BIO 259 Exam 4 Study Guide: Nervous System, Special Senses, and Related Structures

Graded Potentials

Characteristics and Importance

Graded potentials are changes in membrane potential that vary in size, as opposed to the all-or-none nature of action potentials. They play a crucial role in initiating action potentials and in sensory processing.

• Definition: Graded potentials are local changes in membrane potential that occur in the dendrites and cell body of a neuron.

• Distinct from Action Potentials: Graded potentials can be depolarizing or hyperpolarizing, are not propagated over long distances, and their amplitude depends on the strength of the stimulus.

• Importance: Graded potentials are essential for triggering action potentials when they reach the axon hillock and exceed threshold.

• Sensory Receptors: Sensory receptors are specialized cells that respond to specific stimulus types (e.g., mechanoreceptors, thermoreceptors, photoreceptors).

• Stimulus Coding: Action potentials code for stimulus intensity by frequency (rate coding) and by recruiting more neurons (population coding).

• Example: A strong touch generates a larger graded potential, leading to a higher frequency of action potentials.

Spinal Nerves

Anatomy and Function

Spinal nerves are mixed nerves that connect the central nervous system (CNS) to the peripheral nervous system (PNS), transmitting sensory and motor information.

• CNS Structures: Brain and spinal cord.

• PNS Structures: Cranial nerves, spinal nerves, ganglia.

• Dorsal Roots: Carry sensory (afferent) information into the spinal cord.

• Ventral Roots: Carry motor (efferent) information out of the spinal cord.

• Mixing of Axons: Sensory and motor axons mix in the spinal nerve after leaving the spinal cord.

• Branching: Spinal nerves branch into dorsal and ventral rami; ventral rami form nerve plexuses that innervate limbs.

• Nerve Plexuses: Networks of nerves (e.g., brachial, lumbar plexus) that provide redundancy and control to limb muscles.

• Innervation Actions:

  • Radial nerve: Extends wrist and fingers.

  • Ulnar nerve: Flexes wrist and fingers.

  • Femoral nerve: Extends knee.

  • Obturator nerve: Adducts thigh.

  • Sciatic nerve: Flexes knee, extends hip.

• Neuromuscular Junction: Synapse between motor neuron and muscle fiber; site of end plate potential.

• End Plate Potential: Local depolarization at the neuromuscular junction, leading to muscle contraction.

• Efferent Synapses: Efferent nerves synapse with smooth muscle/glands via varicosities, releasing neurotransmitters.

Reflexes

Types and Mechanisms

Reflexes are rapid, automatic responses to stimuli, classified as intrinsic or learned, and as monosynaptic or polysynaptic.

• Intrinsic Reflex: Inborn, such as the knee-jerk reflex.

• Learned Reflex: Acquired through experience, such as driving.

• Modification: Intrinsic reflexes can be modified by higher brain centers.

• Monosynaptic Reflex Arc: Involves one synapse between sensory and motor neuron (e.g., stretch reflex).

• Polysynaptic Reflex Arc: Involves multiple synapses and interneurons (e.g., withdrawal reflex).

• Extrafusal vs. Intrafusal Fibers:

  • Extrafusal: Main contractile fibers of muscle.

  • Intrafusal: Specialized fibers within muscle spindles, detect stretch.

• Muscle Spindles: Provide information about muscle length and rate of change via action potentials to CNS.

• Stretch Reflex: Muscle spindle detects stretch, triggers contraction; reciprocal inhibition prevents antagonist contraction.

• Example: Knee-jerk reflex involves monosynaptic arc and reciprocal inhibition.

The Brain

Development, Structure, and Functions

The brain develops from primary and secondary vesicles, with distinct regions responsible for various functions.

• Primary Vesicles: Prosencephalon, Mesencephalon, Rhombencephalon.

• Secondary Vesicles: Telencephalon, Diencephalon, Mesencephalon, Metencephalon, Myelencephalon.

• Developmental Origins:

  • Cerebral hemispheres: Telencephalon

  • Thalamus: Diencephalon

  • Midbrain: Mesencephalon

  • Cerebellum: Metencephalon

  • Medulla oblongata: Myelencephalon

• Diencephalon Structure: Walls: thalamus; floor: hypothalamus; roof: epithalamus; center: third ventricle.

• Cerebrospinal Fluid (CSF): Found in ventricles and subarachnoid space; secreted by choroid plexus; functions include cushioning, nutrient delivery, and waste removal.

