CH 9. HISTOLOGY & PHYSIOLOGY OF MUSCLES

**major characteristics of skeletal, smooth, and cardiac muscle**

skeletal muscle shape:

very long & cylindrical

**major characteristics of skeletal, smooth, and cardiac muscle**

skeletal muscle structure:

* voluntary
* elongated cells
* composed of myocytes
* striations & multinucleated
* contract rapidly but tire easily

**major characteristics of skeletal, smooth, and cardiac muscle**

skeletal muscle function:

* responsible for most body movements
* maintains posture
* stabilizes joints
* generates heat

**major characteristics of skeletal, smooth, and cardiac muscle**

skeletal muscle location:

attached to bones

**major characteristics of skeletal, smooth, and cardiac muscle**

smooth muscle shape:

spindle shaped

**major characteristics of skeletal, smooth, and cardiac muscle**

smooth muscle structure:

* contracts involuntarily
* unstriated
* single nucleus in center
* gap junctions join visceral cells together

**major characteristics of skeletal, smooth, and cardiac muscle**

smooth muscle function:

* regulates flow through blood vessels
* helps maintain blood pressure
* squeezes or propels substances through organs

**major characteristics of skeletal, smooth, and cardiac muscle**

smooth muscle location:

* most widely distributed type of muscle in body
* in walls of hollow organs & tubes
* interior of eye
* walls of blood vessels

**major characteristics of skeletal, smooth, and cardiac muscle**

cardiac muscle shape:

* cylindrical
* branched

**major characteristics of skeletal, smooth, and cardiac muscle**

cardiac muscle structure:

autorhythmic = contract spontaneously at somewhat regular intervals

* involuntary
* striated
* intercalated disks
* single nucleus, centrally located

**major characteristics of skeletal, smooth, and cardiac muscle**

cardiac muscle function:

* pumping blood
* contractions provide force for propelling blood through blood vessels

**major characteristics of skeletal, smooth, and cardiac muscle**

cardiac muscle location:

only in heart

**list functions of muscular system:**

* bodily movement
* maintenance of posture
* production of body heat
* heart beat
* respiration
* communication
* constriction of organs and vessels

**functions of muscular system**

bodily movement:

contraction of skeletal muscle is responsible for overt movements of the body

**functions of muscular system**

maintenance of posture:

skeletal muscles constantly maintain tone and keep us sitting or standing erect

**functions of muscular system**

production of body heat:

when skeletal muscle contract, heat is released as a by-product

**functions of muscular system**

heart beat:

contraction of cardiac muscle results in the heart beat and allows blood to be pumped to the rest of the body

**functions of muscular system**

respiration:

skeletal muscles of thorax responsible for movements necessary for respiration

**functions of muscular system**

communication:

skeletal muscles involved in all aspects of communication

* (writing, speaking, facial expression, etc.)

**functions of muscular system**

constriction of organs & vessels:

construction of smooth muscle within walls of organs & vessels causes constriction

**what are the four functional properties of muscle tissue?**

* contractility
* excitability
* extensibility
* elasticity

**four functional properties of muscle tissue**

contractility:

ability to shorten forcibly 

**four functional properties of muscle tissue**

excitability:

ability to receive & respond to stimuli

**four functional properties of muscle tissue**

extensibility:

ability to be stretched or extended

**four functional properties of muscle tissue**

elasticity:

ability to to recoil & resume original resting length after being stretched 

**muscular fascia:**

connective tissue sheets that separate and compartmentalize muscles

**types of muscular fascia:**

epimysium, perimysium, & endomysium

**types of muscular fascia**

epimysium:

dense collagenous connective tissue that surrounds the entire muscle; most superficial

**types of muscular fascia**

perimysium:

fibrous connective tissue that surrounds groups of muscle fibers known as fascicles (bundles)

**types of muscular fascia**

endomysium:

fine sheath of connective tissue composed of reticular fibers that surrounds each muscle fiber

**list components of muscle fiber:**

* sarcolemma
* sarcoplasm
* myofibrils
* myofilaments (actin & myofibrils)
* sarcomeres

**components of muscle fiber**

sarcolemma:

plasma membrane of muscle cell

**components of muscle fiber**

sarcoplasm:

cytoplasm of muscle cells 

**components of muscle fiber**

myofibrils:

densely packed, rod-like contractile elements that extend from one end of muscle fiber to the other; make up most of muscle volume

* made of two types of myofilaments = actin & myosin

**myofilaments**

(actin & myosin):

form highly ordered unit called sarcomeres

* sarcomeres are joined end to end to form myofibrils

**myofilaments**

actin:

* thin myofilament
* consists of two helical polymer strands of F actin, tropomyosin, & troponin
* G (globular)actin
* troponin
* tropomyosin

**myofilaments**

actin

* troponin:

molecules are attached at specific intervals along the actin myofilaments & have Ca2+ binding sites

