Peripheral Nervous System

• The peripheral nervous system (PNS) conveys information to and from the central nervous system (CNS), connecting the brain and spinal cord to the rest of the body
• The PNS consists of cranial nerves, spinal nerves, roots and branches, peripheral nerves, and neuromuscular junctions

Afferent (Sensory) Division

• The afferent division converts stimulus information into action potentials and transmits them to the CNS
• Afferent division includes visceral afferent, sensory afferent, and afferent nerves
• Visceral afferent involves visceral pain, nausea, bloating, and dyspnea
• Sensory afferent involves somatic and special senses
• Somatic sensation refers to body sensations like touch, pain, itch, temperature, vibration, and proprioception
• Special senses have specialized organs for vision, hearing, equilibrium, smell, and taste
• Afferent nerves are composed of afferent neurons

Sensory Transduction

• Sensory transduction is the process where a receptor converts the stimulus to an action potential that is transmitted to the CNS
• Conscious awareness of a sensation leads to perception

Amplification of Sensory Information

• Amplification strengthens the stimulus signal, involving signal transduction pathways and accessory structures of the ear
• Key elements to signal transduction pathway include disc and cell membranes, photopigment and cGMP
• Parts of the ear that serve as accessory structures Pinna, temporal bone, malleus, incus, semicircular canal, stapes, round window, cereum, external auditory canal, tympanic membrane, eustachian tube, and the cochlea

Sensory Adaptation

• Sensory adaptation decreases the sensory neurons responsiveness to a constant stimulus
• Some receptors are slowly adapting: Constant stimulus slowly decays, firing action potentials
• Example for of slowly adapting receptors are joints and muscles for the maintenance of steady postures
• Rapidly adapting receptors generate action potentials at the onset and the removal of the stimulus
• Example, skin-vibration and pressure

Sensory Receptors

• Sensory receptors convert stimulus to graded potentials (sensory transduction), requiring an adequate stimulus
• Some receptors respond to other forms of stimulus at high enough intensity, like seeing light when pressing the eye
• Receptor potential refers to the change in sensory receptor membrane potential
• Threshold is the minimum depolarization required to trigger action potential
• There are different receptor is afferent neurons, and receptor regulates the afferent neuron

Sensory Receptor Action

• Opening and closing of channels in sensory receptors trigger changes in ion flux and membrane potential
• Passing the threshold triggers action potentials
• Increased stimulus intensity can increase receptor potential, action potential frequency, and neurotransmitter release

Sensory Adaptation

• Sensory adaptation is the decreased action potential frequency despite constant stimulus presence
• Rapidly adapting (RA) receptors give an on response at stimulus onset, and an off response at stimulus removal
• Slowly adapting (SA) receptors cause a constant stimulus that triggers slowly decaying receptor potential and continuous action potentials.

Receptive Fields

• Receptive field is the area that triggers an afferent neuron
• Convergence creates large receptive fields, while sensitive areas have smaller receptive fields

Acuity

• Acuity is the precision of stimulus location, influenced by sensory unit size and density
• The information from a neuron indicates the stimulus location more precisely if the neuron's receptive field is smaller, with greater sensory unit concentration

Signal Intensity

• Signal intensity is demonstrated by action potentials in an afferent fiber that lead from pressure receptors of an adapting, single sensory unit which increase in frequency

Lateral Inhibition

• Sensory information of adjacent neurons are inhibited by the afferent neuron at the stimulate area's center
• Afferent neurons activate inhibitory neurons that suppress action potential of the adjacent neurons
• Afferent neurons at the stimulus center have a higher initial potential frequency
• Lateral Inhibition acts to localize the site of a stimulus

Sensory Receptors in the Skin

• Free nerve endings respond to temperature, noxious stimuli, and hair movement.
• Meissner's corpuscles respond to flutter and stroking.
• Pacinian corpuscles respond to vibration.
• Ruffini corpuscles respond to stretch.
• Merkel receptors respond to steady pressure and texture.

Sensation

• Somatic sensations include pain and itch
• Special sensations include smell, taste, vision, hearing and equilibrium

Pain

• Pain is subjective and multidimensional and has nociceptors - free nerve endings with little or no myelination, found in skin, joints, muscles, bones, and internal organs
• Nociceptors include mechanical, thermal, and polymodal
• Mechanical nociceptors respond to strong mechanical deformation
• Thermal nociceptors respond to extreme temperatures below 10°C or above 45°C
• Polymodal nociceptors respond to mechanical, temperature, and chemical stimuli from damaged or immune cells
• Fast pain is sharp and localized using A-delta fibers with myelinated axons at 12-30 m/sec
• Slow pain is aching and not well localized using C-fibers with unmyelinated axons at 0.5-2m/sec
• Main neurotransmitters from first-order pain neurons are glutamate and substance P, and activate the next neuron in the pathway

Pain Pathways

• Spinal reflex pathways activate reflexive, protective responses unconsciously.
• Ascending pathways to the cerebral cortex create conscious sensations of pain/itch
• The process is that painful stimuli activates the Afferent pain fiber sending pain signals from the periphery to the CNS
• The signal is then relayed to the somatosensory cortex

