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: IF NEURON TO NEURON: another neuron An effector cell (muscle or gland) IF NEURON TO NEURON: Presynaptic neuron – Postsynaptic neuron –synapse

Synapse NEURON TO NEURON Axodendritic Axosomatic 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) - - - - - - - - - - - - - -

Inhibitory postsynaptic potential (IPSP) 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