YR 1 - HUMAN PHYSIOLOGY CHPTR 9

Question 1

What are the major functions of muscle?

Answer 1

1. Movement of the body
2. Maintenance of posture
3. Respiration
4. Production of body heat
5. Communication
6. Constriction of organs and vessels
7. Contraction of the heart

Question 2

What are the major functional properties of muscle tissues?

Answer 2

1. Contractility- the ability for muscle to shorten forcefully or contract.When muscles contract it either causes the structure to move or increase pressure inside a blood vessel or a hallow organ.
2. Excitability- the capacity of muscles to respond to an electrical stimulus. The stimulus is from nerves that is consciously controlled. Smooth muscle and cardiac muscle also respond to stimulation by nerves and hormones but can contract spontaneously.
3. Extensibility- a muscle can be stretched beyond its normal resting length and still will contract.
4. Elasticity- the ability of muscle to spring back to its original resting length after its been stretched.

Question 3

Describe the skeletal muscle fiber anatomy

Answer 3

Skeletal muscle fibers are very unique cells, they develop from the fusion of several hundred embryonic cells which are called myoblasts.

Most skeletal muscle fibers range in size from 1mm to 4mm.

Large muscles contain many large-diameter muscle fibers, whereas small, delicate muscles contain many small diameter muscle fibers.

Question 4

Electrical component structures:

The 3 muscle fiber components that respond and transmit electrical signals

Answer 4

1. The sarcolemma- is the plasma membrane of muscle fibers.
2. Transverse tubules or T tubules- these are tubelike inward folds of sarcolemma. The sarcolemma forms T tubules by projecting and extending into the interior of the muscle fiber. They also carry electrical impulses into the center of the muscle fiber so that the muscle fibers contract in unison.
3. The sarcoplasmic reticulum- this is a specialised smooth endoplasmic reticulum in skeletal muscle fibers that stores high levels of Ca2+. Release of Ca2+ from the sarcoplasmic reticulum is a switch for muscle contraction.

Question 5

Mechanical component structures:

Answer 5

1. Myofibrils- are bundles of protein filaments. Each muscle fiber has numerous myofibrils in its sarcoplasm, the myofibrils are long and threadlike structures. They extend the entire length of the muscle fiber.
2. There are two types of myofilaments in each myofibrils; one is actin and the second is myosin.

Actin filaments are thin filaments and are approx 8 nanometers in diameter and 1000nm in length.

Myosin myofilaments are thick filaments and approx 12 nanometer in diameter and 1800nm in length.

The actin and myosin myofilaments are arranged into highly ordered units called sarcomeres.

The sarcomeres are the structural and function units of skeletal muscles. The myofilaments in the sarcomeres provide the mechanical aspect of muscle contraction.

Question 6

Describe sarcomeres;
What are they?
Talk about Z disks, A disks and I disks
Also H ZONE AND M LINE

Answer 6

Sarcomeres joined to end forming the myofibrils.
The sarcomere is the smallest portion of a muscle that can contract.

Filamentous networks of proteins called Z disks for a stationary anchor for actin myofilaments. A sarcomere extends from one Z disk to the next Z disk.

There are 2 light staining regions called I bands and a central darker staining region called an A band.

The I bands include a Z disk and extend to ends of myosin filaments, I bands contain only actin myofilaments and they appear lighter staining.

The darker staining band in the center of each sarcomere is called an A band.Every A band has a smaller band and it it called H zone. This contains only myosin myofilaments.

The H zone has a dark line called the M line and the M line consists of delicate filaments that hold the myosin myofilaments in place.

Question 7

What are the 3 separate proteins actin myofilaments are composed of?

Answer 7

1. Globular- G actin molecules are globular subunits that form a long chain of 200 subunits. Chain of 200 G actin subunits form into a strand called fibrous (F) actin. Each G actin subunit has an active site for myosin myofilaments binding during muscle contraction.
2. Tropomyosin- this is a long fibrous protein that lies in the groove along the fibrous actin strand. When a muscle is relaxed tropomyosin is covering the active sites on the G actin subunits. For a muscle to contract the tropomyosin moves to uncover the active sites.
3. Troponin- consists of 3 subunits
   - a subunit that anchors the troponin to the actin
   - a subunit that prevents the tropomyosin from uncovering the G actin active sites in a relaxed muscle
   - a subunit that binds Ca2+

Question 8

What are the important properties of myosin heads?

