AP 1, Unit 3, Ch 9 - Muscle Histology and Physiology

Organization of a muscle, from simple to complex

-myofilament

-myofibril

-muscle fiber

-muscle fascicle

-whole muscle

What are the functions of the muscular ssystem?

1) movement of the body

2)maintenance of posture

3) respiration

4) production of body heat

5) communication

6) constriction of organs and vessels

7) contraction of the heart

What are the three types of muscle?

1)skeletal

2)smooth
3)cardiac

where is skeletal muscle located?

attached to bones

where is smooth muscle located?

walls of hollow organs, blood vessels, eyes, glands, and skin

where is cardiac muscle located?

the heart

striated appearance

skeletal muscle

spindle-shaped appearance

smooth muscle

striated but less muclei

cardiac muscle

What are the four properties of muscle tissue? (explain)

1. Contractability - ability to shorten/contract

2. excitability - ability to receive an electrical stimulus

3. extensibility - the ability to stretch beyond normal length

4. elasticity - the ability to return to normal length after stretching

What is a muscle cell called?

a muscle fiber

what are the three layers of CT in skeletal muscle? (explain)

1. epimysium - forms a CT sheath that surrounds each skeletal muscle

2. perimysium - separates whole muscles into bundles of muscle fibers

3. endomysium - separates individual muscle fibers

What nerve cells are responsible for stimulating skeletal muscle contraction

motor neurons

What is hypertrophy in skeletal muscles?

The increase in size of muscle fibers, not in number of muscle fibers.

What are the two main components in a skeletal muscle?

1. electrical

2. mechanical

What are the electrical components of a skeletal muscle? (explain)

1. sarcolemma - plasma membrane of muscle fibers

2. transverse tubules - inwards folds of the sarcolemma, forms T tubules that carry electric signals into the muscle so everything contracts at once

3. sarcoplasmic reticulum - smooth ER that releases ca2+ as a “switch” for contraction

What is a “triad” in electrical components of skeletal muscle?

the terminal cisternae at the ends of the sarcoplasmic reticulum that run alongside T tubules, critical component of muscle contraction

sarcoplasm

the cytoplasm of a muscle fiber

What are the mechanical components of a muscle fiber? (explain)

1. myofibril - bundles of protein filaments which interact to shorten the muscle during contraction

2. myofilaments - two different types in each myofibril: actin myofilaments (thin filaments) and myosin myofilaments (thick filaments) arranged into ordered units called sarcomeres

sarcomere

join end to end to form the myofibril

What are Z disks in a sarcomere?

delicate structure found at the end of a sarcomere to which myofilaments attach

What are the three regions of a sarcomere? (explain)

I band, darker A band, I band. Each I band includes a Z disk, I bands have only actin myofilaments.

What is in the A band? (explain)

H zone, which has a darkened line called the M line. A band is made up of actin and myosin filaments, but H zone is only myosin filaments. M line holds myosin filaments in place.

What does the protein titin do in skeletal muscle?

give ability to stretch (extensibility) and recoil (elasticity)

Actin myofilament structure (explain)

1. Globular (G) actin - chain of 200 Gs creates fibrous (f) actin. Each G subunit has a active site for myosin myofilaments to bind during contraction

2. tropomyosin - long fibrous protein that lies in the groove along F actin. In relaxed muscle, it covers G binding sites.

3. troponin - consists of a subunit that anchors to actin, subunit that prevents tropomyosin from uncovering G binding sites, and another subunit that binds Ca2+

What are myosin myofilamets composed of?

Myosin head, rod portion

action potentials

electrical signals from motor neurons which cause action potentials in the muscle fibers. Occurs when an excitable cell is stimulated, changes the membrane potential so that the inside of the membrane becomes positively charged compared to the outside.

What is in a neuromuscular junction?

axon terminals and the area of the muscle fiber sarcolemma they innervate

neuromuscular junction and synapses

the point of contact between a motor neuron and muscle fiber

presynaptic terminal

anlarged axon terminal or terminal bouton

synaptic cleft

space between the presynaptic terminal and the muscle fiber

postsynaptic membrane/ motor-end plate

membrane around the muscle fiber at the neuromuscular junction

synaptic vesicles

released from the presynaptic membrane and diffuse across the synaptic cleft to the muscle fiber

acetylcholine (ACh)

neurotransmitter for motor neurons

sliding filament model

model for shortening or contracting muscle (H zone dissapears)

resting membrane potential (explain)

the charge in an unexcited cell. The concentration of K+ inside the membrane is higher than outside, the concentration of Na+ outside is higher than inside, and the membrane is more permeable to K+ than Na+. (sodium-potassium pump, ch 3)

What are the two phases of an action potential?

