ch 10 Sensory Physiology Chapter

Chapter 10: Sensory Physiology

10.1 General Properties of Sensory Systems

• The afferent division of the nervous system is divided into:

  • Special senses: Have specialized sense organs (e.g., vision, hearing).
  • General (somatic) senses: Felt throughout the body (e.g., touch, pain).

• Signal Transduction: All sensory receptors utilize signal transduction, which is the process that converts the energy of a stimulus into a neural signal suitable for transmission to the central nervous system (CNS).

• Neural Receptors:

  • Neural receptors are actual neurons that function as sensory receptors. They use their dendrites to detect stimuli, creating action potentials within the neuron.
  • Examples:
  • Free nerve endings (detect pain, temperature).
  • Olfactory sensory neurons (detect smell).

• Nonneural Receptors:

  • Any other type of cell functioning as sensory receptors, which synapse with sensory neurons to transmit their signals to the CNS.
  • Examples:
  • Hair cells in the inner ear (hearing).
  • Gustatory epithelial cells (taste).

• An adequate stimulus is the type of stimulus that a receptor is most responsive to:

  • Examples:
  • Temperature for thermoreceptors.
  • Light for photoreceptors.

• Alternate Stimuli Response: Many sensory receptors also respond to alternative stimuli; however, regardless of the stimulus type, the sensation is perceived uniformly.

  • Examples:
  • Menthol binds to cold thermoreceptors causing depolarization.
  • Strong physical impacts to the head can activate retinal neurons causing visual perception (“seeing stars”).

• Thresholds: A weak stimulus may not activate a receptor, while those that exceed the threshold trigger sensations. The threshold is defined as the minimal stimulus required for activation.

• Graded Potentials and Action Potentials:

  • Stimuli create graded potentials in neural receptors that may trigger action potentials.
  • Non-neural receptors release neurotransmitters creating graded potentials in sensory neurons, potentially triggering action potentials.

• Integration in CNS:

  • Sensory signals processed in the brain do not all create perceptions.
  • The brain filters out non-critical sensations (e.g., background noise) before they reach the cerebrum; sensations that do not reach the cerebral cortex cannot create perceptions.
  • Examples: Visceral sensations like blood pressure do not reach the cerebral cortex.

• Receptive Fields: Each sensory receptor possesses a receptive field, which is the specific area where stimuli can activate the receptor.

  • The ability to localize stimuli depends on the size of the receptive field (smaller fields are more precise).

• Neuronal Convergence: In general sensory pathways:

  • General sensory information travels from first-order to second-order, and then to third-order neurons before reaching the cerebral cortex.

  • Convergence occurs when multiple first-order neurons synapse on a single second-order neuron, creating larger receptive fields.

  • Example: Two sub-threshold stimuli can sum to create an action potential at the second-order neuron.

• Lateral Inhibition: When two nearby receptive fields are stimulated, stronger stimuli can inhibit weaker ones, enhancing the localization of sensation.

  • Mechanism: Inhibitory axon collaterals release inhibitory signals to less stimulated receptors.

• Action Potentials and Sensory Identity: All action potentials are uniform, and the brain determines the type of sensation based on the type of receptor that generated the signal.

  • Examples:
  • Signals from nociceptors are perceived as pain.
  • Tactile input from the index finger is perceived as touch due to the location of receptors.

• Sensation Strength: The brain assesses sensation intensity based on the frequency of action potentials. Strong stimuli cause higher frequencies, leading to increased neurotransmitter release and heightened perception.

10.2 Somatic Senses

• General Senses (somatic senses): Include temperature, pain, touch, pressure, vibration, and proprioception (body position).

• General sensory receptors are distributed in the skin and internal organs.

  • First-order neurons originate in the dorsal root ganglia, sending signals to second-order neurons in the brainstem/spinal cord, which then communicate with third-order neurons in the thalamus, finally relaying information to the cerebral cortex.

