Download Muscle Histology Contraction

MUSCLE HISTOLOGY: CONTRACTION
How do your muscles work?
Neuron (nerve cell)
Muscle fiber (cell)
Gross anatomy of
skeletal muscle:
Connective tissue
coverings
• Epimysium
-wrap of the muscle
-connective tissue
-connections with tendons
• Endomysium
-surrounds each muscle cell
-thin connective tissue
Microanatomy of muscle
Skeletal muscle cell (fiber)
-Sarcolemma
-membrane (surrounds sarcoplasm)
-Sarcoplasmic reticulum
-muscle’s endoplasmic reticulum (holds calcium)
- T-tubes
-runs transversely through sarcoplasm
-perpendicular to S. R.
- myofibrils
- myofilaments
Myofilaments
-2 types of myofilaments
1) Thick myofilaments
-protein molecules:
- myosin
2) Thin myofilaments
-protein molecules (3):
- actin
- tropomyosin
-troponin
Muscle Anatomy
Parts of the Neuron
1 Perikaryon
-cell body of a neuron (soma)
-complex endoplasmic reticulum & ribosomes for
protein synthesis
2 Cytoplasmic Processes
1) peripheral process
-extends from dendrites to soma
2) central process
-extends from soma to terminal branches (axon)
Dendrites
-connect directly to soma (can have many branches), receives nerve impulses
Axon
-long thin process extending from the some (only one branch), sends nerve impulses
Neuron
Physiology
Resting Potential of neuron membrane
-7OmV inside membrane
-higher [K+) potassium ions; lower [Na+] sodium ions
+35mV outside membrane
-higher [Na+) lower [K+]
Na+/K+ Concentration
-extracellular [Na+] is about 2Ox greater than intracellular
[Na+]
-intracellular [K+] is about 25x greater than extracellular
[K+)
-large # of negatively charged protein molecules within
intracellular fluid
-Polarized membrane- negative inside & positive outside
Nucleus
Dendrites
Cell
body
In saltatory conduction, the
nerve impulses jump from one
node of Ranvier to the next.
Nodes of Ranvier
Node of
Ranvier
Schwann cell
Myelin sheath
-numerous small
constrictions of the myelin
sheath of axon
-impulses jump from one
node to another
-ALWAYS ONE-WAY
TRANSMISSION
Action Potential (excitability):
-impulse nerve conduction very fast
-0.5 meters - 120 meters/second
-wave of polarization (wave of negativity)
-rapid sequence of depolarization & repolarization
Depolarization
-extracellular sodium ions [Na+] flood in creating a +3OmV
intracellular charge
Repolarization:
-intracellular potassium ions [K+] go out of the cell
creating a negative intracellular charge
-after wave of polarization sodium/potassium
pump kicks in to restore the original resting
potential [Na+ & K+]
Action Potential
Physiology of
skeletal muscle
Step 1: A nerve impulse reaches
the neuromuscular junction.
Motor neuron
Axon terminal
Synaptic vesicle
Synaptic cleft
Step 2: Acetylcholine is released
at the neuromuscular junction.
Step 3: When acetylcholine binds to receptors
on the plasma membrane of the muscle cell, an
electrochemical message is generated.
Acetylcholine
Electrical impulse
neuromuscular junction
Plasma membrane
T tubule
Myofibril
-simulated to contract by nerve impulses
-electrical gradients through neurons to
muscles (across the neuromuscular
junction
Neuromuscular junction
Sacroplasmic
reticulum
Exciting of muscle cells
(Innervation)
Step 4: The electrochemical message spreads
through T tubules, causing
the release of calcium ions
from the sarcoplasmic
reticulum.
-as action potential reaches sarcolemma it
spreads through T-tubes
-sarcoplasmic reticulum releases calcium
ions (Ca2+)
- Ca2+ act as mediators between excitation
& contraction
excitation in muscle fiber
Myofilaments
-2 types of myofilaments
1) Thick myofilaments
-protein molecules:
- myosin
2) Thin myofilaments
-protein molecules (3):
- actin
- tropomyosin
-troponin
Sarcomere
(functional aspect of muscle tissue)
-individual myofilament
-actual contraction pieces
-Z Line to Z line
-separations of sarcomeres
-A Band
-myosin & actin
-middle broad section of sarcomere
-I Band
-near the Z Lines
-overlapping sarcomeres along the myofibril
-only thin filaments
-H Zone
-only thick (myosin) filaments
-middle of A Band
The Sliding Filament Theory
Contraction of muscle
cells
•Sliding filament theory
-muscle shortening occurs when
thin filaments slide over thick
filaments
-actin over myosin
-actin is a globular protein:
arranged as two strings of beads
wound in a spiral to form an actin
filament
-myosin molecule is much larger
with globular head & a long tail
piece
•Sarcomere shortens
Cross Bridge Cycle
Return to Rest
• When the action potential passing down the motor neuron stops, Ach release also stops. However, the stimulus does not end until all remaining molecules of Ach on the motor end plate (sarcolemma) are inactivated
• The inactivation is done by an enzyme in the sarcolemma called Acetylcholinesterase
(AchE).
• Immediately following Ach inactivation, Ca++ ions are returned to the SR by active transport, which requires more energy in the form of ATP.
• With the absence of Ca++ ions, the original shape of the thin filaments is restored. • The muscle fiber is ready for the next stimulus because ATP is quickly regenerated by the mitochondria.
Let’s review…
Major Events of Muscle Contraction
– Stimulation occurs when Ach is released from the end of a motor neuron.
– Ach diffuses across gap at neuromuscular junction.
– Sarcolemma is changed and an impulse travels deep into the fiber through the T-­‐
tubules
– Ca++ ions are released from SR and bind to troponin molecules.
– Tropomyosin molecules move and expose specific sites on actin filaments.
– Cross bridges form between actin and myosin filaments.
– Actin filaments slide inward along myosin filaments.
– Muscle fiber shortens as contraction occurs.
Let’s review…
Muscle Fiber Relaxation
– Achesterase causes Ach to decompose, and sarcolemma is no longer stimulated.
– Ca++ are actively transported into SR.
– Cross bridges between actin and myosin filaments are broken.
– Actin and myosin filaments slide apart.
– Muscle fiber lengthens as it relaxes and its resting state is reestablished.
– Troponin and tropomyosin molecules inhibit the interaction between actin and myosin filaments.