chapter 11 pt 2

Question 1

Nerve cells called somatic motor neurons stimulate muscle fibers via

Answer 1

their axons

Question 2

Skeletal muscle never contracts unless

Answer 2

stimulated by a nerve

Question 3

A muscle becomes paralyzed when

Answer 3

nerve connections are severed or poisoned

Question 4

denervation atrophy

Answer 4

shrinkage of paralyzed muscle when connection is not restored

Question 5

nerve cells whose cell bodies are in the brainstem and spinal cord that serve skeletal muscles

Answer 5

somatic motor neurons

Question 6

motor unit

Answer 6

one nerve fiber and all the muscle fibers innervated by it

Question 7

muscle fibers of one motor unit

Answer 7

dispersed throughout the muscle, contract in unison, produce weak contraction over wide area, provides ability to sustain long-term contraction as motor units take turns contacting, effective contraction of several motor units at once

Question 8

synapse

Answer 8

where a neuron meets a target cell

Question 9

on average how many muscle fibers are innervated by each motor neuron?

Answer 9

200

Question 10

Small motor units are present where

Answer 10

fine control is needed, such as in the muscles of eye movement (3 to 6 muscle fibers per neuron).

Question 11

Large motor units are present where

Answer 11

strength is more important than fine control, such as in the gastrocnemius (1,000 muscle fibers per neuron).

Question 12

AChE

Answer 12

acetylcholinesterase

Question 13

ACh

Answer 13

acetylcholine

Question 14

Muscle fibers and neurons are considered

Answer 14

electrically excitable cells

Question 15

Why are muscle fibers and neurons considered electrically excitable cells?

Answer 15

because their plasma membranes exhibit voltage changes in response to stimulation

Question 16

electrophysiology

Answer 16

The study of the electrical activity of cells

Question 17

A difference in electrical charge between two points is called

Answer 17

voltage or electrical potential

Question 18

–90 mV

Answer 18

Resting Membrane Potential (RMP)

Question 19

The resting membrane potential is maintained by the

Answer 19

sodium–potassium pumps.

Question 20

four major phases of muscle contraction and relaxation

Answer 20

excitation; excitation–contraction coupling; contraction; and relaxation

Question 21

Excitation is

Answer 21

the process in which action potentials in the nerve fiber lead to action potentials in the muscle fiber

Question 22

5 steps of excitation

Answer 22

1. A nerve signal arrives at a synaptic knob and stimulates voltage-regulated Ca2+ gates to open; calcium ions enter the synaptic knob.
2. Ca2+ stimulates exocytosis of synaptic vesicles, which release ACh into the synaptic cleft.
3. ACh diffuses across the synaptic cleft and binds to receptor proteins on the sarcolemma.
4. The receptors are ligand-gated ion channels that bind two ACh molecules to open. 5. Areas adjacent to the NMJ have ion-specific voltage-gated ion channels that open in response to the EPP, allowing flow of Na+ in and K+ out, generating an action potential

Question 23

Excitation–contraction coupling refers to

Answer 23

the events that link the action potentials on the sarcolemma to activation of the myofilaments.

Question 24

4 steps of excitation-contraction coupling

Answer 24

A wave of action potentials spreads from the end plate in all directions, and enters the T tubules, continuing down them into the sarcoplasm.

2. Action potentials open voltage-gated ion channels in the T tubules 3. Calcium binds to the troponin of the thin filaments.
3. The troponin–tropomyosin complex changes shape, exposing active sites on the actin filaments that can bind to myosin heads.

Question 25

Contraction is

Answer 25

the step in which the muscle fiber develops tension and may shorten

Question 26

4 steps of contraction

Answer 26

1. Myosin ATPase hydrolyzes ATP that is bound to the myosin head
2. With ADP and phosphate still bound, the activated myosin head binds to an exposed active site on the thin filament, forming a cross bridge.
3. Myosin releases the ADP and phosphate and flexes into a bent, low energy shape, tugging the thin filament along with it
4. Upon binding to another ATP, myosin releases the actin

Question 27

When stimulation ceases

Answer 27

a muscle fiber relaxes and returns to its resting length

Question 28

5 steps of relaxation

Answer 28

1. the synaptic knob stops releasing ACh.
2. ACh dissociates from the receptor, AChE breaks it down; the synaptic knob reabsorbs the fragments, but now no new ACh replaces that which is broken down.
3. Active transport pumps in the SR begin to pump Ca2+ from the cytosol back into the cisternae.
4. As Ca2+ dissociates from troponin, it is pumped into the SR and not replaced.
5. Tropomyosin moves back into position, blocking the active sites of the actin filament and preventing myosin binding.

Question 29

The two forces that aid a muscle to return to its resting length

Answer 29

gravity, contraction of the antagonist

Question 30

The amount of tension a muscle generates depends on

Answer 30

how stretched or contracted it was before it was stimulated; this principle is termed the length–tension relationship.

Question 31

A nerve–muscle preparation can be used to record

Answer 31

a chart of stimulation and muscle contraction called a myogram