Download Muscle - Midlands State University

Myology
Obert Tada
Department of Livestock & Wildlife Management
Midlands State University
Muscle
A contractile form of tissue.
It is one of the four major tissue types,
the other three being epithelium, connective tissue and nervous
tissue.
Muscle contraction is used to move parts of the body,
as well as to move substances within the body.
Types of Muscles
There are three general types of muscle:
Cardiac muscle is a specalized kind of muscle found
only within the heart.
Skeletal muscle or "voluntary muscle" is anchored by
tendons to bone and is used to effect skeletal
movement such as locomotion.
Smooth muscle or "involuntary muscle" is found within
structures such as
the intestines,
throat and
blood vessels.
Types of Muscles (cont’d)
Cardiac and skeletal muscles are "striated" in that
they contain sarcomeres and are packed into highly
regular arrangements of bundles;
smooth muscle has neither.
Striated muscle is often used in short, intense bursts,
whereas smooth muscle sustains longer or even
near-permanent contractions.
Skeletal muscle is further divided into two subtypes:
Skeletal Muscle Divisions
Type I;
slow oxidative, "slow twitch", or "red" muscle
dense with capillaries and is rich in mitochondria and myoglobin,
giving the muscle tissue its characteristic red color
can carry more oxygen and sustain aerobic activity
are associated with endurance; produce ATP more slowly.
Type II;
glycolytic, "fast twitch", or "white" muscle
less dense in mitochondria and myoglobin
can contract more quickly and with a greater amount of force than
Type I muscle
metabolize ATP more quickly
can only sustain short, anaerobic bursts of activity before a buildup of lactic acid in tissue which begins to interfere with muscular
contraction and causes pain.
Characteristics of muscle types
Fibre Type
Type I fibres
Type II A fibres
Type II B fibres
Contraction time
Slow
Fast
Very Fast
Size of motor neuron
Small
Large
Very Large
Resistance to fatigue
High
Intermediate
Low
Activity Used for
Aerobic
Long term anaerobic
Short term anaerobic
Force production
Low
High
Very High
Mitochondrial density
High
High
Low
Capillary density
High
Intermediate
Low
Oxidative capacity
High
High
Low
Glycolytic capacity
Low
High
High
Major storage fuel
Triglycerides
CP, Glycogen
CP, Glycogen
Anatomy
Muscle is composed of muscle cells (also called "muscle
fibers").
Within the cells are myofibrils; myofibrils contain
sarcomeres, which are composed of actin and myosin.
Individual muscle cells are lined with endomysium.
Muscle cells are bound together by perimysium into
bundles called fascicles;
the bundles are then grouped together to form muscle,
which is lined by epimysium.
Muscle Fibre
each muscle fibers contains:
an array of myofibrils that are stacked lengthwise and run
the entire length of the fiber.
mitochondria
an extensive endoplasmic reticulum
many nuclei (becoz each muscle fiber develops from the
fusion of many cells, the myoblasts).
not a single cell, its parts have special names:
sarcolemma for plasma membrane
sarcoplasmic reticulum for endoplasmic reticulum
sarcosome for mitochondrion
sarcoplasm for cytoplasm
Muscle Fibre
striated appearance of the muscle fiber is created by
a pattern of alternating
dark A bands (bisected by H zone) and
light I bands (bisected by Z line).
each myofibril is made up of arrays of parallel
filaments.
thick filaments, 15nm diameter, composed of the
protein myosin.
thin filaments, 5nm diameter composed of protein
actin along with smaller amounts of two other
proteins (troponin and tropomyosin).
Anatomy (cont’d)
Muscle spindles are distributed throughout the muscles
and provide feedback sensory information to the central
nervous system.
Skeletal muscle is arranged in discrete groups, examples
of which include the biceps brachii.
It is connected by tendons to processes of the skeleton.
In contrast, smooth muscle occurs at various scales in
almost every organ,
from the skin
in which it controls erection of body hair
to the blood vessels and digestive tract
in which it controls the caliber of a lumen and peristalsis.
