RCSI Sensory Receptors and Pathways Quiz PDF

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Sensory Receptors and Pathways Class Med Year 2 Semester 1 Module Central Nervous System Code CNS Title Synaptic Transmission Lecturer Dr. Colin Greengrass (RCSI-BH) [email protected] D...

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Sensory Receptors and Pathways Class Med Year 2 Semester 1 Module Central Nervous System Code CNS Title Synaptic Transmission Lecturer Dr. Colin Greengrass (RCSI-BH) [email protected] Dr. Niamh Connolly (RCSI-IE) [email protected] Date 30th October 2024 KEY – DESCRIBES HOW TO USE INFORMATION IN THIS LECTURE THIS INFORMATION ONLY PROVIDES CONTEXT. YOU DON’T NEED TO LEARN THIS FOR THE EXAM. YOU MIGHT NEED THIS FOR OTHER SUBJECTS IF THIS SIGN IS FOUND ON A DIAGRAM YOU DON’T HAVE TO LEARN THE DETAILS THIS INFORMATION IS IMPORTANT. YOU NEED TO LEARN ALL THE CONTENT FOR THE EXAM THE CONCEPT OR MECHANISM IS IMPORTANT. YOU DON’T HAVE TO LEARN ALL THE DETAILS FOR THE EXAM Learning outcomes Describe the characteristics and classification of sensory receptors Describe how sensory receptors transduce stimuli into propagated electrical signals Describe how sensory information is transmitted to the brain, with Dorsal column-medial lemniscal pathway particular emphasis on: Spinothalamic pathway Describe how the brain interprets electrical signals it receives in terms of modality, location, intensity and duration of stimulus Recognise the impact of sensory pathway lesions on sensation, and define dissociated sensory loss What is Sensation? Briefly introduce the concept of sensation and its role in the body's ability to interact with its environment. Vital for survival Sensation is the process by Locating food, avoiding which specialized sensory danger receptors detect and respond to physical stimuli from the environment, converting them into neural signals. Sensation & Environmental Interaction Underlies Provides complex feedback perceptions, mechanism learning and for motor memory functions  Conscious Perception This is the process by which sensory input reaches the cerebral cortex, where it becomes part of our conscious experience. At this level, the brain interprets sensory information in a way that allows for intentional responses. Subconscious Sensory Modulation This refers to the automatic regulation of sensory information without reaching conscious awareness. These processes occur in lower brain centers, such as the brainstem and spinal cord, and enable continuous monitoring and modulation of sensory input to maintain homeostasis and support automatic functions.  Subconscious Sensory Modulation Many sensory processes are continuously regulated without entering conscious awareness Proprioceptive Our sense of body position is continually adjusted by proprioceptors to maintain balance and coordination Modulation Without conscious awareness or control Temperature The hypothalamus integrates temperature signals to modulate body temperature homeostasis Regulation Without conscious intervention, triggering responses like sweating or shivering. Some brainstem and spinal cord mechanisms modulate pain intensity based on Pain Modulation context Pain perception can be inhibited through descending control mechanisms. Sensory Processing Somatic Special senses Vision senses Touch Conscious Hearing Taste Pressure Pain Smell Proprioception Balance Temperature Visceral stimuli Somatic Blood pressure, stimuli Subconscious blood & CSF pH, Muscle Blood pO2, length, internal temperature, tension blood glucose, Proprio- lung inflation ception Definitions Receptors: Transducers that convert external or internal stimuli into electrical potentials Sensory pathways: Neural pathways that carry information from the receptors to the central nervous system integrating centres Typical Sensory Pathway  Proprioception (Mechanical Stimuli) Proprioception is the sense of body position and movement, based on mechanoreceptors located in muscles, tendons, and joints. Muscle spindles and Golgi tendon organs detect muscle stretch and tension, allowing us to maintain posture, balance, and coordinated movement without consciously focusing on each motion. Receptor Classification – By Structure Specialized Highly specialized structures located in complex sensory organs Designed to detect specific sensory modalities with high sensitivity Sense Receptors (light, sound). Simple Neural Basic, often unencapsulated receptors consisting of free nerve endings. Receptors Detect general stimuli like pain or temperature and are widely distributed, especially in the skin. Complex Neural Encapsulated and structurally intricate receptors. Detect more nuanced sensory information, such as touch, pressure, Receptors and vibration, with enhanced sensitivity and specificity.  