2. SENSORY SYSTEMS

3. Sensations
 Four types: superficial, deep, visceral, and special.


4. Superficial sensations
 Touch
 Pain
 Temperature
 Two -point discrimination.

5. Deep sensations
 Muscle and joint position sense (proprioception)
 Deep muscle pain
 Vibration sense.

6. Visceral sensations
 Hunger
 Nausea
 Visceral pain

7. Special senses
 Smell
 Vision
 Hearing
 Taste
 Equilibrium

8. 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

9. 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.

10. 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

11. Ending Type Receptor Responds to
Location
Type
Encapsulated Meissner Changing
endings corpuscles Glabrous skin touch
Skin, joints, Vibration
deep
Pacinian
connective
corpuscles
tissue
Muscle Skeletal Muscle stretch
spindles muscle
Golgi tendon Muscle- Muscle
organs tendon tension
junction

12. Ending Type Receptor Location Responds to
Type
Endings with Merkel Glabrous and Touch
accessory endings hairy skin
structures
Endings Hairy skin Touch
around hairs
Free nerve Ubiquitous Pain,
endings temperature,
light touch

13.  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

14.  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.

15.  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.

16. 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.

17. 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

18. 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

19. Merkel endings
 basal layer of the epidermis
 sensitive, slowly adapting receptors important for
detecting the shape and texture of stationary objects
touching the skin.

20. 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.

21.  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

22.  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.

23.  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

27. CONNECTIONS
 A chain of three long neurons and a number of
interneurons conducts stimuli from the receptor or
free ending to the somatosensory cortex

28. First-Order Neuron
 The cell body lies in a dorsal root ganglion or a
somatic afferent ganglion (eg, trigeminal ganglion) of
cranial nerves.

29. 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.

30. 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.

33. Dorsal column-medial lemniscus
pathway

37. Spinothalamic
tracts/Anterolateral system

42. Variable Dorsal Column- Anterolateral
Medial Lemniscus Pathway
Pathway
Course in spinal cord Dorsal and Anterior and
dorsolateral funiculi anterolateral funiculi
Specificity of signal Each sensation Multimodal (several
conveyed carried separately; sensations carried in
precise localization of one fiber system)
sensation
Diameter of nerve Large-diameter Small-diameter
fiber primary afferents primary afferents
Sensation transmitted Fine touch, joint Pain, temperature,
sensation, vibration crude touch, visceral
pain
Synaptic chain Two or three synapses Multisynaptic
to cortex
Speed of transmission Fast Slow

43. Spinocerebellar tracts