Chapter 12: Muscle Flashcards
Skeletal muscles are composed of
individual muscle fibers that contract
Skeletal muscles are stimulated by the
Somatic motor neurons
Skeletal muscles are usually attached to bone on each end by
tough connective tissue tendons
What causes movement of the bones at a joint?
The tension placed on tendons and attached bones by muscle contraction
What is an insertion?
The more movable bony attachment of a muscle. MOVEMENT IS TOWARD INSERTION
Less movable attachment of a muscle
Origin
What do flexor muscles do?
Decrease the angle of a joint
What do extensor muscles fo?
Increase the angle of their attached bones at the joint
Agonist muscle
The prime mover of any skeletal movement
What is the agonist muscle in flexion?
The flexor muscle. THIS IS THE MAIN MUSCLE RESPONSIBLE FOR MOVEMENT IN A GIVEN DIRECTION. ALSO KNOWN AS PRIME MOVER
Antagonist muscles
Flexors and extensors that act on the same joint to produce opposite actions.
What does an abductor muscle do?
Moves limb away front he midline of the body
What does an adductor muscle do?
Moves limb toward the midline of the body
What does a legator muscle do?
Moves insertion upward
What does a depressor muscle do?
Moves insertion downward
What does a rotator muscle do?
It rotates a bone along its axis
What does a sphincter muscle do?
Constricts an opening
What is the epimysium?
A sheath of muscle tissue proteins within tendons which extend around the muscle in an irregular arrangement
What is a fascicle?
Connective tissue from the epimysium extending into the body of the muscle
What is the perimysium?
Tissue sheath surrounding fascicles
Sarcolemma
Plasma membrane surrounding the muscle fibers or myofibers
What is the endomysium?
Connective tissue layer surrounding sarcolemma
What is the only difference between any other cells in the body and the muscle cell?
The muscle cell is multi nucleated, syncytium
Syncytium
Formed from the union of several embryonic myoblast cells
Most distinctive feature of skeletal muscle fibers
Striated appearance
Striated appearance of muscle cells are caused by :
Dark bands: A bands (think A in dark)
Light bands: I bands (think I in light)
Thin dark lines between A band and I band: Z lines (disks)
What is a motor unit?
A single motor neuron and all the muscle fibers it innervates. They all contract at once. Each consist of branched motor axon
Graded contractions
Varied contraction strength due to different numbers of motor units being stimulated
Nueromuscular junction
Site where a motor neuron stimulates a muscle fiber
Motor end plate:
Area of the muscle fiber sarcolemma where a motor neuron stimulates it using the neurotransmitter, acetylcholine
Control of motor unit
Contraction strength comes from motor unit recruitment
Fine muscle control requires
smaller motor units with fewer muscle fibers
How many muscle fibers/motor units in the eye?
~23 muscle fibers/units
Motor end plate
The specialized region of the sarcolemma of the muscle fiber at the neuromuscular junction, where acetylcholine is released for contraction of the muscle
Where is the cell body of a somatic motor neuron located?
In the ventral horn of the gray matter of the spinal cord, gives rise to a single axon
Recruitment of motor units
Process used when contractions of greater strength are required, larger motor units are activated
Process involved with increase force of muscle contraction:
- motor unit stimulated asynchronously at greater frequency
- recruitment of additional larger motor units with more muscle fibers per motor neuron to increase the force of contraction.
What are myofibrils?
Subunits packed densely in each muscle fiber which run the length of the muscle fiber
What are myofibrils composed of?
Thick and thin myofilaments (the smallest unit)
What does each muscle fiber contain?
Nucleus, striations, sarcolemma, sarcoplasm, myofibrils, myofilaments
I band contains:
Only thin filaments, primarily of the protein ACTIN
A band contains:
All of the thick filaments with some thin filament overlap; the thin filament is the protein MYOSIN
H band are:
The center of the A band with no thin filament overlap
Z discs (lines) are found:
In the center of the each I band
What is the sarcomere?
The basic subunit of striated muscle contraction. Forms hexagonal pattern
What does a sarcomere contain?
Area from a Z disc to the next. It contains the A band, H band, and I band
What does titin do?
Protein which runs from the Z disc to the M line and allows elastic recoil
Where are M lines found?
What do they do?
In the center of each A band
Help hold down thick filament
Sliding Filament Theory of contraction
When muscle contacts, sarcomere shorten.
A band do not shorten but move closer together!!
I band do shorten!! but thin filaments do not!
Thin filaments slide toward the H band
H band shortens or disappears ( I and H bands shorten)
How is the sliding of the filaments accomplished?
Asynchronous power strokes of myosin cross bridges, which pull the thin filaments (actin) over the thick filaments (myosin)
Shortening of the sarcomeres is accomplished by
Sliding of the myofilaments-the length of each filament remains the same during contraction
What are thick filaments composed of?
Myosin
Myosin proteins contain
Two globular heads with actin-binding sites and ATP-binding sites (these are responsible for moving the thin filaments in contraction)
Thin proteins are composed of/?
