Health Science 3020: Human Physiology - Peripheral Nervous System and Receptor Physiology

Health Science 3020: Human Physiology - Peripheral Nervous System and Receptor Physiology

Introduction to Receptor Physiology

  • Definition: Receptors are part of the peripheral nervous system and are critical in perceiving and receiving information about the external environment, influencing both conscious and subconscious processes.
  • Conscious Perception: Examples include pain, proprioception (awareness of body position), and special senses such as vision, hearing, taste, and touch.
  • Subconscious Processes: Examples include gastric motility and visceral nervous system signals related to bodily states like satiety.

Structure and Functional Classes of Neurons

  • Peripheral Nervous System (PNS): Contains both afferent (sensory) and efferent (motor) neurons.
  • Afferent Neurons: Carry sensory information to the central nervous system and make synapses in the brain or spinal cord with interneurons.
    • Types of Interneurons:
    • Inhibitory GABAergic interneurons (short and inhibitory).
    • Non-inhibitory interneurons connecting to other networks of neurons.
  • Efferent Neurons: Their axons are located in the PNS even if their cell bodies are in the CNS.
    • Examples:
    • Alpha motor neurons originate in the spinal cord and extend to skeletal muscle.
    • The sympathetic nervous system connects spinal cord neurons to sympathetic postganglionic neurons in the PNS.

Key Concepts of Afferent and Efferent Neurons

  • Afferents:
    • Sensory neurons bringing information inward (e.g., touch sensation).
    • Activated by stimuli such as heat, pain, or mechanoreception.
  • Efferents:
    • Motor neurons projecting outward to organs/muscles.
    • Can be somatic (voluntary muscle control) or autonomic (involuntary bodily functions like blood pressure regulation).

Necessary Components for Sensory Perception

  1. Stimulus: Examples include heat, mechanoreceptors (pressure), or other specific stimuli.
  2. Detector: Receptors, notably thermoreceptors for temperature detection, which are protein structures.

Metabolism and Receptor Activity

  • Receptors:
    • Comprised of proteins made in the cell body of sensory neurons.
    • Synthesized in ribosomes and processed in the Golgi apparatus before being transported along microtubules to axon terminals.
  • Energy Use: The process uses metabolic energy for protein synthesis and transmission of signals.

Sensory Reception and Signal Transmission

  • Sensory Reception:
    • Activation of sensory receptors leads to the generation of sensory potentials (the initial electrical signal).
  • Adequate Stimulus: The specific stimulus-matching receptor combination necessary for effective signal generation.

Organization of the Nervous System

  • Divisions Under Review:
    • Afferent (Sensory) Division:
    • Sensory stimulation processes and visceral signals.
    • Following Topics:
    • Special senses and efferent division.

Role of Receptors in Physiology

  • Action Potentials: Critical in transmitting information from sensory receptors to the CNS.
  • Receptor Potentials: Created through the activation of receptors in response to adequate stimuli.
  • Encoding Information:
    • The intensity of stimuli correlates with the frequency of action potentials, not their amplitude.
  • Sensory Fibers:
    • Afferent neurons originating from skin or internal tissues lead to CNS with sensory electrical potentials.

Types of Receptors

  1. Photoreceptors: Detect visible light.
  2. Mechanoreceptors: Respond to mechanical stimuli (e.g., touch, pressure, vibration).
  3. Thermoreceptors: Sense changes in temperature.
  4. Osmoreceptors: Detect osmolarity (concentration of solutes within a solution).
  5. Chemoreceptors: Specialized to detect specific chemicals (e.g., oxygen, carbon dioxide).
  6. Nociceptors: Pain receptors that detect harmful stimuli.

Sensory Transduction

  • Definition: The conversion of stimulus information into an electrical signal (sensory potential) that can form action potentials.
  • Receptor Potentials: Graded responses influenced by stimulus strength; larger stimuli lead to larger receptor potentials—distinct from all-or-none action potentials.

Types of Sensory Receptor Responses

  • Specialized Nerve Ending Receptors: Directly respond to stimuli; e.g., mechanoreceptors open sodium channels upon mechanical disturbance.
  • Specialized Cells: Release neurotransmitters that modulate sensory neuron excitation in response to stimuli.
  • Action Potential Threshold: A sufficient receptor potential leads to activation of voltage-gated channels, firing action potentials.

Intensity of Stimulus and Action Potential Generation

  • Encoding Intensity: Increased stimulus intensity results in a higher frequency of action potentials, not their size.
  • Phasic vs. Tonic Receptors:
    • Phasic (rapidly adapting): Respond to changes in stimulus effectively but not continuously.
    • Tonic (slowly adapting): Maintain a constant response to a continuous stimulus.

Conclusion and Key Points for Future Discussion

  • Understanding of the relationship between stimulus intensity, receptor potential, and action potential frequency is crucial.
  • Next lectures will cover specific tactile and visceral afferent fibers to further comprehend their physiological roles and mechanisms.