Neuromuscular 1 Flashcards
1
Q
Muscular force is essential to the control of posture and movement
A
Muscular force is essential to the control of posture and movement
- Controlling our posture and movement depends on controlling how we apply forces to our skeleton and world around us
- F = m x a
- Muscle action generates force joint
- Maximum muscle forces for a single muscle range between ~1-1000 N.
- In the upright stance, ‘anti-gravity muscle’ develop the force necessary to stabilise joints and prevent collapse.
- ‘Active’ movement of body parts occurs when muscle forces exceed forces opposing the movement
2
Q
Three types of muscle action and muscle types
A
Actions
- Concentric = shortening
- Eccentric = lengthening
- isometric = no change
Muscle types
- Skeletal muscle action stretches tendons, applies force to bones and fixates or moves bones about a joint
- Cardiac muscle action squeezes blood, eject blood from the heart and creates blood flow
- Smooth muscle action constricts an aperture and restricts flow of a substance, such as in the circulation (e.g., blood, lymph) or GI tract (e.g. food)
3
Q
Human skeletal muscle and motor units
A
- Human skeletal muscles have many muscle cells (myocytes) - up to a million myocytes!
- Each myocyte is innervated by several types of nerve cells (neurons)
- Type of neuron which initiates muscle action is called an α-motoneuron
- Motor unit (MU) is single α-motoneuron and the muscle fibres it innervates
- MU number is ~100-1000 and proportional to muscle size …but…
- …relative to size, smaller muscles have more MUs but less myocytes per MU.
- MU size refers to size of motor neuron and number of myocytes
- Myocytes of a MU are widely dispersed.
4
Q
Control of force output
A
- Force is developed by muscle fibres; but force development is controlled by activity (firing, impulses) of the α-motoneuron
- The level of force (i.e. force output) from a single muscle can be varied in two ways: by varying 1) the number of MUs ‘recruited’ and 2) the firing rate of α motoneurons
5
Q
Motor unit firing rate and control of force
- Twitch and tetanus
A
- Force can be adjusted by varying the ‘firing rate’ (or ‘stimulation frequency’) of MUs
- ‘Firing’ refers to depolarisation of the neuron (‘impulse’)
- During a short period of firing (e.g., 1s), impulses result in muscle twitches (1 hz), unfused tetanus or fused tetanus.
- Firing at low rate (~1-2 Hz) and ultimately a tetanic contraction (>20 hz)
- Varying the firing rate of a single MU can alter the force considerably
6
Q
Motor unit recruitment and control of force
- Henneman’s ‘size principle’.
A
- Force is also adjusted by varying the number of MUs recruited
- MUs are generally recruited from smallest to largest as the force is increased- Henneman’s ‘size principle’.
Progressive MU recruitment leads to a progressive increase in force
7
Q
Motor unit type and speed (and velocity): slow to fast ‘twitch’
HINT: intrinsic shortening of velocity
A
- Movement velocities are influenced by intrinsic shortening velocity of myocytes, MU recruitment speed, and MU firing rate
- Motor unit and myocytes are classified according to twitch speed: slow twitch (type 1) vs fast twitch (type 2)
- Left figure: S=slow, FR=fast fatigue-resistant, FF=fast-fatigable
- An α-motoneuron innervates one type of myocyte: slow to slow; fast to fast
- Slower MUs are usually smaller, weaker and shorten more slowly
8
Q
MU pool and its recruitment across the maximum range of force
A
- A motor unit ‘pool’ consists of all α- motoneurons which project to a muscle
- Force depends on how much (%) of the MU pool is ‘recruited’
- Maximum force is generated by recruiting 100% of the MU pool
- Adjustments in force or velocity depend on graded variations in MU recruitment
- Therefore, the type of MUs and myocytes involved in movement depends on the force and/or speed of the task
- As the task become more powerful, you progressively recruit faster MU
- The slower MU are also used
9
Q
Central’ neural connections with muscle via alpha motoneurons
- a-motoneurons 3 sources of neural input
A
- α-Motoneurons receive neural input from three sources:
1. efferent neuron from the brain;
2. afferent (sensory) neuron from tissues;
3. spinal neuron - Efferent (1) and afferent (2) neurons also synapse with spinal neurons (3) and influence input from spinal neurons to the α- Motoneurons
- Neural input to α-Motoneurons can be excitatory (1,2,3) or inhibitory (2,3)
10
Q
3 Neural processes influencing MU recruitment and muscle activity
Hint: what do they provide to the motor neurons
A
- Motor neurons are switched on (‘recruited’) or off by three major factors.
1. Efferent neurons originating from the brain provide ‘supraspinal input’ to switch on Motor neurons s and vary their level of activity. They initiate and change movement.
2. Afferent neurons originating from muscles, joints and skin provide ‘sensory input’ to motor neurons. They help modify MU recruitment and control movement.
3. Spinal neurons, especially those which form the circuits of central pattern generators, provide rhythmic input to Motor neurons. They sustain rhythmic movement.
11
Q
Motor command and motor unit recruitment
- Hint: Muscle action….
A
- Muscle action is initiated by supraspinal input from the brain.
- This input is a motor command.
- Motor command descends down spinal cord to α-motoneurons.
- Level of motor command can be varied, reflected in 1) rate of impulses travelling along descending neurons and 2) number of neurons involved.
- The extent of motor unit recruitment is proportional to the level of motor command.
12
Q
Brain and areas of motor command
- 3 main areas
A
- Several major areas of cerebral cortex involved in movement: premotor cortex, supplementary motor area and primary motor cortex.
Premotor cortex
- Coordinates sensory cues (e.g., bright light) with the required movement (e.g., neck and spinal extension).
Supplementary motor area
- Programming the motor plan, particularly for more complex tasks.
Primary motor cortex
Action: initiating and executing movement
- Areas of the primary motor cortex project to distinct parts of the body (bottom figure)
- These motor regions represented in both hemispheres as mirror images.
13
Q
Motor cortex controls MU recruitment
Hint: Corticospinal neurons
A
- Supraspinal control of limb movement involves neurons which originate within motor cortex and project along the spinal cord - corticospinal neurons.
- Corticospinal neurons run along corticospinal tracts and project to different spinal levels.
- At the base of the medulla, these neurons decussate (cross over) to the other side.
- Corticospinal neurons connect directly with a-MNs and help control MU recruitment and firing rate.
- Corticospinal neurons also connect with spinal interneurons (CPGs) which helps integrate supraspinal control with spinal control of MU recruitment and muscle action.
14
Q
Stimulation of motor cortex evokes contraction on the contralateral side
- TMS
A
- TMS – magnetic pulse creates an electrical field and stimulates corticospinal neurons.
- Evokes contraction of muscles on the contralateral side.
- Power of this effect is small in the absence of any voluntary activation of muscle.
15
Q
Cerebrovascular disease and motor cortical dysfunction
A
Stroke - blockage of blood vessel in the brain from blood clot Symptoms - F: Face (slopes) - A: Arms (can't rise) - S: Speech (slurped) - T: Time to call 911
