A&P Chapter 11 Fundamentals of the Central Nervous System and Nervous Tissue - Notes Part 2 of 2

Chemical synapses transmit signals from one neuron to another using neurotransmitters.

1. Action potential arrives at axon terminal. 2. Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. 3. Ca2+ entry causes synaptic vesicles to release neurotransmitter by exocytosis. 4. Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane. 5. Binding of neurotransmitter opens ion channels, resulting in graded potentials. 6. Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse.

A Synapse

A junction where information is transferred from one neuron to the next neuron or from a neuron to an effector (muscle or gland). Most common synapses: Axodendritic - between axon endings of one neuron and dendrites of next neuron. Axosomatic - between axon endings of one neuron and cell body of next neuron.

EPSPs and IPSPs can be Generated at the Same Time.

A postsynaptic neuron can receive excitatory and inhibitory inputs at the same time. EPSPs summate and IPSPs summate but they can cancel each other out.

Synaptic Events are Integrated and Modified

A single EPSP does not induce an AP in postsynaptic neuron. EPSPs must add together or summate. Two types of summation: Temporal summation, Spatial summation.

Continuous Conduction Occurs in Nonmyelinated Axons.

AP propagation in nonmyelinated axons is very slow. Voltage-gated ion channels must open along the entire length of the axon.

Absolute and Relative Refractory Periods.

Absolute refractory period: the period of time from when Na+ channels open until the Na+ channels reset to their original resting state. No stimulus, no matter how strong can generate another AP during this time. Relative refractory period: period of time following absolute refractory period where most Na+ channels have returned to their resting state, and repolarization is still occurring. A stronger than normal stimulus is required to reopen Na+ channels to generate another AP.

Classification of Neurotransmitters by Chemical Structure.

Acetylcholine - Neuromuscular junctions. Biogenic Amines (based on amino acid structure)- Catecholamines: dopamine, norepinephrine and epinephrine. Indolamines: serotonin and histamine. Imbalances are associated with mental illness. Amino Acids. Peptides (strings of amino acids) - Include the endorphins. Purines (nitrogen-containing chemicals) - ATP is a major neurotransmitter (usually excitatory). Adenosine is usually inhibitory and caffeine blocks adenosine receptors. Gases and Lipids - Are not stored in vesicles like other neurotransmitters. Bind to intracellular receptors. NO is thought to increase synaptic strength, involved in memory and learning. Excessive release of NO is found in stroke patients.

The Rate of AP conduction depends on Axon Diameter and Degree of Myelination of Axon.

Axon diameter - the larger the axon's diameter, the faster the impulse travels. Degree of myelination - the presence of a myelin sheath makes AP conduction much faster. Myelin insulates axon and prevents charges from leaking from the axon.

Types of circuits in neuronal pools.

Diverging circuit: One input, many outputs. An amplifying circuit. Example: A single neuron in the brain can activate 100 or more motor neurons in the spinal cord and thousands of skeletal muscle fibers Reverberating circuit: Signal travels through a chain of neurons, each feeding back to previous neurons. An oscillating circuit. Controls rhythmic activity. Example: Involved in breathing, sleep-wake cycle, and repetitive motor activities such as walking. Parallel after-discharge circuit: Signal stimulates neurons arranged in parallel arrays that eventually converge on a single output cell. Impulses reach output cell at different times, causing a burst of impulses called an after-discharge. Example: May be involved in exacting mental processes such as mathematical calculations.

Neurotransmitters can be Classified by Function by Two Broad Ways.

Effects: Excitatory versus Inhibitory. Excitatory cause depolarization. Inhibitory causes hyperpolarization. Some neurotransmitters can do both depending on which type of receptor they interact with. Example: ACh is excitatory in skeletal muscle and inhibitory in cardiac muscle. Actions: Direct versus Indirect. Those that act directly bind to and open ion channels: channel-linked receptors. Fast synaptic signals. Indirectly - bind through second-messenger molecules (think: a game of telephone). Broader, longer-lasting effects.