https://pressbooks.ccconline Classification of Sensory.org/bio106/chapter/nervous- sensory-functions/ receptors - structure Stimulus to which the receptor is most sensitive e.g.. chemoreceptors, thermoreceptors Structure Free nerve endings Specific histological structures Special senses receptors Simple neural receptor Complex neural receptor Special senses receptor e.g.. pain receptors e.g.. mechanoreceptors e.g. hair cells in the (nocireceptor) auditory system Stimulus Detection Each sensory modality has specialized receptors. Within each modality, there are often distinct submodalities, such as brightness, color, and motion in vision. Specific receptors or processing systems are specialized to detect each submodality. Sensory receptors Classification based on stimulus type: 1.Mechanoreceptors: Respond to mechanical pressure or displacement. (e.g.., touch, pressure) 2.Thermoreceptors: Detect changes in temperature. 3.Photoreceptors: Respond to light (e.g.., rods and cones in the eyes). 4.Nociceptors: Detect pain due to damaging stimuli. 5.Chemoreceptors: Respond to specific chemical compounds (e.g.., taste, smell).  Classification of sensory receptors - stimulus Type of Receptor Modality Receptor Location Touch Pacinian Corpuscle Skin Audition Hair Cell Organ of Corti Mechanoreceptor Vestibular Hair Cell Macula, semicircular canal Photoreceptor Vision Rods and Cones Retina Olfaction Olfactory receptor Olfactory mucosa Taste Taste buds Tongue Chemoreceptors Arterial PO2 Carotid and aortic body pH of CSF Ventrolateral Medulla Thermoreceptors Temperature receptors Cold/warm receptors Skin Extremes of Thermal nociceptors Skin Nociceptors pain/temperature Polymodal nociceptors Skin  Sensory INTEROCEPTORS receptors that detect internal stimuli Pathophysiology: Examples include Implications in stretch receptors conditions like in the stomach, dysautonomia, Interoceptors baroreceptors in hypertension, and blood vessels, digestive and disorders osmoreceptors Functions: Monitor and regulate internal bodily functions, contribute to homeostasis EXTEROCEPTORS Sensory receptors that receive external stimuli Exteroceptors Functions: Subtypes include Facilitate mechano- perception of receptors, external thermoreceptors, environment, photoreceptors guide interactions Sensory Transduction ALO 38 – Describe how sensory receptors transduce stimuli into propagated electrical signals Definition: Receptors convert stimuli into receptor potentials, which, if they reach threshold, trigger an action potential. Process: 1.Stimulus application on receptor. 2.Activation of ion channels: Stimulus causes channels to open or close. 3.Generation of receptor potential: Graded change in membrane potential due to ion flow. 4.Threshold attainment: If the receptor potential is strong enough (reaches a threshold), an action potential is generated. 5.Propagation of electrical signals along the sensory neuron. Sensory Transduction Process Signal transduction Generation of action potential in specialise d afferent ending  Signal transduction pathway Sensory receptor: Synapse: Generator potential NT release Primary afferent neuron 2nd order neuron (CNS) Adequate stimulus Adequate Stimulus Detection: A sufficient stimulus interacts with a sensory receptor specialized to detect a particular type of input generated action potentials threshold Generator potential EPSPs  Signal transduction pathway Sensory receptor: Synapse: Generator potential NT release Primary afferent neuron 2nd order neuron (CNS) Adequate stimulus The receptor responds by generating a generator potential, a graded electrical change in the receptor cell – like an EPSP generated action potentials threshold Generator potential EPSPs  Signal transduction pathway Sensory receptor: Synapse: Generator potential NT release Primary afferent neuron 2nd order neuron (CNS) Adequate stimulus If the generator potential is strong enough to reach the threshold level, it triggers action potentials in the primary afferent neuron. The strength of the stimulus affects the frequency of action potentials but not their size. generated action potentials threshold Generator potential EPSPs  Signal transduction pathway Sensory receptor: Synapse: Generator potential NT release Primary afferent neuron 2nd order neuron (CNS) Adequate stimulus Action potentials travel along the primary afferent neuron towards the central nervous system (CNS). The action potentials reach the end of this neuron at a synapse, where it connects with a second-order neuron. generated action potentials threshold Generator potential EPSPs  Signal transduction pathway Sensory receptor: Synapse: Generator potential NT release Primary afferent neuron 2nd order neuron (CNS) Adequate stimulus Arrival of action potentials at the synapse causes the release of neurotransmitters. These neurotransmitters bind to receptors on the second-order neuron, generating excitatory postsynaptic potentials (EPSPs). generated action potentials threshold Generator potential EPSPs  Signal transduction pathway Sensory receptor: