Sensory Receptors and Neural Pathways

• Sensory receptors transduce various forms of energy from the real world into nerve impulses.
• Different modalities of sensations, like sound, light, and pressure, result from variations in neural pathways and synaptic connections.
• The brain interprets impulses based on the nerve delivering it; for example, an impulse from the optic nerve is perceived as light.

Functional Categories Of Sensory Receptors

• Chemoreceptors detect chemicals in the environment (taste, smell) or in blood.
• Photoreceptors detect light.
• Thermoreceptors respond to cold or heat.
• Mechanoreceptors are stimulated by mechanical deformation, such as touch and hearing.
• Nociceptors are pain receptors that depolarize in response to tissue damage
• Nociceptor stimuli consist of heat, cold, pressure, or chemicals.
• The primary neurotransmitters for nociceptors include glutamate and substance P.
• Emotions, concepts, and expectations can heighten pain perception.
• Pain reduction relies mainly on endogenous opioids.
• Proprioceptors, found in muscles, tendons, and joints, provide a sense of body position and enable fine muscle control.
• Cutaneous (skin) receptors perceive touch, pressure, heat, cold, and pain.
• Special senses include vision, hearing, taste, smell, and equilibrium.
• Exteroceptors respond to stimuli from outside the body, such as cutaneous receptors and special senses.
• Interoceptors respond to internal stimuli, monitoring blood pressure, pH, and oxygen concentrations within organs.

Tonic and Phasic Receptors

• Receptors are categorized by their response to stimuli.
• Phasic receptors respond with an initial burst of activity, then quickly adapt and decrease their response, giving on and off information.
  • Phasic receptors alert us to environmental changes, enabling sensory adaptation where we cease to pay attention to constant stimuli.
  • Examples of phasic receptors include those for smell, touch, and temperature.
• Tonic receptors maintain a high firing rate as long as a stimulus is applied; an example is pain.
• Information from a given nerve fiber is experienced as one stimulus type.
• Brain perception depends on the adequate or normal stimulus
• Paradoxical cold refers to the perception of cold at different temperatures.

Generator (Receptor) Potential

• Receptors behave similarly to neuron dendrites.
• Stimuli cause depolarizations called generator potentials, similar to EPSPs.
• A light touch on a pacinian corpuscle in the skin creates a small generator potential.
• Increasing pressure raises the magnitude of the generator potential until threshold is reached and an action potential is generated.
• The generator potential is a graded response.
• Pacinian corpuscles are phasic receptors; if pressure is maintained, the generator potential diminishes due to the structure of the onion skin covering of the receptor.
• In tonic receptors, the generator potential is proportional to the stimulus intensity where increased intensity results in an increased action potential frequency after reaching threshold.

Cutaneous Sensations

• Pain, cold, and heat receptors are naked dendrites of sensory neurons.
• Touch and pressure receptors have special structures around their dendrites.
  • These structures include Merkel’s disks, Meissner’s corpuscles, Pacinian corpuscles, and Ruffini corpuscles.
• There are more cold receptors than hot receptors.
• Cold receptors are located close to the epidermis, stimulated by cold and inhibited by warmth, and some respond to menthol.
• The response temperature range for cold receptors is 8 – 28°C.
• Cold receptors serve as ion channels for sodium and calcium using a transient receptor potential (TRP) channel.
• Warm receptors are located deeper in the dermis, are excited by warming and inhibited by cooling and are different from receptors detecting painful heat.
• The special nociceptor sensing pain from a hot stimulus is named capsaicin receptor.
• Hot receptors function as an ion channel for sodium and calcium - a TRP channel
• Hot receptors react to capsaicin, a chemical in chili peppers, and activate at 43° C or higher.
• Nociceptors, which can be myelinated or unmyelinated, transmit sudden, sharp pain via myelinated neurons, and dull, persistent pain signals through unmyelinated neurons.
• Nociceptors are activated by chemicals, such as ATP, released from damaged tissues or from pH changes or mechanical stimuli.
• Acute itch arises from histamine release from mast cells and basophils.
• Chronic itch is stimulated by other chemicals and does not respond to antihistamines.
• Itch receptors stimulate unmyelinated sensory axons to the spinal cord.

