Sensory System

• The nervous system consists of sensory functions to obtain information, integration functions for information processing, and motor functions for the body's response.

Sensory Receptors

• Sensory receptors are specialized epithelial cells or neurons that convert environmental signals into neuronal signals, either receptor potentials (RP) or action potentials (AP).
• An adequate stimulus leads to a change in membrane permeability, resulting in a graded change in membrane potential known as the receptor potential, which may then lead to an action potential.

Receptor Potential

• Receptor Potential is a graded response to a stimulus
• Proportional to the intensity of the stimulus
• A change in the membrane potential is caused by a change in ion permeability based on a stimulus
• Receptor Potential can be either depolarizing or hyperpolarizing
• It can trigger action potentials if large enough to bring the membrane to the threshold
• The magnitude of the receptor potential determines the frequency of action potentials

Receptor Specificity

• Each receptor type is generally specialized and more sensitive to one particular kind of stimulus.
• An adequate stimulus is the type of stimulus to which a sensory receptor (or any neuron) is especially tuned.
• An adequate stimulus results when a particular receptor responds in normal functioning, meaning it is the type of energy that has the lowest threshold for receptor activation.

Classification of Receptors

• Receptors are categorised by Complexity, Location, and the Type of Stimulus Detected.
• Receptor complexity includes free nerve endings, encapsulated nerve endings, and sense organs like the eyes and ears.
• Location can be classified into exteroceptors, interoceptors (visceroceptors), and proprioceptors.
• Receptors detect different types of stimuli: mechanoreceptors, photoreceptors, thermoreceptors, chemoreceptors, and nociceptors.

Primary Sensory Coding

• Stimulus type (modality): The activated receptor type determines how the brain recognizes the signal.
• All receptors of a single afferent neuron are sensitive to the same type of stimulus.
• Stimulus intensity influences stronger signals, resulting in a larger receptor potential and a higher frequency of action potentials.
• Stronger stimuli affect a larger area and recruit a larger number of receptors.
• Stimulus location and receptive field are characteristic of the receptor.
• Stimulus duration refers to the use of rapid and slow adapting receptors.

Sensation - Basic Steps

• Stimulation occurs when an adequately strong stimulus is applied.
• Transduction involves converting a stimulus from one form to another which can induce a graded receptor potential and an action potential
• Conduction relays information through a specific sensory pathways to the central nervous system, where a synapse occurs in the thalamus before continuing to the cortex.
• Perception includes detecting an environmental change by the central nervous system (cortex), along with evaluating the nature of change and magnitude, resulting in interpretation

Sensation and Perception

• Sensation is the body's response to the environment through the generation of nerve impulses, which are integrated or summed up by receptors.
• Sensory nerve impulses are then transferred via sensory pathways to specific sensory areas in the cortex.
• Perception is created by the brain based on the information it receives from sensory receptors and includes the interpretation of signals.
• Perception is our conscious interpretation of the external world.

Receptor Adaptation

• Receptor adaptation is a change of sensory receptors' sensitivity to the stimulus.
• Receptor adaptation occurs in all senses, excluding the sense of pain.
• Phasic Receptors (rapidly adapting) quickly convey information about changes in a stimulus, providing dynamic information.
• Tonic Receptors (slowly adapting) slowly convey information about the persistence of a stimulus, providing static information.

Lateral Inhibition

• Lateral inhibition is where information from neurons at the border of a stimulus is inhibited.
• Lateral inhibition pathways closest to the stimulus inhibits neighbors and enhances perception of stimulus

Ascending Neural Pathways

• Specific neural pathways have neurons that convey information about only a single type of stimulus to specific primary sensory areas of the brain cortex.
• Nonspecific neural pathways have information from more than one type of sensory unit to the brainstem and regions of the thalamus
• The nonspecific neural pathways control alertness and arousal and are not part of the specific ascending pathways.

Central Control of Afferent Information

• Much of the incoming afferent information is reduced or abolished by the thalamus and descending pathways.
• Presynaptic inhibition, a descending pathway, is the direct inhibition of the central terminals of the afferent sensory neuron.
• Inhibitory synapses between interneurons also contribute to the reduction of information.

