Sensory Receptors Overview

Sensory Receptors

• Receptors detect stimuli, such as pain and temperature, and are widely distributed, especially in the skin
• Complex neural receptors are encapsulated and structurally intricate
• They detect more nuanced sensory information, including touch, pressure, and vibration with enhanced sensitivity and specificity

Receptor Classification

• Sensory receptors can be classified based on their structure: free nerve endings, specific histological structures, and special sense receptors
• They can also be classified based on the type of stimulus they detect: mechanoreceptors, thermoreceptors, photoreceptors, nociceptors, and chemoreceptors

Stimulus Detection

• Sensory receptors convert stimuli into receptor potentials, which can trigger an action potential if they reach threshold
• Generator potentials are graded potentials produced by sensory receptors, and their amplitude is proportional to the stimulus strength

Mechanoreceptors

• Specialized sensory receptors that detect mechanical changes, such as stretch, compression, or vibration
• Mechanoreceptors contain ion channels that open in response to physical deformation of the cell membrane
• They are located in the skin, muscles, and other tissues, and are essential for sensations of fine touch, vibration, and proprioception

Sensory Transduction

• Sensory receptors transduce stimuli into propagated electrical signals
• The process involves conversion of stimuli into receptor potentials, which, if reaching threshold, trigger action potentials

Sensory Pathways

• Specific pathways are designed for precision and undergo limited convergence to maintain accuracy
• Non-specific pathways are broader, carry multiple types of sensory information, and undergo both convergence and divergence

Sensory Processing

• Integration centers in the brain process sensory information
• The thalamus plays a crucial role in relaying sensory information to the cortex
• Olfactory pathways project directly to the cortex, while equilibrium pathways project to the cerebellum and the cortex via the thalamus
• All other sensory pathways pass through the thalamus before reaching their respective cortical areas
• Cortical association areas integrate sensory information across modalities for complex perception

Sensory Pathways Projection to the Brain

• The eye's retina projects to the primary visual cortex in the occipital lobe via the optic nerve and optic tract
• The ear's cochlea projects to the primary auditory cortex in the temporal lobe via the auditory nerve
• The skin projects to the primary somatosensory cortex in the parietal lobe via the dorsal columns and spinothalamic tract
• The nose's olfactory epithelium projects to the olfactory cortex in the temporal lobe via the olfactory nerve
• The tongue's taste buds project to the primary gustatory cortex in the insular cortex via the facial, glossopharyngeal, and vagus nerves

Primary and Secondary Sensory Processing

• Each sensory system features primary processing areas where basic information is processed and secondary processing areas where more complex features are extracted
• The primary visual cortex (V1) processes basic visual information, while secondary visual areas V2, V3, V4, and V5/MT process color, motion, depth, and complex shapes
• The primary auditory cortex processes basic auditory information, while belt and parabelt regions process complex sound processing and sound localization
• The olfactory bulb detects initial odor, while the piriform cortex handles odor recognition and association with experiences
• The primary gustatory cortex processes basic tastes, while secondary gustatory areas process flavor combinations, taste intensity, and preference
• The primary somatosensory cortex processes basic tactile information, while the secondary somatosensory cortex handles texture discrimination, size, and shape of objects
• The vestibular nuclei process sense of balance and spatial orientation, integrating with visual and proprioceptive inputs
• The primary somatosensory cortex processes limb position and movement, and the secondary somatosensory cortex integrates this information across multiple joints for coordination
• The insular cortex processes internal bodily sensations

Sensory Receptors and Pathways

• Sensation: The process of detecting and converting stimuli into neural signals.
• Importance of Sensation: Essential for interactions with the environment, crucial for survival, enabling locating food and avoiding danger.

Classification of Sensory Receptors

• Receptor Classification Based on Structure:
  • Free nerve endings: Simple neural receptors, e.g., pain receptors (nociceptors)
  • Specific histological structures: Complex neural receptors, e.g., mechanoreceptors for touch, pressure, vibration
  • Special senses receptors: Highly specialized receptors, e.g., hair cells in the auditory system

Sensory Transduction

• Process of Stimulus Detection:
  1. Stimulus application on receptor: Interaction between the stimulus and the specialized receptor.
  2. Activation of ion channels: Stimulus causes channels to open or close, leading to ion flow.
  3. Generation of receptor potential: Graded change in membrane potential due to ion flow.
  4. Threshold attainment: If the receptor potential reaches a threshold, an action potential is generated.
  5. Propagation of electrical signals: Action potential travels along the sensory neuron.

Signal Transduction Pathway

• Sensory receptor: Generates a generator potential (graded electrical change) in response to the stimulus.
• Synapse: Neurotransmitter release occurs.
• Primary afferent neuron: Carries the signal from the receptor to the CNS.
• Second order neuron: Located in the CNS, receives the signal from the primary afferent neuron.
• Adequate stimulus: Specific stimulus that the receptor is most sensitive to.
• Generator potential: Graded electrical change that triggers action potentials if it reaches the threshold.

Stimulus Coding and Processing

• Receptive field: Specific sensory space where a stimulus elicits a response from a sensory neuron.
• Modality: The type of sensory stimulus detected, determined by the specific receptor activated.
• Location: The location of the stimulus is determined by the specific receptive field activated.
• Intensity: The strength of the stimulus, encoded by the number of activated receptors and frequency of action potentials
• Duration: How long a stimulus persists, coded by the duration of action potential firing.
• Sensory Fibre Types:
  • Aα and Aβ fibers: Conduct signals for proprioception, vibration, and discriminative touch.
  • Aδ and C fibers: Carry signals for pain and temperature.

Impact of Sensory Pathway Lesions

• Dissociated sensory loss: Loss of one specific type of sensation while others remain intact.
• Spinoreticular tract: Non-specific pathway, transmits pain and temperature information.
• Two-point discrimination: Ability to distinguish between two separate points of touch.
  • Large receptive field: Many primary sensory neurons converge onto a single secondary neuron, making it difficult to distinguish between close stimuli.
  • Small receptive field: Fewer primary neurons converge onto a single secondary neuron, allowing for better discrimination.

Acuity and Sensory Discrimination

• Receptive field size: The size of the receptive field is inversely proportional to the density of receptors.
• Fingertips: High density of receptors, resulting in small receptive fields and greater acuity.
• Overlapping receptive fields: Interactions between sensory inputs refine sensory discrimination.

Explore the complexities of sensory receptors, their classification, and processes involved in detecting stimuli. This quiz covers various types of receptors, including mechanoreceptors and the conversion of stimuli into receptor potentials. Test your understanding of how our body perceives sensations.