Synapse: Generator potential NT release Primary afferent neuron 2nd order neuron (CNS) Adequate stimulus EPSPs are graded potentials, if they reach threshold, they trigger action potentials in the second-order neuron. These propagate the sensory signal, eventually reaching higher brain centers for processing and perception. generated action potentials threshold Generator potential EPSPs Signal transduction - Generator weak strong Stimuli Generator/receptor potentials & frequency potentials Action potentials Threshold: Minimum stimulus that can be detected modulation Mechanoreceptors Mechanoreceptors are a subtype of sensory receptors specialised in detecting mechanical changes They work through mmechanically gated ion channels Located in skin, muscles, and other tissues Functions: Sensations of fine touch, vibration, proprioception  Mechanoreceptors contain ion channels that open in Mechanoreceptors response to physical deformation of the cell membrane. This deformation can occur due to external forces (touch, pressure) or internal forces (muscle stretch). Mechanoreceptors respond to mechanical deformation of the cell membrane, such as stretch, compression, or vibration. Mechanism When mechanical force is applied to the receptor, it distorts the membrane, opening mechanically gated ion channels. This allows ions (usually sodium and calcium) to flow into the cell, creating a depolarizing receptor potential. If the receptor potential is strong enough to reach threshold, it initiates an action potential that propagates to the CNS.  Mechanoreceptors Different types based on location and function: Merkel cells, Meissner's corpuscles, Ruffini endings, and Pacinian corpuscles  Mechanoreceptor Structure Mechanism of Action Function Specialised epithelial Respond to sustained Discrimination of shapes Merkel Cells Mechanoreceptors cells in the skin pressure and touch and textures Respond to low- Encapsulated sensory Detecting light touch and Meissner's Corpuscles frequency vibrations and nerve endings in skin texture discrimination rapid skin indentation Highly sensitive to high- Detecting deep pressure Large, onion-like Pacinian Corpuscles frequency vibrations and and high-frequency structures deep in skin rapid pressure changes vibrations Encapsulated receptors Perception of skin stretch Respond to skin stretch Ruffini Endings in dermis and and grip force and sustained pressure subcutaneous tissues maintenance Associated with hair Detect mechanical Detecting light touch and Hair Follicle Receptors follicles movements of hair shafts hair movement Found within skeletal Respond to changes in Aid in maintaining muscle Muscle Spindles muscles muscle length tone and limb position Located in tendons near Respond to changes in Protect muscles and Golgi Tendon Organs muscle-tendon junctions muscle tension tendons from damage Auditory Hair Cells Located in the cochlea of the inner ear, hair cells detect sound waves through the deflection of their stereocilia. When sound-induced vibrations displace these hair cells, mechanically gated ion channels open, allowing K⁺ and Ca²⁺ to enter, leading to depolarization. This depolarization triggers neurotransmitter release, sending auditory signals to the brain. Nociceptors 1. Detect pain due to damaging stimuli. 2. Specialised sensory neurons that detect noxious stimuli 3. Types based on stimulus modality: thermal, mechanical, and chemical 4. Located in skin, muscles, joints, and some internal organ Nociceptor Type Location in the Body Mechanism of Action Function Skin, mucous Respond to temperature Detect noxious thermal Thermal Nociceptors membranes, organs extremes (hot or cold) stimuli Respond to mechanical Detect tissue injury, Mechanical Nociceptors Skin, muscles, joints damage, pressure, or pressure, or mechanical stretching stress Respond to chemical Detect tissue Chemical Nociceptors Throughout the body irritants, inflammatory inflammation, chemical mediators irritants Respond to various Widespread Detect multiple types of Polymodal Nociceptors noxious stimuli (thermal, distribution tissue damage and injury mechanical, chemical) Somatosensory System and Intensity Different forms of touch are detected by specific receptor types Touch: Involves mechanoreceptors in the skin that respond to light contact or tactile pressure. Meissner’s corpuscles and Merkel cells, for instance, detect light touch and texture. Somatosensory System and Intensity Different forms of touch are detected by specific receptor types Pressure: Involves deeper mechanoreceptors, such as Pacinian corpuscles, which respond to sustained pressure and vibration. These receptors detect more intense or sustained mechanical deformation. Touch receptors in the superficial layers of the skin have smaller receptive fields than those in the deep layers.  