The Nervous System

1. The Nervous System

2. The Neuron • Consists of a cell body with a nucleus and other organelles and 2 types of extensions: dendrites and axons • Dendrites: sensory; they receive incoming messages from other cells and carry the electrical signal to the cell body. Each neuron can have 100s! • Axon: only one per neuron; can be several feet long (in large mammals). Transmits an impulse from the cell body outward to another cell. • See figure

3. Structure of a vertebrate neuron

4. The Neuron • Synapse: the space between 2 neurons • Neurotransmitters: chemical messengers sent across the synapse from one neuron to another

5. Resting Potential • Describes the difference in electrical charge across a cell membrane • Resting potential is the membrane potential of a neuron at rest; typically -60 to -80mV (millivolts) potential; the inside of the neuron is negative to the outside • Figure 48.6 

6. Membrane Potential

7. Action Potential • An all or nothing response • If a stimulus is great enough to excite a neuron, we say the cell “reaches its threshold” and an action potential will occur

8. Steps in an Action Potential • Depolarization: the stimulus opens Na+ channels in the membrane, allowing Na+ to rush inside the cell, changing the membrane potential to a positive value (from negative at rest) = more + inside, - outside • Repolarization: the membrane potential is restored when K+ channels open and K+ leaves the cell. The charge is restored to resting potential but the ions are on the wrong sides of the membrane • Refractory period: Na-K pump reestablishes the original ion distribution – Na is pumped out of the cell, K is pumped in = ACTIVE TRANSPORT; during this period, a neuron cannot send another impulse

10. Action Potential is like a domino effect down the entire neuron