Skeletal Muscle

1
Skeletal Muscle

• Gross muscle
• Plasma membrane
• Neuromuscular junction
• Action potential

2
Muscle Connective Tissue

• provides structure form to muscle
• allows force to be transmitted to tendons/bones
• three layers of connective tissue--composed
  primarily of collagen fibers
• epimysium (outer layer)
• perimysium (groups fibers into bundles
  (fascicles))
• endomysium (surrounds each fiber)

3
Muscle Connective Tissue
4
Skeletal Muscle
5
Endomysial connective tissue within skeletal
muscle
6
Connective Tissue Functions

• provides scaffolding upon which fibers can form
• holds fibers together
• perimysium provides conduit for
  arterioles/venules and intramuscular nerves
• distributes strain/force over entire muscle
• endomysium conveys part of contractile force to
  tendon
• fibers taper near tendon attachment folding of
  plasma membrane

7
Myon
Myonuclei of skeletal fiber
8
Sarcolemma

• surrounds each fiber and composed of
• basement membrane (outer side)
• plasma membrane
• basement membrane contains
• acetylcholinesterase
• collagen
• functions of basement membrane
• termination of synaptic transmission
• attachment of fiber to endomysium
• scaffolding for muscle fiber regeneration

9
Plasma Membrane

• plasma membrane composed of lipid bilayer
• has fluid properties
• regulates fiber ion concentrations with membrane
  protein pumps and channels

10
Plasma Membrane Proteins

• myonuclei and satellite cells
• bound to inter surface of plasma membrane
• peripheral proteins (plasma membrane receptors)
• associated with surface of bilayer
• e.g., adenylate cyclase, kinases, hormone
  receptors
• integrins
• class of connective proteins
• link basement membrane to plasma membrane and
  cytoskeletal structures
• integral proteins function as gatekeepers
• embedded in phospholipid bilayer
• selectively let ions pass

11
Methods of transport

• osmosis (i.e., water)
• simple diffusion (e.g., O2, CO2)
• facilitated diffusion (e.g., glucose, lactate)
• active transport (e.g., Na, K)

12

• several thousand amino acids arranged in 1 or
  more subunits
• hundreds of sugar residues linked
• controlled by voltage- or receptor-regulated gate

13
Transport Times
14
20 0 -20 -40 -60 -80
Membrane potential (mV)
Time (ms)
K
K
K
Na
Na
K
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na channel
K channel
Na-K exchange pump
ATPase
K
K
K
K
K
K
K
K
K
K
ADP
K
K
K
K
K
K
Na
Pi
Na
Na
Na
intracellular
ATP
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Resting Membrane Potentialchannels and pumps
Na
3 Na
K
outside
Na-K ATPase
inside
K
Na
2 K
membrane channel leakiness
Motoneuron resting membrane potential -70
mV Muscle resting membrane potential -90 mV
16
Distribution of Na-K pumps in skeletal muscle
and muscle-nerve bundles (N). Pumps are lit from
exposure to a labeled antibody
17
Action Potential

• results from disturbance (e.g. electrical) to
  membrane
• affects membrane permeability to Na and K
• follows all-or-nothing principle

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Phases of Action Potential
depolarization influx of Na
repolarization efflux of K
hyperpolarization overshoot of K efflux
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Action Potential
20
Motor Unit
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Motoneuron

• inputs to motoneuron are both excitatory and
  inhibitory
• continuous nerve from spinal cord to
  neuromuscular junction
• are all myelinated
• wrapped with myelin (Schwann cells)
• nodes of Ranvier
• AP conducted by saltatory conduction
• greatly ? conduction velocity
• extrafusal motor units innervated by ? motoneurons

22
Motor End Plate
23
Neuromuscular Junction

• AP at motor end plate (active zones) causes Ca2
  influx
• stimulates vesicles to migrate/fuse to membrane
  and release acetylcholine (ACh)
• ACh diffuses across synapse and binds with
  postsynaptic ACh receptors
• most ACh metabolized by cholinesterase
• postsynaptic ACh binding causes Na influx and K
  efflux
• depolarization causes development of APs

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• Curare blocks ACh receptors
• Anticholinesterase drugs (e.g., mustard gas,
  sarin) prevent hydrolysis of ACh
• Botulism (bacterium) blocks release of ACh