Untitled

Sensory Physiology

Overview

  • Lecturer: Dr. R. Ahangari

  • Institution: University of Central Florida, Orlando

  • Textbook: Human Physiology by Linda S. Constanzo

Sensory Systems

  • Definition: Specialized epithelial cells or neurons that transduce environmental signals into neural signals.

  • Environmental Signals Detected:

    • Mechanical force

    • Light

    • Sounds

    • Chemicals

    • Temperature

Types of Sensory Transducers

  1. Mechanoreceptors: Respond to mechanical stimuli.

    • Examples:

      • Pacinian corpuscles

      • Joint receptors

      • Stretch receptors in muscle

      • Hair cells in auditory and vestibular systems

      • Baroreceptors in carotid sinus

  2. Photoreceptors:

    • Rods and cones of the retina

  3. Chemoreceptors: Respond to chemical changes.

    • Examples:

      • Olfactory receptors

      • Taste receptors

      • Osmoreceptors

      • Carotid body O₂ receptors

  4. Nociceptors: Respond to extremes of temperature and pain.

Fiber Types and Conduction Velocity

  • A-alpha fibers:

    • Large alpha-motoneurons

    • Conduction velocity: Fastest

  • A-beta fibers:

    • Responsible for touch and pressure

    • Conduction velocity: Medium

  • A-gamma fibers:

    • Gamma-motoneurons to muscle spindles (intrafusal fibers)

    • Conduction velocity: Medium

  • A-delta fibers:

    • Responsible for touch, pressure, temperature, and pain

    • Conduction velocity: Medium

  • B fibers:

    • Preganglionic autonomic fibers

    • Conduction velocity: Medium

  • C fibers:

    • Responsible for slow pain, postganglionic autonomic fibers

    • Conduction velocity: Slowest

Receptive Field

  • Definition: An area of the body that, when stimulated, changes the firing rate of a sensory neuron.

    • Excitatory Receptive Field: If firing rate is increased.

    • Inhibitory Receptive Field: If firing rate is decreased.

Steps in Sensory Transduction

  1. Stimulus Arrival: Stimulus (e.g., photon of light on the retina, a molecule of NaCl on the tongue) arrives at the sensory receptor.

  2. Ion Channel Opening: Ion channels are opened, allowing current to flow inward, resulting in depolarization of the receptor.

  3. Receptor Potential: The change in membrane potential due to the stimulus is termed the receptor potential, or generator potential.

Adaptation of Sensory Receptors

  1. Slowly Adapting (Tonic) Receptors:

    • Examples: Muscle spindle, pressure, slow pain

    • Function: Respond repetitively to a prolonged stimulus.

  2. Rapidly Adapting (Phasic) Receptors:

    • Examples: Pacinian corpuscle, light touch

    • Function: Show a decline in action potential frequency over time when exposed to a constant stimulus.

Sensory Pathways to the Cerebral Cortex

  1. Sensory Receptors:

    • Activated by environmental stimuli, can be specialized epithelial cells (taste receptors, auditory hair cell) or primary afferent neurons (olfactory chemoreceptors).

    • Transduce stimuli into electrical energy (receptor potential).

  2. First-Order Neurons:

    • Primary afferent neurons that receive transduced signals and transmit information to the CNS.

    • Cell bodies located in dorsal root or spinal cord ganglia.

  3. Second-Order Neurons:

    • Located in the spinal cord or brain stem.

    • Receive information from one or more primary afferent neurons in relay nuclei and transmit it to the thalamus.

    • Axons typically cross the midline in a relay nucleus before ascending to the thalamus (contralateral transmission).

  4. Third-Order Neurons:

    • Located in the relay nuclei of the thalamus.

    • Transmit encoded sensory information to the cerebral cortex.

  5. Fourth-Order Neurons:

    • Located in the specific sensory area of the cerebral cortex.

    • The information received results in conscious perception of the stimulus.

The Somatosensory System

  • Function: Processes information regarding touch, pain, and temperature.

  • Somatosensory Pathways:

    1. Dorsal column system

    2. Anterolateral system

Types of Somatosensory Receptors

  1. Mechanoreceptors: For touch

  2. Thermoreceptors: For temperature

  3. Nociceptors: For pain

Pathways in the Somatosensory System

  1. Dorsal Column System:

    • Processes sensations of fine touch, pressure, two-point discrimination, vibration.

    • Course:

      • Primary afferent neurons have cell bodies in the dorsal root.

      • Axons ascend ipsilaterally to the nucleus gracilis and nucleus cuneatus in the medulla.

      • From the medulla, second-order neurons cross midline and ascend to contralateral thalamus, synapsing on third-order neurons.

      • Third-order neurons ascend to somatosensory cortex, where they synapse on fourth-order neurons.

  2. Anterolateral System:

    • Processes sensations of temperature, pain, and light touch.

    • Course:

      • Composed primarily of a group of fibers that enter the spinal cord and terminate in the dorsal horn.

