Chapter 8 A&P

Transverse Tubules (T-Tubules) def

(T-tubules) run at right angles, and connect the sarcoplasmic reticulum. Depolarization from neuron at neuromuscular junction carries on through the T-tubules

Sarcoplasmic Reticulum Contains

Ca 2+ . This is stored in enlarged sections, known as terminal cisternae, bracketing T-tubules.

Length-Tension Relationship

Depends on the orientation of the myofilaments. Shortening of the sarcomere can disrupt the 3-D orientation of the filaments, leading to less cross-bridges. Depends on the number of cross-bridges formed. Lengthening of the sarcomere can reduce the zone of overlap, leading to less cross-bridges.

Automaticity (Autorhythmicity)

Due to prepotential (pacemaker potential) Specialized cardiac muscle cells that contain very little myofibrils. These are called the Conducting System or Nodal System. This includes the sinoatrial node, atrioventricular node and the conducting cells. The cells of the sinoatrial and atrioventricular nodes are "leaky" and allow sodium to enter the cell, once it has repolarized. The membrane potential gradually rises toward threshold and spontaneously generates a depolarization. The nervous system is not required to generate action potentials, only to regulate them.

Sarcoplasmic Reticulum and Transverse (T) Tubules description

Each myofibril within a muscle cell is surrounded by a network of tubes and sacs. These tubes & sacs transmit the continuation of the nerve impulse to the muscle cell. Network = specialized membrane-bound organelle called SARCOLASMIC RETICULUM

neuromuscular junction description and process

Junction between neuron and muscle fiber. As with typical neuron, Ca2+ floods into end of neuron at NM-junction, releasing a neurotransmitter - in this case, ACETYLCHOLINE. Just as if it were another nerve, this causes a depolarization at the motor endplate and through the SARCOLEMMA (aka MUSCLE CELL MEMBRANE). The ACTION POTENTIAL is carried though the muscle cell via the T-tubules. ACTION POTENTIAL reaches the SARCOPLASMIC RETICULUM via the T-TUBULES. SARCOPLASMIC RETICULUM releases Ca2+ into MYOFIBRIL

Skeletal Muscle Tissue Description and Fxn

Long slender multinucleated cells (muscle fibers) Contain actin and myosin proteins for contraction "Voluntary" skeletal muscles

sliding filament model

MYOSIN has a tail and a head. The tail is bound together with other myosin tails to form a thick filament. The head is sticking out and facing the thin filaments. When a myosin head rocks back toward the M-line, it attaches to the ACTIN filament, forming a CROSS-BRIDGE. Myosin head NEARLY touches actin, but is prevented from complete attachment by a protein called TROPONIN. Troponin is laced around the actin in a spiral-like manner by a structural protein called TROPOMYOSIN. The Troponin-Tropomyosin complex blocks the complete attachment of myosin and thus the formation of a cross bridge. When Ca2+ floods in, the Ca2+ ions bind to the troponin. This changes the shape of the troponin-tropomyosin complex enough to move it out of the way enough to allow the cross bridges to form and the myosin heads to rock backwards. Actin and Myosin slide PAST one another, shortening the sarcomere, and thus the muscle of which it is a part. When Ca2+ ions move away, the troponin-tropomyosin complex can again form a barrier. It takes ATP not to rock the cross bridges, but to detach & reset them. That's why when death occurs and no ATP is available, you can't move (rigor mortis).

motor unit definition

Many muscle fibers (muscles cells) may be innervated by one motor neuron

General Functions of Muscle

Movement/Stability of Somatopleure Movement of Splanchnopleure Heat Production ("muscular thermogenesis") Coelomic pressurization Heartbeat Structural integrity of blood vessels Communication (facial muscles)

Striated Muscle Cell Description

Multinucleate, Striations

Myofibrils description

Myofibrils are grouped in parallel with one another within a muscle cell. The working units of the muscle cell. What gives 'striated muscle' its striated or striped appearance. Made up of many components laid end-to-end called SARCOMERES. Sarcomeres are made of yet smaller components called MYOFILAMENTS

Intercalated discs

Sarcolemmas of adjacent discs are interdigitating and connected by gap junctions. Signal can travel directly from cell to cell, essentially causing the muscle to act like a single cell (coordinated).

Muscle Cell Types

Striated, Smooth, Cardiac

Myofilaments description

Two major types, THIN AND THICK. Called ACTIN and MYOSIN respectively. They are laid down PARALLEL to one another so they can SLIDE PAST ONE ANOTHER. One bundle of these thick and thin myofilaments is called a SARCOMERE.

Smooth Muscle Cell Description

Uninucleate, no visible striations

Cardiac Muscle Cell Description

Uninucleate, striations

Plasticity

Unlike striated muscle, smooth muscle can stretch and adapt to the new length. This is a result of the arrangement of the myofilaments. This allows smooth muscle to retain function even when stretched, as in the walls of the digestive tract

Smooth Muscle Tissue

Very different from striated skeletal muscle Located in the walls of the viscera and blood vessels as well as forming sphincters for the digestive and urinary systems and within the eyes. The fibers are long and slender, tapering like a football. This is called fusiform. Lacks T-tubules and sarcoplasmic reticulum is in the form of a loose network. They are innervated by the ANS. Contain actin and myosin, just like striated muscle, but the arrangement is different. No sarcomeres or myofibrils Thick filaments are scattered throughout the sarcoplasm and have more myosin heads per filament Thin filaments are attached to Dense Bodies, which are scattered throughout the sarcoplasm and attached to the sarcolemma. When the filaments slide past each other, the cell shortens by twisting like a corkscrew. Dense bodies connect one cell to another transmitting force from cell to cell, coordinating contraction. Contraction is triggered by a G protein coupled receptor (a 2nd messenger system). The activated G protein opens calcium channels in the sarcolemma (allowing extracellular calcium to enter) and releases calcium from the sarcoplasmic reticulum. Unlike striated muscle, the calcium binds to Calmodulin, which activates Myosin Light Chain Kinase. The kinase allows cross bridges to form, thus initiating contraction.

actual contact of the neuromuscular junction

at the motor endplate

Z-Line description and process

endplate of each sarcomere. The thinner ACTIN attaches to these endplates. MYOSIN is laid down between the actin. Portion of the actin that doesn't overlap with myosin shows up as lighter looking, and is called the I-BAND.

cardiocytes

essentially the same as striated, skeletal muscle with some differences. Uninucleate with branching muscle cells. Short, broad T-tubules lacking triads and located at the Z-lines. Sarcoplasmic reticulum lacks terminal cisternae and contacts the sarcolemma Sarcolemma is permeable to extracellular calcium

center of the a-band

m-line. the thick filaments are attached to the m-line at the center of the a-band

Sarcomere description

one bundle of thick and thin myofilaments. the region between two successive Z-lines.

zone of overlap

portion of the a-band outside of the h-band in which the thick and thin filaments overlap

I-band

portion of the actin that doesn't overlap with myosin shows up as lighter looking

a-band

region whtere actin does overlap with myosin shows up as darker looking

H-Band

slightly lighter region with only thick myosin (no actin overlap)

Triad

t-tubules and the adjacent terminal cisternae. located at the zones of overlap of the sarcomere