Biology 105
Biology of Exercise
Spring 2003

2/5/03 - post class outline

Learning objectives:

• How does a reflex pathway work?
• How can precise muscular movements be made?
• How do nerve cells carry signals through the body?

Diagrams

• Reflexes
• Stretch reflex 1
• Stretch reflex 2
• Sodium pump (Na-K-ATPase)
• Channels
• The action potential

The stretch reflex

Proprioreception

• joint receptors
• muscle receptors

The nerve cell and action potentials

• Nerve cells have a cell body and a long axon that often projects to another region of the body.
• The sodium pump creates Na and K gradients
  • In sensory cells and nerve cells, the Na-K-ATPase (sodium pump) moves 3 Na out of the cell, in exchange for two K into the cell.  The sodium pump uses ATP and sets up chemical and electrical gradients.
  • The inside of the cell is negative compared to the outside because 3 Na go out in exchange for only 2 K coming in.
  • Na concentration is low inside the cell, and K concentration is high inside the cell.
• Receptors
  • An electrical signal is produced at a sensory receptor when ion channels are opened, allowing ions to flow across the membrane.
  • Na will flow into the cell through Na channels
  • K will flow out through K channels.
  • For example, a pressure receptor could detect stretch of the nerve cell membrane.  The stretching process will open Na channels, allowing positive charge to flow into a cell.
  • In another example, a chemoreceptor might work in the following way:  A chemical binds to a Na channel on the sensory cell membrane, this causes the Na channel to open and positive charge to flow into the cell.
• Information is carried by means of stereotyped electrical impulses called action potentials. Action potentials are the result of rapid opening of Na channels, causing positive charge to flow into the nerve cell, and then opening of K channels, causing K to flow out of the cell.
• Small electrical events at receptors can trigger action potentials.
• An action potential can trigger action potentials in the adjacent portion of a nerve cell. In this way, action potentials propagate along a nerve cell from one part of the body to another. Importantly, the intensity of the signal is not diminished, because all action potentials are nearly identical.