Chapter 34 Notes (con't)

II. Action Potential: At rest, a neuron is a cell that maintains a net negative charge inside the axon (Sodium pumps keep positive Na ions outside the axon membrane). When a great enough pressure is applied to the dendrites, there is a sudden rush of sodium ions into the axon by means of transmembrane protein gates. As the book explain (page 578), this is a positive feedback mechanism in which there is a threshold level that determines if a neuron ‘fires.’ After firing, a neuron returns to its original resting state by use of sodium/potassium channels (both of these are cations, or positive ions).

A. Threshold: The minimum change in voltage needed to cause a neuron to a neuron to ‘fire.’

B. All or nothing: Like a row of dominos, a neuron fires with the same intensity once threshold is reached. It is basically ON/OFF switch, not a guitar amp that goes up to 11.

C. Restored resting potential: Shortly after firing, potassium ions flow out of the axon and sodium/potassium pumps will then restore resting potential.

III. Synapses: Gap that exists between the axon terminal of one neuron and the dendrites of another. Where two neurons meet, a synaptic gap exists. One neuron has a number of vesicles within the axon terminal, which are release when an action potential reaches the end of the axon (these vesicles leave through exocytosis). The vesicles release neurotransmitters, chemicals useful in helping two neurons communicate. The neurotransmitter will be accepted by the dendrite of the postsynaptic neuron, if the dendrite has the correct protein receptors for the specific neurotransmitter.

 

A. Neurotransmitter: Chemical responsible for bridging the synaptic gap between two adjacent neurons.

1. Excitatory: Neurotransmitters that cause the adjacent neuron to fire.

2. Inhibitory: Neurotransmitters that prevent the adjacent neuron from firing (as the book notes, this may be the mechanism by which many antianxiety drugs act on the human brain).

B. Dendritic receptors: The transmembrane proteins embedded within the dendrites. These have specific shapes/active sites that match with given neurotransmitters.

C. "Resting" synapse: the neurotransmitters should be degraded or moved to a neighboring neuroglia cell. Cocaine acts by preventing the body from degrading the dopamine (a neurotransmitter) in a synapse.

D. Effectors: Some neurons don’t connect to another neuron, but rather to a gland or muscle. In this case, the chemical released will induce a response such as contraction.

IV. Information Flow: There are many details to add to the diagram of the neuron used in class. The first is a myelin sheath, which is made of many neuroglia cells that coat the outside of long neurons. These cells greatly increase the speed of transmission by allowing ions to ‘jump’ along the axon, from one exposed region of sodium/potassium pumps to the next. Also, many reflexes (automatic reactions) don’t involve the impulse going all the way to the brain. A reflex arc is a shortcut in which the sensory neuron and motor neuron join in the spinal column, short-circuiting the brain and thereby speeding up impulse transmission.

A. Myelin Sheath: A neuroglia layer around some long axons. They speed up impulse transmission (100-200 times).

B. Reflex Arc: Responsible for fast reflexes. Involve sensory/motor connections in spine, not the brain.