Muscle Physiology Flashcards

week 4

1
Q

Compare and contrast the structure and function of skeletal, smooth, and cardiac muscle

A

The three types of muscle in the human body are skeletal, cardiac, and smooth, and they differ in structure, function, and location:

Skeletal muscle
Attached to bones, this muscle is striated in appearance and is under voluntary control. Skeletal muscles are responsible for rapid and powerful movements, such as supporting posture and controlling voluntary movement.

Cardiac muscle
Found in the heart’s walls, this muscle is striated in appearance and is under involuntary control. Cardiac muscles are responsible for pumping blood and maintaining a steady heartbeat.

Smooth muscle
Found in the walls of hollow organs, such as the liver, pancreas, and intestines, this muscle is spindle-shaped in appearance and is under involuntary control. Smooth muscles are responsible for slow, sustained contractions, such as changing the shape of organs to facilitate bodily functions like digestion and blood flow.

Other differences between the three types of muscle include:
~ Speed of contraction
Skeletal muscles contract quickly, while cardiac and smooth muscles contract more slowly.

~Fatigue
Cardiac muscles are more resistant to fatigue than skeletal muscles because they have more mitochondria required to generate energy for cells.

~Nervous system control
The somatic nervous system controls skeletal muscles, while the autonomic nervous system controls cardiac muscles.

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2
Q

Demonstrate a detailed understanding of the sarcomere and outline the functional roles of myofibril proteins

A

Sarcomere is the contractile unit of the muscle fibre.

Sliding filament mechanism
The thin filaments interact with and slide past the thick filaments, thus overlapping. This causes the sarcomere and, ultimately, the muscle to shorten.

The primary function of myofibrils is to enable muscle contraction and relaxation. Myofibrils are organelles found in muscle fibers that are made up of protein bands that slide over each other to cause muscle to contract and relax:

Structure
Myofibrils are cylindrical organelles that are about 1 micrometer in diameter. They comprise two types of myofilaments: thin actin myofilaments and thick myosin filaments.

Appearance
Under a microscope, myofibrils have a banded appearance, with alternating “dark” and “light” striations. These striations are called A-bands and I-bands, respectively.

Contraction
When a muscle contracts, the sarcomeres within the myofibrils shorten, which causes the muscle fibers to contract. This allows muscles to move bones and perform different movements.

The process of muscle contraction involves the following steps:
1. An action potential reaches the axon terminal
2. Calcium enters the axon terminal
3. Acetylcholine is released into the synaptic cleft
4. Acetylcholine binds to receptors on the sarcolemma
5. Sodium ions flow into the muscle cell
6. An action potential travels to the myofibril, causing muscle contraction.

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3
Q

Describe the processes involved in muscle excitation, contraction, and relaxation

A

Muscle excitation, contraction, and relaxation are a series of steps that involve the release of acetylcholine (ACh) at the neuromuscular junction (NMJ) and the movement of calcium (Ca) in and out of the sarcoplasmic reticulum (SR):

Excitation:
1. ACh is released at NMJ and binds to receptors on
the sarcolemma

Contraction:
2. Action potential generated and travels along sarcolemma and T-tubules
3. Sarcoplasmic reticulum (inc terminal cisternae) releases stored calcium
4. Calcium ions bind to troponin. Tropomyosin moves away from binding sites. Activated myosin head binds to actin to form a cross bridge.
5. Myosin pivots to perform a power stroke and pulls actin towards the midline of the sarcomere. The contraction cycle of binding, pivoting, and detachment continues. Thin filaments are pulled past thick filaments.

Relaxation:
6. Neural stimulation ceases. ACh is broken down by acetylcholinesterase (AChE). Ends the AP.
7. Sarcoplasmic reticulum reabsorbs calcium via active transport. Calcium levels in the sarcoplasm decrease quickly.
8. Tropomyosin returns to normal position and the active binding sites on actin are covered again. Myosin disengages from actin (requires fresh ATP)
9. Contraction ends. Without cross-bridge formation, the contraction ends.
10. Sarcomere and muscle returns passively to resting length.

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4
Q

Explain how tension is produced in a muscle, and outline how sarcomere length can determine tension level

isotonic, isometric

A

The amount of tension a muscle produces is determined by the length of its sarcomeres and is known as the length-tension relationship:

Sarcomere length
The ideal length for a sarcomere to produce maximum tension is 80–120% of its resting length. At this length, the actin and myosin filaments overlap the most, which maximizes the number of cross-bridges that can form.

Cross-bridge formation
When a muscle contracts, myosin heads attach to actin to form cross-bridges. The thin filaments then slide over the thick filaments, shortening the sarcomere and creating tension.

Tension production
The force generated by a muscle depends on the number of cross-bridges formed. More cross-bridges mean more force.

Tension loss
If a muscle is stretched past its ideal length, the filaments don’t overlap as much, so less tension is produced. No tension is generated if the muscle is stretched so far that the filaments don’t overlap.

Other factors that affect muscle tension include the cross-sectional area of the muscle fiber and the frequency of neural stimulation.

Types of muscle contractions

Isotonic ‐  length /constant tension (load) (tension rises and falls but the muscle length is constant.)
* Concentric – shortens: Tension exceeds load i.e bicep curl up
* Eccentric – lengthens: Load exceeds tension i.e. bicep curl down

Isometric ‐  tension/constant length i.e. hold weight on straight arm

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