Download The Muscular System

The Muscular System
Chapter 8
Structure of a Skeletal Muscle
Layers of fibrous connective tissue
called fascia separate an individual
skeletal muscle from adjacent
muscles and hold it in position
Fascia may project beyond the end of
the muscle forming a tendon
Cont.
Epimysium – layer of con. tissue that
surrounds a skeletal muscle
Perimysium – extends inward from the
epimysium and separates the muscle
tissue into small compartments
Fascicles – bundles of skeletal fibers
found in compartments
Endomysium – con. tissue that
surrounds individual skeletal fibers within
fascicles
Cont.
Skeletal muscle fiber
– Thin, elongated cylinder with rounded
ends
– The cytoplasm (sarcoplasm) contains
many small, oval nuclei, mitochondria,
and myofibrils
– Myofibrils contain two types of protein
filaments
Myosin (thick)
Actin (thin)
Striation pattern
– I bands (the light bands) are composed
of thin actin filaments directly attached
to Z lines
– A bands (the dark bands) are composed
of thick myosin filaments overlapping
thin actin filaments
– Sarcomere – segment of a myofibril
that extends from one Z line to the next
Z line
– Sarcoplasmic reticulum –
membranous channels that surrounds
each myofibrl and runs parallel to it
– Transverse tubules – extend inward
as invaginations from the fiber’s
membrane and passes all the way
through the fiber; contains extracellular
fluid
– Both activate the muscle contraction
mechanism when the fiber is stimulated
Neuromuscular Junction
Each skeletal muscle fiber connects to an
axon from a nerve cell, called a motor
neuron
The axon extends outward from the brain
or spinal cord
The muscle fiber contracts only when the
motor neuron stimulates it
The connection between the motor neuron
and the muscle fiber is called a
neuromuscular junction
Muscle fiber membrane forms a motor
end plate
The end of the motor neuron
branches and projects into recesses
of the muscle fiber membrane
Cytoplasm at the distal ends of these
motor neuron axons contains
synaptic vesicles that store
chemicals called neurotransmitters
Summary of Muscle Contraction
Nerve impulse travels from brain to
the end of a motor neuron axon;
synaptic vesicles release a
neurotransmitter (acetylcholine)
into the synaptic cleft between the
neuron and the motor end plate of
the muscle fiber stimulating the
muscle fiber to contract.
Role of Myosin and Actin
Myosin – composed of two twisted
protein strands with globular parts
called cross-bridges projecting
outward along their lengths
Actin – a globular structure with a
binding site to which the myosin
cross-bridges can attach
– Composed of troponin and
tropomyosin
Sliding filament model
– Sarcomeres shorten as myosin crossbridge attaches to an actin binding site
that is exposed by the release of
calcium ions and bends slightly, pulling
the actin filament
– Then the head can release, straighten,
combine with another binding site
further down the actin filament, and pull
again
Globular portions of the myosin filaments contain
an enzyme, ATPase
ATPase catalyzes the breakdown of ATP to ADP
and phosphate, releasing energy
This energy puts the myosin cross-bridge in a
“cocked” position
When a cocked cross-bridge binds to actin, it
pulls on the thin filament
Filament moves toward the center of the
sarcomere, and the sarcomere shortens
Cycle repeats as long as ATP is available and
muscle fiber is stimulated to contract
Vocabulary
Actin
Myosin
Cross bridges
Binding sites
Sarcomere
Tropomysium
Troponin
Calcium
Sarcoplasmic
reticulum
Propagation
Action potential
Permeability
Channels
Use the above vocabulary and pictures to explain
the sliding filament model.