1. depolarization

2. repolarization

explain “depolarization” in an action potential

depolarization is when the inside of the membrane becomes more positive, and when the membrane reaches a certain threshold, an action potential is triggered

What is the “treshold” in action potential?

the membrane potential at which voltage gated Na+ channels open

explain “repolarization” in an action potential

the return of the membrane potential to its resting value after opening of voltage gated Na+ channels, depolarization, then opening of K+ channels.

all-or-none principle of action potentials

when a stimulus is applied to a cell, either an action potential is produced or it is not, a cell will either contract all the way or it won’t

action potential frequency

the number of action potentials produced in a certain amount of time, the greater the stimulus to a neuron/muscle fiber the more action potentials will fire

acetylcholinesterase

breaks down ACh so it doesn’t accumulate in the synaptic cleft

excitation-contraction coupling (explain)

begins with neuromuscular junction, action potential propagated along sarcolemma and T tubules. At T tubules, APs cause voltage gated Ca2+ channels to open in terminal cisternae, when they open Ca2+ rapidly diffuses out into sarcoplasmic reticulum. In the sasrcoplasm, , Ca2+ binds to troponin and frees up binding sites for myosin heads to attach to actin myofilaments, muscles then contract when those cross bridges move.

cross-bridge cycling (explain)

Ca2+ binds to troponin and active sites on G heads are exposed. This forms a cross-bridge and triggers movement of myosin heads at their hinge region. this movement is called the power stroke. As long as Ca2+ is present, the cycle repeats.

muscle twitch

the response of a muscle fiber to a single action potential

What are the three phases of a muscle twitch

1. lag (latent phase)

2. contraction

3. relaxation

What is the lag phase of a muscle twitch?

the gap in time between the time of stimulus application to the motor neuron and the beginning of the contraction. The time in which the AP is traveling along the axon, NM junction, and travels the sarcolemma.

What is the contraction phase of a muscle twitch?

starts when Ca2+ is released and the cross bridge cycling occurs

What is the relaxation phase of a muscle twitch?

Ca2+ is pumped from sarcoplasm, Ca2+ levels decline and cross bridge cycling stops

What are the two types of muscle contractions?

1. isometric

2. isotonic

isometric contractions

the muscle does not shorten. Tension in muscle is increased but the length stays the same. (ie lifting something that is too heavy for your or standing straight with erect posture)

isotonic contractions

the muscle shortens. When you lift an object or move your limbs.

motor unit

a single motor neuron and all the muscle fibers it innervates

passive tension

tension applied to the load when a muscle stretches but is not stimulated

total tension

the sum of active and passive tension

muscle tone

when muscles stay contracted for long periods of time

slow twitch muscle fibers

contract slowly, have better developed blood supply, more mitochondria, and more fatigue resistant. Respond slowly to nervous stimulation.

fast twitch

respond rapidly to nervous stimulation, they are able to break down ATP more rapidly. Whitish in appearance bc of worse blood supply.

What are the ATP dependent enzymes in muscle contraction?

1. the myosin head

2. the Na+-K+ pump to maintain membrane potential

3. Ca2+ reuptake pump

Anaerobic respiration

does NOT require O2

Aerobic respiration

Does require O2.

fatigue

temporary state of reduced work capacity

caveolae

shallow invagination in smooth muscle that performs a function similar to T tubules aand sarcoplasmic reticulum

How does smooth muscle contraction work?

• Ca2+ enters sarcoplasm and binds to protein calmodulin

• calcium-calmodulin activates an enzyme myosin kinase

• myosin-kinase transfers a phosphate group from ATP to heads of myosin molecules

• cross bridge cycling occurs but slower

• relaxation occurs when myosin phosphate removes phosphate group from myosin molecules

latch state

sustained tension in smooth muscle

What are the two types of smooth muscle?

1. visceral - occurs in sheets. Includes digestive, reproductive, and urinary tracts

2. multiunit - walls of blood vessels, arrector pili, small bundles, iris of the eye

intercalated disks

cell-to-cell attachments in cardiac muscle include gap junctions for communications