Referred Pain

• Pain felt at sites besides the injury
• Somatic and visceral nociceptors often converge on second-order neurons

Hyperalgesia

• The feeling of pain that remains after the stimulus is gone can be from stimulating chemicals present, or, that the nocieptors adapt slowly or may not adapt at all

Inhibition of Pain

• Analgesia: Pain suppression without losing consciousness and requires the use of pain medications, exogenous analgesia, and mechanical stimuli

Endogenous Analgesia System

• Some descending pathways from the brain suppress pain signal transmission by promoting morphine-like opioid release
• The system's pathways descend from the brain to spinal cord to suppress incoming pain signals from nociceptors

Pain Killer Drugs

• Analgesics such as aspirin and ibuprofen, cause analgesia by blocking formation of prostaglandins and suppressing the transmission from first-order pain neurons to second-order neurons with morphine
• Anesthetics block pain and other sensations and take the form of general anesthesia - for entire body resulting in unconsciousness, or local anesthesia - a specific area that does not affect consciousness and include the use of procaine and lidocaine that block opening of voltage-gated Na+ channels

Mechanical Stimulus & Inhibition of Pain

• Touch, pressure, and vibration can achieve inhibition, acting via the gate control theory of pain
• Transcutaneous electrical nerve stimulation (TENS) via electrodes on skin activates A-beta fibers from mechanoreceptors, inhibiting pain

Nitrous Oxide & Inhibition of Pain

• Nitrous oxide during childbirth induces opioid peptide release in the brain stem
• Promotes endorphin and dopamine release in the brain

Epidural Anesthesia & Inhibition of Pain

• Uses local aesthetic drugs derived from cocaine
• Injected into the epidural space that covers the dura coverings of the spinal cord
• Early use blocked sensory and motor nerves
• Presently used by combining with opiate drugs to reduce motor block, allows for walking after epidural

Itch

• The science behind Itch is not well understood
• May be due to Nociceptors in the skin and stimulated by mechanical and or chemical stimuli

Smell Sensation

• Olfaction is the sense of smell, achieved via a 5 cm² olfactory epithelium
• Olfactory receptor cells (chemical receptors) that contain 'olfactory receptor proteins' in olfactory cilia, detect odorants
• There are roughly 10 million olfactory receptor proteins in humans, with ~ 400 types that react to only a select group of odorants and find only one type in a receptor cell
• Humans can recognize ~ 10,000 odors
• Supporting cells give physical support and detoxify chemicals
• Basal cells are stem cells that create olfactory receptor cells (~2 months lifespan)
• Olfactory glands produce mucus to moisten the olfactory epithelium
• It operates at a low threshold requiring only a few molecules
• Odorant molecules activate a G protein and adenylyl cyclase, producing cAMP that opens cation channels to allow Na+ and Ca2+ to enter the olfactory receptor and generate an action potential down the axon of the olfactory receptor cell, through depolarizing receptor potentials
• Information from Each glomerulus is transferred through one type of olfactory receptor
• Mitral cells are the second-order neurons in this connection

Taste Sensation

• Gustation involves 5 primary tastes: Salty (Na+), sour (H+), sweet (sugar e.g. glucose, fructose, sucrose, saccharin, aspartame, sucralose), bitter (e.g. caffeine, morphine, quinine), & umami (amino acid especially glutamate)
• Taste buds on the tongue (mostly), pharynx & epiglottis have Gustatory receptor cells (~10 days life span) that detect only one primary taste using Tastants
• There are supporting and basal cells
• Basal cells stem cells give rise to supporting cells that in turn become gustatory receptor cells
• Taste is a combination of these primary tastes & accompany odor, tactile, & temperature
• Gustatory receptor cells can respond to only one type of tastant
• Taste transduction of salty or sour tastants involves direct movement, while sweet, bitter, or umani transduce via G coupled receptors
• The gustatory nucleus on the medulla oblongata first processes the taste information
• It relays some info to the limbic system and the hypothalamus
• Some is relayed to the thalamus through gustatory cortex for conscious awareness of sensations

Vision Sensation

• Visible light uses ~400 - 700 nm wavelengths of electromagnetic spectrum
• The color perceived depends on wavelength, and object color the color of the wavelength that the object reflects

Vision Components

• The parts of the eye consist of three layers, a lens, and two cavities

• These layers are the outer, middle, and inner layer

• The outer layer is the Cornea and is a Sclera

• The middle layer is a Choroid, Ciliary body and Iris

• The inner layer is the Retina

• Cornea: admits light into the eye using the Len and cornea to focus light onto the retina

• Sclera: a tough coat of connective tissue, supports the eye's shape

• Choroid: Contains blood vessels nurturing retina using Melanin that absorbs stray light rays preventing reflection and scattering of light within the eye

• Ciliary body: Produce aqueous humor. Zonal fibers: that attach to the lens, altering the shape of lens for viewing nearby and further objects

