pnb 74

Stimulus for movement

action potential in the motor neuron of the cerebral cortex

Corticospinal tract

connections between the brain and spinal cord where action potentials travel in axons

Motor neurons

neurons that carry outgoing information from the brain and spinal cord to synapses at muscles

Motor unit

one motor neuron and all the muscle fibers it innervates

All or none phenomenon

action potentials either happen completely, or not at all

Is muscle contraction an all-or-none phenomenon?

a single motor unit is all or none, however, recruiting more motor units can increase muscle contraction

therefore, as a whole, muscle contraction is not all or none

different muscle fibers have different thresholds

Electromyography (EMG)

measures electrical activity of skeletal muscles in response to a nerve's stimulation of the muscle

Electromyogram

record of the electrical activity in a muscle

gives information about the timing of the muscle contraction, the pattern of recruitment, and the amplitude of force

Power stroke

1. calcium levels increase in cytosol as calcium is released from SR to sarcoplasm

2. calcium binds to troponin

3. troponin-calcium complex fulls tropomyosin away from actin's myosin-binding site

4. myosin binds strongly to actin

5. ATP is hydrolyzed to ADP, Pi is released

6. power stroke is completed

Termination of a muscle contraction

1. sarcoplasmic calcium-ATPase (SERCA) pumps back into the SR

2. decrease in the cytosolic causes calcium to release from troponin

3. tropomyosin covers the binding site

Where should to ground electrode be placed for an EMG?

wrist

Why are two recording electrodes needed to record an EMG?

EMG recordings display the potential difference between the two electrodes

Why does the PowerLab hardware for EMGs amplify the measured current?

the calcium cascade we are measured is quiet faint, so the signal must be amplified to see the contractions better

PowerLab

a multi channel recording instrument for the measurement of electrical signals

performs data acquisition, signal conditioning, and pre-processing

Signal conditioning

recording hardware can modify the signal by amplification and filtering

Isotonic contractions

maintain constant tension in the muscle as the muscle changes length

Concentric contractions

muscle shortens

Eccentric contractions

muscle lengthens

Reciprocal activation

contraction of agonist muscle causes relaxation (lengthening) of its antagonist muscle

occurs in isotonic contractions

minor involuntary activity occurs in the antagonist muscle to support the joint

Coactivation

during reciprocal activation, there is minor involuntary activity in the antagonist muscle to provide support to the joint

Isometric contractions

contractions in which the joint angle and muscle length do not change

Integrated EMG

an average of the raw EMG used for analysis of the data

integrated by Lt KuraCloud software

Autoscale function

should be used when the data appears as a flat line

Horizontal/vertical compression function

should be used when the data appears too zoomed in or zoomed out

While holding different weights, was a change in EMG activity recorded?

yes, EMG activity increases as weight increases due to recruitment of additional motor units to maintain the joint stability/position

When was the largest EMG amplitude recorded?

when holding the heaviest weight

Co-contraction

opposing muscle groups work antagonistically and cooperatively to maintain a joint angle

examples = holding a squat, standing, holding a weight

What type of muscle contraction occurs during a plank?

isometric

What type of muscle contraction occurs during push ups?

isotonic

Neuromuscular junction signaling

1. stimulation of an action potential in a motor neuron

2. calcium influx through voltage gated channels in neuron

3. vesicle docking

4. acetylcholine release into synaptic cleft

5. acetylcholine receptor binding leads to nonspecific cation influx (Na+ flows in, K+ flows out)

6. opening of voltage gated Na+ channels

7. further depolarization

8. activation of motor end plate

9. propagation of depolarization along muscle cell plasma membrane

Force transducer

measures the tension force produced by a muscle

converts mechanical force into an electrical signal that can be interpreted by the software

Muscle twitch

a motor unit's response to a single action potential of its motor neuron

Summation

a second stimulus arrives before the muscle is able to relax, causing a second twitch on top of the first so that greater peak tension is developed

Unfused tetanus

some relaxation occurs between contractions

Fused tetanus

no relaxation occurs before the following contractions

the result is a maximum sustained muscle contraction

Recruitment of motor units in muscles

as the voltage in the stimulator increases (this mimics activation of motor neurons), more muscle fibers will be electrically stimulated, causing increased contraction of the muscle

Why must the force transducer be calibrated before force measurements are taken?

calibration of the transducer ensures that all succeeding measurements are accurate and can be compared to one another

What characteristic of the stimulus can be changed to achieve tetanus?

frequency of stimulus

What characteristic of the stimulus can be changed to achieve maximal contraction?

increase stimulus amplitude

Threshold stimulus

he minimal strength required to cause a contraction

Maximal stimulus

strongest stimulus that increases maximum contractile force

Supramaximal stimulus

any stimulus stronger than a maximal stimulus

these stimuli cannot produce a greater frequency of action potentials than a maximal stimulus

equal to 1.5x the maximal stimulus determined during the lab

Cross bridge formation

1. myosin heads bind to actin, forming a cross bridge

2. myosin cross bridges rotate toward the center of the sarcomere (power stroke)

3. as myosin heads bind ATP, the cross bridges detach from actin

In a state of rigor mortis, the muscles are highly contracted and difficult to manipulate. Why does this phenomenon occur?

lack of ATP in a corpse leads to myosin heads being firmly attached to the actin filament binding site

Why does rigor mortis go away?

decomposition of tissue (release of enzymes over time)

Length-tension curve

demonstrates that optimal tension is developed at one point known as the resting length, the point in its range where peak force is developed

Muscle fatigue

Inability of muscle to maintain its strength of contraction or tension

Causes of muscle fatigue

1. buildup of ADP, Pi, Mg2+, H+ (from lactic acid)

2. H+ competes with Ca2+ binding to troponin

3. conduction failure (lack of power stroke)

4. limited oxygen

5. depletion of glucose/glycogen

Anaerobic respiration

respiration that does not require oxygen

occurs during exhausting exercise

How does anaerobic respiration lead to muscle fatigue?

1. glucose is metabolized to pyruvate then to lactate

2. pH decreases in muscle fibers

3. ability of muscle to produce force is limited

Events at the NMJ

1. action potential reaches axon terminal

2. voltage dependent calcium gates open and calcium enters the terminal

3. acetylcholine vesicles fuse with presynaptic membrane and releases acetylcholine in the synaptic cleft

4. acetylcholine binds to postsynaptic receptors on the sarcolemma

5. binding causes ion channels to open and sodium to flow into the muscle cell

6. the flow of sodium ions into the muscle cell generates an action potential

Giant fibers of an earthworm

collection of nerve cells that propagate action potentials

cells are electrically coupled through gap junctions so each fiber behaves as if a single axon

1 median (larger)

2 lateral (smaller)

How does the lateral fiber differ from the median giant fiber in terms of threshold and conduction?

lateral fibers have a higher threshold and slower conduction

Stimulus artifact

peak that occurs right after the stimulus

not an action potential

Intracellular recording

used to record resting membrane potential