Study guide: Muscular Tissue

📋 Course Outline

1. Muscle Types and Shared Properties
2. Skeletal Muscle Organization
3. Myofilaments and Sarcomere Contraction
4. Motor Units and Neuromuscular Junctions
5. Membrane Potentials and Muscle Excitation
6. Skeletal Fiber Contraction and Relaxation
7. Whole-Muscle Contraction and Force
8. Muscle Energy and Fatigue
9. Fiber Types and Muscle Conditioning
10. Cardiac Muscle Characteristics
11. Smooth Muscle Structure and Types
12. Smooth Muscle Function and Regulation
13. Muscle Disorders

📖 1. Muscle Types and Shared Properties

🔑 Key Concepts & Definitions

• Skeletal muscle: Voluntary, striated muscle usually attached to bones.

★ Must-know

• The three types of muscular tissue are skeletal, cardiac, and smooth muscle.

• All muscle types exhibit these properties:

  • excitability
  • conductivity
  • contractility
  • extensibility
  • elasticity

Further detail

• Skeletal muscle fibers can be as long as 30 cm.

📖 2. Skeletal Muscle Organization

🔑 Key Concepts & Definitions

• Endomysium: Connective tissue surrounding an individual muscle fiber.
• Perimysium: Connective tissue surrounding a muscle fascicle.
• Epimysium: Connective tissue surrounding an entire muscle.

★ Must-know

• A muscle contains fascicles of muscle fibers; each fiber contains myofibrils composed of myofilaments, and sarcomeres are the functional contractile units within myofibrils.

Further detail

• Tendons attach muscle to bone matrix.

• Collagen in muscle connective tissue stretches slightly under tension and recoils when released, helping resist excessive stretch, protect muscle, restore resting length, and contribute to power and efficiency.

📖 3. Myofilaments and Sarcomere Contraction

🔑 Key Concepts & Definitions

• Thick filament: A thick filament consists of several hundred myosin molecules whose heads project outward in a helical array, with a head-free bare zone in the middle.
• Sarcomere: The segment of a myofibril from one Z disc to the next and the functional contractile unit of a muscle fiber.

★ Must-know

• Thin filaments contain two intertwined strands of fibrous actin, whose globular actin subunits have active sites that bind myosin heads.

• Calcium binding changes troponin's shape, moving tropomyosin away from actin's active sites and permitting contraction.

• Striations consist of alternating dark A bands and light I bands; the A band contains thick filaments, while the I band contains thin filaments.

• During skeletal muscle shortening, thick and thin filaments slide past one another, Z discs move closer together, and filament lengths remain unchanged while overlap increases.

Further detail

• Tropomyosin blocks six or seven active sites on actin, and each tropomyosin molecule is associated with a calcium-binding troponin molecule.

• Titin anchors thin filaments to the Z disc and thick filaments to the M line, stabilizes filament position, prevents overstretching, and provides recoil.

• The H band is the central region of the A band containing thick filaments only, and the M line lies at the center of the H band.

• Dystrophin links actin in the outermost myofilaments to membrane proteins connected to the endomysium, transferring contraction forces to connective tissue and ultimately the tendon.

💡 Memory Hook

A bands are dark and contain thick filaments; I bands are light and contain thin filaments.

📖 4. Motor Units and Neuromuscular Junctions

🔑 Key Concepts & Definitions

• Motor unit: One nerve fiber and all the muscle fibers it innervates.
• Neuromuscular junction: The synapse where a nerve fiber meets a skeletal muscle fiber.

★ Must-know

• The neuromuscular junction includes: an axon terminal containing acetylcholine vesicles, a synaptic cleft between the terminal and sarcolemma, a Schwann cell that envelops and isolates the junction

• A nerve impulse triggers acetylcholine release into the synaptic cleft, where it binds receptors on junctional folds of the sarcolemma; acetylcholinesterase in the basal lamina breaks down acetylcholine.

Further detail

• Muscle fibers belonging to one motor unit are dispersed through a muscle and contract in unison, producing a weak contraction over a wide area.

• Small motor units contain three to six muscle fibers per neuron and support fine control in eye and hand muscles.