Somatosensory System and Intensity Pain (Nociception) Involves high-threshold mechanoreceptors (HTMRs) and nociceptors that activate when mechanical pressure reaches a potentially damaging level. When the stimulus intensity is high enough, these receptors trigger the perception of pain to signal harm and initiate protective responses. Proprioceptors Thermoreceptors Specialised sensory neurons for temperature detection Two main types: cold and warm thermoreceptors Distributed in skin, mucous membranes, and some internal organs  Thermoreceptor Location in the Body Mechanism of Action Function Skin (particularly in the Respond to an increase Detecting and signaling Warm Thermoreceptors dermis) in temperature warmth or heat stimuli Thermoreceptors Skin (especially in the Respond to a decrease Detecting and signaling Cold Thermoreceptors epidermis) in temperature cold or cool stimuli Highly sensitive to Signaling extreme cold or Cold Nociceptors Skin and deeper tissues intense cold noxious cold stimuli Activated by high Sensing and responding Distributed in various TRPV1 Receptors temperatures, typically to noxious heat or high tissues above 43°C (109°F) temperatures Activated by Sensing and responding Skin, particularly in cold- TRPM8 Receptors temperatures below to mild to moderate cold sensitive areas 25°C (77°F) temperatures Activated by noxious Detecting extreme cold, Present in various TRPA1 Receptors cold and chemical chemical irritants, and tissues irritants environmental irritants Photoreceptors Photoreceptors in the retina detect light and enable vision by transducing light energy into neural signals. Photoreceptors contain light-sensitive molecules called photopigments When light hits these pigments, it changes their structure, activating a G-protein cascade that closes ion channels. Photoreceptors Specialised neurons in the retina sensitive to light Two main types: Rods (low-light vision) and Cones (colour vision) Photoreceptors Location in the Cell Type Body Mechanism of Action Function Pigment Responsible for Retina (peripheral Highly sensitive to low Rod Cells scotopic (night) Rhodopsin regions) light levels vision Responsible for Opsins (Multiple Retina (central Sensitive to various Cone Cells photopic (day) and types for color region) wavelengths of light color vision discrimination) Involved in regulating circadian Melanopsin- Retinal ganglion Sensitive to blue light rhythms & non- Containing Retinal Melanopsin cells and light/dark cycles visual light- Ganglion Cells mediated responses  Chemoreceptors Chemoreceptors are sensory cells or structures specialised in detecting changes in chemical concentrations or composition in the body or the environment. They play essential roles in various physiological processes, including the perception of smell and taste, the regulation of respiratory rate and blood chemistry, and the control of blood pressure and cardiovascular function. Chemically Gated Ion Channels or G-Protein- Coupled Receptors (GPCRs) Chemical stimuli bind to specific receptors on the chemoreceptor’s cell surface. This binding either directly opens ion channels (in taste receptors) or activates a cascade via G-proteins (in smell receptors)  Chemoreceptor Type Location in the Body Stimuli Detected Function Odor molecules in the Detection and Olfactory Receptors Nasal epithelium (nose) air perception of odours Chemoreceptors Taste buds (tongue and Chemicals in food and Detection and Taste Receptors mouth) beverages perception of taste Carotid bodies (neck) Changes in blood Regulation of Arterial and aortic bodies oxygen, carbon dioxide, respiration and blood Chemoreceptors (thorax and abdomen) and pH chemistry Changes in Regulation of Central Brain (medulla cerebrospinal fluid respiratory rate and Chemoreceptors oblongata) composition blood chemistry Changes in blood gases Regulation of Peripheral Located in various (oxygen, carbon respiration and blood Chemoreceptors tissues dioxide) and pH chemistry Arterial walls Regulation of blood Changes in blood Baroreceptors (baroreceptor reflex) pressure and pressure and other locations cardiovascular function Primary sensory afferent neurons Peripheral neurons which transduce info from sensory receptors to CNS Cell body in dorsal root ganglion Pseudounipolar neurons Single process splits into two, functioning as an axon Connects with sensory receptor on one side Transmits information from the receptor into dorsal horn of spinal cord DORSAL ROOT GANGLION Transmission of Sensory Information to the Brain ALO 39 - Describe how sensory information is transmitted to the brain, with particular emphasis on the dorsal column -medial leminiscal pathway and the spinothalamic pathway. Description of General Pathway Peripheral receptor → Sensory neuron → Spinal cord/brainstem → Thalamus → Cortex. Dorsal Column-Medial Lemniscal Pathway 1. Type of information: Fine touch, vibration, and proprioception. 2. Route: Dorsal column of spinal cord → Nuclei in medulla → Medial lemniscus in brainstem → Thalamus → Somatosensory cortex. 