Chapter 45: Synapses Transmission of Nerve Impulses

Between Neurons

Chad Smurthwaite & Jordan Shellmire

The Chemical Synapse The most common type of synapse

used for signal transmission in the

central nervous system

Anatomy of a Chemical Synapse

Soma - Main body of a neuron

Axon - Extensions from the soma

into a peripheral nerve that

leaves the spinal cord

Dendrites: Numerous branching

projections from the soma

Transmission of Signals

The first neuron converts the electrical signal into a chemical one

The first neuron secretes a neurotransmitter to stimulate the second neuron

The neurotransmitter is sent across the synaptic cleft

Where it binds with proteins in the membrane of the second neuron that stimulates the neuron

It can excite, inhibit, or modify its sensitivity in some other way

One way conduction

This allows for signals to be directed towards specific goals

Presynaptic Terminals

Mitochondria provide ATP for the synthesis of

neurotransmitters

Transmitter vesicles contain transmitter

substances that are released into the

synaptic cleft via exocytosis

The release of neurotransmitters is caused

when an action potential depolarizes the

presynaptic membrane, opening voltage-

gated calcium channels

This activates proteins that promote the fusion

of the vesicles to the membrane,

Which of the following voltage-gated ion channels open in response to an action potential in the presynaptic terminal? A. Potassium B. Sodium C. Calcium D. Chloride

Postsynaptic Potentials

A change in the membrane

potential of the postsynaptic cell

caused by the action of

neurotransmitters released by

the presynaptic cell

Postsynaptic Potentials

Excitatory (EPSP)

Open Na+ channels (influx)

Inhibit K+ & Cl- channels

Begin to depolarize membrane

May lead to action potentials

Inhibitory (IPSP)

Open K+ (outflow) and Cl- (inflow) channels

Hyperpolarizes membrane

Inhibits ability to generate action potentials

Fig 45-9

Fig 45-11

Summation

Spatial

Additive effect of stimuli from various axons

Transmission of an impulse by simultaneous

or nearly simultaneous stimulation of two

or more presynaptic neurons

-Two separate neurons firing separated by

space

Temporal

Additive effect of successive stimuli from an axon

Transmission of an impulse by rapid

stimulation of one or more presynaptic

neurons

-Same neuron firing, separated by time

Fig 45-10

True or False Temporal Summation occurs when a second

postsynaptic potential (excitatory or inhibitory) arrives

before the membrane has returned to its resting level.

Characteristics of Synaptic Transmission

Characteristics of synaptic transmission

Fatigue

When synapses are repetitively stimulated at a rapid

rate, the response of the postsynaptic neuron

diminishes over time, and the synapse is said to be

fatigued

Mainly a result of INCREASED BUILD-UP of CALCIUM

in the synaptic bouton and an inability to replenish

the supply of NT agent rapidly

Effect of PH

More acidic pH values -->

DECREASE excitability

More basic pH values --->

INCREASE neuronal activity

Characteristics of synaptic transmission Effect of hypoxia

Neuronal excitability is highly dependent on an

adequate supply of oxygen. Cessation of oxygen

for only a few seconds can cause complete

inexcitability of some neurons. This is observed

when the brain’s blood flow is temporarily

interrupted, because within 3 to 7 seconds, the

person becomes unconscious.

A decrease in the supply of OXYGEN -->diminishes

synaptic activity

Effect of anesthetics

Most anesthetics increase the neuronal membrane

threshold for excitation and thereby decrease

synaptic transmission at many points in the

nervous system. Because many of the anesthetics

are especially lipidsoluble, it has been reasoned

that some of them might change the physical

characteristics of the neuronal membranes,

making them less responsive to excitatory agents.

Types of Neurotransmitters

Group 1: Small, Rapidly Acting

General mode of action is to either alter ion channel conductance OR stimulate

receptor-activated enzyme systems

Synthesized in the cytosol of presynaptic terminals and stored in secretory

vesicles

Examples: Acetylcholine, biogenic amines, amino acid derivatives, and nitric

oxide

Acetylcholine

Location: All neuromuscular junctions, many CNS neurons, preganglionic

neurons of the ANS, and postganglionic neurons of PSNS, some SNS

Action: Typically excitatory, some inhibitory effects in PSNS

Biogenic Amines

Dopamine

Location: Midbrain

Action: Usually inhibitory

Norepinephrine

Location: Many CNS neurons, also in most postganglionic neurons of SNS

Action: Excitatory or inhibitory depending on the target

Serotonin

Location: Brainstem

Action: Inhibitory

Amino Acids & Derivatives

Glutamate

Location: Secreted by presynaptic terminals in many of the sensory

pathways entering the CNS

Action: Fast excitatory synapses of brain, fast-pain fibers in spinal cord,

plays a role in strokes

Nitric Oxide

Location: Nerve terminals in brain related to long-term behavior and memory

Action: Synthesized as needed (not stored)

Readily diffuses through membranes

Doesn’t significantly directly alter membrane potential

Modifies intracellular metabolic activity of postsynaptic neurons to affect

neuronal excitability

Group 2: Neuropeptides

Often hormones or releasing/inhibiting factors

Characteristics:

More potent than fast acting transmitters

Smaller quantities produced

Actions more prolonged

Synthesis: Synthesized in neuron cell bodies, packaged by Golgi apparatus and

transported down the axon to the terminal

Then stored/exocytosed in secretory vesicles

Clearance of Neurotransmitters

Enzymatic Degradation (example - acetylcholine)

Vesicles are Recycled - Once they release their neurotransmitters, they

temporarily become part of the membrane. The vesicle portion then recedes

back into the presynaptic terminal and pinches off to form a new vesicle

The new vesicle still contains the appropriate materials for synthesizing new

transmitter substances