Actin
Actin proteins contain
Tropomyosin (String) and troponin that prevent myosin binding at rest
Action of sliding is caused by
Several cross bridges that form between myosin and actin
What part of the myosin acts as the ATPase enzyme?
The myosin head, splitting ATP into ADP and inorganic P
What does hydrolysis of ATP cause?
Allows the myosin head to bind to actin binding sites when the muscle is stimulated
What causes the power stroke?
The release of inorganic P upon binding cocks the myosin head to pull the thin filament toward the center
What happens after the power stroke?
The ADP is released and a new ATP binds causing the myosin head to release the actin.
ATP is split again.
Myosin straightens out and rebind to actin further back
Two types of Actin
F (Filamentous) actin: String of many G actin
G (Globular) actin: round subunits
How is actin arranged in thin filaments?
F actin is made up of 300-400 G actin units, arranged in a double row and twisted to form helix
What can block cross bridges?
Tropomyosin (string which moves to attach to myosin head)
Troponin complex composed of :
Troponin I inhibits binding of myosin
Troponin T binds to tropomyosin
Troponin C binds to calcium
Role of Ca:
Muscle cells are stimulated by release or Ca inside the muscle fiber
Some attach to troponin C causing conformational change in troponin and tropomyosin
Myosin is allowed access to form cross bridges with actin
What is the role of sarcoplasmic reticulum in muscle contraction?
SR is modified to store Ca when muscle is at rest.
Most is stored in terminal cisternae.
When muscle fiber is stimulated Ca diffuses out of calcium reuse channels.
At the end of contraction, Ca is actively pumped back into the SR.
What are transverse tubules?
Found in the SR
They are narrow membranous tunnels formed from the sarcolemma
They are open to the extracellular environment and conduct action potentials.
Excitation Contraction Coupling Summary
ACh released from somatic motor neuron
In sarcolemma: ACh binds to nicotinic ACh receptors, opens ligand gated channels -> Na diffuses in, producing depolarizing stimulus -> Action potential is produced
In transverse tubules: Action potentials conducted along transverse tubules -> Action potentials open voltage gated Ca channels.
In sarcoplasmic reticulum: Ca release channels in SR open -> Ca diffuses out into sarcoplasm
In myofibrils: Ca binds to troponin, stimulating contraction
What happens in muscle relaxation?
Action potential cease
Calcium release channels close
Ca ATPase pumps move Ca back into SR
No more Ca is available to bind to troponin C
Tropomyosin moves to block the myosin heads from binding to actin
How is muscle contraction studied?
in vitro where one end of the muscle is fixed and the other is movable. Using electrical stimulations and contractions are displayed in currents
What is a twitch?
When a muscle quickly contracts and relaxes after a single electrical shock of sufficient voltage
What is summation?
When a second shock is applied immediately after the firs, the second twitch will partially piggyback the first. Twitch increases with voltage increase.
What is a latent period?
Time between the stimulus and the contraction. Also referred to as excitation-contraction coupling at cross bridges.
Graded contractions?
Stronger contractions result in recruitment of more fibers, until all fibers are contracting.
Incomplete tetanus
Increasing the frequency of electrical shocks decreases the relaxation time between twitches. Muscle never relaxes enough before being stimulated again
Complete tetanus
At a certain frequency, there will be no relaxation. This is equivalent to a smooth, sustained contraction. The more twitch the better the result
Treppe effect
Staircase effect seen when a fresh muscle is stimulated with several shocks at maximum voltage, each twitch will be progressively stronger. There is a maximum value reached
Types of muscle contraction:
Force velocity curve
Isotonic contractions
Isometric contractions
Force velocity curve
For muscle to contact, they must generate force that is greater than the opposing forces (gravity/weights). The greater the force the slow the contraction.
Isotonic contractions
Muscle fibers shorten when the tension produced is just greater than the load.
Two subcategories of isotonic contractions
Concentric contractions
Eccentric contraction
Concentric contractions
A muscle fiber shortens when force is greater than load
Eccentric contraction
A muscle may actually lengthen, despite contraction, if the load is too great (going down a hill/lowering weight slowly)
Isometric contraction
Muscles can’t shorten because the load is too great. Can be voluntary
Muscle strength is determined by
Number of fibers recruited to contract
Frequency of stimulation
Thickness of each muscle fiber ( the thicker the stronger)
Initial length of the fiber at rest
Tension is maximal when:
Sarcomeres are at normal resting length
Increasing sarcomere length….
Decreases muscle tension. There are fewer interactions between myosin and actin. At some point, no tension will be generated
Decreasing sarcomere length…
Decreases muscle tension due to fiber shortening and thickening. This increases fluid pressure and distance between the actin and myosin
Energy is needed for:
Myosin ATPase (70%) Ca pump to actively return calcium to SR (30%)
At rest and for mild exercise energy comes from?
Aerobic respiration of fatty acids
For moderate exercise energy comes from?
Glycogen storage
For heavy exercise energy comes from?
Blood glucose
With increased intensity or duration, FLUT4 channels are inserted into the sarcolemma to allow more glucose into cells
Types of twitch?