3. Key features: Decussation (crossing over) in medulla, high degree of spatial fidelity. Spinothalamic Pathway 2. Type of information: Pain, temperature, crude touch. 3. Route: Dorsal horn of spinal cord → Decussation at spinal cord level → Ascend via spinothalamic tract → Thalamus → Somatosensory cortex. 4. Key features: Immediate decussation, lateralised sensation. Spinothalamic pathway / anterolateral system 1st order neurons Axons transmit from sensory receptor to spinal cord, via dorsal root Synapse with 2nd order neuron in dorsal horn of spinal cord Spinothalamic pathway / anterolateral system 2nd order neurons Axons cross (decussate) in spinal cord (close to entry point) Ascend in contralateral spinothalamic tract Synapse with 3rd order neurons in the thalamus 3rd order neurons Axons project from thalamus to sensory cortex The spinothalamic tract is classified as a specific sensory pathway. It carries detailed and localized information about pain, temperature, and crude touch from peripheral receptors to specific regions in the brain. This pathway allows for precise localization and discrimination of sensory stimuli Dorsal column-medial lemniscal (ML) system 1st order neurons Axons transmit signal from sensory receptor to spinal cord, via dorsal root Ascend in dorsal columns Synapse with 2nd order neuron in medulla gracile nuclei (lower limb) cuneate nuclei (upper limb) Gracile/cuneate nucleus also called gracile/cuneate fasciculus Dorsal column-medial lemniscal (ML) system 2nd order neurons Axons cross (decussate) in lower medulla to form contralateral medial lemniscus Synapse with 3rd order neurons in the thalamus 3rd order neurons Axons project from thalamus to sensory cortex The Dorsal Column-Medial Lemniscal (DCML) system is also a specific sensory pathway. It transmits highly detailed information about fine touch, vibration, and proprioception  Persistent stimuli & receptor adaptation Different receptors change their firing rate in relation to the constancy of intensity of the stimulus - adaptation Phasic receptors Fire at stimulus onset Adapt or cease to fire when constant (steady state) stimulus Fire when intensity changes This filters out unnecessary stimuli to focus on new, essential information e.g.. smell (olfactory receptors), pressure Phasic When you first jump into cold water, the abrupt change in temperature is detected by phasic receptors Leading to a burst of sensory information being sent to the central nervous system (CNS), this gives you an initial intense cold sensation. As you remain in the cold water, these phasic receptors adapt rapidly to the constant stimulus and either reduce their firing rate significantly or stop firing altogether. This leads to the sensation of "getting used to" the cold water, even though the temperature of the water hasn't changed. Persistent stimuli & receptor adaptation Different receptors change their firing rate in relation to the constancy of intensity of the stimulus - adaptation Tonic receptors are sensory receptors that adapt slowly to a stimulus and continue to produce action potentials over the duration of the stimulus. This means they provide a more constant and sustained rate of firing for as long as the stimulus is present. e.g.. baroreceptors, nociceptors,  Persistent stimuli & Holding a heavy bag in your hand with your arm extended. receptor adaptation Muscles stretch in response. Muscle stretch receptors Muscle spindles within those stretched muscles will (muscle spindles) - continuously send information to the CNS about the degree continuously monitor the of stretch. length and rate of change in length of a muscle. Continuing to hold the bag, these muscle spindles don't "get When a muscle is stretched, used to" the stretch and reduce their firing rate like phasic muscle spindles generate a receptors rate of action potentials proportional to the degree of stretch. They continue to send a relatively constant rate of signals to the CNS, informing it of the sustained stretch in the muscle.  Stimulus coding & processing Receptive field A receptive field is defined as a distinct area in sensory space (e.g.., a portion of the skin or a section of the visual field) where the presence of a stimulus elicits a response from a particular sensory neuron. Modality Each central neuron recognises the receptor type activated and therefore the nature of the sensory stimulus e.g.. photoreceptors in eye are not activated by olfactory Location or auditory stimuli Each sensory pathway projects to a region of the cerebral cortex dedicated to a specific receptive field Intensity Determined from the number of activated receptors & frequency of action potentials in afferent pathway  Different types of stimuli travel in different types of peripheral nerve fibres Proprioception, vibration, discriminative touch travel in Aα, Aβ fibres Axon Type Aα Aβ Aδ C Heavily myelinated Fast conducting Diameter (µm) 13-20 6-12 1-5 0.2-1.5 Speed (m/s) 80-120 35-75 5-35 0.5-2 Pain, temperature travel in Aδ, C fibres Thinly myelinated and unmyelinated Moderate to slowly conducting Sensory Fibre types https://faculty.washington.edu/chudler/cv.html Impact of Sensory Pathway Lesions & Dissociated Sensory Loss ASO 41 – Recognise the impact of sensory pathway lesions on sensation and define dissociated sensory loss. 1. Definition: Dissociated sensory loss refers to the loss of one type of sensation while others remain intact. 