A motor unit is all the muscle cells controlled by one nerve cell. This diagram represents two motor units. Motor unit one illustrates two muscle cells controlled by one nerve cell. When the nerve sends a message it will cause both muscle cells to contract. Motor unit two has three muscle cells innervated by one nerve cell.
A Acetylcholine binds with receptors in the muscle membrane to allow sodium ions to enter the muscle.
The influx of sodium will create an action potential in the sarcolemma. Note: This is the same mechanism for generating action potentials for the nerve impulse. The action potential travels down a T tubule. As the action potential passes through the sarcoplamic reticulum it stimulates the release of calcium ions. Calcium binds with troponin to move tropomyosin and expose the binding sites. Myosin heads attach to the binding sites of the actin filament and create a power stroke. ATP detaches the myosin heads and energizes them for another contraction. The process will continue until the action potentials cease. Without action potentials the calcium ions will return to the sarcoplasmic reticulum.
The actin filaments are moved by the heads of the myosin filaments. In step one the myosin head attaches to an actin filament to create a cross bridge. Step two shows that the attached myosin head bends to move the actin filament. The myosin head has expended energy to create this movement. This is a power stroke or working stroke. Step three shows that energy in the form of ATP will unhook the myosin head. In step 4 the myosin head is cocked and ready to attach to an actin filament to start another power stroke.
We do not have to see our muscles to be sure that they are performing their intended actions. Muscle sense also contributes to our ability to distinguish the shape of objects.
Lect. 8 muscular tissues
Ms. O.HARA M.S.
Muscular TissuesMuscular Tissues
generate motile forces through contraction.
Contractile proteins are MYOSIN and ACTIN
1.Myoepithelial cells* - in secretory glands
function to expel secretions from glandular acini.
1.Pericytes* - surround blood vessels.
2.Myofibroblasts* - have a contractile function as
well as a role in secretion of extracellular matrix.
inconspicuous in normal tissues
becomes dominant cell during scar formation in
Specialized Contractile cells - single cell contractile units
Muscle cells myofibers /
contractile cells that function by forming
multicellular contractile units.
3 types of muscles cells – based on structure,
contractile properties, and control mechanisms
1.Skeletal muscle – for movement of skeleton, eye
globe, and tongue
2.Smooth muscle - visceral muscle (blood
vessels, GIT, uterus and urinary bladder)
3.Cardiac muscle - rhythmic contractions of heart.
Referred as voluntary muscle.
The primary function - to move the skeleton.
The muscle contractions required for movement
also produce heat, which contributes to the
maintenance of a constant body temperature.
Plasma membrane – sarcolemma
Cytoplasm – sarcoplasm
Endoplasmic reticulum – sarcoplasmic
• A muscle is composed of
FASCICLES of fibers,
wrapped by CT-
• Each fascicle is composed of
MYOFIBERS, the cells of
• Each fascicle is
wrapped by CT –
• Each myofiber is
wrapped by CT –
SKELETAL MUSCLES are subdivided by CT
Connective Tissue wrappings
Epimysium - dense, irreg. CT; collagen;
binds the fascicles into a single muscle.
Perimysium – less dense, irreg. CT; collagen;
surrounding individual fasciculi through which larger
vessels and nerves run.
Endomysium - barely visible; delicate support;
mainly of reticulin fibres and a small amount of
surrounding each muscle fiber.
conveys numerous small blood vessels, lymphatics
and nerves throughout the muscle.
Skeletal muscle fibers (muscle
with peripheral nuclei
Multiple nuclei – due to the fusion of muscle cell
precursor myoblasts during the embryonic
Each myofiber contains MYOFIBRILS that extend the length of the fiber
• Orderly arrays of filamentous subunits
Myofibrils are made of MYOFILAMENTS formed by the contractile proteins
• Actin (thin) and Myosin (thick)
IMPORTANT role of CT in muscle
- to mechanically transmit the forces generated by
contracting muscle cells, because in most instances,
individual muscle cells do not extend from one end of a
muscle to the other.
Two orientations when cut:
- transverse and longitudinal muscle fibers
Skeletal muscles are “striated” muscles due to
distinct cross-striation patterns.
In LM, light I bands and dark A bands are in each
Light bands are I bands ; formed by thin actin
Dark bands are A bands ; located in the middle of
A bands are formed by overlapping actin and
I Band is bisected by Z-line that divide each myofibril into numerous
contractile units called SARCOMERE (2 Z-lines).