Brain Cortex and Perceptual Processing

• Higher-order integrative cortices, or cerebral cortex areas outside the primary sensory areas, are the most developed part of the brain in humans.
• Higher-order integrative cortices analyze signals from multiple regions of cerebral cortex and subcortical structures, and are responsible for the integration of information.
• Unimodal and multimodal association areas allow processing of more complex information.
• Multimodal areas project to motor areas, unimodal sensory areas, or both.

Factors Affecting Perception

• Perception can be affected by processing of information along afferent pathways, including adaptation
• Filtration of the incoming information.
• Perception can also be affected by an individuals’ emotions, experiences, and personality.
• A lack of receptors for some types of energy, such as ionizing radiation, affect perception of the environment
• Damaged neural networks, such as a phantom limb, are also a factor
• Affection by drugs (hallucinogens) affect an individual’s perception of their environment

Somatosensory System

• The Somatosensory System consists of sensory receptors, sensory (afferent) neurons, deeper neurons within the central nervous system (medulla oblongata, thalamus), and sensory cortex.

Somatic Senses

• Somatic senses are nervous mechanisms that deal with collecting sensory information from the body.
• Somatic senses can be classified as mechanoreceptive which include tactile and position sensations and are sensed by mechanoreceptors.
• The thermoreceptive senses deal with heat and cold which are sensed by thermoreceptors.
• The pain sense is classified with pain receptors.

Mechanoreceptors

• Tactile sensations sensed by mechanoreceptors can be further broken down into Meissner corpuscles, Merkel's corpuscles, Pacinian corpuscles, and Ruffini corpuscles
• Proprioceptive sensation perceived by mechanoreceptors can be classified as muscle spindles and Golgi tendon organs.

Meissner Corpuscles

• Meissner corpuscles are rapidly adapting mechanoreceptors which contribute to the tactile sensation of texture.
• Endings are from myelinated axons terminating between layers of Schwann cells which is found is non hairy skin and has small receptive fields.

Merkel's Corpuscles

• Merkel's corpuscles are cutaneous mechanoreceptors with axonal branches ending as flattened expansions in the basal layers of the epidermis.
• Merkel's corpuscles are slowly adapting and contribute to both light touch and superficial pressure sensation and have small receptive fields.

Pacinian Corpuscle

• Pacinian corpuscles are rapidly adapting and contribute to the sensation of deep cutaneous pressure and vibration.
• Pacinian corpuscles are lamellated, like leaves of an onion, and are rare in the most areas of the face and mouth, but have large receptive fields.

Ruffini Corpuscles

• Ruffini corpuscles are slowly adapting mechanoreceptors which respond to continuous touch or pressure and register static joint position and joint movements and have large receptive fields
• The receptors location is in the dermis of fingertips and joint capsules
• They also provide information about movement direction and its speed determining the degree joint rotation.

Receptive Fields

• Fast Adapting, type I mechanoreceptors (FAI) and Slow Adapting, type I mechanoreceptors (SAI) have small, sharp borders.
• Meissner corpuscles (FAI) exhibit no static response.
• Merkel's disks (SAI) exhibit irregular firing patterns and show a static response.
• Fast Adapting, type II mechanoreceptors (FAII) and Slow Adapting, type II mechanoreceptors (SAII) exhibit large, obscure borders.
• Pacini corpuscles (FAII) do not exhibit a static response.
• Ruffini endings (SAII) exhibit regular firing patterns and static response.

