Muscle Physiology Quiz

Neuromuscular System & Exercise

• The study covers the neuromuscular system and its relationship to exercise.
• The unit layout outlines the topics covered: Nervous System Organization, Reflexes, Motor Units & NMJ, Sliding Filament Theory, Fatigue and Proprioception, and Muscle Fiber Characteristics.

Nervous System Organization

• Central Nervous System (CNS) includes the brain and spinal cord.
• Peripheral Nervous System (PNS) comprises cranial (12 pairs) and spinal nerves (31 pairs).
• The Somatic Division of the PNS is responsible for voluntary movements.
• The Autonomic Division (Sympathetic & Parasympathetic) controls involuntary functions.
• Neurotransmitters are chemical messengers that facilitate depolarization or hyperpolarization at synapses.

CNS: The Brain

• The brain contains 50 billion neurons, with approximately 10 million sensory (afferent) and 500,000 motor (efferent) neurons.
• The cerebellum is essential for integration of posture, locomotion, balance and reflexes during the exercise routine.
• The telencephalon (cerebrum) orchestrates motor movements.

CNS: The Spinal Cord

• The spinal cord has two main regions in cross-section:
• H-shaped gray matter with sensory, motor, and interneurons.
• White matter, composed of ascending (sensory) and descending (motor) tracts.

Peripheral Nervous System

• 12 pairs of cranial nerves are involved in reflexes (like visual and auditory).
• Motor (efferent) neurons transmit impulses away from CNS to muscles, glands and other effectors. The impulses exit via the ventral root.
• Sensory (afferent) neurons transmit sensory information from receptors to the brain from peripheral sensory receptors, entering the spinal cord via the dorsal root.
• 31 pairs of spinal nerves mediate reflexive actions.

Somatic Nervous System

• The somatic nervous system controls voluntary skeletal muscle.
• Somatic efferent nerve firing initiates muscle action.

Autonomic Nervous System (ANS)

• The autonomic nervous system regulates involuntary muscles in the gut, heart and sweat glands, along with endocrine glands.
• It maintains internal homeostasis.
• The Sympathetic division prepares the body for "fight or flight" responses with nerve fibers that promote stimulation.
• The Parasympathetic division promotes the "rest and digest" response with nerve fibers that promote inhibition.

Sympathetic Nervous System

• Sympathetic neurons exit the spinal cord and supply heart, smooth muscle and sweat glands.
• The system releases norepinephrine and epinephrine during emergency situations.
• The system increases breathing and heart rate, dilates pupils, increases perspiration and redistributes blood flow to the muscles.

Parasympathetic Nervous System

• Neurons in the parasympathetic nervous system arise from the brain stem and sacral spine and supply the thorax, abdomen and pelvic regions.
• The system releases acetylcholine and counters the effects of the sympathetic division.

Reflexes

• Reflexes are automatic responses to stimuli.
• A reflex arc comprises a sensory receptor, sensory root, spinal cord, motor root and effector.
• Monosynaptic reflex arcs have a single synapse (e.g., patellar reflex).
• Complex reflexes involve multiple synapses (e.g., crossed extensor reflex).
• Learned reflexes are developed through practice (e.g., sports skills).

Motor Units

• A motor unit is the basic functional unit of movement.
• It comprises an anterior motor neuron and all the muscle fibers it innervates.
• About 420,000 motor nerves exist and one nerve can supply many fibres. The ratio of muscle fibers to nerves depends on the complexity of the movement.
• The more specialised the movement the less fibres per nerve.

Motor Units: NMJ

• The neuromuscular junction (NMJ) acts as an interface between the motor neuron and muscle fiber.
• Acetylcholine, a neurotransmitter, transmits the nerve impulse to the muscle fiber.

Neural Activation/Facilitation

• Motoneurons generate action potentials by hypopolarizing the neuron, lowering it's threshold for excitation.
• Effective disinhibition activates muscle groups fully during strenuous efforts.
• Enhanced neuromuscular activation contributes to strength gains early in resistance training without enlarging muscles.

