SENSORY SYSTEMS

Sensations Four types: superficial, deep, visceral, and

special.

Superficial sensations

Touch Pain Temperature Two -point discrimination.

Deep sensations Muscle and joint position sense

(proprioception) Deep muscle pain Vibration sense.

Visceral sensations Hunger Nausea Visceral pain

Special senses Smell Vision Hearing Taste Equilibrium

Receptors

Specialized cells for detecting particular changes in the environment

Exteroceptors Proprioceptors Receptors are not absolutely specific for a

given sensation; strong stimuli can cause various sensations, even pain, even though the inciting stimuli are not necessarily painful

Adaptation denotes the diminution in rate of discharge of

some receptors on repeated or continuous stimulation of constant intensity

the sensation of sitting in a chair or walking on even ground is suppressed.

Exteroceptors Affected mainly by the external environment: Meissner's corpuscles, Merkel's corpuscles,

and hair cells for touch Krause's end-bulbs for cold Ruffini's corpuscles for warmth Free nerve endings for pain

Ending Type

Receptor Type

LocationResponds to

Encapsulated endings

Meissner corpuscles

Glabrous skin

Changing touch

Pacinian corpuscles

Skin, joints, deep connective tissue

Vibration

Muscle spindles

Skeletal muscle

Muscle stretch

Golgi tendon organs

Muscle-tendon junction

Muscle tension

Ending Type

Receptor Type

Location Responds to

Endings with accessory structures

Merkel endings

Glabrous and hairy skin

Touch

Endings around hairs

Hairy skin Touch

Free nerve endings

Ubiquitous Pain, temperature, light touch

The receptors that provide the information we use for the discriminative aspects of touch-assessing the shape and texture of objects, and the direction of movement across the skin-all have large-diameter axons and encapsulated endings or endings with accessory structures

Examples -Meissner corpuscles, pacinian corpuscles, and Merkel endings

The packing density of receptors like this, especially the Meissner corpuscles and Merkel endings, determines the tactile acuity of a given area of skin.

This varies – the fingertips and lips;we can distinguish

between two small objects separated by only a few millimeters

the skin of the legs and trunk;objects can be separated by a few cm and still feel like a single object.

Meissner corpuscles, found in the dermal papillae of glabrous

(hairless) skin, are encapsulated structures important for detecting the details of things

moving across the skin. come into play when we move our fingertips

across something, or when something in our grasp begins to slip and distorts the skin.

Pacinian corpuscles rapidly adapting receptor that responds briefly

at the beginning and end of a mechanical stimulus.

good at detecting rapidly changing stimuli, such as vibrations

Free nerve endings unmyelinated (C) fibers respond best to slow, gentle brushing of the

skin. probably more important for the pleasurable

feelings associated with this kind of touch than for its explicit detection

Merkel endings basal layer of the epidermis sensitive, slowly adapting receptors important

for detecting the shape and texture of stationary objects touching the skin.

Nociceptors detect events that damage or threaten to

damage tissue two groups correspond to everyone's experience with

pain as a two-part sensation.. Fast pain is initiated by firing of Aδ nociceptors, and the delayed onset of slow pain is directly related to the slower conduction velocity of the C fibers that mediate it.

A physically painful event (e.g., touching a very hot pot, missing a nail and hitting a finger with the hammer) elicits first a sensation of sharp, well-localized fast pain followed by a dull, poorly localized, aching sensation of slow pain

Fast pain- firing of Aδ nociceptors Delayed onset -slower conduction velocity of

the C fibers that mediate it

Tissue damage is more complicated than something like skin indentation or muscle stretch.

Multiple things can cause it Once it occurs a series of chemical changes in

the damaged tissue ensue.

Nociceptors transduce multiple aspects of painful stimuli.

Aδ nociceptors respond specifically to intense mechanical stimulation (e.g., pinprick), to painful heat or cold, or to both.

C-fiber nociceptors respond to all of these, as well as to a variety of substances released in damaged tissue=polymodal nociceptors

CONNECTIONS A chain of three long neurons and a number of

interneurons conducts stimuli from the receptor or free ending to the somatosensory cortex

First-Order Neuron

The cell body lies in a dorsal root ganglion or a somatic afferent ganglion (eg, trigeminal ganglion) of cranial nerves.

Second-Order Neuron

The cell body lies within the neuraxis -spinal cord or brain stem;

Axons of these cells usually decussate and terminate in the thalamus.

Third-Order Neuron

The cell body projects rostrally to the sensory cortex.

The networks of neurons within the cortex, in turn, process information relayed by this type of neuron; they interpret its location, quality, and intensity and make appropriate responses.

Dorsal column-medial lemniscus pathway

Spinothalamic tracts/Anterolateral system

Variable Dorsal Column- Medial Lemniscus Pathway

Anterolateral Pathway

Course in spinal cord

Dorsal and dorsolateral funiculi

Anterior and anterolateral funiculi

Specificity of signal conveyed

Each sensation carried separately; precise localization of sensation

Multimodal (several sensations carried in one fiber system)

Diameter of nerve fiber

Large-diameter primary afferents

Small-diameter primary afferents

Sensation transmitted

Fine touch, joint sensation, vibration

Pain, temperature, crude touch, visceral pain

Synaptic chain Two or three synapses to cortex

Multisynaptic

Speed of transmission

Fast Slow

Spinocerebellar tracts