + charges can get inside) The Sodium/Potassium pump rapidly moves Sodium out of the cell.  Further creates the (+) charge outside with a (-) charge inside.

Refractory Period  Brief period of time between the triggering of an impulse and when it is available for another. NO NEW action potentials can be created during this time.

Saltatory Conduction  The “jumping” of an impulse between the “Nodes of Ranvier” thus dramatically increasing it’s speed.  Only occurs in axons having Myelin.  2m/s  120 m/s

All or None Response  If an axon is stimulated above its threshold it will trigger an impulse down its length.  The strength of the response is not dependent upon the stimulus.  An axon cannot send a mild or strong response. It either responds or does not!!!

Homework  View ILO 06 – Structure of a Neuron Complete Activity 1 and 2 (Go to Work)  View ILO 07 – Nerve Tansmission Answer Q. 2, 4, 5, 7  Explain the critical role Myelin plays to an axon. Include at least two roles it plays.

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