Chapter 12 Flashcards
What is epimysium?
fibrous connective tissue from tendons that form sheaths that extend around and into skeletal muscle
What are fascicles?
created by connecting tissue dividing muscle into columns
What is perimysium?
connective tissue around fascicles
What is endomysium?
thin connective tissue layer that wraps muscle fibers
What is sarcolemma?
plasma membrane of muscles
What’s the main difference between muscle fibers and other cells?
muscle fibers are multinucleate and striated
What is the most distinctive feature of skeletal muscle?
striations
What is part of the neuromuscular junction?
single synaptic ending of the motor neuron innervating each muscle fiber and underlying specializations of sarcolemma
Where does neuromuscular junction occur on sarcolemma?
motor end plate
What is a motor unit?
one motor neuron and all the fibers it innervates
What happens when a motor neuron is activated?
all muscle fibers in the motor unit contract
What is the innervation ratio?
motor neurons to muscle fibers
- according to degree of fine control capability of the muscle
When does fine control occur?
motor units are small
- 1 motor neuron innervates small amount of fibers
What is recruitment of motor units?
- brain estimates number of motor units required and stimulates them to contract
- keeps recruiting more units until desired movement is accomplished in smooth fashion
- more and larger motor units are activated to produce greater strength
What is the structure of a muscle fiber?
myofibrils (extend length of fiber)
myofilaments (thick and thin filaments that give rise to bands that underlie striations)
What is the A band?
dark
- thick filaments
- mostly myosin
What is the H zone?
light area in the center of A band
- actin and myosin don’t overlap
What is the I band?
light
- contains thin filaments
- mostly actin
What is the Z line/dic?
at the center of I band
- actins attach
What is a sarcomere?
contractile units of skeletal muscle between 2 Z-discs
What are M lines?
structural proteins that anchor myosin during contraction
What is titin?
elastic protein attaching myosin to Z-disc that contributes to elastic recoil of muscle
What happens when muscles contract? How does it work?
myofibrils get shorter
- thin filaments slide over and between thick filaments towards center
- shortens distance between z discs
What happens during the sliding filament theory of contraction?
at muscle contraction
- A bands (with actin) move closer together (not shorten)
- I bands shorten
- H bands (with myosin only) shorten and disappears
What are cross bridges formed by?
heads of myosin molecules that extend toward and interact with actin
What is the sliding of filaments produced by?
actions of cross bridges
- each myosin head contains an ATP-binding site which functions as an ATPase
How does myosin work with actin?
- must be cocked by ATP
- after binding, myosin undergoes conformational change (power stroke) which exerts force on actin
- after power stroke, myosin detaches
How is cross bridge attachment to actin controlled?
troponin-tropomyosin system
- switch for muscle contraction and relaxation
Where is tropomyosin found?
in grove between double row of g-actins (make up actin thin filament)
Where is troponin?
attached to tropomyosin at intervals of every 7 actins
What dose tropomyosin do in relaxes muscles?
blocks binding sites on actin so cross bridges can’t occur
- Ca++ levels are low
When can muscle contraction occur?
when binding sites are exposed
What happens when Ca++ levels rise?
Ca++ binds to troponin causing conformational change which moves tropomyosin and exposes binding sites
- allows cross bridges and contraction to occur
When does the cross bridge cycle stop?
when Ca++ levels decrease
Where does Ca++ go?
continually pumped back into the sarcoplasmic reticulum
- mostly in terminal cisternae (T tubules are alongside)
What causes depolarization of end-plate potentials and action potentials in muscle?
release of ACh at NMJ
How are dihydropyridine receptors stimulated?
action potentials race over sarcolemma and down into muscle via T tubules
How do action potentials in T tubules cause release of Ca++?
Voltage-gated (ryanodyne receptor) and Ca++ induced release channels
- electromechanical release
What is the mechanism for muscle relaxation?