There are Electrical Synapses and Chemical Synapses

Electrical synapses are less common and are a result of direct connections between touching neurons. Ions can flow directly from one neuron to the next. Very rapid transmission. Chemical synapses utilize chemical neurotransmitters. Made of axon terminal with synaptic vesicles with neurotransmitter. A postsynaptic membrane with neurotransmitter receptors.

Postsynaptic potentials can be excitatory or inhibitory.

Excitatory postsynaptic potential (EPSP) - An EPSP is a local depolarization of the postsynaptic membrane that brings the neuron closer to AP threshold. Neurotransmitter binding opens chemically gated ion channels, allowing Na+ and K+ to pass through simultaneously. Inhibitory postsynaptic potential (IPSP) - An IPSP is a local hyperpolarization of the postsynaptic membrane that drives the neuron away from AP threshold. Neurotransmitter binding opens K+ or Cl- channels. EPSP - excitatory postsynaptic potential is a graded potential that results in a net depolarization of the postsynaptic membrane. If strong enough and if it diffuses to axon hillock, will generate an AP on the axon. IPSP - inhibitory postsynaptic potential hyperpolarizes the postsynaptic membrane Makes neuron less likely to generate AP because makes membrane more permeable to K+ (moving out) and Cl- (moving in)

Action potential propagation in nonmyelinated and myelinated axons.

In bare plasma membranes, voltage decays. Without voltage-gated channels, as on a dendrite, voltage decays because current leaks across the membrane.

A Neurotransmitter is Used by Neurons to Communicate with Other Neurons and the Rest of the Body.

More than 50 types of neurotransmitters have been identified. Most neurons make two or more. Neurotransmitters are classified chemically and functionally.

There are Types of Circuits Involving Neuronal Pools.

Neuron pools refer to how many neurons interact together. Patterns of synaptic connections in neuronal pools are called circuits. There are four basic circuit patterns: Diverging, Converging, Reverberating, Parallel after-discharge circuits.

Neural integration of EPSPs and IPSPs.

No summation: 2 stimuli separated in time cause EPSPs that do not add together. Temporal summation: 2 excitatory stimuli close in time cause EPSPs that add together. Spatial summation: 2 simultaneous stimuli at different locations cause EPSPs that add together. Spatial summation of EPSPs and IPSPs: Changes in membane potential can cancel each other out.

Input Processing can be Serial and Parallel.

Serial processing - input travels along one pathway to a specific destination. One neuron stimulates the next, and so on. Spinal reflexes: rapid, automatic responses to stimuli that occur in reflex arcs. Parallel processing - input travels along several different pathways to be integrated in different CNS regions (how the brain processes information).

All APs have the Same Magnitude, so the CNS Changes the Frequency of AP Generation in Relation to Stimulus Strength.

Strong stimuli generate Aps more OFTEN in a given time interval than do weak stimuli. Stimulus intensity is coded by the # of impulses per second or frequency of action potentials.

Temporal Summation Refers to When a Postsynaptic Neuron Receives Many Signals in Quick Succession.

Temporal = time. One or more presynaptic neurons sends many impulses very quickly, one after the other. The first impulse produces an EPSP, and before it dissipates, another forms causing the postsynaptic membrane to depolarize more than from just one EPSP.

Saltatory Conduction is 30 Times Faster than Continuous Conduction.

The AP appears to jump from gap to gap along the axon. Current generated by an AP moves rapidly to the next myelin sheath gap to trigger another AP.

Spatial Summation Occurs When the Postsynaptic Neuron is Stimulated Simultaneously by Many Presynaptic Neurons Spread Around the Postsynaptic Neuron

Usually more than one presynaptic neuron has terminals that synapse with the postsynaptic neuron. If many presynaptic neurons send impulses to one postsynaptic neuron, those EPSPs that result will summate and enhance depolarization.