Stimulus for Contraction
Acetylcholine – neurotransmitter
that stimulates skeletal muscle
fibers
When nerve impulses cease, two
events lead to muscle relaxation
1. Acetylcholine is decomposed by
acetylcholinesterase
2. Calcium ions transported back into the
sarcoplasmic reticulum
Energy Sources for Contraction
Existing ATP molecules
Once used up cells must regenerate ATP
from ADP and phosphate
Creatine phosphate contains high
energy phosphate bonds
– An enzyme in the mitochondria catalyzes the
synthesis of creatine phosphate
– Stores excess energy released from the
mitochondria
Cellular respiration of glucose
Oxygen Supply and Cellular
Respiration
Glycolysis can take place in the absence of
oxygen
More complete breakdown of glucose
(cellular respiration) requires oxygen
Oxygen comes from lungs and is carried
by rbcs and are attached to hemoglobin
Myoglobin is synthesized in muscle cells
and imparts the reddish-brown color of
skeletal muscle tissue; temporarily stores
oxygen
Oxygen Debt
When skeletal muscles are used even for a
minute or two, the muscle fibers must
increasingly use anaerobic respiration to
obtain energy
Pyruvic acid used to regenerate glycolysis,
but produces lactic acid
Lactic acid accumulates in the muscles,
diffuses into the bloodstream, and reaches
the liver
In the liver, reactions occur requiring ATP,
synthesize glucose
Oxygen debt has to be repaid
May take several hours
Metabolic capacity may change with
training (Table 8.2)
Muscle Fatigue
A muscle exercised strenuously for a
prolonged period may lose its ability to
contract, a condition called fatigue
Interruption in the muscle’s blood supply
or lack of acetylcholine in motor neuron
axons may also cause fatigue
Most often caused from the accumulation
of lactic acid which lowers pH and fibers
no longer respond to stimulation
Cramps occur when a muscle undergoes a
sustained involuntary contraction
Heat Production
More than half of the energy
released in cellular respiration is heat
Blood transports heat to other
tissues, which helps maintain body
temperature
Muscular Responses
When an isolated muscle fiber is exposed
to a series of stimuli of increasing
strength, the fiber remains unresponsive
until a certain strength of stimulation is
applied
Minimal strength is called the threshold
stimulus
Skeletal muscle fiber does not respond
partially – all-or-none response
Extent of shortening depends on
resistance
Skeletal muscle removed from an
animal can be used to show how a
whole muscle responds to
stimulation
Muscle is mounted in a device and
then stimulated electrically
When the muscle contracts, it pulls
on a lever, and its movement is
recorded
Resulting pattern is a myogram
Twitch - single contraction that lasts
only a fraction of a second
Latent period – delay between the
time the stimulus was applied and
the time the muscle fiber responded
– Shorter than 0.01 sec. in humans
Period of contraction – muscle
pulls at attachment
Period of relaxation – muscle
returns to its former length
Summation – process that
combines the force of individual
twitches
When the resulting forceful,
sustained contraction lacks even
partial relaxation, it is called a
tetanic contraction, or tetanus
Electromyography
EMG measurements record the
electrical activity associated with
muscle contraction, so it is not a
direct measure of force
Muscle Tone
Def. – muscle undergoes some
sustained contraction when it
appears to be at rest
Response to nerve impulses that
originate repeatedly from the spinal
cord and stimulate a few muscle
fibers
Important in maintaining posture
Smooth Muscle
Lack striations
Can change length without changing
tautness
Can maintain contraction longer with a
given amt. of ATP
Multiunit smooth muscle
– Muscle fibers are separate
– Found in the irises of the eyes and in walls of
blood vessels
– Contracts only in response to stimulation by
motor nerve impulses or certain hormones
Visceral smooth muscle
– Composed of sheets of spindle-shaped
cells in close contact with one another
– Found in the walls of hollow organs
– Fibers can stimulate each other
– Display rhythmic pattern of repeated
contractions
– Peristalsis – wavelike motion that
occurs in tubular organs and helps force
the contents of these organs along their
lengths
Cardiac Muscle
Found only in the heart
Composed of branching, striated cells
interconnected
Twitches are longer than skeletal
twitches
Intercalated discs allow muscle
impulses to pass freely from cell to
cell
Self-exciting and rhythmic
Skeletal Muscle Actions
Origin – immovable end of a muscle
Insertion – movable end of a
muscle
When a muscle contracts, its
insertion is moved toward its origin
Interaction of Skeletal Muscles
Function in groups
Prime mover (agonist) – muscle
responsible for a particular body
movement
Synergists – muscles that contract
and assist the prime mover
Antagonists – muscles that resist a
prime mover’s action and cause
movement in the opposite direction
Muscle disorders
Muscular dystrophies – inherited diseases;
progressive weakness and deterioration; caused
by mutation of gene which codes for dystrophin
Charcot-Marie-Tooth disease - comprises a group
of disorders that affect peripheral nerves which
lie outside the brain and spinal cord and supply
the muscles in the limbs
Myotonic dystrophy – form of muscular
dystrophy; gradual reduction in strength and
control
Hereditary idiopathic dilated cardiomyopathy the heart becomes weakened and enlarged, and
cannot pump blood efficiently
Major Skeletal Muscles
Doc, what’s wrong with me?
Create a doctor/patient interview
about a given muscular disorder
Include symptoms, description,
cause, how diagnosis is made, risk
factors, treatment, and alternate
names
Must have two copies of interview
and one prop