11-3
Conduction Velocities of Axons
• Conduction velocities vary widely
• Rate of impulse is determined by:
– Axon diameter –
– Presence of a myelin sheath –
Saltatory
conduction of the
AP
Saltatory Conduction (sauter = “to
jump (Fr.)”
• Current passes through a myelinated axon
only at the nodes of Ranvier
• Voltage regulated Na+ channels are
concentrated at these nodes
• Action potentials jump from one node to the
next
Synapses
• HOW NEURONS COMMUNICATE WITH:
– another neuron
– An effector cell (muscle or gland)
• IF NEURON TO NEURON:
• Presynaptic neuron –
• Postsynaptic neuron –synapse
Synapse
NEURON TO NEURON
Axosomatic
Axodendritic
Axoaxonic
PREsynaptic neuron / POSTsynaptic neuron
Figure 11.17
Electrical synapse
Electrical Synapses
• Electrical synapses:
– less common
– Correspond to
– Contain intercellular
– Permit ion flow from one neuron to the next
– BI-directional !!!
– Are found in the brain and embryonic tissue
Chemical synapse
Figure 11.18
Chemical Synapses
• release and receive neurotransmitters
• Typically composed of two parts:
Synaptic Cleft
• Fluid-filled space separating the presynaptic
and postsynaptic neurons
• Prevent nerve impulses from directly
passing from one neuron to the next as in
an electrical synapse
• Transmission across the synaptic cleft:
Synaptic cleft
Postsynapse
presynapse
9-23
Synaptic Delay
• Neurotransmitter must be released, diffuse
across the synapse, and bind to receptor
• Synaptic delay – time needed to do this
(0.3-5.0 ms)
excitatory postsynaptic potential
(EPSP)
Internal membrane at rest
(before EPSP)
---
----
----
---
30
Inhibitory postsynaptic potential
(IPSP)
Internal membrane at rest
(before IPSP)
---
----
----
---
Neurotransmitters
• Chemicals used for neuronal communication
with the body and the brain
• 50 different neurotransmitter have been
identified
• Classified chemically and functionally
Excitatory & inhibitory presynaptic inputs
FUNCTIONS
• Two classifications: excitatory and inhibitory
– Excitatory neurotransmitters cause depolarizations
( glutamate)
– Inhibitory neurotransmitters cause
hyperpolarizations
( GABA and glycine)
• Some neurotransmitters have both excitatory
and inhibitory effects
– Determined by the receptor type of the
postsynaptic neuron
– Example: aceytylcholine
• Excitatory at neuromuscular junctions
• Inhibitory with cardiac muscle
Termination of Neurotransmitter
Effects
• Neurotransmitter bound to a postsynaptic
neuron:
– Produces a continuous postsynaptic effect
– Blocks reception of additional “messages”
– Must eventually be removed from its receptor
• Removal of neurotransmitters occurs when
they:
– Are degraded by enzymes
– Are reabsorbed by astrocytes or the presynaptic
terminals
– Diffuse from the synaptic cleft