EXSC2530 Lecture 11: Skeletal Muscle II

The events of muscle contraction are initiated by the __ __

nervous system

Dendrites

* receiving end of a neuron
* receives chemical signals/neurotransmitters from other cells.
* relays info to cell body

Cell body

* integrative center
* receives sensory info from dendrites which form synapses with other neurons
* houses nucleus

Nucleus of cell body

where the events of synthesis and production of neurotransmitters take place

Axon

long projection extending out from cell body

Axon hillock/initial segment

where the action potential of a neuron is initiated

Once created, where does the action potential travel?

down the axon hillock to the axon terminal

Axon terminal

The terminal end of the axon

Axon collateral

branch(es) of the axon

Dendrites and cell body of a motor neuron is housed in the __ __ of the spinal cord

grey matter

An action potential generated by the muscle fiber is needed to initiate the events of __

contraction

What are the different classes of neurons?

* Afferent neuron
* Interneuron
* Efferent neuron

Afferent neuron

conveys info to CNS

Interneuron

connects afferent to efferent neurons within CNS

Efferent/motor neuron

transmits info out to PNS

The dendrites and cell body of an efferent/motor neuron are housed within the ___

CNS

The ability of a motor neuron to generate an action potential is mostly influenced by

interneurons

Neurons communicate with other neurons in other cells through the release of __

neurotransmitters

Synapse

functional communication site between cells

Chemical synapses can form between…

* Neuron → neuron (dendrites/cell body)


* Neuron → cell (ex. myofiber)

communication across synapse is achieved via…

release of neurotransmitter from the pre synaptic neuron

What are the categories of a neurotransmitter?

excitatory neurotransmitter and inhibitory neurotransmitter

Excitatory neurotransmitter

increases the probability of an action potential being generated in the post-synaptic neuron/cell

Inhibitory neurotransmitter

decreases the probability of an action potential being generated in the post-synaptic neuron/cell

Neurotransmitters influence the membrane potential in the axon hillock of the __ __

motor neuron

Space between the axon terminal and cell body or dendrite of motor neuron is the __ __

synaptic cleft

There are __ synapses formed between the interneurons within the CNS and motor neurons

many

Some interneurons will release __ neurotransmitters, while others will be releasing __

excitatory, inhibitory

For a muscle fiber to contract, the motor neuron has to generate an __ __

action potential

What are the types of ion channels?

* Voltage regulated ion channels
* Chemically regulated ion channels

Voltage regulated ion channels

open/close in response to changed in membrane potential (voltage)

Chemically regulated ion channels

open/close in response to neurotransmitters

an action potential is dependent on the __ of ion channels, following their __

opening, closure

What is the location of voltage regulated ion channels on motor neurons?

Axon of motor neuron

What is the location of voltage regulated ion channels on muscle fibers?

Sarcolemma of myofibers

What is the location of chemically-regulated ion channels on motor neurons?

Dendrites and cell body of motor neurons

What is the location of chemically-regulated ion channels on muscle fibers?

Specialized region on sarcolemma called the motor end plate, which is a components of the neuromuscular junciton

Neuromuscular junction

synapse between a motor neuron and a muscle fiber

Events that take place in the dendrites/cell body increase or decrease the __ __ in the axon hillock

membrane potential

An increase in membrane potential, __ the probability of an action potential being generated in axon hillock

increases

A decrease in membrane potential, __ the probability of an action potential being generated in axon hillock

decreases

T/F: The threshold value must be reached for an action potential to be generated

true

IPSP

inhibitory postsynaptic potential

repolarization

EPSP

excitatory postsynaptic potentials

depolarization

Entry of __ into the axon hillock depolarizes the postsynaptic membrane

Na+

Summation is required because…

a single EPSP/synaptic transmission is not sufficient to cause an action potential in the axon hillock of the motor neuron

What are the types of summation?

