Peripheral Nervous System and Sensation

Cranial Nerves: Functions and Damage

The cranial nerves are twelve pairs of nerves that emerge directly from the brain and are responsible for sensory and motor functions of the head and neck.

• Functions: Include sensory input (e.g., smell, vision, taste), motor control (e.g., eye movement, facial expression), and autonomic regulation (e.g., heart rate).

• Damage: Can result in loss of sensation, muscle weakness, or dysfunction of target organs.

• Example: Damage to the optic nerve causes vision loss; damage to the facial nerve causes facial paralysis.

Phrenic Nerve

The phrenic nerve arises from the cervical spinal nerves (C3-C5) and innervates the diaphragm, essential for breathing.

• Function: Controls diaphragm contraction for respiration.

• Damage: Leads to respiratory distress or paralysis.

Reflexes: Inborn vs. Acquired

Reflexes are rapid, automatic responses to stimuli. They can be classified as:

• Inborn (Intrinsic) Reflexes: Present at birth, such as the patellar (knee-jerk) reflex.

• Acquired (Learned) Reflexes: Developed through experience, such as driving or playing an instrument.

General Senses vs. Special Senses

Sensory systems are divided into general and special senses:

• General Senses: Include touch, pain, temperature, pressure, and proprioception; receptors are distributed throughout the body.

• Special Senses: Include vision, hearing, equilibrium, taste, and smell; receptors are localized in specific organs.

Detection of Stimuli and Receptor Specificity

Sensory receptors detect specific types of stimuli and convert them into electrical signals.

• Receptor Specificity: Each receptor responds to a particular stimulus type (e.g., photoreceptors for light).

Simple Receptors for General Senses

General sense receptors are classified by structure and function:

• Free Nerve Endings: Detect pain and temperature.

• Encapsulated Receptors: Detect pressure and vibration.

• Proprioceptors: Monitor body position and movement.

• Nociceptors: Detect pain.

• Thermoreceptors: Detect temperature changes.

• Mechanoreceptors: Detect mechanical forces (touch, pressure).

Receptive Fields

A receptive field is the area monitored by a single sensory neuron. Smaller fields allow for greater discrimination.

• Example: Fingertips have small receptive fields for fine touch.

Tonic vs. Phasic Receptors

Receptors adapt to stimuli at different rates:

• Tonic (Slow Adapting): Nociceptors and proprioceptors; maintain response as long as stimulus is present.

• Phasic (Rapidly Adapting): Mechanoreceptors and chemoreceptors; respond quickly but adapt rapidly.

Peripheral vs. Central Adaptation

Adaptation reduces sensitivity to a constant stimulus.

• Peripheral Adaptation: Occurs at the receptor level.

• Central Adaptation: Occurs in the CNS, ignoring incoming signals.

Higher Order Functions Overview

Higher order functions involve integration and interpretation of sensory information.

• Sensory Receptors: Include chemoreceptors, thermoreceptors, mechanoreceptors, and photoreceptors.

• Transduction: Conversion of stimulus into an electrical signal (graded potential).

• Sensory Perception: Conscious awareness and interpretation of sensory input.

Autonomic Nervous System (ANS)

Sympathetic Division Functions

The sympathetic division prepares the body for 'fight or flight' responses.

• Preganglionic vs. Postganglionic Fibers: Preganglionic fibers are short; postganglionic fibers are long.

• Adrenergic Receptors: Alpha and Beta receptors respond to norepinephrine and epinephrine; effects can be stimulatory or inhibitory.

Parasympathetic Division Functions

The parasympathetic division promotes 'rest and digest' activities.

• Preganglionic vs. Postganglionic Fibers: Preganglionic fibers are long; postganglionic fibers are short.

• Cholinergic Receptors: Muscarinic and nicotinic receptors respond to acetylcholine; effects can be stimulatory or inhibitory.

Muscle Tissue

Functions of Skeletal Muscle

Skeletal muscle performs essential functions for movement and homeostasis.

• Movement

• Maintenance of posture and position

• Support of soft tissues

• Forming sphincters

• Maintenance of body temperature

• Storage of nutrient reserves

Organization of Skeletal Muscle Connective Tissues

Connective tissue layers organize and protect muscle fibers.

• Epimysium: Surrounds entire muscle; dense irregular connective tissue.

• Perimysium: Surrounds fascicles; dense irregular connective tissue.

• Endomysium: Surrounds individual muscle fibers; areolar connective tissue.

• Myosatellite Cells: Stem cells for muscle repair.

• Tendon/Aponeurosis: Connect muscle to bone or other tissues.

Passage of Blood Vessels & Nerves

Blood vessels and nerves penetrate connective tissue layers to supply muscle fibers.

Features of Skeletal Muscle Fibers

• Large cell size

• Multinucleate

Sarcolemma and Transverse Tubules

The sarcolemma is the muscle cell membrane; transverse (T) tubules conduct action potentials into the cell.

Myofibrils, Filaments, and Sarcoplasmic Reticulum

• Myofibrils: Bundles of contractile proteins.

• Thin Filaments: Actin, troponin, tropomyosin, nebulin, titin.

• Thick Filaments: Myosin molecules, cross-bridges.

• Sarcoplasmic Reticulum: Stores and releases calcium ions.

• Terminal Cisternae and Triad: Structures for calcium storage and release.

Sarcomeres: Structure and Components

The sarcomere is the functional unit of muscle contraction.

• A Band: Contains thick filaments; includes M line and H band.

• Zone of Overlap: Area where thick and thin filaments overlap.

• I Band: Contains only thin filaments; includes Z line/disc.

