Pathophys Exam 2 Lecture 1 . Nerves, Synapses and NMJ

What are the four functional zones of a neuron

Input zone (dendrites and soma); Trigger zone (axon hillock); Conducting zone (axon); Output zone (axon terminals)

What is the role of dendrites in the input zone

Receive synaptic inputs; contain ligand-gated ion channels; integrate incoming signals

What is the function of the axon hillock (initial segement)

Initiates action potentials; serves as the trigger zone

What is orthograde axonal transport and what protein mediates it

Movement from cell body to axon terminals; mediated by kinesin

What is retrograde axonal transport and what protein mediates it

Movement from axon terminals to cell body; mediated by dynein

What is Wallerian degeneration

Degeneration of the distal axon segment after axonal injury; due to loss of axonal transport

What is the resting membrane potential of a neuron (not producing electrical signals)

Approximately -70 mV; maintained by Na+-K+ ATPase and ion gradients

what are the excitable tissues?

nerve cells and muscle cells, have the ability to produce rapid transient changes in their membrane potential when excited thay serves as electrical signals.

How does the Na+-K+ ATPase contribute to resting membrane potential

Pumps 3 Na+ out and 2 K+ in; creates net negative charge inside the cell

What equation calculates membrane potential based on ion permeability

Goldman-Hodgkin-Katz equation

(extension of Nernst equation)

changes in membrane voltage are brought about by changes in…

membrane permeability to ions (Pion) via the opening and closing of ion channels.

What equation calculates equilibrium potential for a single ion

Nernst equation

What happens to Vm when PNa increases dramatically

Vm (membrane potential) approaches ENa (equilibrium potential); depolarizes to approximately +61 mV

What happens to Vm when Pk increases dramatically

Vm (membrane potential) approaches Ek (equilibrium potential); hyperpolarizes to approximately -90 mV

what are the two large classifications of membrane channels?

leak channels or gated channels

What are the types of gated ion channels

Voltage-gated; chemically-gated; mechanically-gated; thermally-gated

What are leak channels

Channels that are always open; allow passive ion movement

what are the two primary forms of electric signals?

graded/electrotonic potentials and action potentials

What are graded potentials

Local; non-propagated (do not spread down the membrane) changes in membrane potential; vary in magnitude and duration. short distance signals

What are examples of graded potentials

EPSPs; IPSPs; receptor/generator potentials; pacemaker potentials; slow-wave potentials; end-plate potentials

what are two important charactersistics of graded potentials?

strength and duration of trigerring event.

What determines the amplitude of a graded potential

Strength of the triggering event

(y axis)

What determines the duration of a graded potential

Duration of the triggering event

How do graded potentials spread

Decrementally; diminish with distance from origin

What is an action potential

Rapid; large (100 mV) propagated electrical signal; involves reversal of membrane potential. serve as long distance signals.

propogated nondecrementally/ do not diminish in strength

What ion movement causes the rising phase of an action potential

Na+ influx through voltage-gated Na+ channels

What marks the absolute refractory period?

Closure of the Sodium inactivation gate

What ion movement causes the falling phase of an action potential

K+ efflux through voltage-gated K+ channels

What causes after hyperpolarization

Continued K+ efflux due to slow closing of K+ channels

What is the absolute refractory period

Period during which no new action potential can be initiated; Na+ channels are inactivated

What is the relative refractory period

Period during which a stronger stimulus is required to initiate an action potential; K+ channels still open

What is contiguous conduction

Action potential spreads along every patch of membrane; occurs in unmyelinated axons

What is saltatory conduction

Action potential jumps between nodes of Ranvier; occurs in myelinated axons

how does stronger stimulus influence action potentials?

it does not cause a LARGER action potential but it does icnrease the NUMBER of action potentials per second.

