section 5, chapter 10

Impulse Conduction

&Neurotransmitters

ivyanatomy.com

Impulse ConductionMyelinated 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 AxonMyelin 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!)

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.

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.

The summary of events leading to the release of neurotransmitters. These events are also outlined in chapter 10, section 3

Synaptic Transmission

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.

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

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.

NeurotransmittersThe 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

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

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

Neuronal Pools

End of chapter 10