Download 6.2 Transmission of Nerve Impulses

Chapter 6 The Nervous System
6.2
Transmission of Nerve Impulses
- neurons have the property of irritability similar to muscles
- neurons also have the property of conductivity = the ability to convert a stimulus
into a nerve impulse
A. Action Potentials
1. A neuron at rest has more sodium ions (Na+) outside the membrane than potassium
(K+) ions inside, therefore there is a more negative charge inside the neuron
- The neuron is said to be polarized
2. If stimulus is received it must reach a critical voltage before it will have an effect
on the neuron
- Once critical voltage is reached, gated channels in the neuron’s membrane open
and sodium ions (NA+) rush into the cell . The net effect is depolarization of
the membrane (more positive charge inside the membrane)
- The depolarization travels the length of the neuron creating an “impulse” or
action potential.
- Once depolarization occurs it will always travel the entire length of the neuron
3. After the depolarization travels the length of the neuron, the gated channels close,
ions are pumped out the cell and the membrane becomes repolarized
- The time between completion of the action potential and repolarization is
known as the refractory period, no new impulse can be sent, the neuron is
considered “fatigued”
B. Impulse Transmission
1. The speed that the impulse travels is controlled by three factors:
a. Presence or absence of a myelin sheath
b. Diameter of the axon
c. Temperature of the body
2. In myelinated axons, depolarization only occurs at the Nodes of ranvier, the
impulse is able to jump over the myelinated portions of the axon. This process is
known as salutatory conduction and is MUCH faster impulse transmission
3. In unmyelinated axon, the larger the diameter, the faster the impulse transmission
4. Warmer temperature increase ion diffusion rates, colder impedes ion diffusion
rates. Warmer temperatures increase impulse transmission, cold slows it down
5. Transmission rates:
- Limb position = 119 m/s
- Touch
= 76 m/s
- Pain
= 1 m/s
- Thought
= 20 – 30 m/s
- Anything over 1 m/s needs myelinated axons
6. At the axon terminal, the action potential opens calcium ion gates
- Flood of calcium causes neurotransmitter vesicles to fuse with the cell
membrane and then release them into the synaptic cleft
o Neurotransmitters have either:
1. Excitatory effect (acetylcholine) – activate muscle fibers
2. Inhibitory effect (endorphins) – stop transmission of pain impulses
- Last step is to remove the neurotransmitting substance from the synapse
C. Reflexes – simple, rapid, involuntary, programmed responses
a. Two types
1. Somatic reflexes – those involved with skeletal muscles
2. Autonomic reflexes – those involving cardiac muscle or smooth muscle