Muscular Tissue

Part 2: Support & Movement

Common Traits

Proteins Needed: Actin Myosin

Four Essential Ions Needed: Calcium Sodium Chloride Potassium

Common Characteristics: Excitability Conductivity Contractility Extensibility Elasticity

Skeletal Muscle Tissue

Striations: Alternating light and dark bands seen on skeletal muscle tissue under the microscope. A Bands: The dark bands I Bands: The light bands

Multinucleated: Skeletal muscles to have multiple nuclei in each cell.

Somatic Motor Neurons: Neurons that stimulate/excite the muscle Neuromuscular Junction (NMJ): The point at which

the neuron and muscle communicate

Skeletal Muscle Structures

Muscle Fibers: Composed of individual muscle cells with multiple nuclei.

Sarcolemma: The plasma membrane of each muscle fiber.

Transverse “T” Tubules: Tunnel from the surface to the center of the fibers. Allows for action potentials to spread throughout the

muscle fiber Sarcoplasm: Located within the sarcolemma.

Glycogen: Contained within the sarcoplasm; necessary for ATP production.

Myoglonin: Contained within the sarcoplasm; helps release oxygen during ATP production.

Skeletal Muscle Structures

Sarcoplasmic Reticulum: Surrounds each myofibril and creates a network that acts as a reservoir for Calcium ions. Terminal Cisternae: Sacs of sarcoplasmic reticulum;

store calcium ions in relaxed muscle fibers and release calcium ions during contraction.

Myofibrils: Long protein threads; two types;. Thick Filaments: Made up of myosin and somewhat

L shaped. Thin Filaments: Made up of actin and have an active

site that binds to myosin. Myofilaments: Smaller elements responsible for

muscle contraction; make up the myofibrils.

Nerve-Muscle Interaction

Motor Neuron: Specific neuron with the cell body located in the brainstem and spinal cord; supplies muscles with electrical signals.

Paralysis: Loss of nerve input resulting in a lack of muscular control.

Atrophy: Degeneration of the muscle tissue due to a lack of use.

Motor Units: Consists of one nerve fiber and all of the muscle fibers it innervates. Contracts as one unit; varies in size. Examples: Eyes have 23 fibers per motor unit to

move the eyes, the thigh has over 1,000 fibers per unit.

Neuro-Muscluar Junction

Similar to the neural synapse - This is the junction where innervation happens!

Resting Membrane Potential: The state of a neuron becoming polarized, or having an electrical charge potential. Maintained by Sodium-Potassium Pump. Sodium-potassium pump: When the cell is

stimulated, the ion gates open within the membrane and sodium ions rush in while potassium ions exit, resulting in a depolarized change in electrical potential.

This causes an action potential to be reached.

Neuro-Muscluar Junction

Action Potential (AP): The nerve signal transmitted from the axon of the nerve to the muscle tissue. Acetycholine (Ach): The neurotransmitter responsible for

action potentials being released into the muscle fiber. Triggered by the resting potential being reached.

Synaptic Knob: The set of vesicles responsible for releasing ACh.

Threshold: The minimum voltage necessary to trigger an action potential to produce a contraction.

Twitch: A single action potential one motor neuron resulting in a brief contraction of the muscle fibers,.

Motor End Plate: The point on the muscle where the neural impulse is received.

Contraction & Relaxation of Skeletal Muscle Fibers

Sliding Filament Mechanism: The model that describes the methods of skeletal muscle contraction.

Occurs in 4 steps

Contraction & Relaxation of Skeletal Muscle Fibers

1. Excitation: 1. Action potentials transmit from the nerve to

muscle fibers

2. Motor end plate releases ACh

3. Depolarization occurs through the sodium-potassium pump.

Contraction & Relaxation of Skeletal Muscle Fibers

2. Excitation-Contraction Coupling: Action potentials in the muscle fibers lead to activation of the microfilaments.

1. When the action potential reaches the sarcoplasmic reticulum, Calcium ions are released.

2. Calcium ions bond to troponin in the thin myofilaments.

3. Toponin exposes the active sites on the actin filaments.

4. Myosin filament heads can now bind to the actin filaments.

5. This initiates contraction.

REMEMBER: Actin & Myosin are contractile proteins!

Contraction & Relaxation of Skeletal Muscle Fibers

3. Contraction: The thin myofilaments slide toward the thick myofilaments, causing the muscle fiber to shorten.

1. Myosin molecules release ATP.

2. Myosin heads contact active actin sites, releasing the Power Stroke.

3. Recovery Stroke follows the power stroke, causing the myosin to release the actin and bind to a new ATP molecule.

4. Power Stroke/Recovery Stroke sequence repeated multiple times per muscle contraction.

Contraction & Relaxation of Skeletal Muscle Fibers

4. Relaxation: The muscle relaxes when the nervous stimulation ends.

1. Occurs when acetylcholinesterase breaks down the Ach to cease generation of action potentials.

2. Calcium is carried back to the sarcoplasmic reticulum to be stored for future contractions.

Length-Tension Relationship

Length-Tension Relationship: The force of the muscle contraction depends on the length of the sarcomeres before the contraction occurs.

