SNS, Action Potentials, and Muscle Contractions

Question 1

What is the precentral and postcentral gyrus.

Answer 1

precentral gyrus is the location of the primary motor cortex responsible for voluntary motor commands.

the postcentral gyrus contains the primary somatosensory cortex responsible for perceiving somatic sensations.

Question 2

Explain the functions of the basal nuclei, cerebellum and motor tracts.

Answer 2

basal nuclei (midbrain) is responsible for subconscious control of skeletal muscle tone and coordination

cerebellum is responsible for learned patterns, repetitive movement and posture

motor tracts are bundles of axons in the spinal cord responsible for delivering messages to skeletal muscles through corticospinal tracts

Question 3

Describe the difference motor and sensory neurons.

Answer 3

Motor neurons are for efferent signals. Sensory neurons are for afferent signals, and all information is relayed up to the post central gyrus (somatosensory cortex)

Question 4

Describe the pathway taken by motor neurons.

Answer 4

1. initiated in the primary motor cortex (upper motor neuron)
2. travels down to the pyramids of the medulla where it decussates (crosses over)
3. travels down corticospinal tract (pyramidal tract)
4. upper motor neuron synapses with the lower motor neuron in the anterior horn of the spinal cord

Question 5

Describe the pathways of sensory neurons for its respective functions.

Answer 5

touch and proprioception - travels via dorsal columns of the spinal cord and decussates in the medulla

temperature and pain - travels via spinothalamic columns and decussates in spinal segments

Question 6

Describe the difference between a neuron and a nerve.

Answer 6

neurons communicate through synapses located on dendrites. a nerve is an enclosed, cable-like bundle of neurons found in the peripheral system. there are sensory, motor and autonomic nerves.

Question 7

What is a myelinated axon

Answer 7

An axon that has a myelin sheet along it

Question 8

Describe what an action potential is and the mechanisms of depolarisation and hyperpolarisation.

Answer 8

an action potential is how the signal gets to the muscle. depolarisation is when the membrane potential becomes more positive, and hyperpolarisation is when the membrane potential becomes more negative.

Question 9

What is the neuromuscular junction and describe the steps that occur at the junction as an action potential arrives.

Answer 9

the neuromuscular junction is where the nerve meets the muscle, demonstrating an interaction between the CNS and the PNS.

the action potential comes through the pre-synaptic neuron and signals an influx of Ca2+, which signals the release of ACh in synaptic vesicles into the synaptic cleft. Ach binds to Ach receptors, which propagates the signal into the muscle.

Question 10

Describe the four properties of skeletal muscle.

Answer 10

excitability - ability to receive and respond to a stimulus
contractability - ability to shorten in response to adequate electrical stimulation
extensibility - ability to stretch (when not contracting)
elasticity - ability to recoil and regain the resting length after being stretched

Question 11

Describe the structure of skeletal muscle.

Answer 11

skeletal muscle (organ), muscle fascicle (bundle of cells), muscle fiber (cell)

Question 12

Describe the steps in excitation contraction coupling.

Answer 12

ACh attaches to receptors and the signal is propogated along the sacrolemma to the transverse tubule (T-tubule). the action potential in the T-tubule signals Ca2+ release from the sacroplasmic reticulum, which interacts with actin. myosin binds to actin leading to cross bridge cycling (muscle contraction). Ca2+ returns to SR and muscle relaxes. Ca2+ remains available and the muscle continues contracting.

Question 13

Describe the properties between fast and slow twitch muscle fibers.

Answer 13

fast - use up lots of energy very quickly then get fatigued
slow - use energy slowly and evenly to last a long time

Question 14

Explain how contraction results in a muscle twitch force through the sliding filament theory.

Answer 14

actin binds to myosin. myosin alters the configuration resulting in a ‘stroke’ that pulls on the actin filament and causes it to slide across the myosin filament, shortening the sacromere and muscle.

Question 15

Describe the concept of a space constant.

Answer 15

space constant is how quickly voltage declines as a function of the distance. caused by a leakage of current in nerve cells. issues can arise as some nerves are very long, and messages fade quickly without assistance.

Question 16

Describe nerve conduction.

Answer 16

nerve conduction is measured as a voltage. currents flow through the axoplasma in the center of the neurons, which is insulated by myelin. leakage of this current is dependent on resistance and insulation.

Question 17

Describe charge separation.

Answer 17

charge separation occurs due to differences in ions inside and outside of the cell. this creates a potential difference.

Question 18

Explain how the resting membrane potential is maintained.

Answer 18

resting membrane potential is between -30mV to -90mV, and maintained through ions such as Na, K, Ca, Cl, HCO3, and organic anions in neurons and surrounding fluid. It is more leaky to potassium, therefore the resting membrane potential is closer to potassium than sodium.

