Synapses Figure 11.17

Fluid-filled space separating the presynaptic and postsynaptic neurons Synaptic Cleft Fluid-filled space separating the presynaptic and postsynaptic neurons Prevents nerve impulses from directly passing from one neuron to the next Transmission across the synaptic cleft: Is a chemical event (as opposed to an electrical one) Ensures unidirectional communication between neurons

Synaptic Cleft: Information Transfer Neurotransmitter Ca2+ Na+ Axon terminal of presynaptic neuron Action potential Receptor 1 Postsynaptic membrane Mitochondrion Postsynaptic membrane Axon of presynaptic neuron Ion channel open Synaptic vesicles containing neurotransmitter molecules 5 Degraded neurotransmitter 2 Synaptic cleft 3 4 Ion channel closed Ion channel (closed) Ion channel (open) Figure 11.18

Synaptic Cleft: Information Transfer Ca2+ Axon terminal of presynaptic neuron Action potential 1 Axon of presynaptic neuron Figure 11.18

Synaptic Cleft: Information Transfer Ca2+ Axon terminal of presynaptic neuron Action potential 1 Mitochondrion Axon of presynaptic neuron Synaptic vesicles containing neurotransmitter molecules 2 Figure 11.18

Synaptic Cleft: Information Transfer Ca2+ Axon terminal of presynaptic neuron Action potential 1 Postsynaptic membrane Mitochondrion Axon of presynaptic neuron Synaptic vesicles containing neurotransmitter molecules 2 Synaptic cleft 3 Ion channel (closed) Ion channel (open) Figure 11.18

Synaptic Cleft: Information Transfer Neurotransmitter Ca2+ Na+ Axon terminal of presynaptic neuron Action potential Receptor 1 Postsynaptic membrane Mitochondrion Postsynaptic membrane Axon of presynaptic neuron Ion channel open Synaptic vesicles containing neurotransmitter molecules 2 Synaptic cleft 3 4 Ion channel (closed) Ion channel (open) Figure 11.18

Synaptic Cleft: Information Transfer Neurotransmitter Ca2+ Na+ Axon terminal of presynaptic neuron Action potential Receptor 1 Postsynaptic membrane Mitochondrion Postsynaptic membrane Axon of presynaptic neuron Ion channel open Synaptic vesicles containing neurotransmitter molecules 5 Degraded neurotransmitter 2 Synaptic cleft 3 4 Ion channel closed Ion channel (closed) Ion channel (open) Figure 11.18

Synaptic Delay Neurotransmitter must be released, diffuse across the synapse, and bind to receptors Synaptic delay – time needed to do this (0.3-5.0 ms) Synaptic delay is the rate-limiting step of neural transmission

Postsynaptic Potentials The two types of postsynaptic potentials are: EPSP – excitatory postsynaptic potentials IPSP – inhibitory postsynaptic potentials

Excitatory Postsynaptic Potential (EPSP) Open Na+ channels Figure 11.19a

Inhibitory Postsynaptic (IPSP) Open K+ and Cl- channels Figure 11.19b

Summation A single EPSP cannot induce an action potential Temporal summation Spatial summation Figure 11.20

Chemical Neurotransmitters 50 different neurotransmitters Acetylcholine (ACh) Biogenic amines Amino acids Peptides Novel messengers: ATP and dissolved gases NO and CO

Neurotransmitters: Acetylcholine Degraded by the enzyme acetylcholinesterase (AChE) Released by: All neurons that stimulate skeletal muscle Some neurons in the autonomic nervous system Tetanus – bacterial toxin

Neurotransmitters: Biogenic Amines Catecholamines – dopamine, norepinephrine (NE), and epinephrine Indolamines – serotonin and histamine emotional behaviors

Neurotransmitters: Amino Acids Include: GABA – Gamma ()-aminobutyric acid Glycine Aspartate Glutamate Found only in the CNS

Neurotransmitters: Peptides Include: Substance P – mediator of pain signals Beta endorphin, dynorphin, and enkephalins Act as natural opiates; reduce pain perception Bind to the same receptors as opiates and morphine Gut-brain peptides – somatostatin, and cholecystokinin

Neurotransmitters: Novel Messengers ATP Is found in both the CNS and PNS Produces excitatory or inhibitory responses depending on receptor type Induces Ca2+ wave propagation in astrocytes Provokes pain sensation

Neurotransmitters: Novel Messengers Nitric oxide (NO) Activates the intracellular receptor guanylyl cyclase Is involved in learning and memory Carbon monoxide (CO) is a main regulator of cGMP in the brain

Functional Classification of Neurotransmitters Two classifications: excitatory and inhibitory Excitatory neurotransmitters cause depolarizations (e.g., glutamate) Inhibitory neurotransmitters cause hyperpolarizations (e.g., GABA and glycine)

Functional Classification of Neurotransmitters Some neurotransmitters have both excitatory and inhibitory effects Determined by the receptor type of the postsynaptic neuron Example: acetylcholine Excitatory at neuromuscular junctions with skeletal muscle Inhibitory in cardiac muscle

Neurotransmitter Receptor Mechanisms Direct: neurotransmitters that open ion channels Promote rapid responses Examples: ACh