2. Dorsal column lesion: Loss of fine touch, vibration, and proprioception, but pain and temperature sensation are intact. 3. Spinothalamic tract lesion: Loss of pain and temperature sensation, but touch, vibration, and proprioception are preserved. 4. Clinical implications: Understanding sensory loss patterns helps in diagnosing specific lesions. TRY OUT THE QUIZ AT THE END Convergence and divergence of sensory input Primary Sensory Divergence: Any individual neuron can Neuron make connections to many different postsynaptic neurons in a network Convergence: One postsynaptic cell can Sensory Sensory signal receive input from a number of different signal presynaptic cells Secondary Sensory Neuron Divergence Convergence divergence convergence Convergence and divergence of sensory input Primary Sensory Neuron The arrangement of the spinothalamic pathway gives rise to convergence or divergence of the sensory input This influences the quality of the sensation at Sensory Sensory the conscious or subconscious level within the signal signal CNS Secondary Sensory Neuron Divergence Convergence divergence convergence Convergence Multiple sensory inputs from different body Primary Sensory areas or types of receptors Converge onto a Neuron single neuron. This pooling of inputs can enhance sensitivity or allow for combined sensory Sensory input. signal Convergence reduce the ability to pinpoint Secondary Sensory an exact location of a stimulus Neuron Convergence increases the overall signal Convergence intensity, like in pain perception. Divergence Primary Sensory One sensory input spreads out or Neuron diverges to multiple neurons and areas in the CNS. This allows a single signal to be Sensory signal processed by different brain regions, which enables the brain to create a complex response to a simple stimulus. Secondary Sensory For example, a touch signal might trigger Neuron both a motor response and awareness in Divergence different cortical areas. Specific vs Non-specific Pathways Specific pathways Non-specific pathways Designed for general awareness and more readily Prioritize precision and undergo both convergence undergo limited convergence (pooling various signals) and to maintain accuracy. divergence (broad impact on CNS).  Specific Pathways Specific sensory pathways (primary afferents) relay information about a single type of stimulus from one type of sensory receptor to specific primary receiving areas of the cerebral cortex These pathways relay detailed, precise information about a specific type of sensory stimulus (like fine touch or proprioception) to specific areas of the brain for processing the localization and identification of the stimulus.  Non-Specific Pathways Non-specific pathways are broader and carry multiple types of sensory information to general areas like the reticular formation and thalamus. They are not focused on precise information but contribute to general awareness, arousal, and alertness, rather than the specifics of the sensation. Convergence is more common in non- specific pathways. Spinoreticular Tract is non-specific and transmits pain and temperature information. It plays a key role in arousal, alertness, and the emotional aspects of pain, rather than precise localization of the stimulus. Two-point discrimination Large Receptive Field – Many primary sensory neurons converge onto a single secondary sensory neuron, creating a large receptive field. Single Perceived Point – When two stimuli fall within this large receptive field (e.g.., compass points separated by 20mm), they are perceived as a single point of touch. One Signal to the Brain – Due to convergence, only one signal is sent to the brain, even if there are multiple points of stimulation. Two-point discrimination Small Receptive Fields – Fewer primary sensory neurons converge onto each secondary sensory neuron, resulting in smaller, more specific receptive fields. Distinct Perception of Stimuli – With separate, smaller receptive fields, two stimuli activate distinct pathways. This allows for perception of two distinct points of touch. Two Signals to the Brain: Each stimulus generates a separate signal, allowing the brain to recognize the two points as separate stimuli.  