Sensory Transduction & Sensory Systems

Sensory Receptors

Membrane proteins that bind substances (NT, hormones, ligands), and specialized cells that respond to environmental stimuli.

Specialized Epithelial Cells

Visual, taste & auditory receptors

Somatosensory

Pain, pressure, temperature, proprioception

Sensory Conditioning

Receptors varies to accomodate to stimuli

Sensory Transduction

Energy activating a receptor is converted into information processed by the CNS. Involves ionotropic or metabotropic receptors and initiates receptor membrane potential change.

Transducers

Convert 1 type of stimulus (light, sound, pressure) to another (electrical, AP or receptor potential)

Sensory Receptors Specialized

Produce an electrical signal in response to an adequate stimulus from their environment.

Receptor Potential (RCP)

A depolarizing or hyperpolarizing signal is produced by an adequate stimulus acting on a sensory cell. Amplitude is proportional to stimulus intensity

Initiation of RCP

By a change in permeability of the receptor cell membrane (ion movement)

Receptor Potentials Resemble to Postsynaptic Potential

Conducted, graded, and initiated by a flow of ions through membrane channels.

Receptors with RCPs not APs

Photoreceptors, hair cells of hearing vestibular systems, taste buds

Receptors that generate APs

Olfactory, pain/temperature/pressure receptors, proprioception

Taste Bud

An epithelial cell that produces receptor potential and can trigger APs in the neuron with that it synapses. Sensory receptor that is NOT a neuron

Neuronal Sensory Receptor

Stimulus generates a receptor potential, and that receptor potential is converted to axon potentials as it moves into the axon, especially the axon hillock (trigger zone)

Receptor is a Neuron

Receptor portion of the membrane generates receptor potential which are then converted into APs in the axon.

Types of Receptor

Light, chemical, temperature, tissue damage, mechanical deformation

Light

Photoreceptor in the retina

Chemical

Chemoreceptor in the tongue, nose, carotid bodies, hypothalamus osmolarity, glucose

Temperature

Thermoreceptors in skin and hypothalamus

Tissue Damage

Nociceptor (pain) in the skin, muscle, internal organs

Mechanical Deformation

Mechanoreceptor everywhere

Chemoreceptor

Respond to chemical ligands, pH, taste, olfactory

Mechanoreceptors

Respond to mechanical energy P - skin, heart, blood vessels, sound, vestibular

Thermoreceptors

Respond to temperature

Photoreceptors

Respond to photons (light)

Nociceptors

Responds to extremes of P, temp., and noxious chemicals

Special Senses

Vision, hearing, taste, smell, equilibrium

Simple Receptors

Free nerve endings with unmyelinated axon and cell body (pain and thermoreceptors)

Complex Neural Receptors

Enclosed capsule looking for specific stimulus, with myelinated axon and cell body (pacinian corpuscle, ruffini endings, meissner’s corpuscle)

Special Sense Receptor

Epithelial release NT onto sensory neurons initiating AP (taste and inner ear, photoreceptors)

Intensity Coded

Number of receptors activated, differences in neuron firing rates in the sensory pathway, and activation of different types of receptors

Determining Intensity

1. AP frequency is proportional to stimulus (greater intensity = more APs).

2. Number of receptors activated (greater intensity = greater # of receptors activated)

Receptive Fields

Enhanced by lateral inhibition, increases contrast between an active receptor and inactive (minimally) neighbors. This makes one specific neuron fired more than others for increased specificity on area

Adaptation of Sensory Receptors

Observed when a constant stimulus is applied for a given timeframe. Initially, the frequency of APs is high, but drops with time (even if the stimulus is still there).

Threshold

Minimum stimulus that can be detected. Large enough stimuli create APs.

Duration

Coded by the duration of fired APs. Some receptors can adapt or cease to respond during prolonged stimulation.

Phasic Receptors

Rapidly adapt to constant stimulus and turn off

Types Tonic Receptors

Ruffini endings, merkel discs

Types Phasic Receptors

Meissner corpuscle, hair follicle, krause bulbs, pacinian corpuscle

Tonic Receptors

Slowly adapting receptors that respond for the duration of stimulus.

