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Sensory Pathways & Receptors (Lecture Notes)

Sensory Pathways: Afferent and Efferent

Sensory pathways begin with neurons that relay sensory information from sensory receptors to the central nervous system. Afferent pathways carry information into the CNS: somatic information such as touch, pressure, and temperature is projected to the cerebral cortex, while visceral information like pressure and pain from internal organs is projected to the brainstem. Efferent pathways carry information out of the CNS and control skeletal muscles, enabling motor responses.

Sensory Receptors, Receptors and Transduction

Sensory receptors are specialized cells or neuronal processes that monitor specific conditions and generate action potentials when stimulated. The resulting action potentials propagate along sensory pathways toward the CNS. Receptor specificity means that each receptor has characteristic sensitivity to particular stimuli, and the receptive field is the area monitored by a single receptor. A larger receptive field makes localizing the stimulus more difficult and can be assessed using the two-point touch threshold. A sensory unit comprises the sensory neuron and all of its receptors. The receptor potential is the change in the receptor's membrane potential produced by stimulation; receptors always convert a stimulus into an action potential, and the magnitude of the receptor potential depends on stimulus strength. A depolarizing stimulus brings the membrane potential closer to threshold, whereas a hyperpolarizing stimulus moves it farther from threshold.

Sensation and Perception

Sensation is the arrival of sensory information to the CNS, while perception is the conscious awareness of that sensation and the assignment of meaning or interpretation to it.

General Senses vs Special Senses

General senses include temperature, pain, touch, pressure, vibration, and proprioception (body position). Special senses include olfaction (smell), gustation (taste), vision, equilibrium (balance), and hearing.

Receptive Fields and Sensory Units

Each sensory receptor monitors a specific area, and the size of the receptive field affects localization accuracy. The sensory unit consists of a sensory neuron along with all the receptors it services, and its output reflects the integrated activity from that receptor ensemble. The combination of receptor specificity, receptive fields, and the sensory unit defines how a stimulus is detected and localized.

Receptor Potential and Transduction

When stimulated, receptors change their membrane potential, creating a receptor potential. Receptors convert the stimulus into an action potential, and the size of the receptor potential scales with stimulus strength. A depolarizing stimulus moves the membrane potential toward threshold, increasing the likelihood of firing, while a hyperpolarizing stimulus moves it away from threshold and reduces firing probability.

Sensory Adaptation and Receptor Dynamics

Sensory adaptation is the loss of responsiveness of a sensory receptor in the presence of a constant stimulus. This allows the CNS to focus on novel or changing stimuli rather than constant background input.

Phasic (Fast-Adapting) vs Tonic (Slow-Adapting) Receptors

Phasic receptors are normally inactive but produce a burst of activity when a stimulus is first applied; they quickly adapt and reduce their response, generating action potentials primarily in response to changes in the stimulus. Tonic receptors remain active with a relatively high firing rate as long as the stimulus is applied; their action potential output changes only when the stimulus itself changes.

Sensory Receptor Classification by Location

Sensory receptors can be classified by the location of the stimulus they monitor: exteroceptors provide information about the external environment; proprioceptors report on the position and movement of skeletal muscles and joints; interoceptors provide information about the visceral organs and internal functions.

Sensory Receptor Classification by Nature of Stimulus

  • Thermoreceptors respond to changes in temperature and share pathways with pain sensations; they are found in the dermis, skeletal muscles, liver, and hypothalamus; they are typically phasic, responding most to change.

  • Chemoreceptors detect chemicals dissolved in body fluids (for example, changes in pH) and include the carotid and aortic bodies.

  • Mechanoreceptors respond to mechanical forces via mechanically gated ion channels and include tactile receptors, baroreceptors, and proprioceptors. Tactile receptors detect fine and crude touch and include free nerve endings, root hair plexus, tactile discs, bulbous corpuscles, lamellar corpuscles, and tactile corpuscles. Baroreceptors detect pressure changes in blood vessels and in parts of the GI, respiratory, and urinary tracts, often in elastic tissues within the walls of distensible organs.

  • Proprioceptors monitor the position of joints and skeletal muscles and include muscle spindles (skeletal muscle length and stretch), the Golgi tendon organ (tension at the muscle-tendon junction), and receptors in joint capsules.

  • Nociceptors signal tissue damage or injury and typically have large receptive fields. They are found in superficial skin, joint capsules, periosteum of bone, and vessel walls. Pain from nociception can be fast or slow, acute or chronic, and can be visceral (referred) in some cases.

Proprioceptors and Mechanoreception in Movement

Proprioceptors, including muscle spindles and Golgi tendon organs, provide essential feedback for spatial orientation and motor control. Muscle spindles detect skeletal muscle length and participate in the stretch reflex, while the Golgi tendon organ monitors muscle-tendon tension and helps modulate force production. Receptors in joint capsules contribute additional information about joint position and movement, enabling smooth and coordinated activity.

Nociceptors and Pain Modalities

Nociceptors are specialized for detecting tissue damage and injury. They produce large receptive fields for broad localization of harmful stimuli. Pain can be categorized by speed (fast vs slow) and duration (acute vs chronic) and may be referred from visceral sources, highlighting the complex sensory mapping in the CNS.

Sensory Homunculus and Motor Homunculus

The sensory homunculus is a functional map of the primary somatosensory cortex, with the area devoted to each body region proportional to the density of sensory neurons in that region, meaning highly sensitive areas (like fingertips) occupy disproportionately large cortical areas. The motor homunculus is a functional map of the primary motor cortex, where the size of each region corresponds to the degree of fine motor control available for that body part.

Sensory Pathways & SNS

Across these systems, sensory pathways integrate with broader nervous system function, aligning sensory input with autonomic and somatic outputs as part of the body’s coordinated response to the environment.