Answer 8

1. The heads bind to active sites on the actin molecules to form cross-bridges to contract the muscle
2. The heads are attached to the rod portion by a hinge region that bends and straightens during contraction
3. The heads are ATPase enzymes which break down adenosine triphosphate (ATP) releasing energy. Part of the energy is used to bend the hinge region of the myosin molecule during contraction.

Question 9

What is a neuromuscular junction?

Answer 9

The point of contact of motor neuron axon branches with the muscle fiber is called the neuromuscular junction or synapse.

Question 10

Talk about presynaptic and postsynaptic membrane

Answer 10

Each axon terminal is called the presynaptic terminal. The space between the presynaptic terminal and the muscle fiber is the synaptic cleft.

The muscle plasma membrane in the area of the junction is called the motor end plate or the postsynaptic membrane.

Each presynaptic cleft contains a synaptic vesicles which is spherical sacs and they also contain mitochondria.

The synaptic vesicles contain the neurotransmitter acetylcholine.

A neurotransmitter is a molecule that allows a neuron to communicate with its target. They are released from a presynaptic membrane and diffuse across the synaptic cleft to alter the activity of the muscle fiber.

Question 11

Describe the sliding filament model

Answer 11

The function of skeletal muscle cells is to generate force by contracting or shortening.
The parallel arrangement of myofilaments in a sarcomere allows them to interact, which causes muscle contraction. This interaction is described by the sliding filament model.

Question 12

Talk about (A) Relaxed sarcomere and (B) Fully contracted sarcomere

Answer 12

(A) Relaxed sarcomere
In a relaxed muscle the actin and myosin myofilaments overlap slightly. The H zone is visible, the sarcomere length is at its normal resting length.
As a muscle contraction is initiated actin myofilaments slide past the myosin myofilaments, the Z disks are brought closer together, and the sarcomere begins to shorten.

(B) Fully contracted sarcomere
In a contracted muscle, the A bands which are equal to the length of the myosin myofilaments, do not narrow because the length of the myosin myofilaments does not change nor does the length of the actin myofilaments.

When a muscle contracts, the actin and myosin myofilaments in the sarcomere slide past one another and shorten the sarcomere.

Question 13

Talk about action potentials

Answer 13

An action potential occurs when the excitable cell is stimulated. The action potential is a reversal of the resting membrane potential such that the inside of the plasma membrane becomes positively charged compared with the outside.This charge reversal occurs because ion channels open when a cell is stimulated.
The diffusion of ions through these channels changes the charge across the plasma membrane and produces and action potential.

Question 14

Talk about depolarisation and repolarisation in an action potential

Answer 14

Stimulation os a cell causes depolarisation. Depolarisation occurs when the inside of the plasma membrane becomes more positive. If the depolarisation causes the membrane potential to reach threshold, an action potential is triggered.

Threshold is the membrane potential at which voltage gated Na+ channels open. The depolarisation phase of the action potential is a brief period during which further depolarisation occurs and the inside of the cell becomes even more positively charged.

The charge difference across the plasma membrane is said to be reversed when the membrane potential becomes a positive value.

The repolarisation phase is the return of the membrane potential to its resting value.

Question 15

Talk about depolarisation and the action potential in skeletal muscle

Answer 15

The depolarisation and repolarisation phases are due to the opening and closing of voltage gated ion channels. When K+ moves out of the cell, the inside of the plasma membrane becomes more negative and the outside becomes more positive.

The action potential ends and the resting membrane potential is reestablished by the sodium potassium pump.

Question 16

Talk about action potentials and the plasma membrane

Answer 16

Action potentials occur in one area of the plasma membrane and they travel or propagate along the plasma membrane.

An action potential produced at one location in the plasma membrane stimulates the production of an action potential in the neighbouring section of plasma membrane.

The depolarisation of the membrane in one action potential location triggers the opening of nearby voltage gated Na+ channels.

An action potential at one location stimulates the production of a second action potential in an adjacent location, which in turn stimulates the production of another.