Ca++ from sarcoplasmic reticulum diffuses from troponin to initiate crossbridge cycling and contraction
- when action potential stops, muscles relax
What is a twitch?
single rapid contraction and relaxation of muscle fibers
What is summation?
2nd stimulus occurs before muscle relaxes from the 1st
- 2nd twitch is greater
What are graded contractions?
contrations of varying strength
What is incomplete tetanus?
muscle is stimulated by increasing frequency of electrical shocks
- tension increases to maximum
What is complete tetanus/tetany?
frequency is so fast = no relaxation
- smooth sustained contration
What is the strength of muscle contraction influenced by?
- frequency of stimulation
- thickness of muscle fibers
- initial length of muscle fiber
What is the relationship between tension and cross bridges?
less tension = fewer cross bridges formed
- no overlap force = no cross bridges formed
What are the lower motor neurons and final common pathway?
ventral horn of spinal cord with motor neuron cell bodies
- axons leave at ventral root
What is neural control influenced by?
sensory feedback from muscles and tendons
- faciliatory and inhibitory activity from upper motor neurons
How does the nervous system receive continuous sensory feedback?
- length/stretch of muscle from muscle spinal apparatus
- tension from golgi tendon organs
What is the muscle spindle apparatus made of?
- modified thin muscle cells (intrafusal fibers)
- regular muscle fibers (extrafusal fibers)
- spindles are arranged in parallel with extrafusal fibers (insert into tendons at each end of muscle)
What are the alpha motor neurons?
fast conducting
- innervate extrafusal fibers
- cause muscle contraction
What are the gamma motor neurons?
slower conducting
- innervate and induce tension in intrafusal fibers
- increase sensitivity of muscle to passive stretch
What do upper motor neurons stimulate?
alpha and gamma motor neurons
- coactivation
What does stimulation of alpha neurons result in? How about gamma neurons?
- alpha neurons = muscle contraction and shortening
- gamma neurons: intrafusal fibers to take up slack, provide continued information of stretch length of extrafusal fibers (maintains normal muscle tone)
What does the monosynaptic-stretch reflex consist of?
1 synapse within CNS
What happens when the knee is hit with reflex hammer?
- stretches spindles to activate annulospiral sensory neurons
- synapse on alpha neurons causing them to stimulate extrafusal fibers
What does the golgi tendon organ consist of?
2 synapses in CNS
- disynaptic reflex
How does the golgi tendon organ reflex happen?
- sensory axons from golgi tendon organ synapse on interneurons
- make inhibitory synapses on motor neurons
- prevents excessive muscle contraction or passive muscle stretching
How does cardiac muscle work?
involuntary contraction
- branches, adjacent myocardial cells joined by intercalated disks (gap junctions)
- allows action potentials to spread throughout cardiac muscle
What are the characteristics of smooth muscle?
- no sacromeres
- gap junctions
- contains 16x more actin than myosin (allows greater stretching and contracting
- actin filaments are anchored to dense bodies
What is smooth muscle contraction controlled by?
controlled by Ca++ (but has little SR and no troponin/tropomyosin)
What is the mechanism of smooth muscle contraction?
- Ca++ enters through voltage gated Ca++ channels in plasma membrane to let in extracellular Ca++
- binds to calmodulin
- complex activates myosin light chain kinase to phosphorylate and activate myosin
- myosin forms cross bridges with actin
- myosin ATPase is slow
How does smooth muscle relaxaiton work?
- Ca++ concentration decreases
- myosin is dephosphorylated by myosin phosphatase
- myosin can no longer form cross bridges
- smooth muscle has slower contractions than striated
- can form state of prolonged binding of myosin to actin (latch state)
- maintains force using little energy
What are the characteristics of single smooth muscle?
single unit is spontaneously active
- pacemakers
- gap junctions to spread electrical activity
What are the characteristics of multiunit smooth muscle?
requires nerve stimulation by ANS
- neurotransmitter released along a series of synapses (varicosities) = synapses en passant
