Sensory Systems and Receptors

The Sensory Systems

Somatosensory – sensory innervation to the skin, muscles, joints, epithelial, mucosal surfaces (tongue, oral cavity, etc.)

Visceral sensory – sensory innervation to the cervical, thoracic, abdominal viscera)

Other: Olfactory (smell), Visual, Auditory, Vestiblular, Gustatory (taste)

Basic Plan – all sensory neurons (except optic and olfactory) have same structure

1^st order neuron – primary sensory neuron in PNS (usually dorsal root or cranial nerve ganglion) even if associated with a special sensory neuron

2^nd order neuron – brainstem or spinal cord projects up to thalamus

3^rd order neuron – thalamus projects to cortex

4^th order – within cortex (processing)

Response properties of neurons

1^st order neuron – intrinsic properties of the neuron and associated receptor specialization (i.e. does NOT receive synaptic inputs which modify their properties) so NO synapses in sensory ganglia

Higher order neurons (2^nd, etc.) – intrinsic properties of the neuron AND by its interactions with other neurons (i.e. receive synaptic input form other neurons)

Interneurons – help discriminate stimuli

Contrast enhancement – Ý contrast for a stimulus to primary neurons by inhibition of surrounding higher-order neurons

System	1^st order neuron	2^nd order neuron	3^rd order neuron	Final destination
Spinal
anterolateral	DRG (small diameter)	Spinal cord dorsal horn	Thalamus (VPL)	Cortex
Doral column-medial lemniscal	Dorsal root ganglion (large diameter)	Brainstem dorsal column nuclei	Thalamus (VPL)	Cortex
Brainstem
Trigeminal	Trigeminal ganglion	Main sensory V,Spinal V	Thalamus (VPM)	Cortex
Vestibular	Vestibular ganglion	Vestibular nuclei	Thalmus (VP)	Cortex
Gustatory	Geniculate and Petrosal Ganglia	Nucleus tractus solitarius	Thalamus (VPM), Parabrachial nucleus	Cortex, Amygdala, Hypothalamus
Visceral	Petrosal and Nodose Gangalia	Nucleus Tractus Solitarius	Parabrachial Nucleus Superior Olivary Complex, Nucleus of lateral Lemniscus, Inferior Colliculus	Hypothalamus
Auditory	Spiral Ganglion	Cochlear Nuclei	Thalamus (MG) – multiple	Cortex

Topography

Somatotopic maps – sensory information from specific anatomical areas are delivered to specific areas of the cortex

homunculus – precentral (motor) and postcentral (sensory) gyrus

receptive fields on the body are maintained in the organization of the CNS

CNS lesions have effects on different parts of the body depending on where the lesion is (aids in localizing lesions)

Dorsal Root Ganglion (DRG) neurons – Primary sensory neurons

Primary sensory neurons have different connections; modalities results in different subclasses of sensory fibers

Large diameters (DCML) – glutamate, mechanoreceptors (light touch pressure)

Example: Pacinian corpuscle (receptor responds vib, press, sensation of certain freq, NOT sustained stimulation) Þ innervated by primary sensory Þ nerve turns upstream in spinal cord and synapses in the brainstem

Small diameters (ALST) – glutamate (fast response), subst P (longer acting), nociceptors. synapse in the spinal cord

unmyelinated, transmits slow, aching, burning pain

3 fiber types:

(1) C-fibers–small and unmyelinated

(2) d-fibers–small and thinly myelinated

(3) a and b fibers–large and myelinated

Receptors – "free endings"=unmyelinated terminal neuron branches that ramify widely in upper dermis and epidermis

Nocioceptors (pain) – free endings respond to thermal, mechanical, and chemical noxious stimuli; C + A-delta fibers

Thermoreceptors (temp) – free endings; C, A-delta

Cutaneous mechanoreceptors (touch) – encapsulated A-b fibers, perceptual abilities

vibrating stimuli, different textures – related to intrinsic properties of cutaneous mechanoreceptors
perceptual abilities (two-point discrimination) – density of mechanoreceptor ending in region

Muscle stretch receptor; A-delta – deep touch (crude)

Organization of sensory inputs to spinal cord

Modality specificity – class of sensations (touch, warmth, cold, pain); segregated all the way up to the cortex

pattern of primary afferent projection along the dorso-ventral axis of spinal gray matter

sub-modalities: touch Þ light touch or deep pressure; pain Þ sharp pain or slow burning pain

receptors: some are receptive to a certain type of stimuli, others are multi-modal(fibers receptive to different modalities)

Topographic: receptive field or spatial domain (the area of skin) with which an adequate stimulus activates a given cell

pattern of afferent projection (DRG neuron) along the medio-lateral axis of spinal gray matter