Muscle Tissue: Types, Structure, and Function

Muscle Tissue Types & Properties

• Skeletal muscle is voluntary and consciously controlled.
• Smooth muscle is involuntary.
• Cardiac muscle is involuntary but striated.

Properties of Muscle Cells

• Excitability allows response to stimuli.
• Contractility allows shortening with force.
• Extensibility allows stretching without damage.
• Conductivity allows the transmission of electrical impulses.
• Plasticity is not a property.

Muscle Structure & Proteins

• Fascicle refers to a bundle of muscle fibers.
• Myofibrils are chains of sarcomeres inside fibers.
• Sarcomere constitutes the unit of contraction between Z discs.
• Thick filaments are composed of myosin.
• Thin filaments contain actin, tropomyosin, and troponin.
• Elastic filament contains titin.
• Dystrophin links actin to the sarcolemma; its defects result in muscular dystrophy.

Neuromuscular Junction & Contraction

• The NMJ (neuromuscular junction) facilitates communication between a motor neuron and muscle fiber.
• Acetylcholine (ACh) is released from synaptic vesicles and binds to the motor end plate.
• AChE (acetylcholinesterase) breaks down ACh to halt stimulation.
• Without AChE, spastic paralysis occurs.
• Myasthenia Gravis is an autoimmune condition treated by inhibiting cholinesterase.

Excitation-Contraction Coupling

• Sarcoplasmic reticulum stores Ca2+, which is released upon action potential.
• T tubules conduct the action potential inside the cell.
• Troponin binds Ca2+, which moves tropomyosin, exposing binding sites.
• Myosin binds actin forming a cross bridge once phosphate is released.
• ATP hydrolysis powers myosin head movement.

Electrical Activity & RMP

• Resting Membrane Potential (RMP) is negative at approximately -90 mV inside the cell.
• At rest, the membrane is most permeable to K+.
• Na+ enters during depolarization, leading to muscle contraction.

Muscle Contraction & Twitch

• Threshold is the minimum stimulus required for contraction.
• Latent period is the time between stimulus and contraction.
• Contraction phase involves tension building and muscle shortening.
• Relaxation phase: Ca2+ returns to the sarcoplasmic reticulum (SR), and tension falls.

Twitch Variants

• Incomplete tetanus involves partial relaxation.
• Complete tetanus involves no relaxation.
• Wave summation involves increased tension with repeated stimuli.
• Recruitment involves increasing motor units for more strength.

Isotonic vs Isometric

• Isotonic muscle action: the muscle changes length.
  • Concentric: shortens.
  • Eccentric: lengthens but still contracts.
• Isometric muscle action: tension occurs without changing length.

Muscle Metabolism

• Aerobic respiration yields 32 ATP/glucose (long-term).
• Anaerobic fermentation is short-term and produces less ATP.
• Phosphagen system offers immediate energy via creatine phosphate.
  • Creatine kinase & myokinase are involved.

Adaptations to Exercise

• More mitochondria result in more endurance (aerobic respiration).
• Training at high altitudes results in more red blood cells, which leads to more oxygen.
• Soleus is for endurance and is aerobic, while Gastrocnemius is for power and is anaerobic.

Autonomic Nervous System (ANS) Study Guide

• General Function and Organization
• The ANS controls involuntary functions, such as cardiac muscle, smooth muscle, and glands, excluding skeletal muscle.
• Sympathetic (fight or flight) and parasympathetic (rest and digest) divisions comprise the ANS.
• A preganglionic neuron and a postganglionic neuron are involved in two-neuron pathways.
• Autonomic tone refers to the background rate of activity for both sympathetic and parasympathetic systems.

Reflex Arcs and Responses

• Visceral Reflex Order: Sensory receptor → Afferent nerve fiber → Interneuron → Efferent nerve fiber → Effector (e.g., gland).
• Somatic reflexes are faster because of myelination and fewer neurons/synapses.

Neurotransmitters and Receptors

• Preganglionic neurons release acetylcholine (ACh).
• Postganglionic neurons release:
  • Sympathetic neurons mainly release norepinephrine (NE), and sometimes ACh.
  • Parasympathetic neurons release ACh.
• Nicotinic receptors (on postganglionic neurons) are always excitatory.
• Muscarinic receptors (on effectors) are either excitatory or inhibitory.
• Adrenergic receptors (α and β) bind NE or epinephrine.
• MAO (monoamine oxidase) breaks down NE.
• Caffeine blocks adenosine, which increases ACh release.

Sympathetic Division

• Originates in the thoracic and lumbar spinal cord.
• Preganglionic neurons are short, while postganglionic neurons are long.
• Ganglia are located in the sympathetic chain near the spinal cord.
• Widespread effects happen because of neural divergence.
• Increases heart rate, bronchodilation, and decreases digestion.
• The adrenal medulla mostly secretes epinephrine.

Parasympathetic Division

• Originates in the craniosacral regions (brainstem and sacral spinal cord).
• Preganglionic neurons are long, while postganglionic neurons are short.
• Effects are more localized.
• Maintains resting heart rate (parasympathetic tone).
• Most axons travel via the vagus nerve (CN X).

Autonomic Nerve Pathways

• White rami communicantes carry myelinated preganglionic fibers.
• Gray rami communicantes carry unmyelinated postganglionic fibers.
• Splanchnic nerves bypass sympathetic ganglia and synapse in collateral ganglia (e.g., celiac ganglion).

ANS and Emotions

• Emotions influence ANS activity (e.g., fear increases heart rate).
• Input comes from the hypothalamus, medulla oblongata, and cerebral cortex.

Dual Innervation & Effects

• Most organs receive dual innervation (both sympathetic and parasympathetic).
• Antagonistic effects are opposite outcomes (e.g., sympathetic increases heart rate, parasympathetic decreases heart rate).
• Cooperative effects work together (e.g., parasympathetic initiates erection, sympathetic causes ejaculation).