Muscle Physiology Part Two

1
Muscle Physiology Part Two

• Energetics Neuronal Control of Muscle
  Contraction
• Cardiac Muscle
• Smooth Muscle

2
Energetics of Muscle Contraction

• In muscle contraction, 2 major processes require
  energy expenditure
• Hydrolysis of ATP by myosin cross-bridges as they
  cyclically attach detach from actin thin
  filaments
• Pumping Ca2 back into SR against a Ca2
  concentration gradient (requires 2 molecules of
  ATP for each ion of Ca2)

3
Energetics of Muscle Contraction

• During a twitch, certain amount of Ca2 is
  released following the AP exactly that amount
  must be pumped back into the SR if the muscle
  fiber is to relax (vs. cramp)
• Both ATP Ca2 are required for muscles to
  contract but relaxation occurs only in the
  presence of ATP and absence of Ca2
• Ca2 pump accounts for 25-30 of total ATPase
  activity during muscle contraction

4
Energetics of Muscle Contraction

• As well as ATP, muscle contain 2nd high-energy
  molecule, creatine phosphate (aka
  phosphocreatine)
• Creatine phosphokinase transfers high-energy
  phosphate from creatine phosphate to ADP,
  regenerating ATP so quickly that the ATP
  concentration remains constant even when the
  muscle is using energy at a high rate

5
Energetics of Muscle Contraction

• If muscle runs out of ATP, it goes into rigor
  (rigor mortis rigidity that develops in a dying
  muscle as ATP becomes depleted cross-bridges
  remain attached) rigor is relieved only by
  removal Ca2 addition of ATP fig. 10-17 p. 382
• During high intensity, short-duration activity
  (running down prey or running away from predator)
  ATP may be used up too fast to be replenished by
  ATP hydrolysis therefore, continuous
  rephosphorylation of ADP by creatine
  phosphokinase keeps muscles supplied with ATP for
  a short time animals life may depend on this
  short-lived extra source of energy

6
Diverse Muscle Activities

• Adaptations for
• jumping (frogs)
• running (horse)
• swimming (fish)
• making noise (rattlesnake)
• flying (dragonfly)

7
Neuronal Control of Muscle Contraction

• Movement requires contraction of many fibers
  within a muscle of many muscles within the body
  correctly timed with one another regulating
  the strength of contraction
• Coordination generated within NS most muscle
  contract only when APs arrive at NMJ

8
Motor Control in Vertebrates

• Vertebrate muscles arranged in antagonistic pairs
  (fig 10-45 p. 411)
• E.g. muscle pulls on a joint, causing it to close
  (flexor) its action is opposed by the muscle that
  causes the joint to open (extensor)
• Each vertebrate skeletal muscle is innervated by
  motor neurons whose somata are located in ventral
  horn of gray matter of the spinal or (or some in
  particular parts of brain)

9
Motor Control in Vertebrates

• Axon of spinal motor neuron leaves spinal cord
  through a ventral root, continues to muscle by
  way of a peripheral nerve branches repeatedly
  to innervate skeletal muscle fibers
• Single motor neuron may innervate only a few
  fibers or a 100 or more
• In vertebrates, each muscle fiber receives input
  form only one motor neuron

10
Motor Control in Vertebrates

• Collection of motor neurons that innervate
  particular muscle motor pool
• Somata of each motor pool clustered together in
  ventral horn of spinal cord segment that is
  relatively near the location of that muscle
• Motor unit motor neuron muscle fibers that it
  innervates in vertebrates motor units typically
  consist of 100 muscle fibers

11
Motor Control in Vertebrates

• Size of motor units in a muscle determines the
  precision of movement possible e.g motor units in
  primate fingers human tongue extremely small,
  permitting very finely modulated movements,
  whereas motor units in big muscles of truck are
  very large
• When AP is initiated in a motor neuron, membrane
  excitation spreads to all its terminal branches,
  activating all of its endplates fig. 612 p. 171
  ACh released as noted previously

