Muscular System High Yield Notes

Question 1

Three types of muscle

Answer 1

1. smooth muscle,
2. cardiac muscle
3. skeletal muscle

Question 2

Striated

Answer 2

means the muscle contains sarcomeres .

Question 3

Cardiac muscle

Answer 3

contains intercalated discs, which are made of desmosomes (hold cells together) and gap junctions that connect the cytoplasm of cells together to allow ion exchange and electrical impulse propagation.

Question 4

Skeletal muscle

Answer 4

is composed of many bundles
within bundles .

Question 5

sarcolemma

Answer 5

is the muscle fiber’s cellular membrane, and it protects each muscle fiber.

Question 6

sarcoplasm

Answer 6

is the cytoplasm of the muscle fiber and holds the myofibrils

Question 7

Sarcomeres

Answer 7

inside of myofibrils are the functional unit of muscle fibers and shorten to cause muscle contraction.

Question 8

Myofilaments

Answer 8

are contained within sarcomeres, divided into thin actin filaments and thick myosin filaments . These filaments slide past each other to shorten sarcomeres through the sliding filament model of muscle contraction .

Question 9

Stimulation of a muscle contraction (4 steps)

Answer 9

1. Action potential propagation reaches the end of a motor neuron’s axon.
2. Acetylcholine is released as a neurotransmitter between the presynaptic motor neuron and postsynaptic skeletal muscle fiber at the neuromuscular junction.
3. Acetylcholine binds to ligand - gated sodium channels , causing sodium to enter
   the cell, which creates graded potentials on the muscle fibers.
4. The graded potentials trigger opening of
   voltage-gated sodium channels , which may
   produce action potentials on the muscle if the stimulus is large enough.

Question 10

sarcolemma

Answer 10

is the cell membrane of striated muscle and contains T-tubules invaginations that quicken action potential propagation on the
muscle.

Question 11

sarcoplasmic reticulum

Answer 11

is the endoplasmic reticulum of muscle fibers that releases stored calcium ions into the sarcoplasm through voltage-gated calcium channels when triggered by the depolarization of the muscle cell.

Question 12

calcium ions

Answer 12

bind to troponin , which removes tropomyosin from the myosin-binding-sites on actin , allowing myosin to interact with actin and cause sarcomere shortening, via sliding filaments.

Question 13

Cross bridge cycling

Answer 13

1. Initiation: Calcium ions expose the
   myosin-binding-sites on actin.
2. A cocked back, high energy myosin head
   (containing ADP and P i ) forms a cross bridge with the actin.
3. The myosin head contracts and the power
   stroke occurs, bringing the myosin head back to a low energy state and releasing ADP and P i . As a result, the sarcomere shortens.
4. A new ATP molecule binds to myosin, causing detachment of the myosin head from the actin filament.
5. The myosin head is an ATPase , and it
   hydrolyzes the ATP into ADP and P i . This
   causes the myosin head to re-enter a cocked back, high energy state . (Return to Step 2 if calcium ions present).
6. Termination: Neuronal signaling from motor neurons ends. The sarcoplasmic reticulum pumps calcium back into itself, and troponin brings tropomyosin back to cover

Question 14

Rigor mortis

Answer 14

occurs in dead animals when there is no ATP available to release myosin from the actin.

Question 15

Z lines

Answer 15

are the ends of the sarcomeres.

Question 16

Thin actin filaments

Answer 16

branch from the Z lines towards the middle of the sarcomere.

Question 17

M lines

Answer 17

are the midpoints of the sarcomeres

Question 18

Thick myosin filaments

Answer 18

branch from the M lines towards the ends of the sarcomere.

Question 19

I band

Answer 19

is the area in the sarcomere where only actin filaments are present. (Mnemonic : “I” is a thin letter, representing thin actin filaments)

Question 20

A band

Answer 20

is the area in the sarcomere where actin and myosin overlap.

Question 21

H zone

Answer 21

is the area in the sarcomere where only myosin is present. (Mnemonic : “H” is a thick letter, representing thick myosin filaments)

Question 22

Motor units

Answer 22

make up muscles; a motor unit refers to all the muscle fibers innervated by a single neuron .

Question 23

Small motor units

Answer 23

include only a few muscle fibers and are used in precision movement.

Question 24

Large motor units

Answer 24

include many muscle fibers that are innervated by a single neuron and are used in powerful movements.

Question 25

twitch contraction

Answer 25

is the contraction of a muscle fiber through motor unit stimulation. Each twitch has the same size and duration .

Question 26

all-or-none principle

Answer 26

which states that a depolarization will cause all the muscle fibers to twitch if it is above threshold potential but will not cause any twitching if the depolarization is below threshold potential.

Question 27

Three phases of a twitch:

Answer 27

1. Latent: action potential spreads over sarcolemma and T-tubules, signaling to
   sarcoplasmic reticulum to release calcium.
2. Contraction: formation of cross bridges as a result of calcium ions binding to troponin . H zones shrink and muscle tension increases.
3. Relaxation: calcium is pumped back into the sarcoplasmic reticulum, ending cross bridge cycling and decreasing muscle tension.

Question 28

Summation

Answer 28

is the process by which twitches add up to create a larger overall contraction .

Question 29

There are two types of summation:

Answer 29

1. Wave summation (temporal summation)
2. Motor unit summation

Question 30

Wave summation (temporal summation)

Answer 30

depolarizing a motor unit again during the
relaxation phase. May cause tetanus , which
is when the muscle fibers cannot be further
stimulated due to a lack of relaxation.
Twitches blend together during tetany,
eventually causing fatigue (loss of muscle
contraction).

Question 31

Motor unit summation

Answer 31

different motor units are stimulated at different times to produce the intended amount of muscle contraction. This is also known as the size principle of motor unit recruitment because smaller motor units are stimulated first before larger motor units come in to help.

Question 32

muscle tone (muscle tonus) .

Answer 32

Weak and involuntary twitches in small motor unit groups contribute to maintaining muscle tone (muscle tonus). Fatigue is never reached because different motor units are stimulated at different times.