• Z Line/Disc: Anchors thin filaments.

• M Line: Center of A band; anchors thick filaments.

Sliding Filament Theory and Muscle Contraction

Muscle contraction occurs as thin filaments slide past thick filaments, shortening the sarcomere.

• Tension: Force produced by muscle contraction.

• Load/Resistance: Opposing force to contraction.

• Compression: Pushing force.

Regulation of Muscle Contraction

• Neural control

• Calcium release

• Thick-thin filament interaction

• Muscle fiber contraction

• Tension production

Neuromuscular Junction

The neuromuscular junction is the synapse between a motor neuron and a muscle fiber.

• Components: Axon terminal, synaptic cleft, motor end plate.

Excitation-Contraction Coupling

Process linking action potential in the sarcolemma to contraction.

• Action potential triggers calcium release from SR.

• Calcium binds to troponin, exposing active sites on actin.

Contraction Cycle

• Active-site exposure

• Cross-bridge formation

• Myosin-head pivoting

• Cross-bridge detachment

• Myosin reactivation

Relaxation

• Active transport of Ca2+ into SR

• Detachment of cross-bridges

Tension Production by Muscle Fibers

• Depends on number of pivoting cross-bridges

• Length-tension relationships

• Frequency of stimulation

• Twitch, treppe, wave summation, incomplete and complete tetanus

Tension Production by Motor Units

• Motor Unit: A motor neuron and all muscle fibers it innervates.

• Recruitment: Activation of additional motor units to increase tension.

• Peak Tension: Maximum force produced.

• Asynchronous Motor Unit Summation: Motor units alternate to sustain contraction.

Muscle Tone

Muscle tone is the constant, passive partial contraction of muscles.

Types of Muscle Contraction

• Isotonic Contraction: Muscle changes length; includes concentric (shortening) and eccentric (lengthening).

• Isometric Contraction: Muscle length remains constant; tension increases.

ATP and Muscle Metabolism

• ATP is required for contraction and relaxation.

• Creatine Phosphate: Stores energy for rapid ATP regeneration.

• Creatine Kinase: Enzyme catalyzing ATP regeneration.

• Aerobic Metabolism: Uses oxygen; TCA/citric acid cycle.

• Anaerobic Metabolism: Glycolysis; produces lactic acid.

Muscle Fatigue

• Depletion of reserves

• Damage to sarcolemma & SR

• Decline in pH, acidosis, reduced Ca2+

• Central fatigue (sense of weariness)

• Recovery period: lactic acid removal, oxygen debt (EPOC), heat production

Hormones and Muscle Metabolism

• Growth hormone, testosterone, thyroid hormones, epinephrine

Types of Skeletal Muscle Fibers

• Fast Fibers (White Muscle): Rapid contraction, fatigue quickly.

• Slow Fibers (Red Muscle): Slow contraction, resistant to fatigue.

• Intermediate Fibers: Characteristics between fast and slow.

Muscle Hypertrophy vs. Atrophy

• Hypertrophy: Increase in muscle size.

• Atrophy: Decrease in muscle size.

Physical Conditioning

• Anaerobic endurance

• Aerobic endurance

• Improvement by changes in muscle fiber characteristics and cardiovascular performance

Muscular System

Levers and Muscle Actions

Muscles act on bones as levers to produce movement.

• Purpose of Levers: Amplify force, speed, or range of motion.

• Types of Levers:

  • 1st Class: Fulcrum between effort and load (e.g., head nodding).

  • 2nd Class: Load between fulcrum and effort (e.g., standing on toes).

  • 3rd Class: Effort between fulcrum and load (most body levers).

Group Actions of Muscles

• Agonist (Prime Mover): Main muscle causing movement.

• Antagonist: Opposes the agonist.

• Synergist: Assists the agonist.

• Fixator: Stabilizes the origin.

Special Senses

Olfaction (Smell)

• Olfactory Epithelium: Located in nasal cavity; contains olfactory receptor cells.

• Physiology: Odorants bind to receptors, triggering action potentials.

• Neural Pathways: Olfactory nerves → olfactory bulb → olfactory tract → brain.

Gustation (Taste)

• Taste Buds: Located on tongue, contain gustatory cells.

• Primary Taste Sensations: Sweet, sour, salty, bitter, umami, water.

• Neural Pathways: Cranial nerves VII, IX, X → brain.

Vision

• Tunics of the Eye:

  • Fibrous (cornea, sclera)

  • Vascular (iris, ciliary body)

  • Neural (retina)

• Lens: Focuses light; accommodation adjusts shape.

• Visual Physiology: Rods (dim light), cones (color vision).

• Visual Pathway: Retina → optic nerve → optic chiasm → optic tract → brain.

Equilibrium and Hearing

• Hearing Process: Sound waves → hair cells → action potentials.

• Inner Ear Receptors: Semicircular canals (rotation), otolith organs (gravity/acceleration).

• Neural Pathways: Cochlear and vestibular branches of CN VIII → brain.

Tables

Types of Sensory Receptors

Types of Sensory Receptors by Stimulus

Types of Skeletal Muscle Fibers

Key Equations

ATP Regeneration

Creatine phosphate reaction:

Muscle Contraction Force

Force generated depends on cross-bridge formation:

Length-Tension Relationship

Optimal muscle length produces maximal tension:

Metabolic Pathways

Aerobic metabolism (simplified):

Anaerobic glycolysis:

Additional info:

• Some details inferred from standard Anatomy & Physiology textbooks to provide context and completeness.

• Tables and equations expanded for clarity and exam preparation.