Temporal and spatial summation

Temporal summation

same location (synapse) coming at a different time

T/F: The time between temporal synaptic transmissions is long

false

Spatial summation

different location (synapse), same time

Depolarization

Na+ in

Repolarization

K+ out

Would summation of EPSPs occur in the motor neurons innervating the biceps when you touch a hot stove? Type of summation?

yes, both temporal and spatial summation would occur

Would summation of IPSPs occur in the motor neurons innervating the triceps when you touch a hot stove?

yes, to ensure that triceps don’t contract

Action potential

serves as the stimulus for an adjacent region of the membrane to reach a threshold

Action potential in a neuron is always conducted from axon __ to the axon __

hillock, terminal

Refractory period

degree of readiness of a region of an axon (or plasma membrane) to generate an action potential

Absolute refractory period

incapable of generating an action potential

Relative refractory period

capable of generating an action potential provided that the stimulus is strong

Absolute refractory period corresponds to all of __ and the majority (75-80%) of __

depolarization, repolarization

Absolute refractory period corresponds to a time when a high % of voltage regulated Na channels are in an __ state (i.e. not yet closed; they need to close before they can open)

inactive

Why doesn’t the action potential get conducted back towards the axon hillock?

because the absolute refractory period is active. This makes the Na+ channels inactive until repolarization is complete

myelin

fatty membranous sheath with many layers that wraps around the axon

Nodes of Ranvier

unmyelinated regions of the axon that can detect or measure an action potential

Myelination of axons greatly __ the speed in which the action potential reaches the axon terminal

increase

The axon in the Nodes of Ranvier are exposed to __ __

extracellular fluid

In Nodes of Ranvier, the number of Na+ and K+ voltage regulated channels is __

high

T/F: Nodes of Ranvier increases conduction velocity of the action potential

true

What regions of a myelinated axon will an action potential be generated?

nodes of Ranvier

What regions of an unmyelinated axon will an action potential be generated?

across the entire axon

What is contained in the neuromuscular junction?

axon terminal, synaptic cleft, and motor endplate

Motor endplate

* specialized region of muscle fiber
* contains receptors for neurotransmitter (acetylcholine) that causes muscle fibers to contract
* contains chemically regulated ion channels

There is __ neuromuscular junctions(s) per muscle fiber

one

Where is a neuromuscular junction usually found in a muscle fiber?

the middle

Where is acetylcholine released from?

axon terminal

an action potential on a muscle fiber is generated on the __

sarcolemma

What channels are on the sarcolemma?

voltage regulated ion channels

the events taking place at the neuromuscular junction serve as a __ for the sarcolemma to generate an action potential

stimulus

__ __ serve as an inward path for an action potential to travel

transverse tubule

Around __ vesicles containing acetylcholine with one synaptic transmission

125

__ binds to its receptor on the motor end plate resulting in the opening of chemically regulated ion channels for Na+ and the diffusion of Na+ into the cytoplasm in the motor end plate area

acetylcholine

if the axon hillock generates an action potential, __ regions of the axon will generate an action potential

all

T/F: if the motor neuron generates an action potential, the muscle fiber that’s innervated by that neuron will also generate an action potential

true

To ensure the 1:1 relationship, any acetylcholine that’s in the synaptic cleft will have to be __

removed

if acetylcholine is still present in the synaptic cleft, it will generate another action potential in the sarcolemma, meaning…

there is no longer a 1:1 relationship

Acetylcholine is broken down by __

acetylcholinesterase

Acetylcholinesterase is found on the __ __ __ and within the __

motor end plate, endomysium

Acetate and choline can be __ and __ very quickly if a second action potential quickly follows the first.

recycled, reformed

Transverse tubule

delivers an action potential to the sarcoplasmic reticulum

Dihydropyridine (DHP) receptors

voltage sensing receptors within the transverse tubule membrane

Ryanodine receptor-channels

mechanically linked to calcium release channels in the sarcoplasmic reticulum

Dihydropyridine (DHP) receptors and ryanodine receptor-channels are collectively viewed as…

voltage sensing mechanical gates for calcium

Calcium gates open during…

depolarization

Calcium gates close during…

repolarization

During repolarization, calcium is actively taken up by the *__ __* when there is no longer an action potential

sarcoplasmic reticulum

During repolarization, with calcium no longer bound to troponin, __ slips back to its blocking position over binding sites on actin; contraction end; actin slides back to original resting position

tropomyosin

Motor neuron and sarcolemma both generate action potentials at similar __

times

Muscle twitch

a single momentary muscle contraction, and is the response to a single stimulus

Latent period

* no contractile force/tension
* period of time in which the motor neuron and sarcolemma both generate an action potential

Contraction phase

calcium binds to troponin, myosin binding sites on actin are exposed and cross bridges form, leading to muscle contraction