apc10(2)

The outstanding characteristic of muscle tissue is its \________

ability to shorten or contract

Skeletal Muscle

Muscle tissue mostly attached to bones

Cardiac Muscle

Muscle tissue located on the heart wall

Smooth Muscle

Muscle tissue that surrounds hollow organs and tubes

Functions of muscular tissue

Producing body movements

Properties of muscular tissue

Electrical excitability

Electrical excitability

Ability to receive and respond to stimuli by action potential production

Contractility

Ability to shorten

Extensibility

Ability to stretch

Elasticity

Ability to recoil

Skeletal muscle contains

Muscle fibers: individual muscle cells

Connective tissue: surrounds muscle fiber and whole muslce

Blood vessels and nerves

The connective tissue layers that surround and protect the muscle are

Epimysium

Epimysium

Connective tissue that surrounds the ENTIRE muscle

Perimysium

Connective tissue that penetrates the muscle and separates and surrounds the muscle fibers into bundles of 10-100 fibers

Endomysium

Thin connective tissue extensions enveloping each muscle FIBER

Faschia

Connective tissue sheets which are continuations of the epimysium. There are two types: deep faschia and superficial faschia

Deep faschia

Connective tissue sheets between neighboring muscles (carry nerves

Superficial faschia

(hypodermis) (subcutaneous layer) connective tissue sheets between muscle and skin (adipose)

Tendons

white

Aponeurosis

sheet-like layer of connective tissue joining a muscle to the part that it moves

Origin

the more STATIONARY bone to which the muscle is attached (head) (usually proximal)

Insertion

the more MOBILE end (of bone) (usually distal)

Belly

the bulk of the muscle

Myoblasts

immature cells giving rise to muscle fibers. They fuse together to become multinucleated mature muscle fibers that cannot divide

Satellite cells

Inactive myoblasts that assist with mature muscle fibers; have the potential to divide

Hypertrophy

englargement of existing muscle fibers

Fibrosis

replacement of muscle fibers by fibrous scar tissue following damage

Sarcolema

plasma membrane

Sarcoplasm

Cytoplasm

T (transverse) tubules

in-folding of sarcolemma; carries electrical current (charge) from surface to cell's interior. Leads to Ca++ release from terminal cisternae

Muscle fiber

made up of MYOFIBRILS

Sarcoplasmic Reticulum (SR)

membranous sacs encircle myofibril; stores Ca++

Terminal cisternae

dilated sacs of SR alongside T-tubules

Triad

T-tubule + 2 terminal cisternae on either side

Thick filaments

mainly made up of the contractile protein MYOSIN

Myosin

A large protein molecule with a globular head attached to a long tail. There are 300 of these molecules/ thick filament. The tails like along a long axis and the heads extend outwards to form cross-bridges.

Thin filaments

made up of the contractile protein ACTIN

Actin

subunits contain myosin BINDING site

Tropomyosin

thread-like protein extending end to end along the actin surface (1 per 7 G-actin subunits). It blocks myosin binding sites on actin

Troponin

small protein bound to tropomyosin; can bind Ca++

Striations

Occur due to actin and myosin organization in skeletal and cardiac muscle

A Band striations

dark band; extends over entire length of thick filament. Includes the H-zone and M line

H-zone

lighter region in middle of A band. Has 0 thin filaments

M line

proteins at center of H zone (middle of sarcomere)

I Band striations

lighter band; thin filaments only. Includes a Z disc

Z disc

narrow line bisecting I band. It is the protein to which thin filaments are anchored to

Sarcomere

compartmental arrangement of the filaments. Each segment of myfibril from Z to Z. It is the functional contractile unit of muscle fiber

The main steps of contraction of a skeletal muscle fiber

1. Excitation of a skeletal muscle fiber

Neuromuscular Junction (NMJ)

Synapse between the motor neuron (mn) and the muscle fiber. Usually one per skeletal muscle fiber

Motor Neuron

Somatic nerve cell supplying the neural stimulus for skeletal muscle fiber contraction

Motor end plate (MEP)

Region of fiber's plasma membrane which lies directly under terminal portion of motor neuron axon

Cleft

separates the motor neuron and the motor end plate

Steps of the excitation of a skeletal muscle fiber (indirect stimulation)

1. Release of acetylcholine

2. Acetylcholine binds to acetylcholine receptors on fiber's motor end plate

3. Production of Muscle action potential

4. Termination of Acetylcholine activity

Step 1 of the excitation of skeletal muscle fiber

Release of acetylcholine. The motor neuron transmits electrical impulses (action potentials). The action potentials reach synaptic end bulb. The acetylcholine molecules are released from motor neuron ending by exocytosis. The acetylcholine diffuse across cleft

Step 2 of the excitation of skeletal muscle fiber

Acetylcholine binds to acetylcholine receptors on fiber's motor end plate. Na+ channels open

Step 3 of the excitation of skeletal muscle fiber

Production of muscle action potential. Depolarization of plasma membrane at motor end plate (excitatory postsynaptic potential) due to Na+ channels opening leads to electrical impulses. Action potential propagates (travels) along membrane. Action potential then initiates a series of intracelluar events that lead to the mechanical event of contraction

Step 4 of excitation of a skeletal muscle fiber

Termination of acetylcholine activity. Acetylcholinesterase is an enzyme in clef that breaks down acetylcholine.

