Chapter 10 - Muscular Tissue
In summary, in order to form crossbridges,
ATP must be broken down by enzyme ATPase Calcium ions must be present in the sarcoplasm Myosin binding sites (actin active site) must be exposed
Major functions of smooth muscle:
Altering the diameter of the respiratory passageways Moving food materials along the digestive tract Moving sperms in the male reproductive tract and oocytes in the uterine tract Expelling fetus by contraction of the wall of the uterus Acting as a sphincter
Skeletal muscle is
voluntarily controlled.
Four properties of muscle tissues are
Electrical excitability Contractility Extensibility [property of muscle gives it the ability to stretch without damage] Elasticity [property of muscle gives it the ability to recoil]
The factors that lead to muscle fatigue include
Inadequate release of calcium ions from the SR Depletion of creatine phosphate Insufficient oxygen Depletion of glycogen and other nutrients Buildup lactic acid Failure of action potentials in the motor neurons to release enough Ach
Fast fibers: Fast glycolytic fibers (FG)
Large diameter Produces powerful contraction Less resistant to fatigue Glycolytic fiber: fast fibers have fewer mitochondria and large of glycogen reserves and that why it uses anaerobic respiration low concentration of myoglobin (White meat)
Activities during anaerobic endurance:
Most of the muscle's energy is produced in cytoplasm by glycolysis Glycolysis is a main source of ATP (2ATPs) Oxygen debts are common Dependent on fast glycolytic fibers Oxygen is NOT required Muscle fatigue quickly (not able to have sustained contraction for hours)
Activities during aerobic endurance:
Most of the muscle's energy is produced in mitochondria Krebs cycle is a main source of ATP (36 ATPs) No oxygen debt Oxygen is required Dependent on slow oxidative fibers Fatigue resistant (sustained contraction for hours)
Muscle has three ways to produce ATP:
Producing ATP from creatine phosphate by direct phosphorylation Producing ATP from anaerobic glycolysis Producing aerobic respiration
Major functions of skeletal muscle include:
Producing body movement and storing nutrients as glycogen, proteins Maintaining posture (upright position) Generating heat Controlling entrance and exit
Slow fibers: Slow oxidative fiber (SO)
Small diameter MOST resistant to fatigue Oxidative fibers have many mitochondria. That is a reason why they use aerobic respiration. high concentration of myoglobin (Red meat) Produce slow contraction
Major characteristics of smooth muscle cells:
Smooth muscle cells are uninucleate Smooth muscles lack sarcomeres Thin filaments are attached to dense bodies Transmitting the contractile forces from cell to cells throughout the muscle tissue. Involuntary control (pacesetter cells)
The contraction cycle includes
Step 1: Myosin head hydrolyses ATP and become energized and oriented Step 2: Myosin head binds to actin , forming the cross bridge Step 3: Myosin head pivots, pulling the thin filament past the thick filament toward center of the sarcomere (power stroke) Step 4 : Another ATP binds attches to the cross bridge, myosin head detaches from actin. Then the cycle repeats
What forms a triad?
a T-tubule and two terminal cisterns on either side
Describe a twitch
a brief contraction of all muscle fibers in a motor unit in response to a single action potential moving down the somatic motor neuron.
Describe the cori cycle
a cycle in the liver in which the lactic acid is converted to pyruvic acid and glucose.
Describe the zone of overlap
a dark region where thin and thick filaments lie side by side.
Describe perimysium
a fibrous connective tissue that covers each fascicle of muscle and contains nerves and blood vessels that service the muscle fibers
Describe nebulin
a long, non-elastic protein wrapped around the entire length of each thin filament.
Describe incomplete tetanus (unfused tetanus)
a muscle producing peak tension with rapid cycles of contraction and relaxation.
Describe complete tetanus (fused tetanus)
a muscle that is stimulated so frequently that the relaxation phase is completely eliminated
Each skeletal muscle fiber is controlled by
a neuron at a single neuromuscular junction.
Each thin filament consists of
a pair of protein actin strands wound together into a helix.
Describe the refractory period
a period that the muscle fiber loses its excitability and cannot response to a second stimulus.
Describe a myogram
a record of muscle contraction.
Describe muscle tone
a small amount of tension in the muscle at rest due to weak, involuntary contractions of its motor units.
