Skeletal muscles are the _________ of the _______ ______ division of the _______ system

• effectors
• somatic motor
• nervous

• effectors
• autonomic

• electrically excitable
• can fire action potentials, similar to those of neurons
  • For muscles, the purpose of the electrical excitation is to cause contraction of the cell, to generate force

• Contract to generate force

• a group of hundreds or thousands of muscle cells
  • muscle cells are also called muscle fibers

• run the length of a muscle
• are multi-nucleated
• plasma membrane is called the sarcolemma
• neuromuscular junctions are specialized synapses of neurons onto the muscle fiber
• mitochondrial generate vast quantities of ATP

Muscle fibers

Sarcoplasmic Reticulum

• is a network of membrane enclosed sacs, within the cell

Muscle fibers

Sarcolemma

• plasma membrane in muscle fibers
• transverse tubules, or T tubules, are extensions of the sarcolemma, which extend down into the cell

• are cylindrical bundles of several kinds of protein, which run the length of the muscle fiber

• are the repeating units of the cylindrical myofibril
• Z lines are the boundaries between neighboring sarcomeres

Thin Myofilaments

• directly attached to the Z line
• made of actin and regulatory proteins
  • G actin monomers combine to make F actin filaments
  • each G actin monomer has a myosin binding site
  • a single thin filaments consists of a double helix of F actin, plus regulatory proteins (troponin, and tropomyosin complexes)

• Indirectly attached to Z line, through titin (a protein with elasticity)

Crossbridge cycle

steps 1,2 and 3 of 5

• Step 1:  binding of myosin to actin - This step can occur only in the presence of calcium
• Step 2:  Power stroke - the binding of myosin to actin triggers  the release of the P_i and ADP from the ATPase site
• Step 3: Rigor - low engery form, myosin and actin are tightly bound together-stiffening of the body after death because the cross bridge gets stuck here without ATP

Crossbridge cycle

steps 4 and 5

• Step 4: Unbinding of myosin and actin - a new ATP enters the ATPase site on the mysin head, triggering a conformation change in the head, which decreases teh affinity of myosin for actin, so the myosin detaches from the actin
• Step 5:  cocking of the myosin head - ATP is spilt by hydrolysis into ADP an P_i which releases engery. High engery myosin - if calcium is present the cycle will start over

• Activates the crossbridge cycle
• This is how neurons (motor neurons) signal muscle cells to contract

Excitation Contraction Coupling

Step 1 - 4 of 7

• Step 1: the motor neuron fires an action potential. Ach is released and binds to receptors in the muscle fiber.  This triggers an action potential in the muscle fiber
• Step 2:  Action potential travels along sarcolemma and down T tubules
• Step 3:  The action potential triggers Ca²⁺ release from the SR
• Step 4:  Ca²⁺ enters teh cytoplasm, where it can bind to troponin, exposing myosin binding sites

Excitation contraction coupling

Steps 5 - 7 of 7

• Step 5:  Crossbridge cycle begins and sarcomere contraction occurs
• Step 6:  After the  action potential, C²⁺ is transported back into the SR
• Step 7:  tropomyosin blocks myosin-binding sites - sarcomere relaxes

• ATP powers muscle contraction

• in relaxed muscle, tropomyosin covers up actin's myosin-binding sites, which prevents the crossbridge cycle from occuring
• following their release from the sarcoplasmic reticulum, calcium ions bind to troponin, causing a conformational change in the troponin complex that shifts tropomyosin's position on the actin filament and exposes the myosin-binding sites

• the initial source of ATP reserves in skeletal muscle is creatine phosphate
• muscles next turn to glycogen stores
• if oxygen is plentiful, muscles can use glucose and fatty acids delivered to them in blood

• a motor neuron, and all the muscle fibers it innervates

• The response of the muscle fiber in a motor unit to a single action potential from teh motor neuron
  • or a muscle cell
  • or an entire muscle, depending on the circumstances
• very reporducible, as long as you wait long enough between action potentials

• Action potentials can happen faster than the time required for a twitch
• can add onto each other, simply by causing more calcium release into the cytoplasm, and activating more crossbridges
• this increases the force generated in one motor unit

• the nervous system can recruit multiple motor units within the muscle, to generate more force

• under autonomic control
  • in some cases can spontaneously fire action potentials
• uses thick and thin filaments, and crossbridge cycles
• does not have sarcomeres
  • filaments run in zigzagging patterns
  • not striated
• does not use troponin and tropomysoin system to activate the crossbridge cycle
• muscle cells are interconnected by gap junctions, which causes them to act in groups

• autonomic control
  • can spontaneously fire action potentials
• uses thick and thin filaments, crossbridge cycles, troponin and tropomyosin system
• has sarcomeres
  • straited
• muscle cells are interconnected by gap junctions to coordinate their contraction