Neural Pathways for Somatesthetic Sensations

• Pressure receptors and proprioceptors are carried by large myelinated fibers ascending the dorsal columns of the spinal cord on the ipsilateral side.
• These fibers synapse in the medulla oblongata and their second tier of neurons cross sides to ascend the medial lemniscus to the thalamus, where they synapse.
• The third-order neurons then proceed to the postcentral gyrus in the parietal lobe (somatosensory cortex).
• Heat, cold, and pain receptors are first carried into the spinal cord by thin myelinated and unmyelinated neurons to the lamina of the dorsal gray horns.
• Synapses within the spinal cord occur onto a second-order neuron, then cross sides and ascend the lateral spinothalamic tract.
• Following this, synapses on third-order neurons in the thalamus happen, then continue to the postcentral gyrus.
• Emotional response to pain arises as information travels from the thalamus to the anterior cingulate gyrus that is part of the limbic system.
• Somatic pain information can synapse on the same interneuron as a neuron carrying visceral pain information, and the brain interprets referred pain, such as heart pain as arm pain, or gallbladder pain as back pain.

Sensory Acuity and Inhibition

• The receptive field is the area of skin that, when stimulated, alters the firing rate of a neuron.
  • The size of a receptive field depends on the density of receptors in that skin region.
  • Receptive fields are large in areas with few receptors, such as the back and legs.
  • Regions with many receptors, like fingertips, have small receptive fields.
  • A smaller field correlates with greater tactile acuity (sharpness of sensation) and consequently a larger area of the somatosensory cortex.
• Two-point touch threshold is used to measure receptive fields by gauging the shortest distance at which a person can perceive two separate points of touch and measure tactile acuity.
• This measurement is important in spacing the raised dots in Braille symbols.
• Lateral inhibition happens when touch is strongest prompting stronger receptor stimulation than lighter touch areas.
  • Receptors most strongly stimulated inhibit those around them, leading to well-defined sensations at a single location rather than a "fuzzy" border.
  • Lateral inhibition occurs in the CNS.
• Chemoreceptors are the sensory receptors for taste and smell.
  • Interoceptors are chemoreceptors that detect chemical changes inside the body.
  • Exteroceptors , detect chemical changes from outside, such as taste and smell.
    • Taste responds to chemicals dissolved in food and drink.
    • Smell responds to airborne chemical molecules, and olfaction greatly influences gustation (tasting).

Taste and Smell

• Taste, also called gustation, receptors are in taste buds and consist of 50-100 specialized epithelial cells featuring long microvilli extending into the mouth's environment through a pore in the taste bud.
• Taste buds are situated in bumps on the tongue known as papillae.
  • Fungiform papillae are on the anterior surface where information travels via the facial nerve.
  • Circumvallate papillae are located on the posterior surface where information travels via the glossopharyngeal nerve.
  • Foliate papillae, are on the sides, and information also travels via the glossopharyngeal nerve.
• Taste pathways go through facial and glossopharyngeal nerves, the medulla oblongata, and the thalamus.
• The taste pathways lead to the primary gustatory cortex of insula, somatosensory cortex of parietal lobe, and prefrontal cortex.
• Taste cells of taste buds are specialized epithelial cells behaving like neurons, depolarizing and producing action potentials.
• Cells release neurotransmitters onto sensory neurons.
• Microvilli gather and are sensitive to each category of tastes.
• Each taste bud has taste cells sensitive to each category of tastes.
• The categories of taste include salty, sour, sweet, umami (meaty), and bitter.
• All regions of the tongue have taste buds for all the taste categories.
• Temperature and texture of a substance influence taste.
• The mechanisms of taste involve the entrance of Na+ into taste cells for salty tastes, and H+ for sour tastes, which depolarize it.
• Sugar or glutamate, for sweet and umami tastes, binds to receptors which activates G-proteins/2nd messengers to close K+ channels.
• Quinine, for bitter, binds to receptors and activates G-protein/2nd messengers to release Ca2+ and is very sensitive to low concentrations for protection.
• G-proteins for taste are known as gustducins.
• In the sweet system, sugar triggers adenylate cyclase, producing cAMP.
• IP3 (inositol triphosphate) and DAG (diacylglycerol may also be activated).
• Smell is also called olfaction.
• Olfactory receptors are located in the olfactory epithelium in the nasal cavity.
• Sustentacular cells oxidize hydrophobic volatile odors.
• Basal stem cells replace receptors damaged by the environment.
• Olfactory receptors constitute bipolar neurons equipped with ciliated dendrites that project into the nasal cavity
• Proteins in the cilia of olfactory receptors bind to odors
• Approximately 380 genes code for around 380 different olfactory receptors.
• Generally, a single odorant molecule stimulates one protein.