• Meninges: Dura mater, arachnoid mater, pia mater; surround brain and spinal cord, relate to CSF by enclosing subarachnoid space.

• Major Brain Regions:

  • Medulla oblongata: Controls autonomic functions (breathing, heart rate).

  • Cerebellum: Coordinates movement and balance.

  • Hypothalamus: Regulates homeostasis, endocrine functions.

  • Precentral gyrus: Primary motor cortex.

  • Postcentral gyrus: Primary somatosensory cortex.

Brain Systems

Reticular Formation, Limbic System, and Sleep

Brain systems integrate functions across multiple regions, influencing consciousness, emotion, and memory.

• Reticular Formation: Network in brainstem; regulates arousal and consciousness.

• Limbic System: Emotional processing; includes amygdala, hippocampus, hypothalamus, cingulate gyrus.

• Amygdala: Processes fear and aggression.

• Hypothalamus: Controls autonomic and endocrine functions.

• Psychosomatic Illness: Limbic system links emotions to physical symptoms.

• Hippocampus: Memory formation and spatial navigation.

• Sleep Types: REM (rapid eye movement) and non-REM (deep sleep).

• REM Cycle: Most like being awake; vivid dreams, muscle paralysis.

• Body Changes: REM: increased brain activity, decreased muscle tone; deep sleep: restorative, low brain activity.

Hair Cells and Hearing

Anatomy and Physiology of Hearing

Hair cells are sensory receptors in the ear that convert mechanical stimuli into electrical signals, enabling hearing.

• Hair Cell: Specialized cell with stereocilia; opens mechanically-gated ion channels when bent, generating action potentials.

• Ear Anatomy:

  • Outer ear: Collects sound.

  • Middle ear: Ossicles (malleus, incus, stapes) transmit vibrations.

  • Inner ear: Cochlea, vestibule, semicircular canals; cochlea for hearing.

• Ossicles: Connect tympanic membrane to oval window; amplify sound.

• Inner Ear Structures: Cochlea (hearing), vestibule and semicircular canals (balance).

• Basilar Membrane: Vibrates at specific locations for different pitches; hair cells detect vibration.

• Pitch and Volume: Pitch determined by location of basilar membrane vibration; volume by amplitude of vibration.

Balance

Inner Ear and Equilibrium

Balance is maintained by integrating sensory input from the inner ear, vision, and proprioception.

• Inputs: Vestibular apparatus, visual system, proprioceptors.

• Inner Ear Anatomy: Vestibule, semicircular canals, cochlea.

• Vestibule Structures: Utricle and saccule; detect linear acceleration.

• Semicircular Canals: Detect rotational (angular) acceleration.

• Maculae: Sensory regions in utricle and saccule; oriented horizontally and vertically; detect head position.

• Hair Cells, Otoliths, Otolith Membrane: Hair cells embedded in otolith membrane; otoliths (calcium carbonate crystals) shift with movement, bending hair cells.

• Spinning Acceleration: Detected by movement of endolymph in semicircular canals, bending hair cells in ampullae.

Vision

Anatomy and Physiology of the Eye

The eye is a complex organ with multiple layers and specialized cells for detecting light and color.

• Three Layers: Fibrous (sclera, cornea), vascular (choroid, ciliary body, iris), inner (retina).

• Fibrous Layer: Dense connective tissue; provides protection and shape.

• Vascular Layer:

  • Choroid: Supplies blood.

  • Ciliary body: Controls lens shape.

  • Iris: Regulates pupil size.

• Inner Layer:

  • Retina: Contains photoreceptors (rods and cones).

  • Macula lutea: Area of high visual acuity.

  • Fovea: Center of macula; highest acuity.

  • Optic disk: Blind spot; where optic nerve exits.

• Light Path: Cornea → aqueous humor → lens → vitreous humor → retina.

• Light Modification: Cornea and lens refract light; pupil regulates amount.

• Rods vs. Cones:

  • Rods: Sensitive to dim light, no color.

  • Cones: Sensitive to bright light, color vision.

• Visual Structures: Macula lutea (sharp vision), fovea (sharpest vision), optic disk (blind spot).

Structure	Function	Location
Radial nerve	Extends wrist/fingers	Upper limb
Ulnar nerve	Flexes wrist/fingers	Upper limb
Femoral nerve	Extends knee	Lower limb
Obturator nerve	Adducts thigh	Lower limb
Sciatic nerve	Flexes knee, extends hip	Lower limb

Additional info: Academic context and explanations have been added to ensure completeness and clarity for exam preparation.