12
Summation

• contraction of individual muscle fibers is
  all-or-none - any graded response must come from
  the number of motor units stimulated at any one
  time
• summation adding together of individual muscle
  twitches to make a whole muscle contraction -
  accomplished by increasing number of motor units
  contracting at one time (spatial summation) or by
  increasing frequency of contraction of individual
  muscle contractions (temporal summation)

13
Summation cont

• processes almost always occur simultaneously
  within normal muscle contraction
• Muscle fatigue Prolonged strong contractions
  leads to fatigue due to inability of contractile
  metabolic processes to supply adequately to
  maintain the work load - nerve continues to
  function properly passing AP onto the muscle
  fibers but contractions become weaker due to lack
  of ATP
• Hypertrophy - increase in muscle mass caused by
  forceful muscular activity increase power of
  muscle contraction
• Atrophy - when a muscle is not used for a length
  of time or is used for only weak contractions

14
Motor Control in Vertebrates

• Pattern of muscle contraction around a joint
  depends on patterns of activity in motor pools of
  different muscles when motor pool of flexors
  are active (excited), neurons in motor pool
  controlling antagonistic extensors receive
  inhibitory input if both are active
  simultaneously, position of joint is locked

15
Functional Terms

• Flexor vs. Extensor
• Abductor vs. Adductor
• Levatator vs. Depressor
• Pronator (turn forearm so palm downward) vs.
  Suppinator (turn forearm so palm upward)
• Rotator
• Sphincter
• Dialator

16
Functions of Skeletal Muscle

• provide skeletal movement
• maintain posture and body position
• support soft tissues
• guard entrances and exits
• maintain body temperature

17
Cardiac Muscle - Overview

• found in the walls of the heart
• under control of the ANS
• cardiac muscle cell has one central nucleus,
  unlike skeletal muscle (multinucleated), but it
  also is striated, like skeletal muscle
• is rectangular/elongated in shape tapered at
  both ends
• contraction is involuntary, strong, and
  rhythmical.

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Cardiac Muscle cont

• Individual fibers connected to neighboring fibers
  by gap junctions especially at structures called
  intercalated disks (allow electric current to
  pass unimpeded between cardiac muscle fibers)
• Fibers are tightly bound together by anchoring
  structures desmosomes

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Cardiac Muscle contTypes of Muscle fibers in
Heart

• Contractile striated and contain many
  myofibrils made up of sarcomeres elaborate SR
  T-tubules
• Conducting (include pacemaker fibers) dont
  resemble most muscle fibers dont contract
  lack contractile proteins instead function as
  signal-transmission system rapidly spreading
  electrical signals thru heart by way of gap
  junctions

20
Cardiac Muscle cont

• Contractions are myogenic initiated in muscle
  fibers themselves
• Electrical signal arises endogenously in
  pacemaker fibers spreads as APs thru heart
• Cardiac muscle dont depend on neuronal input to
  initiate or sustain contraction - cardiac muscle
  fibers do receive input from neurons of SNS PSN
  however, it produces no discrete postsynaptic
  potentials but serves a modulatory role
  strength rate of cardiac muscle contractions
  are increased by input form SNS decreased by PNS

21
Cardiac Muscle cont

• AP here differs from skeletal muscle in that
  rather than being very brief, it is longer has
  a plateau phase hundreds of milliseconds long
  following the upstroke this prevents tetanic
  contraction permits muscle to relax
• Regularly paced, prolonged APs heart contracts
  relaxes at a rate suitable for its function as
  a pump

22
Cardiac Muscle cont

• Contraction activated when cytosolic Ca2
  concentration rises which in cardiac muscles
  depends on influx of Ca2 across plasma membrane
  plus its release from SR (Elaborate SR T-tubule
  system)
• Long plateau of AP depends on influx of Ca2
  through voltage-gated Ca2 channels
• Intracellular Ca2 determined by depolarization
  AND other factors including action of
  catecholamines (epinephrine norepinephrine)