• Quadriceps femoris and gastrocnemius motor units can have 1,000 muscle fibers per neuron and support powerful contractions.

📖 5. Membrane Potentials and Muscle Excitation

🔑 Key Concepts & Definitions

• Action potential: The rapid voltage shift of depolarization followed by repolarization in a stimulated excitable cell.

★ Must-know

• The resting membrane potential of a skeletal muscle cell is about −90 mV and is maintained by the sodium–potassium pump.

• During depolarization, sodium gates open and sodium enters the cell; sodium gates then close, potassium gates open, and potassium exits to repolarize the membrane.

📌 Spastic paralysis is sustained muscle contraction, whereas flaccid paralysis is limp muscle that cannot contract.

Further detail

• At rest, the cell interior is negative relative to the exterior, with excess sodium ions in extracellular fluid and excess potassium ions in intracellular fluid.

• An action potential propagates along a cell membrane because excitation at one point triggers another action potential immediately ahead, creating an impulse.

• Tetanus toxin blocks glycine release in the spinal cord, causing motor-neuron overstimulation and spastic paralysis.

• Curare competes with acetylcholine for receptor sites without stimulating muscle, while botulinum toxin blocks acetylcholine release; both can cause flaccid paralysis.

📖 6. Skeletal Fiber Contraction and Relaxation

🔑 Key Concepts & Definitions

• Length–tension relationship: How the tension a muscle generates depends on its length before stimulation.

★ Must-know

• Skeletal muscle contraction proceeds through excitation, excitation–contraction coupling, contraction, and relaxation.

• An overly shortened muscle produces weak contraction because thick filaments butt against Z discs, while an overstretched muscle produces weak contraction because limited filament overlap permits few cross-bridges.

Further detail

• A muscle at its optimum resting length produces its greatest force, and muscle tone helps keep resting muscles near that length.

• Rigor mortis begins 3–4 hours after death, peaks at about 12 hours, and diminishes over the next 48–60 hours.

• After death, calcium released from deteriorating sarcoplasmic reticulum activates actin–myosin cross-bridging, but ATP depletion prevents muscle relaxation until the myofilaments decay.

💡 Memory Hook

Excitation → coupling → contraction → relaxation.

📖 7. Whole-Muscle Contraction and Force

🔑 Key Concepts & Definitions

• Threshold: The minimum voltage needed to generate a muscle-fiber action potential and produce contraction.
• Twitch: A brief cycle of contraction and relaxation produced by a stimulus at threshold or above.
• Recruitment: The increase in contraction strength achieved by activating more motor units as stimulus strength rises.

★ Must-know

📌 An isometric contraction develops internal tension without changing muscle length, whereas an isotonic contraction changes muscle length while tension remains constant.

📌 A concentric contraction shortens a muscle while it maintains tension, whereas an eccentric contraction lengthens it while it maintains tension.

Further detail

• A muscle twitch has a latent period, a contraction phase, and a relaxation phase; its total duration varies from 7 ms to 100 ms.

📌 The size principle recruits small motor units with weak stimuli and recruits both small and large motor units with strong stimuli.

• Higher stimulus frequency produces temporal summation; partial relaxation between stimuli causes incomplete tetanus, while very high frequencies can produce complete fused tetanus.

💡 Memory Hook

Isometric builds tension without shortening; isotonic changes length while maintaining tension.

📖 8. Muscle Energy and Fatigue

🔑 Key Concepts & Definitions

• Muscle fatigue: Progressive weakness resulting from prolonged muscle use.
• Anaerobic threshold: The point at which blood lactate begins to rise above resting levels.
• VO2 max: The point at which oxygen consumption plateaus and no longer increases with added workload.

★ Must-know

• Anaerobic fermentation produces little ATP and lactate without oxygen, whereas aerobic respiration requires oxygen and produces far more ATP without generating lactate.

• The phosphagen system combines ATP and creatine phosphate to supply nearly all energy for short bursts of activity, providing enough energy for about 6 seconds of sprinting.

• During anaerobic glycolysis, each glucose molecule yields a net 2 ATP and is converted to lactate; the glycogen–lactate system supports 30–40 seconds of maximum activity.