Threshold Pressure

• Threshold Pressure is the lowest pressure to trigger the tactile sensation.
• Threshold Pressure is smallest on the tip of the tongue and nose, but highest on the back and soles of the legs.
• Low-threshold, or high sensitivity, mechanoreceptors are called low threshold because weak stimulation causes action potentials
• These mechanoreceptors provide information about touch, pressure, vibration, and skin tension and are innervated by large myelinated axons

Punctate Localization of Skin Sensation

• Punctate localization of skin sensation is coded by site of the stimulated receptor and is negatively correlated with (1) the amount of convergence in ascending pathways.
• There is a negative correlation of the skin sensation localization and (2) size of the receptive field and (3) amount of overlap with adjacent receptive fields.
• Response is highest at the center of receptive field due to a high receptor density; increased acuity is due to lateral inhibition

Two-Point Discrimination

• Two-point discrimination is a measure of how far apart two stimuli must be before they can be distinguished as separate.
• A similar principle explains our ability to determine accurate location on the skin.

Receptive Field

• A receptive field is an area of the body that, when stimulated, leads to activity of the neuron.
• The receptive fields are regionality different
• The sensory unit includes the receptive field, primary sensory neuron, and secondary sensory neuron.

Regional Differences in Receptive Field Size

• Receptive fields are highly accurate on tips of finger, lips, and tongue
• Decreased overlap of receptive fields results in increased discrimination.
• Lips and fingertips receptive field sizes are from 2-4 mm on the lips and fingertips
• Palms receptive field sizes are approximately 8-15 mm.
• Shins and back receptive field sizes are approximately 30-40 mm on the shins or back

Free Nerve Endings

• Free nerve endings are the simplest, most common sensory receptor.
• They detect sensations such as touch, pressure, and stretching
• They are responsible for the detection of touch, pain, pressure and temperature and are scattered throughout most of the body
• Visceroceptors are of this type.
• As temperature receptors they detect warm stimuli
• They detect cold stimuli 10-15x more than warm
• As pain receptors they detect extreme heat or cold

Muscle Spindle

• Muscle spindles are small sensory organs enclosed within a capsule located in parallel with extrafusal fibers.
• Muscle spindles are formed by several small muscle fibers (intrafusal fibers) with contractile proteins just at both ends.
• Nuclear bag fibers and nuclear chain fibers are the main types of muscle spindle fibres.
• The muscle spindle central region of the sensory dendrites is wrapped by sensory fibres

Golgi Tendon Organ

• Golgi Tendon Organs localization spans the origins and insertion of skeletal muscle fibers into the tendons of skeletal muscle.
• Golgi Tendon Organs structure feature strands of collagen connected at one end to the muscle fibers and at the other into the tendon proper, and each is enclosed in a fibrous capsule.
• It is innervated by nerve fibers that perforate the side of the capsule and lose their medullary sheaths
• Innervation also has nerve endings that subdivide and end between the tendon fibers with irregular disks or varicosities

Thermoception

• Thermoception refers to the perception of different gradations of cold and heat, that range from freezing cold to burning hot.
• Thermoception includes the ability to detect heat and cold and to adapt to temperature.
• The neutral, or comfort, zone of skin temperature is between 30-36 °C.
• Sensitivities in different sites of integument vary
• The body is covered by epithelial tissue, which covers the internal and external body surfaces
• Epithelial tissue has a varying amount of thickness, a degree of keratinization and a level of hydration, influencing its thermal conductivity

Thermoreceptors

• Thermoreceptors are free nerve endings located in the epidermis, near the vessels, spinal cord, and hypothalamus.
• Thermoreceptors are excited by cooling or warming stimulation and associated with ion channels, known as transient receptor potential channels
• Afferent neurons respond with small-diameter, A delta and C fibers.
• Types include sensitive low-threshold thermoreceptors for temperatures where cold is (10-30°C) and warm is (36-45°C).
• Small receptive fields of low-threshold thermoreceptors are approximately Ø300 μm and highly sensitive to rapid temperature change
• High-threshold thermoreceptors receive information about temperatures that have the potential for tissue damage.
• Large receptive fields of high-threshold are aproximately 2-4 mm².

Somatic Sensory Afferents

• Meissner corpuscles, Merkel's corpuscles, Pacinian corpuscles, and Ruffini's corpuscles are innervated by myelinated type II fibers (Aß).
• Mechanosensitive and thermosensitive nociceptors are innervated by myelinated fibers type III (Αδ).
• Polymodal nociceptors are innervated by nonmyelinated fibers type IV (C).