* attached to tropomyosin

**myofilaments**

actin

* tropomyosin:

* molecules located along the groove between twisted strands of F actin
* covers active sites on G actin when Ca2+ is not bound to troponin

**myofilaments**

myosin:

* thick myofilament: 
* composed of myosin molecules 
* contains:
* myosin head
* hinge region
* rods

**myofilaments**

myosin

* myosin head:

contains ATPase, which breaks down ATP

**myofilaments**

myosin

* hinge region:

enables head to move

**myofilaments**

myosin

* rods:

attach each other & are arranged so that heads of myosin molecules are located at each end of myofilament

**myofibrils**

sarcomeres:

smallest individual contractile unit within a muscle

* bound on either end by Z disks (attach to actin myofilaments)
* myofibrils appear striated due to the organization of actin and myosin myofilaments with sarcomeres

**myofibrils**

sarcomeres structure:

* A band
* Z disc
* H zone
* M line

**sarcomeres structure**

A band:

extends the length of the myosin myofilaments within a sarcomere

**sarcomeres structure**

Z disc

coin-shaped sheet of proteins (connectins) that anchors actin & connects sarcomeres to one another

**sarcomeres structure**

H zone:

center of each "A" band where the myofilaments

* only myosin present
* DO NOT OVERLAP

**sarcomeres structure**

M line:

dark line in middle of H zone

* delicate filaments that attach to the center of the myosin myofilaments (helps hold the myosin in place)
* appear darker due to presence of the protein desmin

**sliding filament model**

during contraction:

actin slides past myosin to shorten the sarcomeres

* actin & myosin do not change in length, only position

**sliding filament model**

during relaxation:

sarcomeres lengthen b/c of external force, like contraction of antagonistic muscles 

* as actin myofilaments slide over myosin myofilaments
* "H" zone &"I" bands narrow
* "A" bands stay

**sliding filament model**

fully contracted muscle:

the ends of the actin myofilaments overlap at center of sarcomere; "H" zone disappears

**what happens to length of A band, I band, H zone** __**during contraction?**__

length of A band:

do not narrow b/c length of myosin myofilaments do not change

**what happens to length of A band, I band, H zone** __**during contraction?**__

length of I band:

band narrows

**what happens to length of A band, I band, H zone** __**during contraction?**__

H zone:

zone narrows

**resting membrane potential:**

* charge difference across plasma membrane
* inside = negative
* outside of plasma membrane = positive (Na+/K+)
* difference must exist for action potentials to occur 

**action potential:**

* nervous system stimulates muscle to contract through electrical signals
* reversal of the resting membrane potential; inside of plasma membrane becomes positive (depolarizes)

**role of ion channels in production of action potential:**

* assist with production/spread of action potentials
* two types of gated ion channels help produce action potentials:
* ligand-gated channels
* voltage-gated channels

**role of ion channels in production of action potential:**

ligand-gated channels

* ligand:

molecule that binds to a receptor located in plasma membrane

**role of ion channels in production of action potential:**

ligand-gated channels

* receptor:

protein or glycoprotein with a receptor site to which a ligand can bind

**role of ion channels in production of action potential:**

ligand-gated channel:

an ion channel that is stimulated to open by the binding of a ligand to a receptor

* **(three step process)**

**role of ion channels in production of action potential:**

voltage-gated channel:

open and close in response to small voltage changes across plasma membrane

* **(two steps)**

**production of action potential; depolarization & repolarization**

depolarization:

* results from an increase in permeability of plasma membrane to 
* if depolarization reaches threshold, an action potential is produced
* results from opening of many Na+ channels, creating inside of membrane more positive

**production of action potential; depolarization & repolarization**

repolarization phase:

* occurs when Na+ channels close & K+ channels open
* Na+ movement into cells stops, & K+ movement out of cell increases, causing repolarization

**all-or-none principle:**

* subthreshold stimulus:
* produces no action potential
* threshold stimulus:
* stimulus at or above threshold will produce an action potential

**neuromuscular junction:**

* presynaptic terminals
* motor end-plate

**neuromuscular junction**

presynaptic terminals:

* axonal endings
* located in invagination of sarcolemma
* synaptic vesicles contain ACh

**neuromuscular junction**

motor end-plate:

* specific part of sarcolemma in area of synapse
* contains ACh receptors

**how action potential is transmitted across junction:**

(1) calcium enters terminal, releasing ACh into the synaptic cleft

(2) ACh binds to ligand gated Na+ channels

(3) sodium ions diffuse into muscle fiber causing depolarization that exceeds threshold causing an action potential

**excitation-contraction coupling:**

mechanism by which an action potential in sarcolemma causes contraction of muscle fiber

* in order contract a skeletal muscle must:
* be stimulated by a nerve ending
* propagate and electrical current (action potential) along it's sarcolemma
* have rise in intracellular Ca2+ levels