Further detail

• Myokinase transfers a phosphate group from one ADP molecule to another, converting the recipient ADP into ATP.

• Creatine kinase transfers a phosphate group from creatine phosphate to ADP to form ATP.

• After about 40 seconds of exercise, oxygen delivery becomes fast enough for aerobic respiration to meet most muscle ATP demand, and aerobic respiration yields another 30 ATP per glucose beyond glycolysis.

• After 3–4 minutes of exercise, oxygen consumption reaches a steady state; for 30 minutes energy comes equally from glucose and fatty acids, after which fatty acids become the more significant fuel as glucose is depleted.

• High-intensity fatigue is associated with potassium accumulation in T tubules and excess ADP with slow cross-bridge movement, which inhibit calcium release and reduce myofibril force production.

• Proposed contributors to low-intensity, long-duration fatigue include: fuel depletion, electrolyte loss through sweat, reduced motor signaling from the brain, inhibition of brain cells by ammonia released by exercising muscles

• Excess postexercise oxygen consumption is the elevated oxygen-consumption rate after exercise above the usual resting rate and can reach six times basal consumption and last an hour.

💡 Memory Hook

Short burst: phosphagen; next: glycogen to lactate; prolonged work: aerobic respiration.

📖 9. Fiber Types and Muscle Conditioning

★ Must-know

📌 Slow oxidative type I fibers resist fatigue through aerobic ATP production, whereas fast glycolytic type II fibers support quick, powerful responses using glycolysis and anaerobic fermentation.

• Factors affecting muscular strength include:
  • muscle size
  • fascicle arrangement
  • active motor-unit size
  • multiple motor-unit summation
  • temporal summation
  • resting muscle length
  • fatigue

Further detail

• Slow oxidative fibers are thin and rich in mitochondria, capillaries, and myoglobin, and are common in posture-maintaining muscles such as erector spinae and soleus.

• Fast glycolytic fibers are thick, rich in glycogen and creatine phosphate, have little myoglobin, and are prominent in eye and hand muscles, gastrocnemius, and biceps brachii.

• Fast-twitch intermediate type IIA fibers are fatigue resistant and are common in humans.

• A muscle can exert a tension of 3 or 4 kg/cm2 of cross-sectional area, and pennate fascicles are stronger than parallel fascicles, which are stronger than circular fascicles.

• Resistance training stimulates muscle-fiber enlargement through synthesis of additional myofilaments and myofibrils, making the fibers thicker.

• Endurance training increases slow-twitch fiber mitochondria, glycogen, and capillary density, and improves skeletal strength, red blood cell count, oxygen transport, and cardiovascular, respiratory, and nervous system function.

💡 Memory Hook

Slow oxidative fibers favor endurance; fast glycolytic fibers favor quick, powerful movement.

📖 10. Cardiac Muscle Characteristics

★ Must-know

• Cardiac and smooth muscle cells are shorter, single-nucleated myocytes, are involuntary, and receive autonomic rather than somatic motor innervation.

• Cardiac muscle contracts rhythmically, works continuously, resists fatigue, and coordinates cells within each heart chamber to contract in unison long enough to expel blood.

• Intercalated discs join cardiomyocytes; their gap junctions electrically connect neighboring cells, while their mechanical junctions prevent cells from pulling apart.

• A built-in pacemaker generates electrical waves that allow cardiac muscle to contract rhythmically without nervous stimulation.

Further detail

• Cardiomyocytes are striated, shorter and thicker than skeletal muscle fibers, and have less-developed sarcoplasmic reticulum and larger T tubules that admit extracellular calcium.

• Autonomic input can increase or decrease heart rate and contraction strength, while cardiac twitches last about 200–250 ms to allow time for blood ejection.

• Cardiac muscle uses aerobic respiration almost exclusively and is rich in myoglobin and glycogen; mitochondria occupy about 25% of cardiomyocyte volume compared with 2% of skeletal muscle-cell volume.

• Damaged cardiac muscle cells are repaired by fibrosis, and functional cardiac muscle is not regenerated after a heart attack.