Question 19

What is depolarisation and describe the process that occurs.

Answer 19

depolarisation is when the voltage of the membrane is increasing.

Na+ voltage gated ion channels open quickly resulting in a rapid influx of Na+ into the cell. The membrane potential moves more towards that of Na+, simultaneously, K+ channels open but much more slowly.

Question 20

What is repolarisation and describe the process that occurs.

Answer 20

repolarisation is when the voltage of the membrane is decreasing.

as Vm reaches that of Na+, the influx of Na+ decreases. the earlier opening of K+ channels and increase in K+ conductance slows the upswing of Vm towards Na+ even more. Na+ channels start to close and more K+ channels open, bringing Vm down towards the membrane potential for K+.

Question 21

What is hyperpolarisation and describe the process that occurs.

Answer 21

hyperpolarisation is the refractory period. it must be met at -55mV to propagate the signal

K+ channels are slow to close, leading to Vm reducing below RMP. an even greater stimulus will be required for the propagation of action potentials.

Question 22

Define contraction, force, load, power, and energy as they relate to muscle contraction.

Answer 22

contraction - physiological response to stimuli that results in shortening in length and development of force
force - amount of overall tension that is produced by increasing motor unit recruitment
load - the force exerted on the muscle by an object
power - amount of work produce by a muscle over a period of time
energy - the power or metabolic heat that is created and used to produce work

Question 23

Define a muscle twitch.

Answer 23

It is the contraction-relaxation sequence of a muscle fibre in response to a single action potential.

Question 24

What are the two different types of muscle contractions?

Answer 24

isometric - produces a force without a change in the length of muscle due to immobile attachment points. a stimulus causes an increase in tension but no muscle shortening occurs,

isotonic - shortening or lengthening of muscle with a constant load applied to it

Question 25

Describe the length-tension relationship in isometric contractions.

Answer 25

at maximum tension, there is a max overlap of actin and myosin filaments, and the max number of cross-bridges that can be formed.

at longer than optimum, actin is pulled out from between thick filaments due to force.

at shorter than optimum, thin filaments overlap between sides and thick filaments are forced against the z-line.

Question 26

Describe the force-velocity relationship in isotonic contractions.

Answer 26

the velocity of muscle contraction is dependent on the load amount. it slows as the muscle shortens.

max velocity is only possible when load = 0.

max load is too heavy to allow contraction shortening. the muscle contracts isometrically (v=0). as load continues to increase, the muscle fibers lengthen.

Question 27

Explain how a muscle action potential can result in multiple action potentials and what this means.

Answer 27

the muscle action potential is over before the muscle has started to contract. a contraction lasts around 100ms, therefore it is possible to have many action potentials while the muscle is still contracting.

Question 28

Explain the purpose of increasing the endurance of a muscle contraction.

Answer 28

during sustained contraction, tension is maintained by asychronus motor unit summation resulting in unfused tetanus. individual muscle fibres therefore have enough time to replenish their ATP stores during prolonged contraction of the muscle.

Question 29

Describe motor units and motor unit recruitment.

Answer 29

a motor unit is when all muscle fibers are innervated by one alpha-motor neuron from a motor neuron pool. motor unit recruitment happens alongside the grading of muscular force. at lower levels of force, motor unit recruitment occurs first followed by increasing stimulus frequency to increase force.

Question 30

Describe the function of asychronus recruitment and how it relates to the size principle.

Answer 30

asychronus recruitment prevents fatigue, and is shifted amongst different motor units to conserve energy. it only occurs for submaximal force production.

size principle states that the smallest and slowest motor units are activated first followed by longer and faster ones.

Question 31

Compare and contrast the different types of muscle tissue.

Answer 31

skeletal muscle tissue is responsible for voluntary control.cardiac and smooth muscle tissue is responsible for involuntary control.

Question 32

What is innervation?

Answer 32

innervation is the process in which the fiber type is determined by the alpha-motor neuron.

Question 33

Compare the properties of slow and fast twitch fibers.

Answer 33

slow twitch stains dark due to the presence of oxygen for oxidative pathways. it has smaller cell bodies due to lower thresholds. there is a lower motor unit to fiber innervation ratio.

fast twitch stains lightly due to glycolytic pathways. it has larger cell bodies due to higher thresholds. there is a higher motor unit recruitment to fibre innervation ratio.

Question 34

Describe the characteristics of cardiac muscle.

Answer 34

cardiac muscle relies on pacemaker cells that generate aps, which can be influenced by external stimuli. there is a stroke volume-end diastolic relationship. the heart continues beating and does not reach fused tetanus.

Question 35

Describe the characteristics of smooth muscle.

Answer 35

there is always a background level of activity due to smooth muscle tone. a twitch may last several seconds. stretching can induce muscle contractions. it relies on aerobic metabolism, and some anaerobic in peak activity.