Biology 107

Lecture Notes: Nervous System I.

Reading: Chapter 48 in text.

Overview of the nervous system

• The nervous system has three overlapping functions

• Sensory input

• Integration

• Motor output

• These functions involve both parts of the nervous system:
• Central nervous system (CNS)

• Peripheral nervous system (PNS)

• The nervous system is composed of neurons and supporting cells
• Neurons

• Dendrites

• Axons

• axon hillock

• Synapse

• Neurotransmitters

Functional organization of neurons

• There are three major classes of neurons

• Sensory neurons

• Interneurons

• Motor neurons

• Clusters of neurons

• nucleus

• ganglion

• Supporting cells

• Glia

• Astrocytes

• Oligodendrocytes

• Schwann cells

• Myelination

The nature of nerve signals

• All cells have an electrical membrane potential or voltage across their plasma membranes

Ion	Conc. Inside cell	Conc. Outside cell	Difference
· Na^+	· 15 mM	· 150 mM	10 X outside
· K^+	· 150 mM	· 5 mM	30 X inside
· Cl^-	· 10 mM	· 120 mM	12 X outside
Anions in cell	· 100 mM	· --------	· ------

• The selective permeability of the plasma membrane maintains ionic differences

• Ion channel

• Voltage gated

• Ligand gated

• An action potential is an all-or-none change in the membrane potential

• excitable cells

• resting potential

• graded potentials

• threshold

• An action potential is the rapid change in the membrane potential of an excitable cell, caused by stimulus-triggered selective opening and closing of voltage-gated ion channels

• An action potential has four phases

1. Resting

2. Large depolarization phase

3. The steep repolarizing phase

4. The undershoot phase

Action potentials “travel” along an axon because they are self propagating

· Factors affecting the speed of action potential propagation

· Saltatory conduction