Micro-structure of Muscle
Skeletal Muscle Attachment
Physiology
The three types of muscle have significant differences, but all
use the movement of actin against myosin to produce
contraction and relaxation.
In skeletal muscle, contraction is stimulated by electrical impulses
transmitted by the nerves, the sensory nerves and motoneurons in
particular.
All skeletal muscle and many smooth muscle contractions are
facilitated by the neurotransmitter acetylcholine.
Muscles and muscular activity account for most of the body's
energy consumption.
Muscles store energy for their own use in the form of glycogen,
which represents about 1% of their mass.
This can be rapidly converted to glucose when more energy is
necessary.
Nervous Control (sensory leg)
Vertebrates move muscles in response to voluntary and
autonomic signals from the brain.
Deep muscles, superficial muscles, muscles of the face and internal
muscles all correspond with dedicated regions in the brain.
muscles react to reflexive nerve stimuli that do not always send signals all
the way to the brain.
Nerves that control skeletal muscles in mammals correspond
with neuron groups along the primary motor area of the brain's
cerebral cortex.
Commands are routed though basal ganglia and modified by
input from the cerebellum before being relayed through the
pyramidal tract to the spinal cord and from there to the motor
end plate at the muscles.
Deeper muscles such as those involved in posture often are controlled
from nuclei in the brain stem and basal ganglia.
Nervous Control (motor leg)
Muscle memory,
Proprioception is the "unconscious" awareness of where
the various regions of the body are located at any one
time.
Several areas in the brain coordinate movement and
position with the feedback information gained from
proprioception.
The cerebellum and nucleus ruber in particular
continuously sample position against movement and
make minor corrections to assure a smooth projection.
Muscle contraction
Occurs when a muscle cell (a muscle fiber) shortens.
Locomotion is possible only through the repeated
contraction of many muscles at the correct times.
For most muscles, contraction occurs as a result of
conscious effort originating in the brain.
The brain sends signals, in the form of action
potentials, through the nervous system to the motor
neuron that innervates the muscle fiber.
However, some muscles (such as the heart) do not
contract as a result of conscious effort (autonomic).
Muscle contraction
Also, it is not always necessary for the signals to
originate from the brain.
Reflexes are fast, unconscious muscular reactions
that occur due to unexpected physical stimuli.
The action potentials for reflexes originate in the
spinal cord instead of the brain.
There are three general types of muscle contractions,
skeletal muscle contractions, heart muscle
contractions, and smooth muscle contractions.
Skeletal muscle contractions
Steps:
1.
2.
3.
4.
An action potential reaches the axon of the motor
neuron.
The action potential activates voltage gated calcium ion
channels on the axon, and calcium rushes in.
The calcium causes acetylcholine vesicles in the axon to
fuse with the membrane, releasing the acetylcholine into
the cleft between the axon and the motor end plate of
the muscle fiber.
The acetylcholine diffuses across the cleft and binds to
nicotinic receptors on the motor end plate, opening
channels in the membrane for sodium and potassium.
5.
6.
7.
Sodium rushes in, and potassium rushes out.
because sodium is more permeable, the muscle fiber
membrane becomes more positively charged, triggering an
action potential.
The action potential on the muscle fiber causes the
sarcoplasmic reticulum to release calcium.
The calcium binds to the troponin present on the thin
filaments of the myofibrils. The troponin then allosterically
modulates the tropomyosin.
Normally the tropomyosin physically obstructs binding
sites for cross-bridge; once calcium binds to the troponin,
the troponin forces the tropomyosin move out of the way,
unblocking the binding sites.
The cross-bridge (which is already in a ready-state) binds to
the newly uncovered binding sites.
It then delivers a power stroke.
9.
ATP binds the cross-bridge, forcing it to change conformation
in such a way as to break the actin-myosin bond.
Another ATP is split to energize the cross bridge again.