Slow twitch (Type I) Fast twitch (Type IIx) Intermediate twitch (Type IIa)
Slow twitch
Type I: Slower contract speed; can sustain contraction for Longer periods without fatigue
Rich capillary supply
More mitochondria
More respiratory enzymes
More myoglobin
ALSO CALLED RED FIBERS OR SLOW OXIDATIVE FIBERS
Fast twitch
Type IIx: Faster contraction speed
Fatigue fast
Fewer capillaries, mitochondria, respiratory enzymes and myoglobin
Intermediate twitch
Type IIa: Fast twitch but with high oxidative capacity; called fast oxidative fibers
What reduces the ability to generate force?
Muscle fatigue
Fatigue can be due to
Reduced SR Ca release Lack of ATP Buildup of ADP Fatigue in CNS (Also called central fatigue) Lactic acid accumulation/lower pH Depletion of glycogen
What causes muscle decline with aging?
Reduced muscle mass (Type II fibers)
reduction in capillary blood supply
Fewer satellite cells
Increased myostatin production
How do muscle cells repair themselves?
Using stem cells called satellite cells located newer muscle fibers
Lower motorneurons
Neurons whose axons innervate skeletal muscles
Cell bodies in ventral horn of spinal cord or brain stem
Travel down via ventral root or spinal nerves
Influenced by sensory feedback and stimulation from higher motor neurons or brain
Higher motorneurons
Neurons in the brain that are involved in the control of skeletal movements and that act by facilitating or inhibiting the activity of the lower motor neurons.
Alpha motorneurons
Lower motor neurons whose fibers innervate ordinary (extrafusal) muscle fibers.
Gamma motorneuons
Lower motor neurons whose fibers innervate the muscle spindle fibers (intrafusal/active stretch)
Agonis/Antagonist muscles
A pair of muscles or muscle groups that inset on the same bone, the agonist being the muscle of reference
Synergist
A muscle whose action facilitates the action of the agonist
Ipsilateral/contralateral
Ipsilateral is located ion the same side, or the side of reference
Contralateral is located on the opposite side
Afferent/Efferent
Afferent neurons are sensory
Efferent neurons are motor
Golgi tendon organs respond to:
Tension a muscle puts on a tendon
Constantly mojito tension in tendons
Sensory neuron stimulates interneuron in spinal cord, interneuron inhibits motor neuron, tension in tedon is reduced
Muscle spindle apparatus responds to :
Muscle length
Muscles that require more control have more spindles
Stretching a muscle causes spindles to stretch
Two types of muscle sensory organs?
Intrafusal fibers (Thin) Extrafusal fibers (thick)
Which is the simples reflex?
Monosynaptic stretch reflex
Only involves one sensory neuron synapsing on one motor neuron in the spinal cord
Stimulated by striking the patellar ligament in the knee
Knee-Jerk Reflex:
- Striking patellar ligament stretches tendon and quadriceps femoris muscle
- Spindle is stretched, activating sensory neuron
- Sensory neuron activates alpha motor neuron
- Alpha motorneuron stimulates extrafusal muscle fibers to contract
Crossed extensor reflex:
Type of double reciprocal innervation seen when you step on a tack;
- Flexor contracts and extensor relaxes to withdraw foot
- Extensor contracts and flexor relaxes in contralateral leg to support weight
Upper Motor Neuron Control parts
Precentral gyrus
Neurons through extrapyramidal tracts
Cerebellum
Basal nuclei
Role of pre central gyrus:
Sends neurons through pyramidal tracts by the lateral and ventral corticospinal tracts
Role of neurons through extrapyramidal tracts
Reticulospinal tracts inhibit lower motor neurons
Role of cerebellum
Receives info from muscle spindles and golgi tendon organs as well as other senses
also inhibits region sod the basal nuclei; red nuclei and vestibular nuclei
Role of basal nuclei
Act to inhibit motor activity through the rubrospinal tract
Types of paralysis
1. Flaccid paralysis: caused by damage to lower motor neurons. Reduced muscle tone Depressed stretch reflexes Atrophy 2. Spastic paralysis: caused by damage ot upper motor neurons. Increased muscle tone Exaggerated stretch reflexes Hyperactivity
Babinski’s Reflex
Extension of the great toe when the sole of the foot is stroked along the lateral border
Spastic paralysis
High muscle tone and hyperactive stretch reflexes; flexion of arms and extension of legs
Hemiplegia
Paralysis of upper and lower limbs on one side (stroke)
Paraplegia
Paralysis of the lower limbs on both sides as a result of lower spinal cord damage
Quadriplegia
Paralysis of upper and lower limbs on both sides as a result of damage to the upper region of the spinal cord or brain
Chorea
Random uncontrolled contractions of different muscle groups as a result of damage to basal nuclei
Resting tremor
Shaking of limbs at rest; disappears during voluntary movements; produced by damage to basal nuclei
Intention tremor
Oscillations of the arm following voluntary reaching movements; produced by damage to cerebellum
Cardiac muscles are:
Involuntary
Regulated by autonomic nervous system
Contraction if due to myosin/actin cross bridges in sarcomeres
Stimulated by Ca
Striated
Fibers are short, branched and connected via gap junctions called INTERCALATED DISCS
