Sarcolemma and Muscle Cell Structure

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.