Characteristics of Afferent Fibers

• Type Αα fibers are the largest, with a diameter of 13-20 μm and speed 80-120 m/s and found in proprioceptors.
• Type Αβ fibers have a diameter of 6-12 μm and speed 35-75 m/s and found in mechanoreceptors of skin.
• Type Αδ fibers have a diameter of 1-5 μm and speed 5-30 m/s and relates to pain and temperature.
• Type C fibers are the smallest, with a diameter of 0.2-1.5 μm and speed 0.5-2 m/s and relates to pain and temperature.

Segmental Innervation - Dermatomal Map

• A dermatome is a "segmental field" of the skin innervated by one spinal.
• The borders between the dermatomes are blurred

Pathways of Somatosensory System

• Conscious pathways of the somatosensory system project to the cerebral cortex and includes the Lemniscal, Anterolateral, and the Trigeminothalamic systems.
• The unconscious pathways of the somatosensory system does not project to cerebral cortex and includes the Spinocerebellar pathways
• The unconscious pathways includes influence over muscle tone and postural adjustment

Lemniscal System

• Posterior column pathway is tactile-based for upper extremity proprioception and consciousness
• Lateral column pathway is used for lower extremity proprioception and consciousness

Anterolateral System

• The anterolateral system is responsible for thermosensation and pain with somatotopic organization.
• It contains three tracts including Spinothalamic Tr.(main; than to cortex)
• Spinoreticular Tr. (→RF → nonspecific thalamus → cortex)
• Spinomesencephalic Tr. (pain; connection to ANS and emotion and motivation cent. of limbic system).

Posterior Column Pathway

• Posterior Column Pathway processes a set of signals within the Dorsal Root Ganglion (DRG), which run to the Dorsal Column nuclei
• Within the Dorsal Column nuclei is the gracile and cuneate nc. and the overall data set crosses to the oppisite side
• From there Medial Lemniscus conveys the data to the Ventral Posterolateral Nucleus with finally processing in the Primary Sensory Cortex.

Spinothalamic Tract

• The Spinothalamic Tract processes a set of signals within the Dorsal Root Ganglion (DRG) and transmits through the Dorsal Horn of the spinal cord
• The processing signal travels up the spinal cord before sending an axon across the midline toward the VPL ncl.
• From there a signal is sent to reach the postcentral gyrus
• This functions to transmit thermoceptive and fast pain information.

Trigeminal Nuclear Complex

• The Trigeminal Nuclear Complex functions as a central path for tactile information from the face
• The signal starts with the Trigeminal Nerve to the Principal sensory trigeminal nucleus then relays the signal to the Ventral posteromedial nucleus of thalamus
• From there the signal gets sent to the Primary Sensory Cortex for final processing

Trigeminal Neural System Breakdown

• The Trigeminal Neural System is responsible for conveying tactile, pain, and temperature impulses from the skin of the face, the mucous membranes of the nasal and oral cavities.
• It also relays and processes information from the facial and masticatory muscles
• The Anterior Trigeminalthalamic tract relays information regarding discriminative information from the head, face, and oral cavity
• The Posterior Trigeminalthalamic tract relays discriminative tough and conscious proprioception

Somatosensory Cortex

• The Primary Somatosensory Cortex is located in the postcentral gyrus of the parietal lobe.
• It follows a somatotopic organization
• Contains dense tactile innervation of the orofacial region, leading to a large representation related from trigeminal nerve leading to large representation within the primary Somatosensory system
• The Secondary Somatosensory Cortex or Brodmann area 40 presents bidirectional connection with other somatosensory cortices
• Also has bidirectional connections with ventrobasal nuclear complex of thalamus
• Is responsible for tactile learning and memory
• Somatosensory Association Cortex located in Posterior Parietal Lobe

Damages to Somatosensory Cortex

• Damage to the primary somatosenory cortex can produce weakened sensation.
• Primarily effect both tactile sensation and proprioception as well as increase threshold pressure, worsens the movement sensation and position sensation of extremities
• Does not affect nociception