Question 17

What is an action potential frequency?

Answer 17

An action potential frequency is the number of action potentials produced in a certain amount of time and this is usually seconds.

As the strength of the stimulus applied to a neuron of a muscle fiber increases, the number of action potentials fired increases. The action potential frequency can affect the strength of a muscle contraction.

The nervous system controls muscle contractions by sending action potentials along axons, which then cause action potentials sent to the muscle fibers can result in stronger muscle contraction.

Question 18

Excitation-contraction coupling (intro)

Answer 18

Action potentials produced in the sarcolemma of a skeletal muscle fiber can lead to contraction of the muscle fiber.

The contraction of the fiber is due to the mechanical component.

The sarcoplasmic reticulum actively transports Ca2+ into its lumen, the concentration of Ca2+ is approximately 2000 times higher within the sarcoplasmic reticulum than in the sarcoplasm of a resting muscle fiber.

Question 19

Excitation contraction coupling (second part) + more detail

Answer 19

Excitation contraction coupling begins at the neuromuscular junction with the production of an action potential in the sarcolemma.

The action potential is propagated along the entire sarcolemma of the muscle fiber and into the T tubules.

The T tubules wrap around sarcomeres where actin and myosin overlap and carry action potentials into the interior of the muscle fiber.

The action potential causes the voltage gated Ca2+ channels in the terminal cisternae of the sarcoplasmic reticulum to open.

When the Ca2+ channels open Ca2+ rapidly diffuses out of the sarcoplasmic reticulum and into the sarcoplasm surrounding the myofibrils.

Question 20

Cross-bridge movement

Answer 20

The mechanical component of muscle contraction is called cross bridge cycling. This rapid sequence of events will cause the sarcomeres to shorten and the muscle will contract.

The energy from one ATP molecule is required for each cross bridge cycle. Before each cycle, the myosin head is in its resting (high energy) position. The myosin head stores energy from ATP breakdown that occurred during the previous cycle.

The myosin head will remain in the resting position until the muscle fiber is stimulated by a motor neuron initiating the events of excitation contraction coupling.

Once the Ca2+ binds to the troponin and the active sites on the G actin are exposed, the myosin heads quickly bind to them.

Question 21

Muscle relaxation

Answer 21

Muscle relaxation occurs when acetylcholine is no longer released at the neuromuscular junction.

The cessation of action potentials along the sarcolemma stops Ca2+ release from the sarcoplasmic testicular and Ca2+ is actively transported back into the sarcoplasmic reticulum.

Question 22

What are the 3 major ATP-dependent events that are required for muscle relaxation?

Answer 22

1. After an action potential has occurred in the muscle fiber, the sodium potassium pump must actively transport Na+ out of the muscle fiber and K+ into the muscle fiber to return to and maintain resting membrane potentials.
2. ATP is required to detach the myosin heads from the active sites for the recovery stroke.
3. ATP is needed for the active transport of Ca2+ into the sarcoplasmic reticulum from the sarcoplasm

A muscle fiber takes at least twice as long to relax as it does to contract.

Question 23

What is cardiac muscle?

Answer 23

Cardiac muscle tissue is striated but each cell usually contains one nucleus located near the center.

Question 24

What are some properties of smooth muscle?

Answer 24

1. Smooth muscle can contract auto rthymically in response to stretch or hen stimulated by the autonomic nervous system or hormones.
2. Smooth muscle maintains a steady tension for long periods
3. The force of smooth muscle contraction remains nearly constant, despite changes in muscle length
4. Smooth muscle does not develop an oxygen deficit

Question 25

What is a muscle twitch?

Answer 25

1. A muscle twitch is the contraction of a single muscle fiber or a whole muscle in response to a stimulus
2. A muscle twitch has lag, contraction, and relaxation phases

Question 26

Describe force of contraction in whole muscles

Answer 26

1. Multiple motor unit recruitment results in more motor units responding to greater stimuli
2. Concentric contractions cause muscles to shorten and tension to increase
3. Eccentric contractions cause muscle to lengthen and tension to decrease gradually
4. Muscle tone is the maintenance of steady tension for long periods
5. Asynchronous contractions of motor units produce smooth, steady muscle contractions