Lecture 6 - Skeletal Muscle: Anatomy and Excitation

Question 1

3 subtypes of muscle tissue and their functions

Answer 1

1. Skeletal muscle tissue (Striated muscle) - moves the skeleton
2. Cardiac muscle tissue (Striated muscle) - pumps blood through the heart
3. Smooth muscle tissue - located at blood vessels, digestive tract, etc.

Question 2

How are skeletal muscles organized?

Answer 2

Within layers of fasciae

Question 3

How are each skeletal muscle connected to the skeletal system

Answer 3

They are connected via tendons (narrow and/or rounded) or aponeuroses (broad/flat)

Question 4

What type of connective tissue is the epimysium and what does it do

Answer 4

Dense irregular connective; it surrounds the whole muscle and connects to tendon

Question 5

What type of connective tissue is the endomysium and what does it do

Answer 5

Loose/areolar connective; surrounds each muscle fibre

Question 6

What type of connective tissue is the perimysium and what does it do

Answer 6

Dense elastic connective; surrounds a fasicle

Question 7

Nerve fibres

Answer 7

activate and monitor muscle contractions

Question 8

Myofibre vs. myofibrils and their function

Answer 8

Myofibers (aka muscle fibre, muscle cell) contains myofibrils and is associated with supporting cells and tissues

Question 9

Myosatellite cells

Answer 9

Stem cells for muscle tissue (within endomysium)

Question 10

Describe the organization of a skeletal muscle from the most outer structure to the most inner

Answer 10

Muscle –> Fascicle –> Myofibre (muscle fibre, muscle cell) –> Myofibril

Reminder: the epimysium surrounds the muscle, the perimysium surrounds a fasicle, the endomysium surrounds a myofibre

Question 11

Myofibre

Answer 11

A single cell that contains many nuclei and stretches from the muscle origin to its insertion (tendon-to-tendon). Nuclei are all superficial

Question 12

How are myofibres produced and how do they repair?

Answer 12

They are produced by the fusion of many myoblasts during development, and can be repaired in adulthood by myosatellite cells (stem cells) proliferating and fusing to existing myofibres

Note: adults don’t produce myofibres

Question 13

What is a motor unit made up of?

Answer 13

Motor unit = mature myofibre + motor neuron (which consits of one neuromuscular junction (NMJ))

Each myofibre always only has ONE NMJ, but a single motor neuron can make NMJs with anywhere between 1 and 1000 myofibres (motor neuron branches off into multiple NMJs that only attach to 1 myofibre)

Question 14

What is a neuromuscular junction (NMJ) made up of?

Answer 14

Axon terminal, the synaptic cleft, and the motor end plate

Question 15

Myofibrils

Answer 15

Are assemblies of special protein filaments that allow muscles to contract

Question 16

Sarcolemma and sarcoplasm

Answer 16

Sarcolemma is the plasma membrane of the muscle cell and the sarcoplasm is the cytoplasm of the muscle cell

Question 17

The 3 repeating organelles of myofibres

Answer 17

1. Myofibril - allow muscles to contract
2. T-tubules - infoldings of the sarcolemma
3. Sarcoplasmic reticulum - network of tubes that surround all myofibrils and stores and releases calcium

Question 18

Excitation

Answer 18

The sarcolemma generating an electric signal that precedes contraction

Question 19

Contraction

Answer 19

Myofibres/myofibrils becoming shorter or producing tension

Question 20

E-C coupling

Answer 20

The mechanism by which excitation triggers contraction

Question 21

Steps of excitation of a muscle cell occurring at the NMJ

Answer 21

1. Myofibre is at rest
2. There is a release of neurotransmitter (acetylcholine)
3. Neurotransmitter binds to the ACh-receptor membrane channel, which allows the uptake of Na+ into the myofibre
4. The excitation will spread throughout the myofibre
5. Then there will be enzymatic removal of the NT (Ach)

Question 22

When is contraction triggered?

Answer 22

Contraction of a skeletal muscle fibre is only triggered when a command is received from the nervous system, activating the motor neuron, which excites the muscle fibre

Question 23

What are excitable cells and give 2 examples

Answer 23

Excitable cells can rapidly change their transmembrane potential to send and receive signals. All cells have transmembrane potential, but only excitable cells can change them

Ex. muscle cells and neurons

Question 24

How is membrane potential formed?

Answer 24

It arises from the unequal distribution of ions and unequal permeability of the membrane. The ICF and ECF contain different ion concentrations

Note: at rest, Na+ cannot move through ion channels in the sarcolemma, but K+ can

Question 25

What causes ion channels to open and what does this allow to happen?

Answer 25

Motor neurons release a chemical neurotransmitter (acetylcholine) and allows ion channels to open, which allow Na+ into the myofibre

Question 26

Where does excitation propagation occur and how?

Answer 26

It occurs through the myofibre from the NMJ due to voltage-gated ion channels, which are found throughout the sarcolemma. This spreading depolarization can cause extra voltage-gated ion channels permeable to Na+ to open, creating an action potential through the entire length of the sarcolemma. This is very rapid

Question 27

T-tubules’s role in triggering Ca2+ release

Answer 27

T-tubules bring the sarcolemma’s action potential into the vicinity of the sarcoplasmic reticulum, which triggers Ca2+ release

Question 28

What does the action potential result in?

Answer 28

A muscle action potential spreads throughout the length of the myofibre, and triggers the release of Ca2+ ions from the sarcoplasmic reticulum