10. Steps 7 and 8 repeat as long as calcium is present on thin
filament.
11. All the time, the calcium is actively pumped back into the
sarcoplasmic reticulum.
When no longer present on the thin filament, the
tropomyosin changes conformation back to its previous
state, so as to block the binding sites again.
The cross-bridge ceases binding to the thin filament, and
the contractions cease.
8.
Smooth muscle contraction
1. Contractions are initiated by an influx of calcium
which binds to calmodulin.
2. The calcium-calmodulin complex binds to and
activates myosin light-chain kinase.
3. Myosin light-chain kinase phosphorylates myosin
light-chains, causing them to interact with actin
filaments.
•
This causes contraction.
Smooth muscle contraction
The calcium ions leave the troponin molecule in order to
maintain the calcium ion concentration in the sacoplasm.
As the calcium ions are being actively pumped by the
calcium pumps present in the membrane of the
sarcoplasmic reticulum creating a deficiency in the fluid
around the myofibrils.
This causes the removal of calcium ions from the
troponin.
Thus the tropomyosin-troponin complex again covers the
binding sites on the actin filaments and contraction
ceases.
Control of Cardiac Muscle Contraction
Cardiac or heart muscle resembles skeletal
muscle in some ways:
it is striated and each cell contains sarcomeres
with sliding filaments of actin and myosin.
myofibrils of each cell (and cardiac muscle is
made of single cells — each with a single
nucleus) are branched.
The branches interlock with those of adjacent
fibers by adherens junctions.
These strong junctions enable the heart to
contract forcefully without ripping the fibers apart
Control of Cardiac Muscle
action potential that triggers heartbeat is
generated within the heart itself.
Motor nerves (of the autonomic nervous system) run
to the heart but simply modulate — increase or
decrease — the intrinsic rate and the strength of the
heartbeat.
even if the nerves are destroyed (as they are in a
transplanted heart), the heart continues to beat.
action potential that drives contraction of heart
passes from fiber to fiber through gap junctions.
Control of Cardiac Muscle
Significance:
All the fibers contract in a synchronous wave that
sweeps from the atria down through the ventricles
and pumps blood out of the heart.
Anything that interferes with this synchronous
wave (such as damage to part of the heart muscle
from a heart attack) may cause the fibers of the
heart to beat at random — fibrillation.
the refractory period in heart muscle is longer
than the period it takes for the muscle to contract
(systole) and relax (diastole).
Control of Cardiac Muscle
Cardiac muscle has a much richer supply of
mitochondria than skeletal muscle.
reflects its greater dependence on cellular
respiration for ATP.
Cardiac muscle has little glycogen and gets little
benefit from glycolysis when the supply of oxygen is
limited.
thus anything that interrupts the flow of oxygenated
blood to the heart leads quickly to damage — even
death — of the affected part (heart attack).
Control of Smooth Muscle
Smooth muscle is made of single, spindle-shaped cells.
each smooth muscle cell contains thick (myosin) and thin
(actin) filaments that slide against each other to produce
contraction of the cell.
thick and thin filaments are anchored near the plasma
membrane (with the help of intermediate filaments).
Smooth muscle (like cardiac muscle) does not depend on motor
neurons to be stimulated.
However, motor neurons (of the autonomic system) reach
smooth muscle and can stimulate it — or relax it — depending
on the neurotransmitter they release (e.g. noradrenaline or
nitric oxide, NO).
Control of Smooth Muscle
Smooth muscle can also be made to contract by other
substances released in the vicinity (paracrine stimulation)
e.g.: release of histamine causes contraction of the
smooth muscle lining air passages (triggering an attack of
asthma)
by hormones circulating in the blood
E.g.: oxytocin reaching the uterus stimulates it to contract
to begin parturition.
The contraction of smooth muscle tends to be slower than
that of striated muscle.
It also is often sustained for long periods.
This, too, is called tonus with a mechanism not like that in
skeletal muscle.