1. Biology 211 Anatomy & Physiology I
Muscle Histology

2. Three types of Muscle Tissue
Skeletal Cardiac Smooth
Muscle Muscle Muscle
Very long
Unbranched
Shorter
Branched
Short, Unbranched
Spindle-shaped
Hundreds per cell
Peripheral
One or two per cell
Central
One per cell
Yes No
Voluntary
Involuntary
Myocytes
Nuclei
Striations

3. Muscle: Special Terminology:
Prefixes =
Cell =
Plasma membrane =
Endoplasmic reticulum =
Cytoskeletal Filaments =
Myo- / Sarco-
Myocyte ("Fiber")
Sarcolemma
Sarcoplasmic
reticulum
Myofilaments

4. We will discuss cardiac muscle and smooth muscle in later lectures.
This week: Let’s focus on Skeletal Muscle:
Always voluntary: Each
myocyte connected to and
controlled by an axon from a
motor neuron.

5. We will discuss cardiac muscle and smooth muscle in later lectures.
This week: Let’s focus on
Skeletal Muscle:
High metabolism
Myocytes very close
to capillaries.
Myocytes are all oriented parallel to long axis of muscle
which is
Parallel to direction which muscle pulls when it contracts
& lengthens when it relaxes

6. Skeletal Muscle Anatomy:
Each myocyte surrounded by, and firmly attached to, layer of loose connective tissue called endomysium
Endomysium

7. Skeletal Muscle Anatomy:
Myocytes grouped together into bundles called fascicles; each fascicle surrounded by, and firmly attached to, layer of dense irregular connective tissue called perimysium
Perimysium

8. Skeletal Muscle Anatomy:
Entire muscle surrounded by, and firmly attached to, layer of dense irregular connective tissue called epimysium
Epimysium

9. Skeletal Muscle Anatomy:
All three layers of connective tissue blend together at each end of muscle. Thus, force is transmitted from:
Myocytes
Endomysium
Perimysium
Epimysium
Tendon, bone,
etc.

10. Muscle
Myocyte
Myofibril
Myofilaments

13. I A I A I A I H Z Z Z Z
Sarcomere

15. Myocyte Contraction Requires:
Stimulus from motor neuron (nerve cell)
Spread of stimulus along sarcolemma and into myocyte through transverse tubules
Release of Ca++ from sarcoplasmic reticulum into cytoplasm of myocyte
Binding of calcium to thin myofilaments
Formation of cross-bridges between thin myofilaments and thick myofilaments
Let’s look at each of those steps

16. Myoneural junction or
Neuromuscular junction

17. Stimulation of the myocyte begins when the neuron releases a chemical neurotransmitter from its synaptic vesicles into the synaptic cleft between the neuron and the myocyte.
The neurotransmitter diffuses across the synaptic cleft and binds onto receptors located on the sarcolemma of the myocyte.
This causes an electrical change of the sarcolemma called “depolarization”

18. When resting, the sarcolemma of the myocyte is polarized.
Sodium ions concentrated on
its outer surface & potassium
Ions concentrated on its inner
surface.
Large negative ions (proteins,
phosphate, sulfate, etc) also
concentrated on the inner surface.
Net effect: outer surface of sarcolemma more positive
inner surface of sarcolemma more negative
Sodium channels and potassium channels are closed.

19. When neurotransmitter binds onto its receptors on the
sarcolemma, sodium channels
open. (For now, don't worry about
what causes this to happen).
Sodium ions, carrying
their positive charges, flow
into the cell, making the inner
surface of the sarcolemma more positive.
The sarcolemma has begun to depolarize.

20. A few milliseconds later,
potassium channels on the
sarcolemma open while the
sodium channels close.
Potassium ions, with their
positive charges, flow out
of the cell, again making the
outer surface of the
sarcolemma more positive.
but
Sodium and potassium ions are mixed together on both
sides of the sarcolemma

21. After both the sodium channels and the potassium channels have closed, “sodium-potassium pumps”
- Push sodium ions back to the
outside of the sarcolemma
and
- Push potassium ions back to
the inside of the sarcolemma.
This returns the sarcolemma to its original “polarized” condition.
It is ready to depolarize again.

22. This depolarization / repolarization starts at the myoneural junction and spreads along the sarcolemma in all directions

23. When the depolarization /
repolarization signal reaches
the openings of transverse
tubules, these carry it deep
into the myocyte
As the signal travels along the
transverse tubules, it stimulates
the sarcoplasmic reticulum to
release large amounts of calcium
ions (Ca++) into the cytoplasm of
the myocyte

24. This calcium binds onto troponin of the thin myofilament
which moves the tropomyosin
to expose active sites on actin
Myosin head groups can now bind to the actin, forming cross bridges, after which they flex to move the thin filament

25. These cross-bridges form between myosin molecules (thick filaments) and actin molecules (thin filaments) where these overlap in the A-band

26. As long as calcium ions bind to troponin of the thin myofilament:
- Cross-bridges will continue to form between thin and
thick myofilaments, so
- Myocyte will remain contracted
Therefore:
To make myocyte relax (no crossbridges form), calcium ions must be removed from thin myofilaments
Which means
The calcium ions must be removed from the cytoplasm of the myocyte

27. Relaxation occurs when calcium ions are removed from the cytoplasm.by
being pumped into the sarcoplasmic reticulum (thus out of the cytoplasm), where it is not available to bind to troponin.
Sarcoplasmic reticulum takes up calcium ions only if no electrical signals are travelling down transverse tubules
which
Happens only if the sarcolemma is not being stimulated by a motor neuron

28. Quick Summary
Contraction Relaxation
Motor neuron stimulates sarcolemma of myocyte
Stimulus spreads along sarcolemma & into myocyte through transverse tubules
This causes release of Ca++ from sarcoplasmic reticulum into cytoplasm of myocyte
Calcium binds to thin myofilaments
Cross-bridges can now form between thin myofilaments and thick myofilaments
Motor neuron stops stimulating sarcolemma
Sarcolemma stays polarized
Sarcoplasmic reticulum pumps calcium back into its lumen, removing it from the cytoplasm
Thin myofilaments change shape
Cross-bridges break between thin and thick myofilaments

29. One motor neuron +
All myocytes it
innervates =
one Motor Unit

30. 1. Myocytes and muscles always pull (exert force by
Skeletal Muscle:
1. Myocytes and muscles always pull (exert force by
contraction), they never push. They usually, although
not always, pull on bone through a tendon.
2. If a sarcomere shortens, it always does so completely
"All-or-none"
3. All of the sarcomeres in the entire myocyte shorten at
the same time. "All-or-none"
But:
4. All of the myocytes in a muscle don't always contract at
the same time. No "all-or-none"

31. Skeletal Muscle:
The total force produced by a myocyte is equal to the sum
of the forces produced by individual sarcomeres.
Thus: More sarcomeres in a myocyte = more force
The total force produced by a muscle is equal to the sum of the forces produced by all sarcomeres in its cells.
Thus: More myocytes in a muscle = more force