Sarcolemma and Muscle Cell Structure

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Questions and Answers

What is the main function of sarcolemma?

Synthesis of muscle proteins

Regulation of muscle contraction

Regulation of calcium storage and release

Propagation of action potential to initiate contraction (correct)

What is the function of sarcotubules or T-tubules?

To synthesize muscle proteins

To regulate muscle contraction

To allow action potential to pass rapidly to the interior of the cell (correct)

To store calcium ions

What is the site of calcium storage and release for excitation-contraction coupling?

Sarcoplasmic reticulum (correct)

Sarcolemma

Sarcomere

Myofibrils

What is composed of actin and myosin filaments along with support proteins?

Myofibril Signup and view all the answers

What is the basic contractile unit of muscle fiber?

Sarcomere Signup and view all the answers

What is the appearance of skeletal muscle due to the arrangement of thick and thin filaments?

Cross-striated Signup and view all the answers

What is the function of troponin I in muscle contraction?

Inhibits the interaction between myosin and F-actin during rest Signup and view all the answers

What type of protein is tropomyosine?

Helical protein Signup and view all the answers

What is the function of troponin C?

Initiates contraction by binding to calcium Signup and view all the answers

What is the function of the myosin head?

Binds to actin and contains an ATPase portion Signup and view all the answers

What is the percentage of myofibrillar proteins composed of myosin?

40% Signup and view all the answers

What is the function of troponin T?

Binds the other two troponin sub-units to tropomyosin Signup and view all the answers

What is the function of the tail portion of the myosin filament?

Dimerizes and assembles into bipolar thick filaments Signup and view all the answers

What happens to the H band and I band during muscle contraction?

They both shorten Signup and view all the answers

What happens to the muscle during an eccentric contraction?

It elongates while generating force Signup and view all the answers

What is the purpose of a voluntary eccentric contraction?

To slowly lower a heavy weight Signup and view all the answers

What is a motor unit composed of?

A motor neuron and the skeletal muscle fibers it innervates Signup and view all the answers

What happens when a motor unit is activated?

All of its fibers contract Signup and view all the answers

How is the force of a muscle contraction controlled?

By the number of activated motor units Signup and view all the answers

What is a characteristic of muscles that act on larger body masses?

They have more muscle fibers in each motor unit Signup and view all the answers

What type of muscle fibers are found in a motor unit?

All fibers in a motor unit are of the same type Signup and view all the answers

What is the term for all the motor units within a muscle?

Motor pool Signup and view all the answers

What is responsible for dephosphorylation of myosin light chains in smooth muscle?

Myosin light chain phosphatase Signup and view all the answers

What is the result of dephosphorylation of myosin light chains in smooth muscle?

Muscle relaxation Signup and view all the answers

What is formed via nitric oxide synthase in endothelial cells?

Nitric oxide Signup and view all the answers

What is the effect of nitric oxide on smooth muscle cells?

It leads to muscle relaxation Signup and view all the answers

What is unique about smooth muscle action potentials?

Membrane potential acts to initiate or modulate contraction Signup and view all the answers

What is the effect of calcium channels in smooth muscle cells?

They open slower than skeletal muscle cells Signup and view all the answers

What is the result of slow repolarization of smooth muscle cells?

Long-lasting action potentials Signup and view all the answers

What can stimulate graded membrane response in smooth muscle cells?

Multiple factors, including local humoral factors, circulating hormones, or mechanical stimulation Signup and view all the answers

What is a characteristic feature of single-unit smooth muscle?

It consists of multiple cells connected through connexins. Signup and view all the answers

What is the role of connexins in single-unit smooth muscle?

They allow for cell-to-cell communication between groups of cells. Signup and view all the answers

What is the difference between single-unit and multi-unit smooth muscle?

Multi-unit smooth muscle receives its own synaptic input. Signup and view all the answers

What is the function of smooth muscle in the gastrointestinal tract?

Propulsion of the food bolus. Signup and view all the answers

What is required for smooth muscle contraction?

