Autonomic Nervous System, Sensory Systems, and Skeletal Muscle Physiology: Study Guide

Autonomic Nervous System

Afferent vs. Efferent Neurons

The nervous system is composed of different types of neurons, each with specific functions in transmitting information.

• Afferent neurons: Also known as sensory neurons, these carry information from sensory receptors toward the central nervous system (CNS).

• Efferent neurons: Also known as motor neurons, these transmit signals from the CNS to effectors such as muscles and glands.

• Example: Touching a hot surface activates afferent neurons, which send signals to the CNS; efferent neurons then stimulate muscles to withdraw the hand.

Peripheral Nervous System Control of Organs

The peripheral nervous system (PNS) regulates involuntary functions by innervating various organs.

• Autonomic division of the PNS controls smooth muscle, cardiac muscle, and glands.

• These tissues are found in organs such as the heart, digestive tract, and blood vessels.

Autonomic Neurons and Ganglia

Autonomic neurons are classified based on their location and function within the nervous system.

• Preganglionic neurons originate in the CNS and synapse in autonomic ganglia.

• Postganglionic neurons originate in autonomic ganglia and innervate target organs.

• Sympathetic ganglia are located near the spinal cord (thoracolumbar region).

• Parasympathetic ganglia are located near or within target organs (craniosacral region).

• Example: The sympathetic chain ganglia run parallel to the spinal cord.

Functions of Sympathetic and Parasympathetic Divisions

The autonomic nervous system is divided into sympathetic and parasympathetic branches, each with distinct roles.

• Sympathetic division: Prepares the body for 'fight or flight' responses (increases heart rate, dilates pupils).

• Parasympathetic division: Promotes 'rest and digest' activities (slows heart rate, stimulates digestion).

Autonomic Neurotransmitters

Neurotransmitters mediate communication between neurons and target cells in the autonomic nervous system.

• Sympathetic division: Uses acetylcholine (ACh) at preganglionic synapses and norepinephrine (NE) at postganglionic synapses.

• Parasympathetic division: Uses acetylcholine at both preganglionic and postganglionic synapses.

Adrenergic Receptors

Adrenergic receptors respond to catecholamines such as norepinephrine and epinephrine.

• Types: Alpha (α) and beta (β) receptors, each with subtypes (e.g., α1, α2, β1, β2).

• Location: Found on target cells in organs such as the heart, lungs, and blood vessels.

• Function: Mediate effects like increased heart rate (β1) or vasoconstriction (α1).

Somatic Motor System and Neuromuscular Junction

Somatic Motor Neurons and Neuromuscular Junction

Somatic motor neurons control voluntary muscle movements by synapsing at the neuromuscular junction.

• Neuromuscular junction (NMJ): Specialized synapse between a motor neuron and a skeletal muscle fiber.

• Neurotransmitter: Acetylcholine (ACh) is released at the NMJ.

• Receptor: ACh binds to nicotinic receptors on the muscle cell membrane.

Synaptic Transmission at the NMJ

Transmission at the NMJ involves a sequence of events leading to muscle contraction.

• Action potential arrives at the axon terminal.

• ACh is released into the synaptic cleft.

• ACh binds to nicotinic receptors, causing depolarization of the muscle membrane.

• Depolarization triggers muscle contraction.

Sensory Systems: Vision and Hearing

Photoreceptors in the Retina

The retina contains specialized cells that detect light and enable vision.

• Types: Rods and cones.

• Rods: Responsible for vision in low light; sensitive to light but do not detect color.

• Cones: Responsible for color vision and visual acuity; function best in bright light.

Light Sensitivity and Adaptation

Photoreceptors adapt to varying light conditions to maintain optimal vision.

• Light adaptation: Process by which eyes adjust to bright light.

• Dark adaptation: Process by which eyes adjust to low light.

• Photopigments in rods and cones regenerate to maintain sensitivity.

Pupil Diameter Regulation

Pupil size is controlled by autonomic input to smooth muscle in the iris.

• Sympathetic stimulation: Dilates the pupil (contracts radial muscles).

• Parasympathetic stimulation: Constricts the pupil (contracts circular muscles).

Accommodation in the Eye

Accommodation refers to the adjustment of the lens to focus on near or distant objects.

• Parasympathetic activation: Causes the lens to become more rounded for near vision.

• Changes: Ciliary muscles contract, reducing tension on the lens.

Sound Transmission and the Cochlea

Sound waves are transmitted through the ear and converted to electrical signals in the cochlea.

• Ossicles: Malleus, incus, and stapes; transmit vibrations from the tympanic membrane to the oval window.

• Cochlea: Contains three chambers: scala vestibuli, scala media, and scala tympani.

• Basilar membrane: Supports hair cells that detect sound vibrations.

• Hair cells: Convert mechanical vibrations into electrical impulses.

Brain Regions and Functions

Primary Functions of Brain Regions

Different regions of the brain have specialized functions.

• Hypothalamus: Regulates homeostasis, endocrine functions, and autonomic responses.

• Thalamus: Relay station for sensory information.

• Cerebellum: Coordinates movement and balance.

• Cerebrum: Responsible for higher cognitive functions.

• Brainstem: Controls basic life functions (breathing, heart rate).

Skeletal Muscle Structure and Contraction

Organization of Skeletal Muscle

Skeletal muscle is organized into myofibrils, sarcomeres, and myofilaments.

• Myofilaments: Actin (thin) and myosin (thick) filaments.

• Sarcomere: The functional unit of muscle contraction, defined by Z disks.

• Myofibril: A bundle of sarcomeres within a muscle fiber.

• Transverse tubules (T-tubules): Invaginations of the muscle membrane that transmit action potentials.

Contractile Proteins and Filament Organization

Contractile proteins interact to produce muscle contraction.

• Actin: Forms thin filaments; binds myosin during contraction.

• Myosin: Forms thick filaments; has heads that bind to actin.

• Cross-bridge: Connection formed between myosin head and actin filament.

• Power stroke: Movement of myosin head that pulls actin filament.

Excitation-Contraction Coupling

Muscle contraction is triggered by a sequence of molecular events.

• Action potential travels down T-tubules.

• Calcium ions ($Ca^{2+}$) are released from the sarcoplasmic reticulum.

• $Ca^{2+}$ binds to troponin, causing tropomyosin to move and expose binding sites on actin.

• Myosin heads bind to actin, forming cross-bridges.

• ATP hydrolysis powers the power stroke.

• $Ca^{2+}$ is removed by active transport back into the sarcoplasmic reticulum.

Role of DHP and Ryanodine Receptors

These receptors are essential for calcium release during muscle contraction.

• DHP receptor: Voltage sensor in T-tubules; triggers opening of ryanodine receptors.

• Ryanodine receptor: Calcium channel in the sarcoplasmic reticulum membrane.

Types of Skeletal Muscle Cells

Skeletal muscle fibers are classified based on their contraction speed and metabolic properties.

Type	Contraction Speed	Metabolism	Fatigue Resistance
Type I (Slow-twitch)	Slow	Oxidative	High
Type IIa (Fast-twitch, oxidative)	Fast	Oxidative/Glycolytic	Intermediate
Type IIb (Fast-twitch, glycolytic)	Fast	Glycolytic	Low

Example: Type I fibers are abundant in postural muscles; Type IIb fibers are common in muscles used for rapid, powerful movements.