Each A band is bisected by the lighter H band and the darker M line
• As the muscle contracts the sarcomere shortens the Z
lines are drawn closer together -- the thick and thin filaments
slide past each other.
• I and H bands shorten, while A bands stay the same.
• SLIDING FILAMENT THEORY
Muscles are arranged around the skeleton so
as to bring about a variety of movements.
The two general types of arrangements
• muscles that have opposing or opposite functions.
- cooperative synergists
• muscles with the same function or those that work together to
perform a particular function.
of the brain
The contraction of
depends on the brain.
The nerve impulses
for movement come
from the frontal lobes
of the cerebrum.
Motor areas of the frontal lobes generate electrochemical
impulses that travel along motor nerves to muscle fibers,
causing the muscle fibers to contract.
Motor Unit - Muscles are innervated by large motor
nerves; supplying electrical impulse into the muscle and
groups of muscle fibers (neuromuscular junctions).
- muscle cells controlled
by one nerve cell.
Each muscle fiber has its own motor nerve
ending; the neuromuscular junction is where
the motor neuron terminates on the muscle fiber
sarcomere in a repeating
fashion are the tubules of
and mitochondria T tubule is
each side by the
cisternae of the
SR and form
are stored in the
1. ACTIN – Thin filament
- composed of two chains of G-actin (globular) that
forms F-actin (filamentous) arranged in helix.
• Tropomyosin - stabilizes the troponin and
attaches to actin filament.
• Troponin complex (TnT, TnC, TnI) –
initially covers the F-actin during muscle
• TnT – attaches to tropomyosin
• TnC – (+) receptor for calcium ions
• TnI – inhibits contraction
a. The heavy chains of myosin molecules form the core of a thick filament.
b. Structure of a myosin
2. MYOSIN – Thick filament
Before the arrival of the nerve impulse to the muscle, the
muscle is relaxed and the calcium ions are stored in the
cisternae of the sarcoplasmic reticulum.
In summary, a nerve impulse causes depolarization of a
muscle fiber, and this electrical change enables the
myosin filaments to pull the actin filaments toward the
center of the sarcomere, making the sarcomere shorter.
Acetylcholine / Ach (neurotransmitter) is released from
motor neuron. Ach binds with receptors in the muscle
membrane opens Na+
Channel to allow sodium to enter.
Sodium influx causes the sarcolemma to depolarize,
becoming negative outside and positive inside.
This will generate an action potential in the
Action potential is propagated along the entire
length of the sarcolemma and transmitted
deep to every myofiber by the network of the
The T tubules bring the action potential to the
interior of the muscle cell.
Cisternae of the SR release calcium ions into the individual
sarcomeres and the overlapping thick and thin myofilaments of
At each triad, the action potential is transmitted from the T tubules
to the sarcoplasmic reticulum (SR) membrane and stimulates the
release of calcium ions.
Calcium binds with troponin to move the troponin,
Binding sites in the actin filament are exposed
Myosin head attach to binding sites and create a
ATP detaches myosin heads and energizes them
for another contaction
When action potentials cease the muscle stop
SLIDING FILAMENT THEORY
Initiated once calcium ions is released from the
Calcium ions activate binding between actin and
myosin that results in their sliding past each other
and muscle contraction.
When the stimulus subsides and the membrane
is no longer stimulated, calcium ions are actively
transported back into and stored in the cisternae
of the sarcoplasmic reticulum, causing muscle
is the brain’s ability to
know where our
muscles are and what
they are doing,
without our having to
consciously look at
Stretch receptors / muscle
Detect stretching of muscles and generate
impulses, which enable the brain to create a
mental picture to know where the muscles are
and how they are positioned.
Conscious muscle sense is felt by the parietal
Unconscious muscle sense is used by the
cerebellum to coordinate voluntary movements.
Junquiera LC, Carneiro J. 2005. BASIC
HISTOLOGY : TEXT AND ATLAS 11th
McGraw-Hill’s ACCESS MEDICINE.
Young B. 2009. WHEATER’S FUNCTIONAL
Edition. UK: Churchill
Livingstone. Distributor: Phils: C & E Publishing,
1. Explain the consequence of TETANUS AND
BOTULISM in skeletal muscle contraction.
What is BOTOX and how does it work?
1. Describe MYASTHENIA GRAVIS and give its
consequence to the process of muscular