Lecture 10/15: Electrophysiology II

Question 1

Skeletal Muscle

Answer 1

Has a characteristic deep-red color and surface striations with alternating dark and light bands, hence skeletal muscle is also known as striated (striped) muscle. The surface striations in skeletal muscles are composed of muscle fibers. Skeletal muscle fibers develop from precursor cells myoblasts, which fuse into muscle fibers. Each muscle fiber has many nuclei derived from all of the cells that fused to form the fiber.

The muscle fiber cell membrane is called the sarcolemma. Within the cytoplasmic space of the fiber (known as the sarcoplasm), muscle cells produce filaments that are the basis of the molecular motors that power muscle contraction.

The outer surface of muscle fibers is innervated by a motor neurons at a special neuromuscular junction (NMJ).

Question 2

Graded Potential

Answer 2

A change in membrane potential in response to the binding of ligand to the outer membrane of the cell. This potential is graded because it increases with increasing ligand.

Local change in resting membrane potential but never enough to hit threshold. Compared to action potential, which is an all-or-none response, and hits threshold.

• Amplitude varies with stimulus strength
• Can be summed
• No threshold or refractory period
• Conducted decrementally (slows down with time)
• Can be depolarizing or hyperpolarizing
• Initiated by stimulus to receptor; neurotransmitter at synapse, or spontaneously
• Mechanism: Ligand-gated channels or other physical/chemical changes

Question 3

Hyperopolarization

Answer 3

A decrease in the membrane potential toward more negative values. Inhibits activity of neurons and muscles.

Occurs when…

1) K+ leaves the cell , makes potential more negative.
2) Allow Cl- into cell. Same thing.

Question 4

Depolarization

Answer 4

A change in the membrane potential towards more positive values.

Question 5

Phases of Action Potential

Answer 5

(Rest)
Depolarize to threshold
Depolarization
Repolarization
After-hyperpolarization
Return to Rest

Question 6

Action Potentials

Answer 6

“All-or-none”
- Cannot be summed

Threshold

Refractory period

Non-decremental conduction

Depolarizing

Initiated by graded potential

Mechanism: Voltage-gated channels

Question 7

Comparison of Skeletal, Cardiac, and Smooth Muscle Cells

Answer 7

The contractile cells of the body can be classified into 3 major groups based on their shape, number and position of nuclei, presence of striations, and whether they are under voluntary or involuntary control.

Skeletal

• Elongated cell.
• Multiple peripheral nuclei.
• Visible striations.
• Voluntary

Cardiac

• Branching cell.
• Single central nucleus.
• Visible striations.
• Involuntary.

Smooth

• Spindle-shaped cell.
• Single central nucleus.
• Lacks visible striations.
• Involuntary.

Question 8

Internal Structure of a Skeletal Muscle

Answer 8

Skeletal muscle is composed of an orderly arrangement of connective tissue and contractile cells.

• The entire muscle is surrounded by an external connective tissue wrapping called the epimysium.
• Skeletal muscle is made up of fasicles, which are bundles of individual muscle cells.
• Each fascicle is surrounded by a connective tissue layer called the perimysium.

Question 9

Fasicle

Answer 9

Bundles of muscle cells.

Within the fasicle, the third connective tissue layer, the endomysium, separates and electrically insulates the muscle cells from each other.

All 3 connective tissue layers bind the muscle cells together, providing strength and support to the entire muscle. They merge at the ends of the muscle and are continuous with the tendons.

Question 10

Muscle Fibers

Answer 10

Skeletal muscle cell.

Nucleus - The cellular structure that contains the genetic material of the cell.

Sarcolemma - plasma membrane of the cell.

Sarcoplasmic Reticulum - endoplasmic reticulum of the muscle cell. Its interconnecting tubules surround each myofibril like the sleeve of a loosely knit sweater.

Terminal Cisternae - the sac-like regions of the sarcoplasmic reticulum lying adjacent to the T tubules; serve as specialized reservoirs of calcium ions.

T Tubule - an invagination of the sarcolemma that projects deep into the muscle cell’s interior.

Triad - three unit group consisting of one T tubule lying between two adjacent terminal cisternae.

Mitochondrion - the cytoplasmic organelle that is the site of ATP synthesis.

Cytosol - the intracellular fluid in which the organelles are suspended and molecules and ions are dissolved.

Myofibril - a cylindrical bundle of contractile filaments within the skeletal muscle cell.

Question 11

Myofibril

Answer 11

Composed of individual contractile proteins called myofilaments.

Two types of myofilaments:

• the thin filament is compsoed mainly on the protein actin.
• the thick filament is made up chiefly of the protein myosin.