Describe flaccid
a state of limpness in which muscle tone is lost.
Tetanus is due to
a toxin that makes the muscle cell membrane more permeable to calcium ions (Ca2+), causing powerful tetanic contraction.
Describe aponeurosis
a wide and flat tendon.
Describe myasthenia gravis
an autoimmune disorder that targets the ACh receptors at the NMJ and ultimately reduces the number of available receptors. Treat the patient with a drug that inhibits the activity of acetylcholinesterase will increase contraction.
Creatine phosphate acts as
an energy reserves in muscle tissue.
Describe spasticity
an increased muscle tone associated with an increase in tendon reflexes.
Describe rigidity
an increased muscle tone in which tendon reflexes are not affected (tetanus).
Describe cross bridges
are portions of thick filament (myosin head) act as (an enzymes) ATPase during the contraction cycle of muscle generate force
Both cardiac and skeletal muscle tissues
are striated and smooth muscle is not.
Thin and thick filaments do not extend the entire length of muscle fiber. Instead, they are
arranged in compartments called sarcomere.
Describe action potentials
conducted into a skeletal muscle fiber by transverse tubules (T-tubules).
The epimysium, perimysium, and endomysium are all
continuous with the connective tissue to form a rope like tendon that attaches a muscle to the periosteum of the bone.
Describe myofibrils
contractile organelles of muscle cell.
Actin and myosin are
contractile proteins of the muscle fiber.
After prolonged strenuous exercise has stopped, heavy breathing will often continue for several minutes in order to provide the oxygen needed to
convert the lactic acid produced during exercise back into glycogen resynthesize creatine phosphate replace oxygen displaced from muscle myoglobin
A Troponin molecule consists three globular subunits:
Unit 1 binds to tropomyosin Unit 2 binds to G-actin Unit 3 binds to calcium ions.
In skeletal muscles, the combined amounts of
creatine phosphate and ATP provide enough energy for the muscle to contract maximally for approximately 15 seconds.
Describe a motor unit
describes a somatic motor neuron and all the skeletal muscle fibers it stimulates.
Describe terminal cisterns
dilated end sac of the SR.
Aerobic respiration supplies enough ATP for muscles
during periods of rest or moderate exercises (running, jogging, swimming, and walking)
Describe myoblasts
embryonic cell (stem cell) that forms muscle fiber.
Smooth muscle cells are small but
form thick layers of hollow organs.
Sarcoplasm stores
glycogen and myoglobin, a red-colored protein that binds O2.
At rest, the tropomyosin molecule is
held in place by troponin.
Describe the M line
in sarcomere, thick and thin filaments are linked laterally by filaments
There are approximately 40 million receptors for Ach that are found
in the motor-end -plate.
Both cardiac and smooth muscle tissue are
involuntary and auto-rhythmic.
A myogram of twitch contraction includes the
latent period, the contraction period, and the relaxation period.
Mitochondrial activity in skeletal muscle cells are usually efficient, but can have
limited ATP production if there is limited O2 availability
Describe fascia
lines the body wall and limbs that surround and support muscles.
The cytoplasm of the synaptic terminal at NMJ contains
mitochondria and vesicles filled with neurotransmitter acetylcholine
At peak levels of exertion,
mitochondria can provide only one-third (33%) of the ATP needed. The remainder comes from glycolysis.
Skeletal muscle cells have
more than one nucleus, are large and run parallel to each other.
In multi-unit smooth muscle tissue,
muscle cells have few gap junctions with neighboring cells and thus must be excited by their own motor neuron terminal.
When energy reserves in a muscle are exhausted and lactic acid levels increase,
muscle fatigue occurs.
Rigor mortis is a condition in which
muscles are in a state of rigidity 3-4 hour after death and last about 24 hours.
The ratio of motor neurons to muscle fiber is the greatest in
muscles that control the eye and the tongue.
The protein of M line is
myomesin
Describe Z line or Z disk
narrow, plate-shaped region of dense protein material that separates one sarcomere to another.
Somatic motor neurons (voluntary control) provide
nerve impulses that stimulate skeletal muscle to contract.
Receptors for acetylcholine are located
on the motor-end plate (the region of the sarcolemma opposite the synaptic end)
Cardiac muscle cells usually have
only one, sometimes two, centrally located nuclei and are branched.