23
Smooth Muscle - Overview

• found in the walls of the hollow internal organs
  (e.g. blood vessels, bladder, GIT, and uterus)
• under control of the ANS cannot be controlled
  consciously involuntary (one exception may be
  urinary bladder?)
• non-striated (I.e. smooth), spindle-shaped cells
  with one central nucleus
• Lack sacromeres
• contracts slowly and rhythmically

24
Smooth Muscle cont

• Primarily supports visceral functions rather than
  locomotion other behavior
• Some similarities differences from both cardiac
  and skeletal muscles - can be divided into
  subclasses each different
• Some can produce more force per cross-sectional
  area than striated muscles and some can generate
  prolonged contractions that require much less
  energy per unit time than striated muscle

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Smooth Muscle cont

• Sliding-filament theory applies
• Generally with little or no SR lack T-tubules
• Rather than sacromere organization gathered
  into bundles of thick and thin filaments anchored
  in structures called dense bodies or connect to
  inside surface of plasma membrane at sites called
  attachment plaques

26
Smooth Muscle Categories

• Single-unit smooth muscles small, elongated and
  tapered at both ends coupled with one another
  thru electrically conducting gap junctions
  activation of a few fibers can generate
  contraction that moves thruout the entire organ
  in a wave e.g. peristalsis in GIT pushing food
  along called single-unit because entire set of
  fibers behaves as a unit rather than a set of
  independently controlled fibers

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Smooth Muscle Categories cont

• Multi-unit smooth muscles act independently
  contract only when stimulated by neurons or in
  some cases hormones contraction is neurogenic
  not coupled to one another by gap junctions e.g.
  muscles regulate diameter of pupil in iris and
  those in walls of blood vessels

28
Smooth Muscles cont

• Synapses of autonomic neurons with smooth muscle
  fibers are different form endplates formed by
  motor neurons NT released from many swellings
  called varicosities along length of autonomic
  axons diffuses over some distance encountering
  many smooth muscle cells along the way receptor
  molecules on smooth muscle cells appear to be
  distributed diffusely over the cell surface

29
Regulation of Smooth Muscle Contraction

• As in striated muscles, cyclic binding
  unbinding of myosin actin myofilaments depends
  on presence of free Ca2 in cytoplasm
• Contract relax more slowly than striated
  muscles are capable of more sustained
  contraction
• Slow release uptake of Ca2 associated with
  relatively underdeveloped SR (composed only of
  smooth, flat vesicles located close to inner
  surface of plasma membrane)

30
Regulation of Smooth Muscles cont

• because of elongated shape no point in
  cytoplasm is gt few micrometers away from plasma
  membrane diffusion of Ca2 between membrane
  myofilaments is sufficient for regulating slow
  contraction plasma membrane cells performs
  Ca-regulating function similar to those of SR in
  striated muscle

31
Regulation of Smooth Muscle cont

• In striated muscle, troponin tropomysin control
  access to myosin binding sites however, smooth
  muscles lack troponin but have the filamentous
  protein caldesmon binds to thin filaments
  preventing binding between myosin actin
• Caldesmon removed by 2 mechanisms
• Calmodulin Ca2 binding protein when
  calmodulin/Ca2 complex binds to caldesmon,
  myosin cross-bridges are permitted to bind to
  thin filaments OR
• Caldesmon may be phosphorylated by protein kinase
  C (when phosphorylated cant bind to thin
  filaments doesnt inhibit myosin-actin
  interactions)

32
Unusual Features

• Sensitive to mechanical stimulation e.g.
  stretching smooth muscles can cause
  depolarization this accounts for auto
  regulation seen in small arterioles increase in
  BP stretches smooth muscles in walls of
  arterioles leading to muscle contract helps to
  maintain relatively constant blood flow in
  peripheral tissues muscles in GIT performing
  peristalsis also rely in part on stretch-induced
  contractions of single-unit smooth muscle
  stretch activation

33
Unusual Features cont

• Smooth muscles are specialize to maintain
  contracted state for long periods of time while
  expending minimum amount of energy (here rate of
  cross-bridge cycling drops radically when
  contraction is prolonged drastically reducing
  energy cost unlike skeletal muscles) called
  latch (vertebrates) or catch (invertebrates)