• Sensory Pathways:

  • Fine touch, vibration, and proprioception pathways cross midline in the medulla.
  • Pain and temperature sensations cross midline in the spinal cord.

• Discriminative Sensation: Sensations perceived in the primary somatic sensory cortex and sensory pathways sync in the thalamus.

10.3 Chemoreception: Smell and Taste

• Olfaction: The sense of smell detects airborne chemicals that interact with mucus in the nasal cavity.

• Olfactory Sensory Neurons: Neural receptors utilize receptor proteins to identify combinations of odorant molecules, leading the brain to perceive distinct smells.

• Neural Pathways and Emotional Response: Olfactory pathways activate the amygdala and hypothalamus (limbic system), establishing an emotional-link to smells.

• Gustation: The sense of taste detects chemical stimuli dissolved in saliva, utilizing gustatory epithelial cells found in taste buds on the tongue.

• Types of Gustatory Epithelial Cells:

  • Type I: Support cells (detect salty via Na+).
  • Type II: Taste receptor cells (detect sweet, bitter, umami).
  • Type III: Presynaptic cells (detect sour via H+).

• Basic Taste Sensations:

  • Sweet, salty, sour, bitter, umami (savory).
  • Taste receptors are highly sensitive to unfavorable tastes (e.g., acid, toxins).

• Taste Signal Transduction: Ligands activate taste cells, leading to specific ion channel opening and the release of neurotransmitters, enabling action potentials to be sent to the brain.

10.4 The Ear: Hearing

• Hearing: The perception of pressure waves in the air, interpreted by the brain as sound.

• Cochlear Anatomy:

  • Contains three parallel, fluid-filled channels: scala vestibuli, scala tympani, and cochlear duct.
  • The spiral organ (organ of Corti) on the basilar membrane has hair cells (nonneural receptors) for sound detection.

• Sound Wave Transmission: Vibrations from sound waves transduced by tympanic membrane are transferred through the auditory ossicles to the cochlea, leading to fluid wave initiation within the cochlear duct.

• Stereocilia and Signal Generation: Movement of the fluid causes stereocilia on hair cells to bend, altering ion channel state, subsequently leading to neurotransmitter release and action potential creation in auditory neurons.

• Cochlear Adaptation: Hair cells serve as tonic receptors, continuously signaling at low sound levels. The degree of stereocilia movement influences the strength of the produced signal.

10.5 The Ear: Equilibrium

• Equilibrium: The sense responsible for maintaining body balance, sensed using hair cells located in the vestibular complex of the inner ear.

• Vestibular Complex Function: The interconnected, fluid-filled chambers (semicircular canals, saccule, utricle) provide information about head movement and position.

• Cerebellar Integration: Most equilibrium sensations are sent to the cerebellum, with a minimal amount reaching the cerebral cortex for conscious perception.

10.6 The Eye and Vision

• Vision: The ability to see relies on nonneural sensory receptors (photoreceptors) that detect light and formulate visual images.

• Three Steps to Vision:

  1. Light enters through the cornea and pupil and is focused on the retina.
  2. Photoreceptors detect light and generate neural signals.
  3. Visual signals are processed by neural pathways in the retina and visual cortex for perception.

• Lens Adaptation: The lens refracts light to ensure the focal point is accurately aligned with the macula of the retina. Through accommodation, the lens adjusts to focus on near or distant objects.

• Refractive Errors: Conditions such as hyperopia, myopia, and astigmatism can lead to blurry vision, usually resolved through corrective lenses.

• Phototransduction: In rods and cones, the binding of light to opsins triggers the closure of cation channels, which leads to hyperpolarization and reduced neurotransmitter release, signaling light detection to the brain.

• Visual Processing: Signals processed in the retina utilize converging pathways to enhance contrast and detail in vision. This involves multiple photoreceptors synapsing with bipolar and ganglion cells to create receptive fields.

  • Types of Ganglion Cell Responses: On-center/off-surround and off-center/on-surround ganglion cells modulate signal processing, contributing to contrast detection and overall visual acuity.