Human Physiology: Study Guide for Chapters 11-13 (Nervous and Muscular Systems)

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Chapter 11: The Efferent Peripheral Nervous System

Overview of the Efferent Peripheral Nervous System

The efferent peripheral nervous system is responsible for transmitting signals from the central nervous system (CNS) to the body's effectors, such as muscles and glands. It is essential for initiating and regulating voluntary and involuntary actions.

Definition: The part of the peripheral nervous system that carries motor commands away from the CNS.
Divisions: Includes the somatic nervous system (controls skeletal muscles) and the autonomic nervous system (controls smooth muscle, cardiac muscle, and glands).

Autonomic Nervous System (ANS): Subdivisions and Functions

Sympathetic Division: Prepares the body for 'fight or flight' responses (e.g., increases heart rate, dilates pupils).
Parasympathetic Division: Promotes 'rest and digest' activities (e.g., slows heart rate, stimulates digestion).
Anatomic Distinction: Sympathetic nerves typically originate from the thoracolumbar region, while parasympathetic nerves arise from the craniosacral region.
Functional Distinction: The two divisions often have opposing effects but can also cooperate to maintain homeostasis.

Autonomic Reflexes and Control Centers

Autonomic Reflexes: Involuntary responses regulated by the ANS, such as heart rate and blood pressure adjustments.
Medulla: Directs many autonomic reflexes, especially those related to cardiovascular and respiratory function.
Hypothalamus: Integrates autonomic functions with endocrine and behavioral responses.

Antagonistic Control in the ANS

Definition: The phenomenon where sympathetic and parasympathetic divisions exert opposite effects on the same organ (e.g., sympathetic increases heart rate, parasympathetic decreases it).
Examples: Heart rate, pupil diameter, gastrointestinal motility.

Two-Neuron Efferent Pathway

Structure: Consists of a preganglionic neuron (originates in CNS) and a postganglionic neuron (extends to target tissue).
Location of Ganglia: Sympathetic ganglia are near the spinal cord; parasympathetic ganglia are near or within target organs.

Neurotransmitters in the ANS

Sympathetic: Preganglionic neurons release acetylcholine (ACh); postganglionic neurons release norepinephrine (NE).
Parasympathetic: Both pre- and postganglionic neurons release acetylcholine (ACh).
Receptors: Cholinergic receptors (bind ACh) and adrenergic receptors (bind NE and epinephrine).

Varicosities and Synapses en Passant

Varicosities: Swellings along autonomic axons that release neurotransmitters over a broad area, allowing widespread effects.
Synapses en Passant: Synaptic connections formed at varicosities rather than at traditional synaptic terminals.

Adrenergic Receptors and Stimulation

Adrenergic Receptors: Found on target tissues of the sympathetic nervous system; include alpha (α) and beta (β) subtypes.
Adrenergic Stimulation: Effects vary by tissue and receptor subtype (e.g., β1 increases heart rate, β2 relaxes bronchial smooth muscle).

Chapter 12: Muscle Physiology and the Neuromuscular Junction

Muscle Structure: Sarcoplasm, Sarcolemma, and Tubules

Sarcoplasm: The cytoplasm of a muscle fiber, containing organelles and dissolved substances.
Sarcolemma: The plasma membrane of a muscle fiber.
Transverse (T) Tubules: Invaginations of the sarcolemma that conduct action potentials into the muscle fiber.
Channels in T Tubules: Include voltage-gated sodium and calcium channels.

Muscle Fiber Components: Myofibrils, Myofilaments, and Regulatory Proteins

Myofibrils: Bundles of contractile proteins within muscle fibers.
Myofilaments: Include thick filaments (myosin) and thin filaments (actin, troponin, tropomyosin).
Regulatory Proteins: Troponin and tropomyosin regulate the interaction between actin and myosin.

Sarcomere Structure and Bands

Sarcomere: The functional contractile unit of muscle, defined by Z-disks.
A-bands: Dark bands containing thick filaments.
I-bands: Light bands containing thin filaments.
H-zone: Central region of A-band with only thick filaments.
M-line: Center of the sarcomere, where thick filaments are anchored.