Boltulinum toxin

blocks exocytosis of acetylcholine from motor neuron

Curare

blocks acetylcholine receptors

Anticholinesterases

slow acetylcholinesterase which leads to an increase in acetylcholine in the cleft which leads to muscle strength. It slows the breaking down process

Excitation-Contraction Coupling

Series of events by which a propagated action potential leads to thick and thin filament interaction (contraction)

Steps to excitation-contraction coupling

1. propagated action potential passes from the plasma membrane (sarcolemma) along the t-tubule
2. as the action potential passes along t-tubules it causes the opening of Ca++ channels in the sarcoplasmic reticulum (SR) which leads to Ca++ entering the cytosol
3. Ca++ binds to troponin causing troponin to change shape
4. This conformation moves tropomyosin away from the myosin binding site on actin which leads to cross bridging between thick and thin filaments. This leads to muscle contraction

Contraction Cycle

Events that cause the filaments to slide

Steps of Contraction Cycle

1. ATP hydrosis (energizes the myosin head)
2. /Crossbridges (myosin head attaches to actin)
3. /Power stroke (crossbridges rotate toward center of sarcomere)
4. /Detachment of mypsin from actin (due to ATP binding)

Sliding Filament Theory

The force of the myosin heads of the thick filaments pulling on the thin filaments is transmitted to the plasma membrane of the fiber and ultimately to the load. If the muscle tension \> load this leads to thin filaments being pulled past thick filaments. This leads to Z lines drawn closer together and fiber shortening.

Relaxation

Step of contraction of a skeletal muscle fiber in which Ca++ is actively pumped back into sacroplasmic reticulum. Troponin strengthens attachment with actin leading to tropomyosin moving back into blocking positition until another action potential arrives causing Ca++ release

3 ways a muscle fiber can form ATP

1. Phosphorylation of ADP by creating phosphate (creatine phosphate) (unique to muscle fibers)
2. /Oxidation phosphorylation of ADP in the mitochondria (aerobic metabolism)
3. /Substrate phosphorylation of ADP (anaerobic metabolism)

Creatine phosphate

Immediate energy. Phosphorylation of ADP by this is a rapid means of forming ATP. It provides only enough ATP to support muscle contraction during strenuous exercise for a few additional seconds. Supports activities that require short bursts of intense muscle contraction

Nutrients involved with creatine phosphate

Glucose

Aerobic Metabolism (respiration)

Long term energy. It occurs if sufficient O2 is available to muscle. It produces ATP by breaking down glycogen

Anaerobic Metabolism (respiration)

Short term energy. Breaks down glycogen and glucose into lactic acid (non-O2 utilizing). It occurs during periods of intense muscular activity when O2 cannot be supplied fast enough. Can produce more ATP than aerobic metabolism over a limited time. Uses a lot of glucose (or glycogen) and generates H+

Muscle Fatique

inability of a muscle to maintain a particular strength of contraction over time. May be due to a decrease in nutrients

Oxygen debt

difference between the resting rate of O2 consumption and the increase rate following exercise. "recovery oxygen uptake"

Contractile elements

those muscle structures actively involved in contraction. example: thick and thin filaments

Series elastic elements

structures that resist stretching (but can be stretched). It is located between the contractile elements and load including connective tissue (ex. tendon)

Internal tension

the force generated by the contractile elements

External tension

force exerted on load

Tension

force exerted by contracting muscle on an object

Load

force exerted by the object

Isotonic contraction

constant tension in muscle while length changes. Example: when muscle is moving a load. To move a load

Concentric isotonic contraction

constant tension while muscle shortens

Eccentric isotonic contraction

constant tension while muscle lengthens

Isometric contraction

Muscle develops tension

Twitch contraction

The mechanical response of a muscle fiber or motor unit to a single action potential. Muscle contracts rapidly

Latent period of twitch contraction

Phase of twitch contraction when there is a delay immediately following stimulus arrival; short (a few msecs); associated with excitation - contraction coupling processes

Contraction period of twitch contracation

Phase of twitch contraction when tension develops; cross bridges form

Relaxation period

Phase of twitch contraction when there is decreased tension

Two factors that influence the development of muscle tension

Frequency of stimulation and length

Why does frequency of stimulation influence the development of muscle tension

Since a single action potential in a skeletal muscle fiber lasts 1-2 ms

Twitch

a single stimulus (action potential) that leads to contraction and relaxation

Wave summation

leads to an increase in the mechanical response of a muscle fiber to a second action potential occurring during the mechanical response produced by a previous action potential. In other words

Tetanus

maintained contraction in response to repetitive stimulation. Two types: incomplete (unfused) and complete (fused)

Incomplete tetanus

partial muscle relaxation between stimuli

Complete tetanus

stimuli arrive so rapidly that there is no relaxation (normal physiology)

Length influences the development of muscle tension because

the length at which the fiber develops the greatest tension is optimal length (lo). If the muscle is overstretched no thick and thin filaments overlap which means crossbridging does not occur and there is no fiber contraction. If the muscle is compressed the thin filaments overlap which causes interference which leads to a decrease in active tension.