Muscle fibers have the most tension when optimal overlap between thick and thin filaments occurs.

If the muscle becomes overly stretched, there is little to no overlap and the muscle cannot contract.

Rigor Mortis

Rigor Mortis: The rigidity of muscle tissue beginning 3-4 hours after death.The result of leaky cellular membranes

causing Calcium ions to flow into the cytosol and myosin heads to bind to actin.

Muscles are then in a perpetual state of contraction, causing them to be rigid.

Muscle Tension

Muscle Tension: The small amount of taughtness or tension in the muscle due to weak & involuntary contractions of the motor units.

Contraction Strength: The strength of the contraction of the whole muscle is increased by the number of motor units activated. Higher-frequency stimulation results in more units

participating and a stronger contraction. Treppe: AKA the staircase effect… The graduated

series of increasingly stronger contractions as a result of the muscle being exposed to a series of signals of the same strength.

Probably due to the buildup of calciun and the inability of the muscle cells to return to homeostasis.

Refractory Period

Refractory Period: A period after each muscle twitch where the muscle cannot respond to another impulse. Wave Stimulation: Stimuli arriving at different times

result in stronger contractions - any further stimulation just causes the muscle to become more tense.

Incomplete or Unfused Tetanus: If stimulation continues without the muscle being given enough time to completely relax the muscle maintains a sustained but wavering contraction.

Can occur at 20-30 stimuli per second. Complete or Fused Tetanus: If the muscle is not

given any period to rest between stimulations, the muscle becomes completely rigid.

Can occur at 80-100 stimuli per second.

Types of Contractions

Isometric Contractions: Contraction results in muscle tension, but produces no change in length & no movement. E.g. holding a book steady with an outstretched arm.

Isotonic Contractions: Contraction results in a change of muscle length (movement) but no change in tension. 2 forms: Concentric Contractions: The muscle shortens as it

contracts. Eccentric Contractions: The muscle lengthens as it

contracts.

Muscle Metabolism

Aerobic Respiration: Oxygen exchange typically supplies the energy for muscle contraction.

Anaerobic Fermentation: If aerobic respiration is insufficient, the muscle catalyzes stored creatine phosphate to create ADP, which in turn yields ATP to fuel the muscle cells.Results in lactic acid buildup on the muscle

cell which prevents oxygen exchange.

Muscle Metabolism

Muscle Fatigue: A muscle’s inability to contract after a period of prolonged exercise. Due to…. Increased Calcium ion levels Buildup of lactic acid Insufficient oxygen Depletion of glycogen Inadequate release of acetylcholine

Oxygen Debt: The amount of oxygen that must be replenished after periods of exercise. Occurs when the body is still recovering and the heart

and lungs are still working harder. AKA Recovering Oxygen Uptake.

Types of Muscle Fibers

Fast Twitch Fibers (Glycolytic Fibers): Produce quick energy necessary for stop-and-go activities. Larger sized, produce short & powerful

contractions. Low in myoglobin, causing them to appear

white.Few mitochondria. Used for intense anaerobic movements, such

as weight lifting, sprinting, or throwing an object.

Types of Muscle Fibers

Slow Twitch Fibers (Oxidative Fibers): Produce smaller, longer contractions used for sustained activity. Have a richer blood supply Increased mitochondria Increased myoglobin (used to store oxygen) giving a

reddish appearance Called red fibers because of this

Use aerobic metabolism to function Resist fatigue Produced sustained contractions Used in activities such as long-distance running.

Muscle Conditioning & Exercise

Resistance Exercise or Conditioning: Involves contracting muscles against a load that resists movement.Stimulates muscle growth by enlarging

existing muscle cells. Endurance or Aerobic Exercise:

Improves muscle resistance to fatigue. Increases the density of slow-twitch fibers. Increases blood and oxygen supply.

Cardiac Muscle

Cardiac Muscle: Similar to skeletal muscles, with a few differences… Fibers are branched and interconnected Intercalated Discs: Gap junctions passing electrical

impulses from cell to cell Autorhythmicity: Beats continuously and

rhythmically without stimulation from the nervous system… But is still affected by the nervous system.

Involuntary: We cannot control this muscle type! 25% of the muscle fibers composed of mitochondria

to meet energy needs.

Smooth Muscle

Smooth Muscle: Composed of both thick & thin filaments, but are not aligned with each other to produce striations. Single nucleus per cell InvoluntaryVisceral Muscle: Does not attach to boneNo Z-discs or T-tubulesVery little sarcoplasmic reticulumCalcium for contractions comes mostly from

the extracellular fluid

Smooth Muscle

Can be found in blood vessels, the digestive, respiratory, urinary, & reproductive tracts.

Can remain partly contracted for long periods without stimulation – can maintain muscle tone easily.

Contracts more slowly and for longer overall periods.

Can stretch more than the other muscle types and still maintain its function.

E.g. the uterus & bladder muscles.