Amorphosynthesis

• Amorphosynthesis is damage in the somatosensory association cortex
• Inability to recognize complex object and complex forms by the process of feeling them

Somatosensory Summary

• Receptors send data through myelinated or unmyelinated primary afferent nerve fibers.
• Primary afferent neuron: cell body in DRG or ncl. of cranial nerves
• Secondary neuron: cell body in spinal cord or brain stem
• Ascending axon crosses to opposite side of CNS; axons conveying conscious sensation terminate in thalamus
• Tertiary Neuron cell body in: thalamus projection to: primary somatosensory cortex (conscious)
• UNCONSCIOUS SENSATION (proprioception) is perceived: to cerebellum
• Side of the brain receiving somatosensory neuron are contralateral to source area

Nociception

• Nociception is to protect against bigger damage
• Nociception is two component. Including: Sensroy and Motivationally-Affective

Etiology of Pain

• Nociceptive or Physiologic pain which is from functioning
• Neuropathic or Pathologic pain that is linked to trauma

Nociceptors

• Nociception - Detection of pain stimuli
• Pain stimulus: stimulus that is caused by tissue damage
• Receptors that detect pain stimulus are nociceptors
• Opening of mechanosensitive K+ channels is the result of stimulus
• Depolarization: is the action to prepare the signal for transmission
• Ca2+ channels: channels that can help modulate the signal
• Release of NT: signals must be able to arrive
• Nerve fiber Stimulation: post-synaptic response

Nociceptive Signals

• NOCICEPTIVE-SPECIFIC NEURONES:
• Nociceptors, pure pain only with small area. In spinothalmic track
• WIDE DYNAMIC RANGE NEURONES: Wide range, can distinguish intensity of pain on spinoreticular track
• Source/Pathways/Perception and treatment of these two types of pain need to be addressed differently
• A delta fibers:* Lightly myelinated, and good at: high-threshold and mechanosensitive
• Small receptive field - allows alert to the presence of pain well isolated
• C fibers:* Unmyelinated axons in which there is a polymodal in which allow large receptor Small receptive field to allow pain relay

Hyperalgesia

• Hyperalgesia is an Increased pain response to a noxious stimulus. Resulting from sensitization of tissues
• Primary and secondary areas can be affected in a stimulus
• Primary: pain from damage source.
• Secondary: pain in other areas near damage due to damaged tissues there

Visceral Pain

• Only pain receptors that cause sensation in the viscera
• Referred pain = the feel that pain is being sent to the wrong origin
• Cause: same nerve pathways the organs are in

Central Pathways in Nociceptive

• Anterolateral System processes data with a: Spinothalamic Tract, Spinoreticular Tract or Spinomesencephalic Tract
• Anterior Trigeminal Thalamic Tract assists

The Spinothalamic Tract Path

• Processing of signal happens in the DRG dorsal horn + in dorsal horm signals cross to the VPL
• VPL leads information to - post central information, thus leads thermoceptive and fast pain

Spinoreticular Function

• In dorsal horn the signal crosses the midline/ axon and is released to: hypothalamus, thalamus brain, (Motivated)

Anterolateral Path

• Signal start :Trigeminal ganglion( first order neurons)
• Then goes to Trigeminal brainstem- sensory nuclear complex ( second order
• Then is passed to the contralateral side
• then passed to :VPM : VPM Nucl and ends with : cerebral ( and limbic brain matter)

Cortical Processing

• Somatosensory: Sensory discrimination in level of pain
• Hypothalamus affect and motivation

Pain Treatment options

• Treatment by both: sensory inhibition and stimulation
• Gate control / opoids / nerve block can all modulate in some way the perception

Pain Modulation circuits

• Spinal neuron can block signals from small fiber = non noxious signal.
• Stress release can inhibit pain
• The body can release opioid that relieve signals and can help modulate brain

Chemical Senses

Chemoreceptors

• They occur in the body as chemical sensors to detect :chemical skin, ingested material
• Detects : Carbondioxide level in blood / acrity of muscles
• Parallel processing in corte