How Smell Works

• The process of smell is G-protein coupled.
• Odor binding leads to the activation of adenylate cyclase, resulting in the production of cAMP and PPi (pyrophosphate).
• The production of cAMP causes Na+ and Ca2+ channels to open.
• The opening of Ca2+ and Na+ channels prodcues a graded depolarization which stimulates the action potential.
• Because of up to 50 G-proteins to one receptor protein there is great sensitivity through amplification
• Olfactory neurons are unmyelinated and synapse on a glomerulus in the olfactory bulb.
  • Each type of olfactory receptor synapses on one glomerulus.
  • A flower comprised of complex odor molecules may excite multiple types of odor receptors.
  • This is coded according to which glomeruli are stimulated.
  • Odor identification improves with lateral inhibition.
• The mitral and tufted neurons of the glomeruli in the olfactory bulb synapse on the primary olfactory cortex of the frontal and parietal lobes.
• Interconnections are made with the amydgala, hippocampus, and limbic system through the piriform cortex
• The prefrontal cortex also receives taste information, to connect the two senses

Vestibular Apparatus

• The vestibular apparatus provides a sense of equilibrium.
• It's located in the inner ear.
• The apparatus consists of otolith organs and semicircular canals.
  • Otolith organs, containing the utricle and saccule, detect linear acceleration such as in the utricle (horizontal), and the saccule (vertical).
  • Semicircular canals recognize rotational acceleration.
• The inner ear includes a bony labyrinth encircling a membranous labyrinth.
• Between the two labyrinths is perilymph.
• The membranous labyrinth holds endolymph, which features an unusually high K+ concentration, leading to depolarization.
• Sensory hair cells are modified epithelial cells having 20−50 hairlike extensions called stereocilia (not true cilia) and one kinocilium (true cilium).
• When stereocilia bend toward the kinocilium, K+ channels open, causing K+ to rush into the cell and depolarizing it.
• Cells then release a neurotransmitter that depolarizes sensory dendrites in the vestibulocochlear nerve.
• Bending away from the kinocilium hyperpolarizes sensory dendrites, allowing for a coding system to detect direction.
• The macula, a specialized epithelium, houses hair cells.
  • Stereocilia are lodged inside the gelatinous otolithic membrane.
  • The gel contains calcium carbonate crystals known as otoliths (ear stones).
• Each canal contains a semicircular duct filled with endolymph.
  • Each duct’s base features an enlarged region recognized as the ampulla.
  • Hair cells are positioned in the crista ampullaris, where stereocilia are stuck into a gelatinous cupula.
• Rotation makes the endolymph circulate, pushing the cupula and bending the hair cells.
• The vestibulocochlear nerve synapses in the vestibular nuclei of the medulla and in the cerebellum.
• The medulla directs neurons to the oculomotor area of the brain stem to regulate eye movements, and down the spinal cord to influence body movements.
• Nystagmus is a jerky eye movement from vertigo, a loss of equilibrium.
  • When a person's body is spinning, eye movements shift toward the opposite direction of the spin to maintain a fixation point.
  • When the body stops, the cupula bends due to fluid inertia and affects eye movement.
  • Nystagmus can be accompanied by dizziness, pallor, sweating, nausea, and vomiting.