Depolarization, hormones, or neurotransmitters. Signup and view all the answers

What is the role of L-type channels in smooth muscle contraction?

They allow calcium to enter the smooth muscle cell. Signup and view all the answers

What is the function of smooth muscle in the respiratory tract?

Regulation of bronchiole diameter. Signup and view all the answers

What is the function of smooth muscle in the sensory system?

Dilation and constriction of the pupil as well as changing lens shape. Signup and view all the answers

Study Notes

Muscle Structure and Function

The sarcolemma is responsible for propagating the action potential to initiate contraction.
The sarcoplasm (cytoplasm) contains many nuclei, mitochondria, and sarcotubules (T-tubules) that allow the action potential to pass rapidly to the interior of the cell.
The sarcoplasmic reticulum is the site of Ca++ storage and release for excitation-contraction coupling, and contains Ca++-ATPase (Ca++ pump), which transports Ca++ from the intracellular fluid into the interior.

Myofibrils and Sarcomeres

Bundles of myofibrils compose each muscle fiber, with myofibrils consisting of actin (thin filaments) and myosin (thick filaments) along with support proteins.
The thick and thin filaments are arranged longitudinally in sarcomeres, causing the cross-striated appearance of skeletal muscle.
A sarcomere is the basic contractile unit of muscle fiber, composed of two main types of protein filaments: contractile filaments (actin and myosin) and non-contractile filaments (titin and actinin) for stabilization.

Contractile Filaments

Thin filaments (actin) are double helical structures composed of monomeric units of F-actin, Tropomyosine, and troponin.
F-actin exhibits polarity, creating a positive and negative end within the sarcomere, with the positive end situated toward the terminal end of the sarcomere.
Tropomyosine is a helical protein that runs along the actin double helix, lying on top of the active sites of the actin strands in the resting state.
Troponin is a complex of three loosely bound protein sub-units (I, C, and T) that play specific roles in controlling muscular contraction.

Thick Filaments (Myosin)

Myosin filaments are composed of 4 light chains and 2 heavy chains, accounting for more than 40% of myofibrillar proteins in skeletal muscles.
The light chains are the location of the power stroke, while each heavy chain is further subdivided into two regions: the myosin head (binds actin and contains an ATPase portion) and the tail (dimerizes and assembles into bipolar thick filaments).

Muscle Contraction Types

Concentric contraction: muscle shortens while generating force (e.g., lifting a heavy weight).
Eccentric contraction: muscle elongates while generating force; can be voluntary (controlled lowering of a heavy weight) or involuntary (muscle is slowly lowered while under tension).

Motor Unit and Muscle Fibers

A motor unit consists of a motor neuron and the skeletal muscle fibers innervated by that motor neuron's axonal terminals.
Groups of motor units often work together to coordinate the contractions of a single muscle.
All muscle fibers in a motor unit are of the same fiber type, and when a motor unit is activated, all of its fibers contract.
The force of a muscle contraction is controlled by the number of activated motor units.

Smooth Muscle

Smooth muscle contraction is dependent on calcium influx, which increases within the smooth muscle cell through depolarization, hormones, or neurotransmitters.
Cross-bridge cycling leads to muscle tone, while dephosphorylation of myosin light chains terminates smooth muscle contraction.
Nitric oxide (formed via nitric oxide synthase in endothelial cells) induces relaxation by stimulating cGMP-dependent protein kinase, which activates MLCP, leading to dephosphorylation of myosin light chains.

Smooth Muscle Action Potentials

Smooth muscle action potentials are unique in that membrane potential acts to initiate or modulate contraction.
Graded membrane responses can be stimulated by local humoral factors, circulating hormones, or mechanical stimulation like stretching of the cells.
Action potentials in smooth muscle cells are slower than skeletal action potentials, and they can last almost fifty times as long.

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Description

This quiz covers the structure and function of sarcolemma, sarcoplasm, and other components of muscle cells, including their role in muscle contraction.