1. Histology of Muscle

2. Skeletal Muscle Tissue

3. Muscle Histology Elongated cylindrical cells = muscle fibers
Plasma membrane = sarcolemma
Transverse (T) tubules tunnel from surface to center of each fiber
Multiple nuclei lie near surface of cell
Cytoplasm = sarcoplasm

5. Muscle Histology Throughout sarcoplasm is sarcoplasmic reticulum
Stores calcium ions
Sarcoplasm contains myoglobin
Red pigmented protein related to Hemoglobin that carries oxygen
Along entire length are myofibrils
Myofibrils made of protein filaments
Come in thick and thin filaments

7. The Sarcomere Filaments overlap in repeating patterns
Unit structure is called sarcomere
Separated by Z discs
Darker area = A band associated with thick filaments
H zone has no thin filaments
I band has thin filaments no thick filaments

10. Functional Anatomy
Thick filament (myosin) has moveable heads (like “heads” of golf clubs)
Thin filaments (actin) are anchored to Z discs
Contain myosin binding sites for myosin head
Also contain tropomyosin & troponin
Tropomyosin blocks myosin binding site when muscle is at rest

11. Sliding Filament Mechanism
During contraction myosin heads bind actin sites
Myosins pull and slide actin molecules (and Z discs) toward H zone
I bands and H zones become more narrow
Sliding generates force and shortens sarcomeres and thus fibers

12. Neuromuscular Interaction
Nerve signal triggers muscle action potential
Delivered by motor neuron
One neuron can trigger 1 or more fibers at the same time
Neuron plus triggered fibers = motor unit

13. Neuromuscular Interaction
Neuronal ending to muscle fiber = neuromuscular junction (NMJ)
Synaptic end bulbs (at neuron terminal)
Release neurotransmitter
Muscular area = Motor end plate
Between is synaptic cleft

14. 1
Axon terminal
Axon collateral of
somatic motor neuron
Sarcolemma
Myofibril
ACh is released
from synaptic vesicle
ACh binds to Ach
receptor
Junctional fold
Synaptic vesicle
containing
acetylcholine
(ACh)
Synaptic cleft
(space)
Motor
end
plate
(a) Neuromuscular junction
(b) Enlarged view of the
neuromuscular junction
(c) Binding of acetylcholine to ACh receptors in the motor end plate
Synaptic
end bulb
Neuromuscular
junction (NMJ)
Synaptic end bulb
Motor end plate
Nerve impulse
Muscle action
potential is produced
Na+ 2 3

15. Action at NMJ Release of acetylcholine (ACh)
Diffuses across cleft
Activation of ACh receptors
Generation of Muscle Action Potential
Repeats with each neuronal action potential
Breakdown of ACh

16. Contraction Trigger
Muscle action potential → Ca2+ release from Sacroplasmic Reticulum (SR)
Ca2+ binds to troponin →
Moves tropomyosin off actin sites →
Myosin binds & starts cycle

17. Contraction Cycle
Myosin binds to actin & releases phosphate group (forming crossbridges)
Crossbridge swivels releasing ADP and shortening sarcomere (power stroke)
ATP binds to Myosin → release of myosin from actin
ATP broken down to ADP & Pi → activates myosin head to bind and start again
Repeats as long as Ca2+ concentration is high

19. Relaxation Breakdown of ACh to stop muscle action potentials
Ca2+ ions transported back into SR lowering concentration →
This takes ATP
Tropomyosin covers actin binding sites

20. ACh diffuses across
synaptic cleft, binds
to its receptors in the
motor end plate, and
triggers a muscle
action potential (AP).
Nerve impulse arrives at
axon terminal of motor
neuron and triggers release
of acetylcholine (ACh).
Synaptic vesicle
filled with ACh
ACh receptor
Acetylcholinesterase in
synaptic cleft destroys
ACh so another muscle
action potential does not
arise unless more ACh is
released from motor neuron.
Ca2+
Muscle action
potential
Nerve
impulse SR
Contraction: power strokes
use ATP; myosin heads bind
to actin, swivel, and release;
thin filaments are pulled toward
center of sarcomere.
Troponin–tropomyosin
complex slides back
into position where it
blocks the myosin
binding sites on actin.
Muscle relaxes.
Ca2+ active
transport pumps
Ca2+ release channels in
SR close and Ca2+ active
transport pumps use ATP
to restore low level of
Ca2+ in sarcoplasm.
Ca2+ binds to troponin on
the thin filament, exposing
the binding sites for myosin.
Muscle AP travelling along
transverse tubule opens Ca2+
release channels in the
sarcoplasmic reticulum (SR)
membrane, which allows
calcium ions to flood into the
sarcoplasm.
Elevated Ca2+ 1 2 3 4 9 5 6 7 8
Transverse tubule

21. Muscle Tone
Even at rest some motor neuron activity occurs = Muscle Tone
Keeps muscle in a state of readiness
If nerves are cut fiber becomes flaccid (very limp)

22. Production of ATP for Muscle Contraction

23. Aerobic Cellular Respiration
Production of ATP in mitochondria
Requires oxygen and carbon substrate
Produces CO2 and H2O and heat.

24. Fatigue Inability to contract forcefully after prolonged activity
Limiting factors can include:
Ca2+
Creatine Phosphate
Oxygen
Build up of acid
Neuronal failure

25. Oxygen Use After Exercise
Convert lactic acid back to glucose in liver
Resynthesize creatine phosphate and ATP
Replace oxygen removed from myoglobin

26. Control of Muscle Contraction
Single action potential(AP) → twitch
Smaller than maximum muscle force
Total tension of fiber depends on frequency of APs (number/second)
Maximum = tetanus
Total tension of muscle depends on number of fibers contracting in unison
Increasing numbers = Motor unit recruitment