CNS Interpretation

Distinguish the different adequate stimuli from one another and the intensity of stimuli (AP frequency is proportional to intensity)

Tuning

Receptors are tuned to specific adequate stimuli. A specific receptor type is normally only stimulated by one stimulus, the adequate stimulus (wavelengths of photons for receptor to be stimulated)

Label Line Principle

Axons carrying APs from specific sensory stimuli go only to specific areas of the brain. Each sensory stimulus has a specific pathway into and through the CNS. Specific areas of the brain each produce a unique subjective sensation that we interpret as a stimulus.

Taste Buds

Modified epithelial cells (destroyed and replaced by basal cells). Molecules we perceive as having a taste bind to membrane receptors on microvilli of receptor cells (flavors). It can either depolarize or hyperpolarize.

Anterior 2/3 of Tongue

Soft palate by Facial (CN VII) - cell bodies in geniculate ganglion

Posterior 1/3 of Tongue

Glossopharyngeal (CN IX) - cell bodies in the petrosal (inferior) ganglion

Epiglottis

Vagus (CN X) - cell bodies in the nodose ganglion

Teeth Sensation

Pain

Facial Nerve (CN VII) Cell Body

Geniculate Ganglion

Glossopharyngeal (CN IX) Cell Body

Petrosal (inferior) Ganglion

Vagus (X) Cell Body

Nodose Ganglion

Sensory Fibers of Mouth

Trigeminal Nerve (V2 & V3), maxillary and mandibular branches - cell bodies located in semilunar or trigeminal ganglion and terminate in pons

Trigeminal Nerve (V) Cell Body

Semilunar or Trigeminal Ganglion - Terminates in Pons

Pulpal Sensitivty

Pain forces on an individual tooth from causes such as cracked tooth, tooth decay, or grinding teeth

Dentinal Sensitivity

More widespread pain from the enamel damage or when stimuli such as heat, cold, or acid reach the nerve endings in the dentin

Synesthesia

Perceptual condition in which stimulation of one sense or cognitive pathway leads to involuntary associaton of another sense or pathway.

Mechanoreceptors

Touch - Merkel Cells, Ruffini Endings, Meissner Corpuscle, Pacinian Corpuscle

Nocireceptor

Pain & Temp. - Free Nerve EndingsM

Meissner’s Corpuscle

Rapidly adapting mechanoreceptor - touch and pressure (phasic)

Merkel’s Corpuscle

Slowly adapting mechanoreceptor - touch and pressure (tonic)

Free Nerve Endings

Slowly adapting nocireceptors, itch receptors, thermoreceptors, and mechanoreceptors (tonic)

Pacinian Corpuscle

Rapidly adapting mechanoreceptor for skin stretch (phasic)

First Order Sensory Afferent Neurons

Cell bodies outside of the CNS are in the dorsal root ganglia. Receive information through receptors and then transmit it.

1st Order Pain Fibers

Nociceptors deliver pain to the spinal cord, with cell bodies in dorsal root ganglia. Terminate in dorsal horn at level of entry and synapse.

Second-Order Sensory Afferent Neurons

Relay nuclei (spinal cord or brainstem), and decussate, mostly relaying information to the thalamus. Interneurons that are excitatory or inhibitory

Third-Order Sensory Afferent Neurons

The thalamus, where all sensory information is processed before going to the cerebral hemispheres.

Fourth-Order Sensory Afferent Neurons

Cerebral cortex, integrate complex sensory information

Spinothalamic Tracts

Carry pain, pressure, and crude touch input to the brain

Brown-Sequard Syndrome

Injury to ½ of the spinal cord (R or L), loss of function will occur on the ipsilateral side for nerve tracts that crossed in the brainstem, and on the contralateral side of fibers that cross at lower levels in the spinal cord. Causes paralysis and loss of proprioception on same side, and loss of pain and temperature on contralateral sides.