📖 11. Smooth Muscle Structure and Types

★ Must-know

• Smooth muscle lacks striations, can undergo mitosis and hyperplasia, regenerates well after injury, and contracts more slowly than skeletal or cardiac muscle while sustaining contraction without fatigue.

• Smooth muscle myocytes are fusiform, have one central nucleus, and contain thick and thin filaments that are not aligned into sarcomeres.

📌 Multiunit smooth muscle has independently contracting motor units whose nerve branches synapse with individual myocytes, whereas single-unit smooth muscle electrically couples cells through gap junctions so many contract together.

Further detail

• Smooth muscle has scant sarcoplasmic reticulum, no T tubules, and obtains contraction calcium from extracellular fluid through sarcolemmal calcium channels.

• Smooth muscle lacks Z discs, and dense bodies serve as anchoring sites for filaments and are linked between neighboring cells.

• Multiunit smooth muscle occurs in some large arteries and air passages, piloerector muscles, and the iris, while single-unit smooth muscle is more common in blood vessels and digestive, respiratory, urinary, and reproductive tracts.

💡 Memory Hook

Multiunit cells contract independently; single-unit cells couple and contract together.

📖 12. Smooth Muscle Function and Regulation

🔑 Key Concepts & Definitions

• Stress–relaxation response: A brief contraction followed by relaxation when a hollow organ is stretched, helping the organ fill gradually without emptying.
• Plasticity: Smooth muscle's ability to adjust its tension to the degree of stretch.

★ Must-know

• Smooth muscle can respond to:

  • autonomic activity
  • hormones
  • carbon dioxide, oxygen, and pH
  • temperature
  • stretch
  • autorhythmicity

• Calcium entering smooth muscle binds calmodulin, which activates myosin light-chain kinase to phosphorylate a regulatory protein on myosin; myosin ATPase then hydrolyzes ATP and enables filament sliding.

• Smooth muscle can contract powerfully over lengths from half to twice its resting length because it lacks Z discs, does not organize filaments into sarcomeres, and has myosin heads along the entire thick filament.

Further detail

• Smooth muscle contraction puckers and twists the cell as thick filaments pull thin filaments, which pull on dense bodies and membrane plaques.

• Smooth muscle contraction has a 50–100 ms latent period, reaches peak tension at about 500 ms, and tension declines over 1–2 seconds.

• In the latch-bridge mechanism, myosin heads remain attached to actin without consuming additional ATP, sustaining smooth muscle tone with resistance to fatigue.

• Stretch can open mechanically gated calcium channels and trigger smooth muscle contraction; peristaltic waves then propel contents along an organ.

💡 Memory Hook

Picture a hollow organ gently stretching, contracting to propel contents, then relaxing to accommodate more.

📖 13. Muscle Disorders

🔑 Key Concepts & Definitions

• Muscular dystrophy: A group of hereditary diseases in which skeletal muscles degenerate and weaken and are replaced by fat and fibrous scar tissue.
• Myasthenia gravis: An autoimmune disease in which antibodies attack neuromuscular junctions and cluster acetylcholine receptors, reducing muscle-fiber sensitivity to acetylcholine.

📝 Essential Points

• Duchenne muscular dystrophy is an X-linked recessive disease caused by a dystrophin-gene mutation, affects about 1 of 3,500 live-born boys, and is usually diagnosed between ages 2 and 10.

• In Duchenne muscular dystrophy, absent or defective dystrophin disrupts the actin–sarcolemma link, damages cell membranes during contraction, and leads to necrosis and scar tissue.

• Facioscapulohumeral muscular dystrophy is autosomal dominant, affects both sexes equally, and affects facial and shoulder muscles more than pelvic muscles.

• Myasthenia gravis often first affects facial muscles and can cause drooping eyelids, double vision, difficulty swallowing, and limb weakness.

• Treatments include:

  • cholinesterase inhibitors
  • immunosuppressive agents
  • thymectomy
  • plasmapheresis

💡 Memory Hook

Myasthenia gravis weakens signaling at ACh receptors; muscular dystrophy damages skeletal muscle fibers.

📊 Synthesis Tables

Major Muscle Fiber Types