What factors affect conduction velocity of action potentials

Myelination (saltatory conduction) and axon diameter (larger the diameter, larger the speed)

What is the role of myelin in conduction

Increases speed; enables saltatory conduction. jump between nodes of ranvier

What cells produce myelin in the PNS

Schwann cells

What cells produce myelin in the CNS

Oligodendrocytes

What is multiple sclerosis

Autoimmune demyelinating disease; affects CNS myelin

what is the importance of the absolute refractory period?

it forces action potentials to move in one direction

What is the all-or-none law of action potentials

Action potentials either occur fully or not at all; no partial responses

How is stimulus strength encoded in neurons

By frequency of action potentials; not amplitude

What are the types of nerve fibers and their conduction velocities

Aa (70–120 m/s); Aß (30–70 m/s); Ay (15–30 m/s); Aδ (12–30 m/s); B (3–15 m/s); C (0.5–2.3 m/s)

take home that diamter increases speed of action potential

what structures can a neuron terminate in

muscle, gland, or another neuron

What is a synapse

Junction between two neurons; site of neurotransmitter release

What are the two types of synapses

Electrical and chemical

How do electrical synapses transmit signals

Via gap junctions; bidirectional; rapid; unregulated

Where are electrical synapses found

Retina; tooth pulp; rare in human nervous system

How do chemical synapses transmit signals

Via neurotransmitter release; unidirectional; regulated

What is the synaptic cleft

Space between presynaptic and postsynaptic neurons; prevents direct electrical transmission

action potentials must be propogated by chemical means.

What triggers neurotransmitter release at chemical synapses

Ca2+ influx into presynaptic terminal via voltage-gated Ca2+ channels

describe the steps at a synaptic cleft in a chemical synapse

What is synaptic delay

Time required for neurotransmitter release and receptor binding

what are the two types of post synaptic potentials?

EPSP and IPSP

What are EPSPs

Excitatory post-synaptic potentials; caused by Na+ influx

What are IPSPs

Inhibitory post-synaptic potentials; caused by K+ efflux or Cl- movement

Can a neurotransmitter cause both EPSPs and IPSPs

Yes; depends on receptor type and location

e.g norepinephrine can be an EPSP at one synapse and an IPSP at another

BUT the response to a given neurotransmiter receptor combination is ALWAYS the same

What are common neurotransmitters

Acetylcholine; dopamine; norepinephrine; serotonin; histamine; glycine; glutamate; aspartate; GABA

What determines the postsynaptic response

Sum of all EPSPs and IPSPs; graded potential summation (two forms, temporal and spatial)

What is temporal summation

Repeated stimulation of one presynaptic input; EPSPs add together

What is spatial summation

Simultaneous stimulation of multiple presynaptic inputs; EPSPs add together

What is EPSP-IPSP cancellation

Simultaneous excitatory and inhibitory inputs cancel each other

What is presynaptic inhibition

Inhibitory neuron reduces neurotransmitter release from excitatory neuron; axo-axonic synapse

What is postsynaptic inhibition

Inhibitory neuron releases neurotransmitter directly onto postsynaptic neuron

What is the neuromuscular junction (NMJ). What is unque about motor neurons?

Synapse between motor neuron and skeletal muscle fiber

As a motor neuron approaches a muscle it divides and lsoes its myelin sheath to form numerous axon terminals that innervate multiple muscle fiber cells that make uo the muscle.

what is the terminal button?

axon terminal ends that have enlarged knoblike structures

What is the motor end plate

Specialized region of muscle membrane under terminal button; contains ACh receptors

What neurotransmitter is released at the NMJ

Acetylcholine (ACh)

What triggers ACh release at the NMJ

Ca2+ influx into terminal button via voltage-gated Ca2+ channels

Describe the steps at the neuromuscular junction

Are EPPs? excitatory or inhibitory?

ALWAYS excitatory

Unlike synapses in the CNS, the NMJ is designed to be reliably excitatory—there’s no inhibitory counterpart here. One motor neuron firing = one muscle fiber contracting. Efficient and direct.

What is the end-plate potential (EPP)

Graded depolarization of muscle membrane; caused by ACh binding to receptor-channels

Why is the EPP larger than typical EPSPs

Multiple release sites due to multiple terminal buttons; more neurotransmitter; more receptors; larger surface area thus larger grader potential

Does the motor end plate have a threshold potential?

NO, so action potentials ar enot initated at this site. current from the EPP flows between the depolarized end plate which is usually in the kiddle of a long musclew fiber into the adjacent resting cell membrane in all directions.

THESE adjacent areas get to threshold and a subsequent action potential

How does EPP lead to muscle action potential

Local current flow opens voltage-gated Na+ channels in adjacent membrane; initiates AP

What terminates the muscle response to ACh

Acetylcholinesterase degrades ACh in the synaptic cleft

How does AP propagate in muscle fibers

By contiguous conduction; bidirectional from NMJ

How does NMJ differ from a synapse

NMJ is always excitatory; one-to-one transmission; no inhibition at NMJ