Human Physiology Unit 2 Flashcards

what kind of receptors do neuromuscular junctions contain?

nicotinic

Explain the somatic motor pathway

extends from CNS and releases ACh that binds to nicotinic receptor on target skeletal muscle

what are NaChR channels of skeletal muscke similar to?

N_NACh receptors on neurons

what kind of channels are NaChR channels?

chemically-gated channels with 2 binding sites for ACh

are NaChR channels inhibitory or excitatory?

always excitatory and create muscle contractions

what is the synapse of a somatic motor neuron on a muscle fiber called?

neuromuscular junction (NMJ)

what does the NMJ consist of?

axon terminals, motor end plates, schwann cells, synaptic cleft, postsynaptic membrane

what do the motor end plates on muscle membrane in the NMJ contain?

high concentrations of ACh receptors and lies opposite to axon terminal that is modified into motor end plate

what is the synaptic cleft between the axon and muscle in the NMJ filled with?

fibrous matrix whose collagen fibers hold the axon terminal and motor end plate in proper alignment

what does the matrix in the synaptic cleft in the NMF contain?

acetylcholinesterase

acetylcholinesterase

enzyme that rapidly deactivates ACh by degrading it into acetyl and choline

what do nicotinic ACh receptor (nAChR) channels do in the active zone?

cluster together

what are the steps that lead to muscle contractions?

1. Action potentials arrive at axon terminal and open voltage-gated calcium channels in membrane

2. calcium diffuses into cell down its electrochemical gradient to trigger release of ACh containing synaptic vesicles

3. ACh diffuses across synaptic cleft and combines with NAChRs on skeletal muscle membrane

4. chemically gated ion channels with 2 binding sites for ACh

5. when ACh binds to receptor, channel gate opens and allows monovalent cations to flow through

   1. in skeletal muscle, net entry into muscle fiber depolarizes it, triggering action potential that causes contraction of the skeletal muscle cell

what are the 3 types of muscle?

skeletal, cardiac, smooth

what kind of muscle is the skeletal muscle and what is it attached to?

striated muscle attached to bones of skeleton

what do skeletal muscles do?

control body movement and responsible for voluntary control

what neurons do skeletal muscles respond to?

somatic motor neurons

what kind of fibers are in skeletal muscles?

large, multinucleate cells that appear striped/striated under miscroscope

what kind of muscle is the cardiac muscle and what is it attached to?

striated muscle and found only in heart

what do cardiac muscles do?

move blood through circulatory system and responsible for involuntary control

what do cardiac muscle respond to?

autonomic innervation, spontaneous contraction, modulated by endocrine system

what kind of fibers are in cardiac muscle?

striated, smaller, branched, and cells are uninucleate and joined in series by junctions

what are intercalated discs

junctions that join cells in the fibers of cardiac muscle together in series

what kind of muscle is the smooth muscle and what is it attached to?

primary muscle of internal organs and tubes

what do smooth muscle do?

influence movement of material into, out of, and within body and responsible for involuntary control

what do smooth muscle respond to?

autonomic innervation, spontaneous contraction, modulated by endocrine system

what kind of fibers are in smooth muscle

small fibers that lack striations

in skeletal muscles, thick filament is…

actin

in skeletal muscles, thick filament is…

myosin

in thick filament, heavy chains are…

motor domain and have myosin ATPase

in thick filament, light chains are…

regulatory function

what are the regulatory proteins in skeletal muscle?

tropomyosin and troponin

what are the accessory proteins in skeletal muscle?

titin and nebulin

what do T-tubules do?

allow rapid AP movement from cell surface into interior of fiber

where do t-tubules transmit action potentials for synchronous contraction?

to regions near the sarcoplasmic reticulum

how do t-tubules help with synchronous contraction?

allow for coordinated calcium release throughout fiber and leads to uniform contraction

what are t-tubules?

extensions of cell membrane (sarcolemma) that associate with the ends (terminal cisternae) of the sarcoplasmic reticulum

myofibrils

muscle fiber contractile structures

sarcomere

contractile unit of microfibril

sarcomere: z discs

have filaments between them that have accessory proteins that link think filaments together

sarcomere: I band

made of thick filaments (actin)

sarcomere: A band

made of darker regions where both light and heavy vilaments overlap

sarcomere: H zone

clear band in middle of A band and has heavy filaments only

sarcomere: M line

proteins to which heavy filaments attach

what happens to sarcomeres overall during contraction?

shorten

in sarcomeres, what happens to actin and myosin during contraction?

don’t change length but slide past one another in an energy-dependent process

in sarcomeres, what happens to the H zone, I band, and A band during contraction?

