Biology 211
Anatomy & Physiology I
Dr. Thompson
Histology of Muscle
Three types of Muscle Tissue
Skeletal
Muscle
Myocytes Very long
Unbranched
Nuclei
Cardiac
Muscle
Shorter
Branched
Hundreds per cell One or two per
Peripheral
cell
Central
Striations Yes
Voluntary
Smooth
Muscle
Short, Unbranched
Spindle-shaped
One per cell
Central
Yes
No
Involuntary
Involuntary
Muscle: Special Terminology:
Prefixes =
Cell =
Myo- / Sarco-
Myocyte ("Fiber")
Plasma membrane = Sarcolemma
Endoplasmic reticulum = Sarcoplasmic reticulum
Cytoskeletal Filaments = Myofilaments
We will discuss cardiac muscle and smooth muscle in
later lectures.
Let’s focus on
Skeletal Muscle:
Always voluntary: Each
myocyte connected to and
controlled by an axon from a
motor neuron.
High metabolism, so 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
Skeletal Muscle Anatomy:
Each myocyte surrounded by, and firmly attached to,
layer of loose connective tissue called endomysium
Endomysium
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
Skeletal Muscle Anatomy:
Entire muscle surrounded by, and firmly attached to, layer
of dense irregular connective tissue called epimysium
Epimysium
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.
I
A
I
A
I
H
Z
Z
Z
Sarcomere
A
I
Z
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
Myoneural junction or
Neuromuscular junction
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”
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.
When neurotransmitter binds
onto its receptors on the
sarcolemma, sodium gates
(or "gated 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.
A few milliseconds later,
potassium gates on the
sarcolemma open while the
sodium gates 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
After both the sodium gates and
the potassium gates 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.
This depolarization / repolarization spreads along the
sarcolemma in all directions away from the myoneural
junction
When the 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
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
These cross-bridges
form between myosin
molecules of the thick
filaments and actin
molecules of the thin
filaments where these
overlap in the A-band
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
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
Quick Summary
Contraction
Relaxation
Motor neuron stimulates
sarcolemma of myocyte
Motor neuron stops
stimulating sarcolemma
Stimulus spreads along
sarcolemma & into myocyte
through transverse tubules
Sarcolemma stays polarized
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
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
One motor neuron
+
All myocytes it
innervates
=
one Motor Unit
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"
Skeletal Muscle:
The total force produced by a myocyte is equal to the sum
of the forces produced by individual sarcomeres.
Thus: More sarcomeres = more force
The total force produced by a muscle is equal to the sum
of the forces produced by individual sarcomeres.
Thus: More myocytes = more force