Ears and Hearing

• Sound wave characteristics include:
  • Frequency, is measured in hertz (Hz), and is related to higher frequencies of sounds having higher pitches where the human hearing range is 20−20,000 Hz.
  • Intensity or loudness is measured in decibels, is related to the amplitude of the wave, within Human optimal range is 0-80 dB.
• Within the outer ear, sound waves are channeled by the pinna (or auricle) into the external auditory meatus, which directs them to the tympanic membrane (eardrum).
• The middle ear includes an air-filled cavity between the tympanic membrane and the cochlea, and ossicles.
  • The ossicles consist of: malleus → incus → stapes
  • Vibrations are transmitted and amplified along the bones.
  • The stapes is connected to the oval window, transferring vibrations into the cochlea.
  • The stapedius muscle dampens the stapes with intense sound.
• The cochlea is the hearing part of the inner ear, and has three chambers: -The upper chamber is a portion of the bony labyrinth called the scala vestibuli.
  • The lower bony chamber is called the scala tympani.
  • Both chambers are filled with perilymph.
• The cochlea includes a portion of the membranous labyrinth called the scala media, or cochlear duct, filled with endolymph where the middle chamber has three turns.
• The helicotrema connects the scala vestibuli to the scala tympani.
• Sound transmission vibrates from the oval window of the middle ear displacing perilymph in the scala vestibuli.
  • Vibrations go through the vestibular membrane into the cochlear duct through the endolymph.
  • Then, vibrations travels the basilar membrane into the perilymph of the scala tympani.
  • Vibrations leave through the round window out of the inner ear
• Sound waves travel through the cochlear duct by locations according to frequency.
  • Low-frequency sounds travel further down the spiral of the cochlea to the apex and high-frequency sounds are closer to the base.
• Sensory hair cells are positioned on the basilar membrane, and project into the endolymph of the cochlear duct.
  • Inner hair cells are for 3500 that form one row along the basilar membrane. Each is innervated by 10−20 sensory neurons of cranial nerve VIII for sound relay.
  • Outer hair cells are 11,500 arranged in rows with 3 rows per turn, and are innervated by motor neurons that shorten with depolarization and elongate when hyperpolarized.
  • Hairs are stereocilia that are large microvilli arranged in bundles.
• Stereocilia in a bundle get larger in size stepwise and interconnect, becoming embedded in the gelatinous tectorial membrane.
• The spiral organ is made of the basilar membrane, inner hair cells with sensory fibers, and the tectorial membrane.
• Upon entry of sound waves into the scala media, the tectorial membrane vibrates, bending stereocilia.
  • The bending opens K+ channels facing the endolymph.
  • K+ travels in, depolarizing the cell.
  • Glutamate is released onto sensory neurons.
  • K+ journeys back to perilymph at the stereocilia base.
• With more basilar membrane displacement and stereocilia bending, more glutamate exits, raising the receptor potential.
• Place Theory of Pitch: hair cells near the location where vibrations are displaced into the scala media are stimulated.
• This theory provides the neural code for pitch discrimination given changes in length.
• Outer hair cells magnify this effect and differentiate between similar pitches.
• Sound localization comes from interaural intensity and time differences.

Neural Pathways for Hearing

• Neural pathways for hearing include: vestibulocochlear nerve → cochlear nuclei in the medulla oblongata & pons → inferior colliculus of midbrain → medial geniculate body of the thalamus → auditory cortex of temporal lobe.
• Cochlear nuclei and the auditory cortex are tonotopic: specific areas represent different sound frequencies.

Hearing Impairment

• Sound waves are not conducted with conduction deafness, impeding hearing.

  • The common causes may be a buildup of earwax, fluid in the middle ear, eardrum damage, or overgrowth of bone in the middle ear.

  • The impairment influences all sound frequencies and the condition responds well to hearing aid devices.

• Impulses are not conducted towards sensorineural/perceptive deafness, which may be due to damaged hair cells (from loud noises). Conduction deafness does not conduct sound to the cochlea.
  • Only particular sound frequencies are impaired, but cochlear implants can aid hearing in the client.
• Presbycusis is an age-related hearing impairment.