Neural Inhibition

• Presynaptic terminals trigger inhibitory impulses by increasing postsynaptic membrane permeability; hyperpolarizing the neuron to make firing more difficult. This is the opposite effect from neural activation.
• Neural inhibition safeguards the body, filtering unwanted stimuli to ensure smooth, deliberate actions.

Motor Unit Characteristics and Muscle Fiber Types

• Motor unit characteristics vary across muscles, exhibiting varied twitch characteristics.
• Different motor units and muscles develop varying amounts of tension.
• Various fiber types exhibit different responses to stimulation in terms of force and speed.
• Three main factors govern tension development: all-or-none principle, gradation of force principle, and level of motor unit recruitment patterns.
• All-or-none principle*: When a stimulus triggers an action potential in a motor neuron, all accompanying muscle fibers contract synchronously.
• Gradation of force principle*: Increasing the number of recruited motor units or the frequency of motor unit discharge alters muscle force.
• Motor unit recruitment patterns*: A progressive recruitment of the motor neuron axons with increasing muscle force results in a selective recruitment and firing pattern for fast-twitch and slow-twitch muscles.

Muscle Gross Structure

• Epimysium surrounds the entire muscle.
• Perimysium circles bundles of muscle fibres.
• Endomysium encloses individual muscle fibres.
• Fascia surrounds and dissects muscles.
• Sarcolemma is the cell membrane of a muscle fiber.

Muscle Microscopic Structure

• Muscle fibers contain myofilaments (actin, myosin, tropomyosin, troponin) which contribute to muscle contraction.
• The sarcomere (region between two Z-lines) is the functional unit of the muscle cell.
• Thousands of sarcomeres in parallel form a muscle.

Sliding-Filament Theory

• Muscle fibers shorten or lengthen due to sliding of thick (myosin) and thin (actin) filaments past each other.
• Myosin cross-bridges attach, rotate, and detach from actin to generate movement.
• ATP provides energy for this process.

Muscle Contraction (Sequence)

• A nerve impulse depolarizes the sarcolemma and initiates calcium release.
• Calcium ions bind to troponin, releasing inhibition and allowing actin-myosin interactions.
• Myosin ATPase hydrolyzes ATP and powers the movement of myosin cross-bridges to cause muscle shortening.

Muscle Fatigue

• Muscle fatigue is a decrease in muscle tension or force generating capacity due to repeated stimulation.
• Four factors affecting fatigue include: neurotransmitter levels (serotonin, dopamine), decreased glycogen and or glucose content or increased lactate within the muscle and fatigue at the neuromuscular junction.

Proprioceptors

• Specialized sensory receptors in muscles, joints, and tendons that detect stretch, tension, and pressure.
• Muscle spindles and Golgi tendon organs are important proprioceptors.
• Muscle spindles respond to stretch to contract the muscle; Golgi tendon organs detect tension, causing relaxation.
• Proprioceptors provide crucial information about muscle dynamics and limb position to the CNS, enabling refined motor control.

Muscle Fiber Classifications

• Muscle fibers are classified by speed and fatigue resistance into Type I (slow-twitch, oxidative), Type IIa (fast-twitch, oxidative-glycolytic), Type IIx (fast-twitch, glycolytic), and Type IIb (fast-twitch, glycolytic).
• Type I fibers dominate in endurance activities; Type IIx is pivotal for short-duration, high-intensity activities.

Changing Muscle Fibres

• Muscle fiber type, percentage of slow-twitch to fast-twitch, generally does not change significantly with training.
• Chronic or specific training may lead to the conversion of Type IIa fibers to Type IIx or vice versa.
• Genetic endowment significantly influences success in specific sports.

Test your knowledge on muscle physiology with this quiz. Explore key principles such as motor unit contraction, methods of increasing muscle force, and the structure surrounding muscles. Perfect for students of physiology or anyone interested in human anatomy.