Skeletal muscle

Question 1

What are smooth muscles?

Answer 1

Smooth mainly line hollow organs e.g. gut and blood vessels and are not under voluntary control. By the action smooth muscle to open up blood vessels or close them to regulate blood flow to organs.

Question 2

What are cardiac muscles?

Answer 2

Cardiac muscle is located only in the heart, it generates force to pump blood around the body and is not under voluntary control. Has a particular arrangement for the sole purpose to deliver its force on the blood that’s inclosed within it. Generate pressure to move blood through body. Indirectly influence (e.g. going on a run)

Question 3

What is skeletal muscle?

Answer 3

Skeletal muscle applies force to the bones to control posture and body movements. Allows environment manipulation. It is under voluntary control, also known as striated muscle or voluntary muscle. Voluntary activation of the skeletal muscle from electrical signals in the brain, resulting in the activation of physical force e.g. walking, standing, chewing, moving.

Question 4

What is the primary job of the skeletal muscle?

Answer 4

The primary job is to develop force. (Not shortening, but resisting a lengthening). This force is mainly used to move and to resist the movement of joints e.g. stop the knee from bending when falling, some cases this is shortening, some case it is passively lengthened by external forces

Question 5

What are some secondary jobs of the skeletal muscle?

Answer 5

Provides support and protection for soft internal organs e.g. organs behind abdominal wall
Provides voluntary control over major openings that allow passage of substances into or out of the body. E.g. mouth, arse
Converts energy to heat which is used to maintain core temp e.g. homeostasis (maintaining core temp). (Active muscles generate heat, shivering is activation on muscles to produce heat)
Provides major storage for energy and proteins.

Question 6

Why are there nerves in muscles?

Answer 6

Muscles don’t do anything on their own. They only do things (generate force) in response to instructions that they get from brain. They get these instructions from the nerve fibres. Along the nerve fibres to the muscle fibres so they can respond, takes the form of electrical currents.

Question 7

What is a muscle fasicle?

Answer 7

A bundle of muscle fibres surrounded by the perimysium

Question 8

What connects muscle fibres to the bone?

Answer 8

Tendons

Question 9

What is the epimysium?

Answer 9

Around outside of muscle. A collection of connective tissue.

Question 10

Inside the epimysium we have a bundle of muscle fibres called…

Answer 10

Muscle fasicles

Question 11

What surrounds the outside of a muscle fascicle?

Answer 11

Perimysium

Question 12

What is the endomysium?

Answer 12

Connective tissue that surrounds individual muscle fibres (myofibrils)

Question 13

How are individual muscle fibres made?

Answer 13

Muscle starts out as individual cells with one membrane, nucleus and cytoplasm. During development cells called myoblasts fuse together and two cells fuse there membranes ending up with one membrane, one cytoplasm but two nuclei. This will continue and the cell will become longer and longer. It will produce one large cell with one cytoplasm, one membrane, but many nuclei (100 - 1000s)

Question 14

Why are muscle fibre cells so long/ have so many nuclei?

Answer 14

The cell may be 10cm long which is good to make proteins which the cell needs a lot of to maintain. If there was only one nucleus in the cell it would not be able to make enough proteins to ship to the other ends. The individual nuclei can control the cytoplasm that is surrounding it.

Question 15

What are the final long muscle cells called?

Answer 15

Sarcolemma

Question 16

How is the sarcolemma functionally important?

Answer 16

We can think of each muscle fibre as a long cylinder cytoplasm. The specific arrangement of proteins in the cytoplasm give muscle the ability to develop physical force in response to electrical signals from the brain.

Question 17

What are myofibrils made up of?

Answer 17

Network if individual filamentous proteins that are called myofilaments.

Question 18

What are the main proteins in myofilaments?

Answer 18

Actin and myosin.

Question 19

What interaction allows the production of force?

Answer 19

Interaction between actin and myosin

Question 20

What gives the skeletal muscle its striated appearance?

Answer 20

The highly organised array of the proteins in myofilaments.

Question 21

What is the arrangement of actin and myosin?

Answer 21

A hexagonal array (each thick filament is surrounded by 6 thin filaments). They have a regular repeated pattern along the length of the muscle fibre.

Question 22

What is a sarcomere?

Answer 22

A functional unit of myofilament. From Z-line to Z-line.

Question 23

What causes the muscles striated appearance?

Answer 23

The dark light pattern of thick and thin filaments.

Question 24

How does the integral structure of the muscle get damaged?

Answer 24

When the contractile proteins generate enough force (a sudden generation of force) to pull itself apart.

Question 25

How does the muscle repair itself?

Answer 25

The nucleus will recognise the damage and takes the damaged material away to recycle and make new proteins. Over a day or two the muscle will be fixed as they are highly vascular.

Question 26

Why does activity of muscle fibres have to be regulated?

Answer 26

As they are voluntary muscles and can turn it off and on when needed.

Question 27

What are the transverse tubules (T-tubules)?