Acuity and sensory discrimination The size of the receptive field varies inversely with the density of receptors e.g.. fingertips have high density of receptors, each with small receptive fields → greater acuity or discrimination ability of the input Overlapping receptive fields (of identical sensory receptors) allows interactions between sensory inputs and refines sensory discrimination Two point discrimination test (e.g.. callipers) Humans normally recognise two points 2-5 mm apart on fingertips, but only 20- 30 mm apart on the back of the hand (2-4 mm on lips; 8-12 mm on palms; 30-40 mm on shins/back)  Somatotopic Organisation Each sensory pathway projects to a region of the cerebral cortex dedicated to a specific receptive field Somatosensory neurons from one side of the body project to the sensory cortex on the opposite (contralateral) side Cortical representation of the body reflects receptor density Clinical relevance: Helps pinpoint lesions based on deficits and guides treatments like deep brain stimulation.  Dermatome A dermatome is a specific area of the skin that receives sensory input from a single spinal nerve. Each spinal nerve (except the first cervical nerve, C1) innervates a particular segment of the skin Diagnostic Use – Sensory Loss Pattern If there is a loss of sensation in a defined dermatome area, it suggests damage to the corresponding spinal nerve or root. By identifying which dermatome area has a sensory deficit, clinicians can trace the issue back to a specific spinal level. Dermatome mapping helps localize spinal injuries or nerve compression, aiding in the diagnosis of conditions like herniated discs or nerve impingements. Cortical sensory homunculus Topographical representation of the sensory distribution of the body found in the cerebral cortex Reflects the relative space occupied on the somatosensory cortex by each part of the body  Somatotopic organisation is not fully formed at birth While the basic architecture for sensory processing is present in newborns, the somatotopic maps in the primary somatosensory cortex and other brain regions undergo refinement during postnatal development. 1.Sensory Experience: Active exploration and sensory experiences during infancy and early childhood play a significant role in refining and solidifying somatotopic organisation. 2.Synaptic Pruning: In the early stages of development, there is an overproduction of synaptic connections. Over time, unused or less frequently used synapses are pruned away  INTEGRATION CENTRES Thalamus Superior Colliculus Posterior Parietal Cortex Sensory Association Cortices Basal Ganglia Processing Limbic System To Primary Visual Cortex To Secondary Visual Area To integration Centres Occipital Middle lobe Temporal + To Primary Auditory Cortex To Secondary Auditory Area To integration Centres Temporal Superior lobe Temporal Gyrus + Reception Primary Secondary Integration of Sensory Sensory Sensory Centres Prefrontal cortex Stimuli Processing Processing Posterior Association Area Location of Primary Sensory Processing Sensory Organ Connecting Pathway Target Brain Area Primary Visual Cortex (Occipital Eye (Retina) Optic Nerve, Optic Tract Lobe) Primary Auditory Cortex (Temporal Ear (Cochlea) Auditory Nerve Lobe) Dorsal Columns, Spinothalamic Primary Somatosensory Cortex Skin Tract (Parietal Lobe) Nose (Olfactory epithelium) Olfactory Nerve Olfactory Cortex (Temporal Lobe) Facial, Glossopharyngeal, Vagus Primary Gustatory Cortex (Insular Tongue (Taste buds) nerves Cortex)  Information Primary Processing Information Secondary Processed Sensory System Area Processed (Primary) Processing Area (Secondary) Secondary Visual Colour, motion, Primary Visual Cortex Basic visual Visual Areas (V2, V3, V4, depth, complex (V1) information V5/MT) shapes Complex sound Primary Auditory Basic auditory Belt and Parabelt Auditory processing, sound Cortex information Regions localisation Odour recognition, Olfactory Olfactory Bulb Initial odour detection Piriform Cortex association with experiences Secondary Gustatory Flavour combinations, Primary Gustatory Gustatory Basic tastes Areas (in Orbitofrontal taste intensity and Cortex Cortex) preference Primary and Secondary Sensory Processing  Primary and Secondary Sensory Processing Information Primary Processing Processed Secondary Information Processed Sensory System Sensory Organ Area (Primary) Processing Area (Secondary) Primary Secondary Basic tactile Texture discrimination, Somatosensory Skin, Muscles Somatosensory Somatosensory information size and shape of objects Cortex (S1) Cortex (S2) Sense of balance Integration with visual Vestibular Areas Vestibular Inner Ear Vestibular Nuclei and spatial and proprioceptive inputs in the Cortex orientation for balance Integration of position Primary Secondary Limb position and and movement across Proprioception Joints, Muscles Somatosensory Somatosensory movement multiple joints, and Cortex (S1) Cortex (S2) coordination Internal bodily Interoception Internal Organs Insular Cortex - - sensations Sensory pathways – projections to the brain via the Thalamus Olfactory pathways from the nose project directly to the cortex