• Iris: eye color, regulates the amount of light that enters the eye by adjusting pupil diameter

• Retina: Pigmented layer similar to chordoid, with a Neural layer a multilayered outgrowth of the brain

• The neural Retina is composed of a Photoreceptor layer, bipolar cell layer and ganglion cell layer

• Fovea is the site of a visual acuity

• Macula lutea is the center of the posterior retina for Central vision, and composed of the Fovea, as the location of highest resolution

• Optic disc that exits eyeballs of optical II nerve contains blind spot where there aren't photoreceptors, and the brain fills in the missing information

Photoreceptors Vision:

• Rods: ~120 million cells for use in dim light and for responding to a single photon for creating photopigments (rhodopsin) that absorb most wavelength and cannot distinguish colors - only blacks and whites
• Cones: ~6 million cells providing Low sensitivity and requiring hundreds of photons for Day vision and of different types
• Cones absorb different wavelengths to see more than 3 colors, and its colors are interpreted in the Brain after processing its three types
• Retinal: light absorbing component
• Opsin: 4 types in human (1 in rods, 3 in cones) for different absorbed wavelength
• When cis-retinal absorbs a photon of light, it converts and isomerizes to the the trans- retinal
• With isomerizing, chemical changes occur in the outer segment that lead to receptor potential
• Pigmented layer: Store a lot of retinal > regenerate rods
• Cones regenerate much faster, and both do not depend on pigment
• Rods have 5-minute regeneration vs cones: 90-seconds

Phototransduction

• Is the process by which light energy converts into a receptor potential and releases the rod cell at the synaptic area

• Requires the processes of hyperpolarization and having a resting membrane with a average -40mV

• In the fovea, convergence is minimal because many cones create lower sensitivity with sharper images

• Because Single photoreceptor has a single bipolar cell, many Bipolar cells lead to single ganglion cells - create higher light sensitivity but lower resolution

Hearing Sensation

• Requires the external, middle, and innerear
• The external ear collects and directs sound inward
• The middle ear conveys sound vibrations to the inner ear
• The parts of the Auditory ossicles: malleus (hammer), incus (anvil), and stapes (stirrup) transmit and amplify the vibrations to the aural windows
• When the Inner ear is faced with loud noises, the inner ear can be protected via skeletal muscle (Tensor tympani muscle), limiting vibrations of the tympanic membrane to prevent the Stapedius mucsles from decreasing movements of stapes
• The inner ear can experience with a Bony labyrinth (cochlea {hearing}and vestibule +semicircular canals {equilibrium}
• Also is a Membranous labyrinth that is Perilymph, fluids similar to cerebrospinal fluid with high K+ for generating auditory signals

Properties of Sound Waves - Pitch and Intensity

• These Sound waves are alternating high- and low-pressure areas that originate from a vibrating object
• The Pitch tone is it's Frequency measured in hertz with normal hearing ranging between 20 to 20,000Hz, accurately between 500 - 5,000Hz
• Intensity is a way to express sound based on the amplitude (decibel) - a different in pressure between areas Larger amplitudes indicate louder sound
• High amplitudes near 140 dB are known to cause pain

Conversion of Mechanics to sound in Hearing

• Requires vibrations of the cilia cells to bend from mechanically gated cation channels open from the hair the high amounts of potassium entering and creates depolarizing receptor actions with channels opening as they are released to send action potentials

Hearing Pitch

• The basilar membrane is "tuned’ to particular pitches or frequencies to perceive the sound

Auditory Pathway

• From the hair cells the auditory information is conveyed along the cochlear branch of the vestibulocochlear (VIII) nerve to reach the brain, including parts of the thalamus and cerebral cortex
• A loss of function is known as deafness. Conduction deafness is where sound can not flow to the cochlea, where as sensorineural inhibits actions to process it including in areas responsible for the ear brain communication, leading to potential damage

Equilibrium Sensation

• Uses parts of vestibular, including: utricle, saccule, and semicircular ducts

Otolithic organs

• Saccule and Utricle are useful during linear acceleration and deceleration
• A Macula in between Saccule and utricle that are used primarily as hair cells
• The Otolithic membrane surrounds the hair cells. The Otolith is a layer of calcium carbonate crystals that are used in upright positioned heads
• The Horizontal Macula of utricle in upright heads accelerates - decelerates, with tilts
• At the Vertically oriented Macula saccule is used for all elevated momentums

Otolithic Organs

• Uses Rotational acceleration with ampula and cristas with various supporting components such that Cupuals to direct gelatinous hair at the opposite ends and also using endolymph to guide this direction
• From hair vestibular ,saccule+semicirculars the vestibular info is conveyed down the vertibrular branch that controls eye movement
• Vestibuli nuclei control info from the vertibulanis, along with somatic receivers in the neck muscles
• Sends commands to Cranial nerve for quick actions
• Vestibular has muscle tones to do its equilibrium
• thalmis>vertibular and have to be constant such as awreness of head