H zone and I band shorten; A band remains constant

what are titin and nebulin together and what do they do?

proteins that ensure proper alignment of filaments in the sarcomere

what is titin responsible for individually?

stabilizes myosin

what is nebulin responsible for individually?

aligns actin

muscle tension

force created by muslces

load

weight/force that opposes contraction

relaxation

release of tension

what are the steps to skeletal muscle contraction

1. events occur at the NMJ

2. excitation-contraction (E-C) coupling

3. contraction-relaxation cycle

in regards to length, what happens to fibrils?

remain at a fixed length

what do myosin crossbridges do?

move actin filaments

myosin crossbridges: power stroke

myosin crossbridge swivels and pulls actin toward M line

myosin crossbridges: end of power stroke

myosin releases actin and resets and binds to another actin (heads are not released in unison)

true or false? the power stroke occurs only in one cycle

False; the powerstroke is repeated multiple times

myosin ATPase

energy places the myosin head into a “cocked" position”)

what happens when myosin comes in contact with actin

myosin ATPase initiates a pull

what does troponin do?

controls position of tropomyosin

what does troponin C do?

reversibly binds to calcium

what does tropomyosin do and why?

partially covers myosin binding site on actin to prevent myosin and actin from interacting

what are the steps of contraction when initiated by calcium signaling?

1. calcium released from terminal cisternae

2. calcium binds troponin

3. troponin pulls tropomyosin from myosin-binding sites on actin

4. myosin binds tightly to and moves actin

5. repeated as long as binding sites uncovered and ATP is available

where do crossbridges form and why?

forms between actin and myosin heads to trigger contraction

After a powerstroke, there is a rigor state where…

myosin is tightly bound to G-actin 

steps of rigor state

1. ATP binds and myosin detaches (ATP decreases myosin affinity for actin)

2. ATP hydrolysis provides energy for myosin head to rotate and reattach to actin (head weakly binds to new actin)

3. powerstroke that begins in response to calcium binding to troponin (release of P_i allows head to swivel, pulling actin toward M line)

4. myosin releases ADP and makes room for next ATP

what does myosin ATPase do?

breaks down ATP to ADP and P_i

E-C coupling steps

1. ACh released from somatic motor neuron

2. ACh initiates AP in muscle fiber (binds to receptors on sarcolemma and causes depolarization of cell for an end plate potential and muscle AP)

3. muscle AP triggers calcium release from sarcoplasmic reticulum

4. calcium combines with troponin to initiate contraction

twitch

single contraction-relaxation cycle in skeletal muscle fiber

relaxation

when calcium is pumped back to sarcoplasmic reticulum using SERCA

timing of E-C coupling: latent period

short delay between muscle action potential and beginning of muscle tension development

what is central fatigue due to?

CNS

what is peripheral fatigue due to?

neuron/muscle

peripheral fatigue: what does submaximal exercise do?

extended submaximal exercise leads to depletion of glycogen stores

peripheral fatigue: what does short-duration maximal exertion do?

leads to increased levels of P_i which can slow P_i release from myosin and decrease calcium release

peripheral fatigue: what does maximal exercise do?

leads to ion imbalances because potassium leaves the muscle fiber, which leads to increased extracellular concentrations of potassium and altered membrane potential; also leads to changes in sodium/potassium pump activity

what are skeletal muscles classified by?

speed and fatigue resistance

skeletal muscle: slow-twitch fibers (ST / type I)

rely primarily on oxidative phosphorylation

skeletal muscle: fast-twitch fibers

develop tension faster (split ATP more rapidly), pump calcium into SR more rapidly

skeletal muscle: fast-twitch oxidative-glycolytic fiber (FOG / type IIA)

use oxidative and glycolytic metabolism

skeletal muscle: fast-twitch glycolytic fibers (FG / type IIB/X)

rely primarily on anaerobic glycolysis

what does myoglobin do?

binds oxygen in muscle so it is readily available for aerobic processes

oxidative fibers have what?

more myoglobin

sarcomeres contract with optimum force under what conditions?

if at optimum length (not too long or short) before contraction begins

what is tension directly proportional to?

number of crossbridges

the force of contraction increases with what?

summation — contractions are stronger when the muscle does not relax completely between action potentials

tetanus

maximal contraction

what is a motor unit?

1 motor neuron + all of the muscle fibers it innervates

what does contraction force depend on?

types and numbers of motor units

what does asynchronous recruitment of motor units help?

helps avoid fatigue

isotonic contractions

move loads

isotonic contractions: concentric action

shortening

isotonic contractions: eccentric action

lengthening 

isometric contractions

force without movement

isometric contractions: what happens to sarcomeres?

shorten while elastic elements stretch, resulting in little change in overall length

What does this graph show and give an example?

phasic smooth muscle that is usually relaxed; esophagus

what does this graph show and give an example?

phasic smooth muscle that cycles between relaxation; intestine

What does this graph show and give an example?

tonic smooth muscle that is usually contracted; sphincter that relaxes to allow material to pass