Muscle Mechanics and Smooth Muscle

• Type 1:
• - Slow oxidative
• - Use cellular respiration
• - High endurance
• Type 2:
• - IIa: fast oxidative
• - IIb: fast glycolytic
• - Use anaerobic metabolism
• - Low endurance

• Type 1:
• Structure:
• - Slow contraction time + high resistance to fatigue
• - Small motor neuron and fiber diameter
• - High mitochondrial and capillary density
• - High myoglobin content
• Energy:
• - Low creatine phosphate
• - Low glycogen
• - Large triglycerides stores
• - Use oxidative metabolism (Krebs + ETC)
• Function:
• - Low force, long duration activities i.e. walking
• Type IIb:
• Structure:
• - Quick contraction + low resistance to fatigue
• - Large motor neuron and fiber diameter
• - Low mitcohondrial and capillary density
• Energy:
• - High creatine phosphate and glycogen levels
• - Low triglycerides
• - Many glycolytic enzymes but few oxidative enzyme
• Function:
• - Short duration, high intensity activity i.e. hurdling

• Outside cell:
• - striated
• - intercalated discs; contain gap junction, desmosomes, fascia adherens
• - branches
• Inside cell:
• - Sarcomeres (requiring AP, Ca, activation of Troponin-Tropomyosin complex)
• - Contract via CICR mechanism
• - T-tubules more developed, less SR

• Function: contraction of cardiac muscle
• Mechanism:
• - depolarization of membrane
• - direct activation DHPR; opening of ion channels
• - Ca+ enters cell; activates RyR; opening of Ca+ channels in SR
• - Contraction occurs
• - Re-uptake of Ca+ by SERCA 2 and Ca/Na exchanger

• Made up of a-motor neuron and all mucle fibers it innervated
• Starts from cell body in ventral horn (or cranial nerve nucleus) end at NMJ’s
• Muscle fibers innervated by a single motor unit are SAME (fast or slow twitch)
• Some units are big (to postural muscles) some are small (to hand muscles)

The systematic transmission of impulses from the motor cortex to motor units, resulting in coordinated muscular contractions

• If one motor neuron fires, ALL the muscle fibers within it fire as well
• In large fibers, there is large contraction, stabalize posture
• In small fibers, there is less contraction, more controlled movement

Gradual recruitment of more motor units to increase force

Sum of action potentials or all the muscle fibres in one motor unit

• Cells that contract and produce force are called twitch fibres
• The contraction produced is called “twitch”
• Number of fibres recruited per impulse is “recruitment gradation”
• Slow twitch fibres are easily recruited
• Fast twitch fibres are mor difficult

• Test that measures the health of muscles and nerves which control those muscles
• 2 types:
• - Needle EMG and Surface EMG
• - displayed on oscilloscope or heard through speaker

• Test muscles at rest (increased electrical acivity in disease)
• Test voluntary contractions

• Time between nerve stimulation and response being recorded
• Measures the conductivity (not velocity) of segment of a nerve

• Stimulate nerve at two points along its course
• V= d/ tp – td
• *d distance between two points
• *tp proximal latency
• *td distal latency

• Repetitive stimulation produce progressively weaker responses
• This is called Decrement of the Compound Muscle Action Potential (CMAP)

• AP reaches nerve terminal; Depolarization at NMJ
• Voltage-gated Calcium channels open; calcium pours in
• Stimulates fusion of s. vesicles with pre s. membrane
• Release of Ach by exocytosis
• Activates NAchR fast ion channels in post s. membrane; (Ach broken down by Ach-esterase)
• Influx of Na down electrochemical gradient
• Local depolarization of sarcolemma (end plate potential)
• Initiates AP which spreads along muscle fibre membrane
• AP carried through muscle fibre along transverse tubules
• Activates Dihydropyradine receptors
• DHPR a-subunit interact with extracellular domain of RyR in SR; opens Ca+ channel
• Calcium ions rush into sarcoplasm surrounding myofibrils
• Muscle contraction occurs
• Re-uptake of Ca ions by SERCA primarily and Calsequestrin

• Name: Sarcoplasmic/endoplasmic reticulum Calcium ATPase
• Function:
• - actively absorb Ca+ back into SR
• - pumps 2 Ca+ for every ATP used
• Location: SR

• Structure:
• - tetramer
• - 3 isomers: RyR1 (SKM); RyR2 (Cardiac); RyR3 (Brain)
• - Largest ion channel known
• - located in terminal cisternae of SR
• Function: release Ca+ from SR into sarcoplasm surrounding myofibrils

• Increased calcium conductance (nervous, hormonal, stretch)
• Calcium binds with calmodulin
• Ca-Cal complex activates MLC Kinase
• Phosphorylates MLC (regulatory chain)
• Interaction between myosin and actin
• Contraction occurs
• Inhibited by Myosin phosphatase (which is itself regulated by Rho-kinase)

• Spindle shaped cells
• Non-striated (actin + myosin form loose bundles)
• Single nucleus
• Attached to dense bodies

• Form attachment between smooth muscle cells
• Spread force ofcontraction from one cell to another

• Multi unit:
• Discrete and seperated
• Contract independently
• Stimulated by nerve impulse ONLY
• Seperated by membrane
• Found in ciliary muscles of eye; piloerector muscles
• Unitary:
• Forms sheets
• Contract as a unit
• Nervous and non-nervous impulses
• Joined together by gap junctions
• Found in walls of viscera + blood vessels

• Summation: stimulation of muscle fiber before full relaxation
• Tetany: high frequency stimulation – no relaxation. Can be incomplete or complete.
• Treppe: low frequency stimulation
• Isometric contraction: tension changes – length constant
• Isotonic contraction:
• - muscle tension constant – length changes.
• - can be eccentric: muscle lengthens
• - concentric: muscle shortens

• Smooth muscle contractions is more prolonged
• Requires less energy
• Produces more force
• Because of slow cycle of actin myosin binding