Describe sarcolemma
plasma membrane of a muscle fiber (muscle cell)
Muscle fibers have many nuclei in order to
produce large amounts of the enzymes and structural proteins needed for contraction.
Describe dystrophin
protein used to reinforce the sarcolemma and help transmit the tension generated by the sarcomeres to the tendons.
Rigor mortis disappears as
proteolytic enzymes from lysosomes digest the cross-bridges)
Describe hypotonia
refers to decreased or lost muscle tone.
Describe hypertonia
refers to increased muscle tone.
Tropomyosin and troponin are
regulatory proteins of the muscle fibers.
Individual who lifts weights build larger muscles because
skeletal muscles increase number of myofibrils but not number of cells.
Describe microfilaments
smaller proteins within myofibrils.
Muscle tissue is capable of undergoing the
stress-relaxation response when stretched is single-unit smooth muscle fibers.
Titin, myomesin, nebulin, α-Actinin, and dystrophin are
structural proteins of the muscle fibers.
An action potential or a (depolarization) or a (nerve impulse) is required for
the SR releases Ca2+ into sarcoplasm because when an action potential travels along the T. Tubule to open the voltage-gated Ca2+ channel allows a large amount of calcium flow out of the SR.
Thin filaments at either end of sarcomere are attached to
the Z line (Z disc).
Describe a sarcomere
the basic functional unit of a myofibril of a skeletal muscle fiber.
When a muscle fiber is relaxed,
the concentration of Ca2+in the sarcoplasm is very slow, but the concentration of Ca2+ in the sarcoplasm reticulum (SR) is very high. As a result, to trigger muscle contraction, Ca2+ must be released from SR.
describe sarcoplasm
the cytoplasm of a muscle fiber.
Describe A band
the darker middle area of sarcomere that contains thick & thin filament.
Describe endomysium
the delicate connective tissue that surrounds the individual muscle fiber and contains a large capillary network.
Describe epimysium
the dense irregular connective tissue that surrounds the entire muscle.
Describe recruitment
the increase in muscle tension that is produced by increasing the number of active motor units.
Describe treppe
the increased strength of a contraction that occurs when a second stimulus arrives after the muscle fiber has relaxed completely following the previous stimulus.
Describe wave summation
the increased strength of a contraction that occurs when a second stimulus arrives before the muscle fiber has relaxed completely.
Describe titin
the largest size protein in the muscle fiber that extends from the Z disk to M line.
Describe I band
the light area of the sarcomere that contains thin filament only.
Describe myosin
the main component of the thick filament and functions as a motor protein of muscle tissue.
Describe actin
the main protein of the thin filament.
Describe eccentric isotonic contraction,
the muscle fiber lengthens. (object is dropped)
After the fusion of myoblasts,
the muscle fiber loses its ability to go through mitosis.
Describe concentric isotonic contraction
the muscle fiber shortens. (object is lifted)
When acetylcholine binds to receptors at the motor-end plate,
the muscle membrane becomes more permeable to sodium ions ( Voltage-gated Na+ channel opens to allow Na+ ions entering inside the sarcolemma)
ATP hydrolysis reaction energizes
the myosin head.
Describe H zone
the narrow area in the center of the A band that contains thick filament only.
Describe the synaptic cleft
the narrow space between the neuron and the muscle fiber.
What makes the entire skeletal muscle fiber appear striated?
the overlapping of thick and thin filaments and the prominent striations of the myofibrils
Cramp occurs because
the relaxation phase of a muscle contraction is often prolonged in a fatigued muscle, raising the likelihood of fused summation of action potentials causing painful disturbances to a skeletal muscle.
Describe excitation-contraction coupling
the sequence of events that links excitation (generation an action potential) to the contraction
Describe sarcoplasmic reticulum (SR)
the series of membraneous channels that surround each myofibril.
Single-unit smooth muscle is found in
the skin and in tubular arrangements that form part of the walls of small arteries and veins and of hollow organs such as the stomach, intestines, uterus, and urinary bladder.
Describe NMJ
the synapse between the terminal end of a somatic motor (axon terminal) neuron and a portion sarcolemma (called Motor-End-Plate) of a skeletal muscle fiber.
