Linking nervous and musculoskeletal system

Question 1

Describe the build up of a motor neuron

Answer 1

Each neuron has a long threadlike extension called an axon that extends from the cell body in the spinal cord to the muscle fiber. It divides at the fiber, giving off several branches that collectively form a neuromuscular junction or motor end plate. The end of the axon (axon terminal) and muscle fiber are separated by a space (synaptic cleft). Within the axon terminal are synaptic vesicles (small membranous sacs containing the neurotransmitter Acetylcholine Ach).

Question 2

Give an overview of how an action potential causes contraction of muscles

Answer 2

Motor neuron fires an AP down its axon
The axon terminal releases Ach into the synaptic cleft
Ach binds to the receptors on the junctional folds of the sarcolemma.
Ach binding causes a local depolarization called an end plate potential (EPP)
EPP triggers AP in adjacent sarcolemma
Sarcoplasmic reticulum releases Ca2+
Ca2+ binds to tropinin, which shifts tropomysoin to uncover the myosin-binding sites on actin.
Mysoin heads bind to actin
Contraction occurs via cross bridge cycling

Question 3

Describe how an end plate potential is made

Answer 3

AP arrives at axon terminal
Voltage gated Ca2+ channels open and Ca2+ enters the axon terminal reducing the electrochemical gradient
Ca2+ entry causes Ach to be released by exocytosis
Ach diffuses across the synaptic cleft and binds to Ach receptors on the sarcolemma
Ach binding opens chemically gated ion channels, that allow Na+ into the muscle fiber and K+ out of the muscle fiber. More Na+ ions enter, producing a change in the membrane potential (end plate potential)

Question 4

Describe the 3 steps of triggering and then propogating an action potential

Answer 4

1. Generation of EPP
   EPP is generated in the neuromuscular junction and causes a wave of depolarization that spreads to the adjacent sarcolemma
2. AP depolarization
   Depolarization of the sarcolemma opens voltage-gated Na+ channels. Na+ enters, down its electrochemical gradient. At a certain membrane voltagen an AP is generated which propagates along the sarcolemma in all directions
3. AP repolarization
   Voltage-gated Na+ channels close and voltage-gated K+ channels open. K+ diffuses put of the muscle fiber, down its electrochemical gradient and restores the negatively charged conditions of the sarcolemma.

Question 5

What causes an action potential?

Answer 5

Action potential is the result of a predictable sequence of electrical changes

Question 6

What is a refractory period in a muscle fiber?

Answer 6

During repolarization, a muscle fiber is said to be in a refractory period, because the cell cannot be stimulated again until repolarization is complete

Question 7

Describe the impulse conduction in unmyelinated fibers

Answer 7

Voltage-gated Na+ and K+ channels regenerate the AP at each point along the axon. Conduction is slow because it takes time for ions and for gates of channel proteins to move.
Called continuous conduction

Question 8

Describe the impulse conduction in myelinated fibers

Answer 8

Myelin keeps current in the axons, so voltage doesn’t decay. AP are generated are only generated in the myelin sheath gaps and appear to jump quickly from gap to gap. Conduction is fast.
Called saltatory conduction

Question 9

Describe the 2 types of contractile filaments

Answer 9

1. Thick filaments; contain bundled myosin molecules

2. Thin filaments; contain actin molecules

Question 10

What is the role of elastic filaments and describe what it is and where it is found

Answer 10

Provide elastic recoil when tension is released and help maintain myofilament organization. It is a large spongy protein called titin, and runs through the core of each thick filament

Question 11

Describe the 4 stages of muscle contraction and relaxation

Answer 11

1. Excitation; AP in the nerve leads to formation of AP in muscle fiber
2. Excitation-contraction coupling; AP in sarcolemma activate myofilaments
3. Contraction; shortening of muscle fiber
4. Relaxation; return of muscle fiber to its resting length.

Question 12

Describe the steps in excitation-contraction coupling

Answer 12

1. AP propogates along the sarcolemma and down the T tubules
2. Causes the voltage-sensitive tubule proteins to change shape. This opens the Ca2+ release channels, allowing Ca2+ to flow into the cytosol.
3. Ca2+ binds to troponin causing it to change shape. This exposes myosin-binding sites on the actin
4. Myosin binds to actin, forming cross bridges and contraction begins

Question 13

What occurs after excitation-contraction coupling

Answer 13

Muscle AP ceases, the voltage-sensitive tubule proteins return to their original shape, closing the Ca2+ channls. Ca2+ levels in the sarcoplasm fall, the blocking of tropomyosin is restored, myosin-actin binding is inhibited and relaxation occurs

Question 14

In what 2 ways can muscle contraction be graded?

Answer 14

1. Changing frequency of stimulation
   Causes temporal summatation. The higher the frequency, the greater the strength of contraction
2. Changing strength of stimulation
   Causes recruitment. The stronger the stimulation, the more motor units are activated and the stronger the contraction.

Question 15

Describe the following terms:

1. Motor unit
2. Twitch
3. Treppe
4. Summation

Answer 15

1. Consists of one motor neuron and the muscle fiber it supplies
2. Stimulus surpasses the threshold intensity and a muscle fiber contraction occurs
3. Higher frequency of stimulation but each twitch develops more tension than the one before
4. Higher frequency of stimulation and a new twitch ‘piggybacks’ on the previous one, developing higher tension

Question 16

Describe the following terms:

1. Length-tension relationship
2. Muscle tone

Answer 16

1. Optimum resting length/ overly contracted/ over stretched

2. Resting muscle in a state of partial contraction

Question 17

Describe the following terms:

1. Isotonic contraction
2. Isometric contraction
3. Concentric contraction
4. Eccentric contraction

Answer 17

1. Contraction with a change in length, but not a change in tension. Movement occurs
2. Contraction without a change in length. No movement
3. Muscle shortens as it maintains tension
4. Muscle lengthens as it maintains tension

Question 18

What are the two types of isotonic contraction?

Answer 18

Concentric and eccentric

Question 19

Briefly describe the nervous systems 3 overlapping functions

Answer 19

1. Sensory input = nervous system uses it sensory receptors to monitor changes
2. Integration = nervous system processes and interprets sensory input to decide what should be done
3. Motor output = nervous system activates effector organs to cause a response

Question 20

Describe the 2 principal parts the nervous system can be broken down into

Answer 20

1. Central Nervous System CNS
   consists of brain and spinal cord. It interprets sensory input and dictates motor output
2. Peripheral Nervous System PNS
   part of the nervous system outside the CNS. Consists mainly of nerves that extend from the brain and spinal cord and ganglia.

Question 21

What are the 2 subdivisions of the PNS?

Answer 21

1. Sensory/afferent division
   consists of nerve fibers that convey impulses to the CNS
2. Motor/efferent division
   transmits impulses from CNS to effector organs

Question 22

What are the 2 main parts of the motor division

Answer 22

1. Somatic nervous system
   composed of somatic motor nerve fibers that conduct impulses from CNS to skeletal muscles, often referred as voluntary nervous system
2. Autonomic nervous system
   consists of visceral motor nerve fibers that regulate the activity of smooth muscles, cardiac muscle and glands. Often called the involuntary nervous system