      • Second-order neurons cross midline to the anterolateral quadrant of the spinal cord and then to contralateral thalamus, synapsing on third-order neurons.

      • Third-order neurons ascend to somatosensory cortex, where they synapse on fourth-order neurons.

Somatotopic Arrangement

  • Information from different body parts is organized somatotopically.

  • Implication: Destruction of thalamic nuclei results in loss of sensation on the contralateral side of the body.

Pain

  • Definition: Associated with the detection and perception of noxious stimuli (nociception).

  • Receptors: Free nerve endings located in the skin, muscle, and viscera.

  • Neurotransmitter: Substance P is released by nociceptors; inhibition of its release is the basis for pain relief by opioids.

Fibers for Pain

  1. Fast Pain:

    • Carried by group A-delta fibers.

    • Characterized by rapid onset/off and precise localization.

  2. Slow Pain:

    • Carried by C-fibers.

    • Characterized as aching, burning, or throbbing, poorly localized.

Referred Pain

  • Definition: Pain originating from visceral sources is perceived at sites on the skin.

  • Dermatome Rule: Sites innervated by nerves from the same spinal cord segment exhibit this phenomenon.

Two-Point Touch Threshold

  • Concept: If each caliper point touches the receptive fields of different sensory neurons, two separate touch sensations will be experienced. If both points touch the receptive field of a single sensory neuron, only one touch sensation will be perceived.

Taste and Smell

  • Category: Gustation (taste) and Olfaction (smell) are forms of Chemoreception.

  • Function: Specialized cells act as receptors for specific chemical compounds, making these senses sensitive to the molecules in food and air.

Gustatory System

  • Mechanism: Taste buds transduce taste signals, conveyed by three main cranial nerves:

    1. Cranial Nerve VII (Facial Nerve): Carries taste sensations from the anterior two-thirds of the tongue and soft palate.

    2. Cranial Nerve IX (Glossopharyngeal Nerve): Carries taste sensations from the posterior one-third of the tongue.

    3. Vagus Nerve: Carries some taste sensations from the back of the oral cavity (pharynx and epiglottis).

  • Functionality: Dendritic endings of these nerves are located around taste buds; sensations of touch and temperature are also relayed.

  • Neural Pathway: Taste sensations are transmitted to the medulla oblongata, synapsing with second-order neurons that project to the thalamus; third-order neurons then project to the postcentral gyrus in the cerebral cortex responsible for tongue sensations.

Types of Taste

  1. Salt:

    • NaCl receptor; Na⁺ entry induces depolarization and subsequent neurotransmitter release.

  2. Sour:

    • Signals presence of acidic compounds (H⁺ ions in solution); involves simple ion channels allowing H⁺ entry.

  3. Bitter:

    • Involves G-protein coupled receptors (GPCRs) activating gustducin upon activation.

  4. Sweet:

    • Similar to bitter taste transduction involving GPCRs.

Disorders of the Tongue

  • Ageusia: Loss of taste due to damage to the facial nerve.

  • Hypogeusia: Decreased taste sensitivity.

  • Hypergeusia: Increased taste sensitivity.

  • Sore Tongue: Usually caused by trauma or irritation; may indicate underlying medical conditions such as diabetes or vitamin deficiencies.

  • Glossodynia: Characterized by a burning sensation on the tongue.

  • Benign Migratory Glossitis: Irregular inflamed patches on the tongue surface with no known cause (also known as geographic tongue).

Olfactory System

A. Receptor Cells: Located in the olfactory epithelium; true neurons conducting action potentials to the CNS.
B. Cranial Nerve I (Olfactory): Transmits information from olfactory receptor cells to the olfactory bulb.

  • Composed of unmyelinated C fibers; the smallest and slowest in the nervous system.

  • Innervated by CN V (trigeminal) detecting painful stimuli (e.g., ammonia).

  • Axons pass through the cribriform plate to the olfactory bulb; fractures can cause reduced (hyposmia) or eliminated (anosmia) sense of smell, preserving ammonia response due to CN V pathway.

Transduction in Olfactory Receptor Neurons

A. Odorant molecules bind to receptors on the cilia of olfactory receptor neurons.
B. Binding activates G proteins (G olf), leading to activation of adenylate cyclase.
C. Results in increased intracellular cAMP, opening Na⁺ channels and producing depolarization (receptor potential).
D. Depolarization generates and propagates action potentials.

Disorders of Olfaction

  • Anosmia: Lack of olfaction; loss of smell.

  • Phantosmia: Smelling odors that aren't present.

  • Dysosmia: When smells are perceived differently than they should be.

Anatomy of the Eye

  • The human eye is an elongated ball approximately 1 inch (2.5 cm) in diameter, protected by a bony socket in the skull.

  • Eye Layers: Comprise three layers or coats making up the eye's exterior:

    1. Sclera:

    • Outer white fibrous layer providing support and protection, maintaining the eye's shape.

    • The front part is transparent and called the cornea, which refracts light rays and serves as the outer window of the eye.