When cross-bridges bind to the active site of actin thin filaments (myosin binding site),
the thick filaments pivot toward the M line.
Describe isometric contraction
the type of contraction in which the muscle fibers produce tension but do not shorten nor lengthen. (Object cannot be moved)
Describe isotonic contraction
the type of contraction in which the muscle fibers produce tension while muscle length is changed.
Multi-unit smooth muscle is found in
the walls of large arteries, in airways to the lungs, in the arrector pili muscles that attach to hair follicles, in the muscles of the iris that adjust pupil diameter, and in the ciliary body that adjusts focus of the lens in the eye.
A muscle fiber develops its greatest tension when
there is an optimal zone of overlap (sarcomere length is 2.0-2.4 μm) between thick and thin filament. This dependency is the length-tension relationship.
There are two types of microfilaments:
thin and thick filaments.
The part of the titin that extends from the Z disc is very elastic. Thus,
titin accounts for much of the elasticity and extensibility of myofibrils.
Cardiac muscle cells also are connected
to each other via gap junctions.
Describe transverse tubules or T tubules
tunnels in from the surface toward the center of each muscle fiber.
Anaerobic glycolysis supplies enough ATP for muscles for
two minutes of maximal activity.
Like cardiac muscle cells, some smooth muscle cells communicate
via gap junctions.
There are two types of smooth muscle single-unit
visceral smooth muscle and multi-unit smooth muscle.
When a skeletal muscle fiber contracts in the sliding filament mechanism,
Z line (Z disk) moves closer together The H zones and I bands get smaller The width of the A band remains constant The zone of overlap gets larger (more zone of overlaps between thin & thick filament)
Describe plasticity
an ability of the smooth muscle to function over a wide range of lengths.
The SR (sarcoplasmic reticulum) releases calcium in response to
an arrival of action potential.
The order of steps in generation an action potential
Step 1: The arrival of a nerve impulse at a motor neuron opens voltage-gated calcium Ca2+ channel in the sarcolemma, allowing Ca2+ enter to the synaptic terminal to trigger the releasing of acetylcholine. Step 2: Ach diffuses across the synaptic cleft by exocytosis and binds to receptors in the motor-end-plate. The binding opens voltage-gated sodium Na+ channel in the sarcolemma, allowing Na+ enters to the sarcoplasm. Step 3: The influx of Na+ produces a depolarization of the sarcolemma called "muscle action potential" Step 4: The muscle action potential that propagates along the T. Tubules triggers the opening of voltage-gated Ca channel in the sarcoplasmic reticulum (SR). SR releases Ca into sarcoplasm Step 5: Termination of Ach by enzyme Acetylcholinesterase (AChE) Step 6: Ca2+ binds to troponin on the thin filament, exposing the active site or myosin-binding site of actin. Step 7: Crossbridges are formed and contraction cycle begins. Step 8: When the generation of action potential ends, Ca2+ is pumped back to the SR by an active transport.
To generate an action potential,
acetylcholine (Ach), a neurotransmitter must be released at the neuromuscular junction (NMJ)
In single-unit smooth muscle tissue,
action potentials are initiated in response to neurotransmitters, hormones, or an auto-rhythmic signal. The action potential spreads throughout the tissue by moving through gap junctions that connect all the muscle cells together within the tissue.
During the refractory period,
additional oxygen is required to metabolize the lactic acid produced during exercise.
Active sites on the actin thin filament become available (exposed) for
binding when calcium ion binds to troponin subunit. Once calcium ion binds to troponin, troponin then moves tropomyosin away from the myosin-binding sites of thin filament.
At rest, active sites of actin (myosin-binding site) thin filament are
blocked (covered) by tropomyosin molecules.
SR stores
calcium ions (Ca2+)
Smooth muscle contracts when
calcium ions Ca2+ interact with calmodulin which activate the enzyme myosin light chain kinase, enabling myosin heads to attach to actin.
The mechanisms of rigor mortis is due to
calcium ions leak out of the SR (allowing myosin heads to bind to actin) and muscle fibers run out of ATP (causing the cross-bridges cannot detach from actin).
Contraction of myofibrils within a muscle fiber begins when
calciumion is released from the terminal cistern of sarcoplasmic reticulum.