**events of excitation-contraction coupling:**

(1) AP propagates along sarcolemma down (T)-tubules

(2) depolarization of (T)-tubules cause gated Ca2+ channels to open, increasing permeability SR to Ca2+; diffuses from SR to sarcoplasma

(3) Ca2+ binds to troponin molecules; Ca2+ releases from SR

(4) myosin heads bind to active sites on G actin forming cross-bridges; movement of cross-bridges results in contraction

**events of cross-bridge movement**

(1) Ca2+ binds to troponin causing tropomyosin to move exposing active sites

(2) myosin heads bind to active sites and a cross-bridge is formed

(3) myosin head moves at the hinge = power stroke, actin slides past myosin and ADP is released

(4) ATP binds to myosin heads so cross-bridge releases, myosin heads separates from actin

(5) ATP is split by myosin ATPase to ADP and phosphate attached to myosin head = hydrolysis of ATP

(6) : recovery stroke, myosin head returns resting position if Ca2+ are still bound to troponin, cross-bridge formation and movement are repeated

**conditions needed for muscle relaxation**

* muscular relaxation:
* occurs as a result of active transport of Ca2+ back into the sarcoplasmic reticulum
* both contraction & relaxation require ATP

**muscle twitch:**

hypothetical contraction of single muscle fiber in response to single action potential

**phases of muscle twitch:**

* lag phase:
* time between application of stimulus to motor neuron & muscle contraction
* contraction phase:
* contraction occurs
* relaxation phase:
* relaxation occurs

**structure of motor unit:**

consists of a single motor neuron and all of the muscle fibers that it innervates

**strength/force of contraction increased in two ways:**

multiple-motor unit summation & multiple-wave summation

**strength/force of contraction**

multiple-motor unit summation:

* increased strength of stimuli increased the number of muscle fibers that respond & contract
* individual motor units & individual muscle fibers follow the all-or-none principle, while whole muscles respond to stimuli in graded manner

**strength/force of contraction**

multiple-wave summation

* increased frequency of stimuli increases force of contraction as muscle doesn’t have time to fully relax

**strength/force of contraction**

all-or-none principle

(muscle fibers)

contraction of equal force is produced in response to action potential

* sub-threshold stimulus:
* no action potential, no contraction
* threshold stimulus:
* action potential, contraction
* stronger than threshold:
* action potential, contraction = to that threshold stimulus

**strength/force of contraction**

whole muscles respond in a ____rather than an all-or-none fashion:

graded manner

**strength/force of contraction**

whole manners

will contract with small or large force depending on the number of motor units stimulated

* more motor units stimulated = force of contraction increases

**what occurs in treppe?**

increase in the force of contraction during first few contractions of rested muscle

**multiple-motor-unit recruitment:**

* nervous system regulates muscle force by increasing number of contracting motor units
* whole muscles are composed of many motor units

**incomplete tetanus vs complete tetanus**

incomplete tetanus:

muscle fibers relax partially between contractions

**incomplete tetanus vs complete tetanus**

complete tetanus:

action potentials produced rapidly; no relaxation occurs

**initial length of muscle & amount of tension produced**

* active tension = as the length of a muscle fiber increases
* if stretched passed optimal length

**isometric vs isotonic contractions:**

isometric contraction:

increase in muscle tension; no change in muscle length

* ex. (plank, wall sit)

**isometric vs isotonic contractions:**

isotonic contractions:

change in muscle length; no change in muscle tension

**concentric vs eccentric phases**

concentric phase:

isotonic contractions in which muscle tension increases & muscle shortens

**concentric vs eccentric phases**

eccentric phase:

isotonic contractions in which tension maintained as muscle lengthens

m**uscle fiber types:**

* fast-twitch oxidative (FOG)
* fast-twitch glycolytic (FG)
* slow-twitch muscle fibers (SO)

**muscle fiber types**

slow-twitch muscle fibers (SO):

* type I
* fatigue-resistant
* aerobic energy sources
* high mitochondrial density
* highly vascularized
* contain myoglobin
* long distance

**muscle fiber types**

fast-twitch glycolytic (FG):

* type IIb
* highly fatigable
* high glycogen concentration
* high intensity
* short duration

**muscle fiber types**

fast-twitch oxidative (FOG):

* type IIa
* fatigue resistance intermediate (btwn SO & FG)
* low mitochondrial density
* use aerobic & anaerobic respiration
* moderate-intensity endurance

**physiological vs psychological fatigue**

physiological fatigue:

* inability of muscle to contract/relax
* results from ATP depletion
* w/o adequate ATP levels, cross-bridges & ion channels do not function properly
* muscle tension declines

**physiological vs psychological fatigue**

psychological fatigue:

* most common type, involves central nervous system
* muscles can still do work, but person perceives it cannot

changes in aging skeletal muscle

* by 80 years of age 50% of muscle mass is gone