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82 Cards in this Set

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  • Back
3 Main functions of Skeletal Muscle:
1. Execution of voluntary mvmts
2. Maintaining posture with respect to gravity
3. Production of heat/metabolic energy
What are skeletal muscles in relation to motor systems?
The effectors
What do muscles do by contracting?
Generate tension - FORCE
3 types of contractions of muscle:
1. Shortening
2. Isometric
3. Lengthening
What is a lengthening contraction?
One where the force of the muscle is less than the force of the load, and results in the muscle being torn.
What is a skeletal muscle made up of?
Parallel bundles of fascicles
What is a fascicle made up of?
Muscle fibers
What are Muscle fibers made up of?
What are synergistic muscles?
Muscles that work together across a joint to produce a similar action.
What are antagonistic muscles?
Those that work in opposite directions - against each other
How do muscles generate tension?
Always by PULLING - never by pushing.
B/c muscles always PULL, what does each joint require?
At least two opposing muscles:
What do flexors do?
Move limbs toward the body
What do extensors do?
Move limbs away from the body
What is the final common pathway for movement?
Lower Motor Neurons!!!!!
What activates a muscle?
A motor unit
What is a motor unit?
One lower motor neuron, and the muscle fibers it innervates.
What does the term "motor unit" allow us to describe?
The smallest unit of muscle under neural control, which provides the smallest increment of tension that can be generated.
What makes the size of a motor unit vary?
The precision of muscle movement in the myofibers innervated.
How many muscle fibers are in an Eye motor unit?
How many muscle fibers are in a motor unit for the digits?
5-10 - for precise control
How many muscle fibers are in a motor unit for the legs?
2000 - gives power
How many muscle fibers are in a typical muscle in general?
How are the LMNs of the motor units that innervate the thousands of fibers in a muscle arranged WITHIN THE SPINAL CORD?
into an elongated motor nucleus in the ventral horn, that may extend over several segments (1-4).
What does 1 LMN normally innervate?
Many muscle fibers in DIFFERENT FASCICLES within one muscle.
What happens during low force movement?
A FEW motor units are recruited, and the entire muscle contracts.
What happens during higher force movements?
More motor units are recruited
Why is motor unit recruitment for increasing force, GRADED?
So limb movement is smooth.
How are different motor units activated (in nature)? Why?
Asynchronously - to produce smooth, fluid movements.
What is indicated by a motor unit innervating many muscle fibers that are all in ONE fascicle?
Pathologic deinnervation - reinnervation
How is deinnervation-reinnervatn manifested in movements?
As large, jerky, and not finely graded movements - too powerful for the force needed.
What are fasciculations?
Irregular, broad, high amplitude multiphasic potentials seen in diseased muscle.
You should look at the graph on pg 5 of this lecture
2 sites where motor neurons are located:
1. Spinal cord (ventral horn)
2. Brain stem (face/tongue)
What is a Motor Nucleus?
A vertical column of motor neurons in the spinal cord that extends 1-4 segments.
How are the motor neurons that make up a motor nucleus related?
They innervate the same muscle
What are the motor neurons that make up a motor nucleus sometimes called?
A motor neuron pool
How are the motor nuclei in the spinal cord ventral horn arranged?
According to individual muscles
What do Medial motor nuclei innervate?
Axial muscles of neck/back
How are the Medial motor nuclei interconnected?
Across many segments by propriospinal neurons with LONG axons and EXTENSIVE BRANCHES.
What do the Lateral motor nuclei innervate?
-Most medial
-Most lateral
Limb muscles:
Medial = proximal limbs
Lateral = distal limbs
How are the Lateral motor nuclei interconnected?
Across a FEW segments by propriospinal neurons, but with SHORT axons and FEWER branches.
Why do the lateral motor nuclei branch less than medial?
Because the distal limbs need finer control of movements than the back/neck trunk muscles.
How are flexor/extensor innervating neurons arranged within the spinal cord?
Flexors = near center
Extensors = more peripheral
Define a lower motor neuron:
Primary motor neuron that originates in the spinal cord or brainstem to directly innervate muscle.
Define an upper motor neuron:
A neuron in the brainstem or motor cortex that synapses ON LMNs to convey descending CONTROL of movement.
Where do motor neurons communicate with muscles?
At the NMJ - neuromuscular junction.
What does a motor axon terminal do?
Branches into synaptic boutons.
What do synaptic boutons contain?
-Synaptic vesicles filled w/ NT
-Many Ca channels
What is the NT at an NMJ?
How wide is the synaptic cleft?
50-100 nm
What kind of receptors are contained in junctional folds at the muscular end plate?
Nicotinic ACh recepors - in high density.
What happens when ACh binds receptors at the NMJ endplate?
It opens voltage-gated Na channels.
How are the ACh-filled vesicles released by the presynaptic terminal?
Via Calcium influx at voltage-gated calcium channels.
What does calcium influx trigger?
Vesicle fusion and release of ACh into synaptic cleft.
What happens when an AP propagates along the muscle fiber as result of neuromuscular transmission?
Ca is released from SR, activates contractile proteins, and the muscle contracts.
How do the muscle fibers that are innervated by the same motor unit contract?
What happens as the force required increases?
Additional motor units are activated so that many fibers now fire ASYNCHRONOUSLY
What is achieved by asynchronous firing of motor units?
Graded, smooth movement.
How is neuromusc transmission STOPPED?
Via AChesterase
What are 3 ways in which different types of motor units and muscle fibers differ?
1. Speed of recruitment
2. Strength in contraction
3. Fatigability
What are slow twitch fibers like? (4 features)
-Slower to recruit
-Smaller force output
-Fatigue resistant
What are slow twitch fibers specialized for?
Endurance - posture, standing and walking.
What are fast twitch fibers like?
-last to recruit
-Larger force of contraction
Why do fast twitch fibers fatigue?
Because glycogen is rapidly depleted
What are fast twitch fibers specialized for?
Strength and speed
-Vigorous exercise
What are intermediate fibers?
Fast Fatigue-resistant fibers! the best of both worlds..
What are Intermediate fibers specialised for?
Marathon running and endurance walking.
What type of fibers do most muscles contain?
A mixture of all 3 types
2 things that Ultimate Force depends on:
-Rate of firing
What is Rate Code?
The frequency of action potential firing in a motor neuron
What is it called when Rate code increases?
Temporal summation - the number of AP's increases in the same amt of time.
What is Recruitment?
Increasing the number of motor units firing
What type of summation is recruitment?
What size of motor units fire first and last?
First = smallest motor units
Last = largest motor units
Why do small motor units fire before large?
Because small have a lower threshold for firing.
So what type of muscle fibers are stimulated first? What type of force do they generate?
Slow twitch - small motor neuron cell bodies, small axons, slower conduction velocity, low threshold - so generate small force!
Why do larger fast-twitch fibers fire last?
Because their larger cell body takes more input - more EPSPs = to reach threshold.
When small force is required, what type of fibers are recruited?
When MORE force is required, what is added?
Fast Fatigue-resistant units (intermediate)
When MAXIMUM force is required, what is added?
Fast Fatigable units
In what order do fibers STOP firing?
Reverse - fast stop first, slow stop last.