This document summarizes key aspects of impulse conduction and neurotransmitters discussed in Chapter 10. It describes how myelinated axons conduct impulses faster than unmyelinated axons via saltatory conduction between nodes of Ranvier. It also explains how neurotransmitters are released at synapses and can either excite or inhibit postsynaptic neurons through EPSPs and IPSPs. Summation of these synaptic potentials determines whether an action potential is triggered in the postsynaptic cell. Examples of different neurotransmitters like acetylcholine, monoamines, amino acids, and gases are provided, along with how neuronal pools process nerve impulses through convergence and divergence.
2. Impulse Conduction
Myelinated axons conduct impulses differently than unmyelinated axons.
Unmeylinated Axon generates a series of action potentials along the
entire axon
The impulse is slow (travels at 1 mile/hour)
Myelinated Axon
Myelin is an electrical insulator and prevents action potentials along
myelinated portions of the axon.
Action potentials are generated only at the Nodes of Ranvier
The impulse travels through the myelinated portions by electrical
conduction.
The impulse is fast (travels at 285 miles/hour!)
3. Saltatory Conduction – Action potentials appear to jump from node to node
on myelinated axons
Figure 10.19. On a myelinated axon, a nerve
impulse appears to jump from node to node.
4. Myelinated Vs. Unmeylinated neurons
Myelinated neurons transmit impulses rapidly whereas
unmyelinated neurons transmit impulses slowly.
Example: Think when you cut yourself with a knife.
The sharp instant pain travels on myelinated neurons. Shortly
after, the slow throbbing pain travels on unmyelinated neurons.
5. Synaptic Transmission
The summary of events leading to the release of neurotransmitters. These
events are also outlined in chapter 10, section 3
6. Synaptic Transmission
Neurotransmitters diffuse across the synapse and bind to receptors
(ligand-gated ion channels) on postsynaptic dendrites.
The neurotransmitters cause changes in local (graded) membrane
potential on postsynaptic neuron = synaptic potentials
The neurotransmitters may either excite the post-synaptic cell or it may
inhibit the post-synaptic cell.
7. Synaptic Potentials
• EPSP = Excitatory postsynaptic potential
EPSPs depolarize the local membrane of the postsynaptic neuron
EPSPs increase the likelihood of generating an action potential.
• IPSP = Inhibitory postsynaptic potential
IPSPs hyperpolarize the local membrane of the postsynaptic neuron
IPSPs decrease the likelihood of generating an action potential
8. Summation of EPSPs and IPSPs
EPSPs and IPSPs are added together in a
process called summation
Summation occurs at axon hillock
The integrated sum of EPSPs and IPSPs
determines if an action potential occurs
If threshold stimulus is reached an action
potential is triggered.
Figure 10.20 The synaptic knobs of many axons
may communicate with the cell body of a neuron.
9. Neurotransmitters
The nervous system produces at least thirty different types of neurotransmitters.
Examples:
1. Acetylcholine – skeletal muscle contractions
2. Monoamines
• Norepinephrine
- in CNS it creates a sense of well-being
- in PNS it may stimulate or inhibit autonomic nervous system
•
Dopamine
- in CNS it creates a sense of well-being
- Amphetamines increase the levels of norepinephrine and dopamine
3. Amino Acids
• GABA – inhibitory neurotransmitter of the CNS
• Many sedatives and anesthesia enhances GABA secretions
• Schizophrenia is associated with a deficiency of GABA
4. Gases
• Nitric Oxide
• Vasodilation in PNS
10. Impulse Processing
Nerve impulses are processed by the CNS in a way that reflects
the organization of neurons in the brain and spinal cord.
Neuronal Pools – organized group of interneurons within the CNS.
Pools are organized as neural circuits that perform a common
function, even though they may be in different parts of the CNS
May have either excitatory or inhibitory effects on effectors or other
pools
11. Neuronal Pools
Convergence – several neurons synapse onto one
post-synaptic neuron
•A neuron may sum impulses from different sources
e.g. Information from various sensory receptors
may converge onto a single processing center
Divergence – impulse spreads from one axon
to several post-synaptic neurons.
• A single neuron may ultimately stimulate
many neurons - Amplifies an impulse
End of chapter 10