Answer 27

Tiny holes in the membrane which lead to a network of tubes which are made of membrane, that run all the way through the sarcolemma. They do so at regular intervals and run across the muscle fibres

Question 28

What are transverse tubules function?

Answer 28

These are the mechanism by which the electrical event (an action potential) is conducted along the surface of the muscle fibre and through the T-tubules so that the wave of depolarisation is conducted away from the surface of the muscle fibre down into the ‘guts’. Every bit of the muscle fibre gets activated by this at the same time.

Question 29

Why are the transverse tubules important to force production?

Answer 29

All of the force producing elements are activated at the same time through the transverse tubules, so there will be synchronised uniform activation of the contractile elements for maximum effect.

Question 30

What is the Sarcoplasmic reticulum?

Answer 30

Extensive network of membranous cavity (tubes) holds calcium within a membranous compartment.

Question 31

What does the SR do?

Answer 31

These have Calcium pumps, high concentration of Ca. Calcium kept inside the cell, but inside the membrane so it can’t interact with the cytoplasm. The job of the SR is to take up and store calcium, then to release Ca into the cytoplasm on receipt of an action potential conducted along the associated T-tubules.

Question 32

Where are the terminal cisternae?

Answer 32

At the end of the SR are terminal cisternae that associate with the membrane of the T-tubule, inside cross section of SR. These for a membranous triplet called a triad.

Question 33

What is the molecular structure of thin filaments?

Answer 33

Actin is a globular protein (G-actin). The globules assemble to form filamentous protein strands (F-actin). Each thin filament is twisted strand of 2 rows of F-actin terminating at one end at the z-line. Associated with each thin filament is a pair of strands of tropomyosin that attaches to the actin at regular sites via the binding at regular intervals of the globular protein, troponin.
Troponin binds tropomyosin and actin, and has an additional important role in regulating the interaction between actin and myosin during the working cycle.

Question 34

What is the molecular composition of myosin?

Answer 34

The thick filaments is primarily myosin. The myosin molecule has a long thin tail and a globular head. The thick filament is formed from arrays of pairs of myosin molecules arranged with the tails pointing towards the M-line and forming a complex double headed structure. The head of the various myosin pairs form a spiral, each facing one of the 6 surrounding thin filaments (recall the hexagonal array of thick and thin filaments of the myofibril). The globular head of myosin has an ATP binding site and has ATPase activity (it can hydrolyse ATP to ADP+Pi, releasing energy in the process).

Question 35

What is the sliding filament theory?

Answer 35

The thick filaments pull the ends of the thin filaments into the middle and they slide over on another. These contractile proteins develop force by triggered molecular interaction that allows association of the myosin head with the nearby thin actin filament followed by the flexing of the myosin head to allow it to “walk” along the thin filament. The arrangement of the myosin heads in a sarcomere means that when this process is activated, the ends of the sarcomere are drawn closer together by the flexing of the myosin heads (the sarcomere Z-lines are drawn closer to the central M-line).

Question 36

What is the neuromuscular junction?

Answer 36

The axon of a motor neuron terminates at a single point of the muscle fibre. This specialised synapse is the NMJ.

Question 37

What is a synapse?

Answer 37

A structure that permits a neuron to pass a chemical or electrical signal to another neuron or to the target effector cell.

Question 38

What is the relationship between a motor neuron and a muscle fibre?

Answer 38

Each muscle fibre only receives contact from one motor neuron. One motor neuron makes contact on many muscle fibres, as the axon branches. (10-1000 connections)

Question 39

What regions in the brain controls the muscles?

Answer 39

The motor cortex.

Question 40

How does an electrical signal get to the muscle fibre?

Answer 40

Motor neurons in the motor cortex become electrically active and send nerve impulses down the nerve fibres and through the spinal cord. The motor neurons will project to the right muscle, in which the muscle will send a impulse down the nerve fibre to release a chemical neurotransmitter substance onto the muscle fibre to cause it to be activated.

Question 41

What is a motor unit?

Answer 41

A motor neuron and all the muscle fibres it controls.

Question 42

Where are motor neuron cell bodies?

Answer 42

In the ventral (anterior) part of the spinal cord.

Question 43

How can the force produced in a muscle vary from one instance to another?

Answer 43

The number of motor units activated at any one time can be varied to change the amount of force produced, this is called recruitment.

Question 44

What is the function of a big motor unit?

Answer 44

A big motor unit is a collection of hundreds of fibres. These are able to develop a lot of force, but do not have much control over small increments in force. (e.g. big muscles of the leg)

Question 45

What is the function of small motor units?

Answer 45

Small motor units (less than 20 fibres) do not develop much force, but provide from fine control over force because activation of each unit in turn adds only a small amount to the total muscle force (e.g. small muscle of the hand)

Question 46

Where do motor axons project out of?

Answer 46

Motor axons project out of the spinal cord to form ventral nerve roots and eventually to form spinal nerves. Axons project together to the target muscle, in the muscle they branch so that each axon innervates many fibres, through each fibre is contacted by only one axon.