Equilibrium pathways project to the cerebellum (with a branch to the cortex via the thalamus) All other pathways pass through the thalamus before they project to their relevant cortical area Sensory Processing Integration Cortical Association Areas integrate sensory information across Essential for complex perception, enabling us to understand the world in a cohesive and meaningful way. These areas are located across various lobes of the cerebral cortex Prefrontal Cortex Located in the frontal lobe, the prefrontal cortex is involved in executive functions like decision-making, planning, and social behavior. Integrating information from various sensory and motor areas, enabling high-level functions like abstract thinking, personality expression, and problem- solving. Posterior Association Area Located at the junction of the occipital, parietal, and temporal lobes, this area is crucial for spatial awareness and object recognition. Integrates visual, auditory, and somatosensory information, allowing us to understand the spatial relationships between objects and our own body position in space. Important for language comprehension as well, particularly in the left hemisphere, where it includes regions involved in language processing (e.g.., Wernicke's area).  Involved in emotional Limbic processing, memory formation, and associative System learning. The limbic structures, such as the amygdala and hippocampus, add emotional context to sensory experiences, which enhances memory encoding and retrieval. Sensory Integration Primary Processing Area Integrated Information Site of Integration Association Areas, Prefrontal Primary Visual Cortex (Occipital Lobe) Visual features and scenes Cortex Primary Auditory Cortex (Temporal Sounds and auditory Superior Temporal Sulcus, Lobe) patterns Prefrontal Cortex Primary Somatosensory Cortex (Parietal Posterior Parietal Cortex, Tactile and temperature info Lobe) Prefrontal Cortex Olfactory Cortex (Temporal Lobe) Smells Orbitofrontal Cortex, Amygdala Primary Gustatory Cortex (Insular Tastes Orbitofrontal Cortex Cortex) Receptive Field Specific region of sensory space where a stimulus elicits neural firing. Generator Localised change in membrane potential in response to a stimulus; if threshold is reached, an Potential action potential ensues. Ipsilateral Pertaining to or affecting the same side of the body in relation to a reference point. Contralateral Relating to the opposite side, especially with reference to bilateral brain structures. Decussation Anatomical crossing of nerve fibres from one side of the body or brain to the other. Acuity Precision of sensory modality; often used in context of spatial resolution or discriminative ability. Sensory Neural capability to distinguish between distinct stimuli based on specific qualities. Discrimination Neurological process where neuronal firing rates decrease in response to a constant or repetitive Adaptation stimulus. Dissociated Neurological deficit where one form of sensation is lost, while another remains intact, often due Sensory Loss to specific lesions. Conscious Sensory information that has been processed and ascends to higher cortical areas for cognitive Perception recognition. Terminology you should understand Factors enhancing or inhibiting perception of sensory stimuli Sensory receptors (at a peripheral level) decrease responsiveness to constant Adaptation or repetitive stimuli, leading to reduced perception. Example: strong odours fade as olfactory receptors adapt. Attention & Selective Perception is influenced by focus; stimuli that are actively attended are Perception perceived more strongly, while unattended ones may go unnoticed. Habituation is a central process in the CNS, involving decreased synaptic Habituation transmission or neural responsiveness due to repetition, which helps the brain filter out irrelevant stimuli from conscious perception. Each sense has a minimum threshold below which stimuli are not consciously Sensory Thresholds perceived, like the minimum light level needed to see an object in vision. Factors enhancing or inhibiting perception of sensory stimuli Psychological Factors Emotions, expectations, and cognitive biases shape perception. For instance, emotional state can amplify or dull pain perception Fatigue can attenuate sensory perception by reducing the responsiveness of sensory receptors and Physiological Factors decreasing the efficiency of neural processing pathways. Diminished arousal level affects the brain’s capacity to process and prioritize incoming sensory inputs effectively as the brain prioritises rest over perception. Environmental The setting influences perception; for example, faint sounds are more noticeable in quiet Factors environments than in noisy ones. Sensory Overload Exposure to too many simultaneous stimuli can overwhelm the senses, reducing the ability to perceive individual inputs, creating a "numbing" effect. Bibliography