Neuromuscular Junction (NMJ)

Three Parts: Synaptic knob (axon terminal), synaptic cleft, and motor end plate (muscle membrane).
Synaptic Vesicles: Contain acetylcholine (ACh), released upon arrival of an action potential.
Voltage-Gated Channels: Open in response to depolarization, allowing Ca2+ influx and neurotransmitter release.
Motor End Plate: Contains ACh receptors; initiates muscle action potential.

Resting Membrane Potential and Muscle Contraction

Resting Membrane Potential: The electrical charge difference across the muscle cell membrane, typically around -70 mV.
Establishment: Maintained by Na+/K+ ATPase pumps and ion channels.

Physiological Steps of Muscle Contraction

Excitation: Action potential arrives at NMJ, ACh is released and binds to receptors.
Excitation-Contraction Coupling: Action potential travels along sarcolemma and T-tubules, triggering Ca2+ release from the sarcoplasmic reticulum.
Contraction: Ca2+ binds to troponin, allowing actin-myosin cross-bridge cycling and muscle shortening.

Key Events:

End Plate Potential (EPP): Local depolarization at the motor end plate.
Threshold and Depolarization: If EPP reaches threshold, an action potential is generated.
Refractory Period: Time during which the muscle fiber cannot be re-excited.
Calcium Cycling: Ca2+ is released into the sarcoplasm, binds to troponin, and is later pumped back into the sarcoplasmic reticulum.
Cross-Bridge Cycling: Myosin heads bind to actin, perform a power stroke, and detach in a cyclic manner.

Muscle Fatigue

Central Fatigue: Originates in the CNS; may involve decreased motivation or altered neurotransmitter levels.
Peripheral Fatigue: Due to changes at the neuromuscular junction or within the muscle fiber (e.g., ion imbalances, depletion of energy stores).

Smooth vs. Skeletal Muscle

Skeletal Muscle: Striated, voluntary, multinucleated, rapid contraction.
Smooth Muscle: Non-striated, involuntary, single nucleus, slower contraction.

Chapter 13: Reflexes and Motor Control

Classification of Neural Reflexes

Monosynaptic Reflex: Involves a single synapse between sensory and motor neuron (e.g., patellar reflex).
Polysynaptic Reflex: Involves one or more interneurons (e.g., withdrawal reflex).
Somatic Reflexes: Affect skeletal muscles.
Autonomic Reflexes: Affect smooth muscle, cardiac muscle, or glands.

Reflex Pathways

Pathway: Stimulus → Sensory receptor → Sensory neuron → Integration center (CNS) → Motor neuron → Effector.
Example: The patellar tendon (knee-jerk) reflex is a monosynaptic reflex.

Autonomic vs. Skeletal Muscle Reflexes

Autonomic Reflexes: Involuntary, regulate internal organs.
Skeletal Muscle Reflexes: Involve voluntary muscles, can be consciously overridden.

Alpha and Gamma Motor Neurons

Alpha Motor Neurons: Innervate extrafusal muscle fibers, responsible for muscle contraction.
Gamma Motor Neurons: Innervate intrafusal fibers within muscle spindles, adjust spindle sensitivity.

Golgi Tendon Organ (GTO) and Muscle Spindles

GTO: Located at the junction of muscle and tendon; senses muscle tension and prevents excessive force.
Muscle Spindles: Found within muscles; detect changes in muscle length and trigger stretch reflexes.
Innervation: Muscle spindles are innervated by sensory (afferent) and gamma motor (efferent) neurons.

Stretch Reflex and Alpha-Gamma Coactivation

Stretch Reflex: Muscle contraction in response to stretching within the muscle (e.g., knee-jerk reflex).
Alpha-Gamma Coactivation: Ensures muscle spindle sensitivity during muscle contraction.

Types of Movement

Reflex Movements: Involuntary, rapid responses to stimuli (e.g., withdrawal reflex).
Voluntary Movements: Initiated consciously (e.g., reaching for an object).
Rhythmic Movements: Repetitive, patterned movements (e.g., walking, breathing).

Table: Comparison of Somatic and Autonomic Reflexes

Feature	Somatic Reflex	Autonomic Reflex
Effector	Skeletal muscle	Smooth/cardiac muscle, glands
Conscious Control	Usually voluntary	Involuntary
Number of Synapses	Mono- or polysynaptic	Polysynaptic
Example	Knee-jerk reflex	Pupillary light reflex

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