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Lecture 15

ANAT 1010 Lecture Notes - Lecture 15: Somatic Nervous System, Sensory Neuron, Itch

Course Code
ANAT 1010
A. Jaffar

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Lecture: General Senses
October 17, 2018
[Overview of Sensations]:
‘sensation’ is the conscious or subconscious awareness of changes that occur in the external or
internal conditions of the body. In order for a sensation to occur, four conditions must be
1. A stimulus, or change in environment, capable of activating certain sensory neurons
must occur.
2. A sensory receptor must convert the stimulus to nerve impulses,
3. The nerve impulses must then be conducted along a neural pathway from the sensory
receptor to the brain.
4. A region of the brain must receive and integrate (process) the nerve impulses,
producing a sensation.
Note: a stimulus may be light, heat, pressure, mechanical energy, or chemical energy. A sensory
receptor will respond to a stimulus by altering the membrane’s permeability to small ions. In
most sensory receptor types, the resulting ion flow across the membrane produces a change
that triggers 1+ nerve impulses, which are then conducted along the sensory neuron to the
General vs Special Senses:
- General Sensory Receptors (Somatic Receptors)
o No structural specializations in the free nerve endings that provide us with pain,
tickle, itch, temperatures
o Some structural specializations in receptors for touch, pressure, and vibration.
- Special Sensory Receptors (Special Sense Receptors)
o Very complex structures vision, hearing, taste, equilibrium, and smell.
Classification of Sensory Receptors:
- Structural classification
- Location of receptors and the origin of stimuli
- Type of stimuli they detect
Structural Classification of Receptors:
1. Free nerve endings (nonencapsulated):
- Bare dendrites that are associated with pain, thermal, tickle, itch, and some touch

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2. Encapsulated nerve endings:
- Dendrites enclosed in connective tissue capsule for pressure. Vibration, and some
touch sensations.
3. Separate sensory cells:
- Specialized cells that respond to stimuli.
- Receptor cells synapse with first-order sensory neurons; located in the retina of the
eye (photoreceptors), inner ear (hair cells), and the taste buds of the tongue
(gustatory receptor cells).
- Vision, taste, hearing, and balance
Classification by Location:
1. Exteroceptors:
- Located at or near the body surface; they are sensitive to stimuli that originate
outside of the body (external environment).
- They provide information regarding the external environment, and will convey
visual, smell, taste, touch, pressure, vibration, thermal, and pain sensations.
2. Interoceptors:
- Located in blood vessels, visceral organs, and nervous system.
- They provide information regarding the internal environment.
- Impulses that are produced are usually not consciously perceived but may
occasionally be felt as pain or pressure.
3. Proprioceptors:
- Located in the muscles, tendons, joints, and the inner ear.
- They provide information regarding body position, muscle length and tension,
position and motion of joints, and equilibrium (balance).
Classification by the Detected Stimuli:
1. Mechanoreceptors:
- Receptors that detect mechanical stimuli.
- They provide sensations of touch, pressure, vibration, proprioception, and hearing
and equilibrium
- They also monitor the stretching of blood vessels and the internal organs.
2. Thermoreceptors:
- Receptors that detect changes in temperature
3. Nociceptors:
- Receptors that respond to painful stimuli resulting from physical or chemical damage
to tissue.

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4. Photoreceptors:
- Receptors that detect light that strikes the retina of the eye.
5. Chemoreceptors:
- Receptors that detect chemicals in the mouth (taste), nose (smell), and body fluids.
Somatic Tactile Sensations:
1. Touch/Vibration:
- Arise from the stimulation of tactile receptors in the skin or subcutaneous layer.
- Fine touch (corpuscles of touch) = specific information about a touch sensation (the
point on body touched)
- Crude touch (hair root plexuses) = ability to perceive that something has contacted
the skin, although its exact location, shape, size, or texture cannot be determined.
2. Pressure:
- A sustained sensation that is felt ofver a larger area than touch, it occurs with
deeper deformation of the skin + subcutaneous layer.
- The receptors that contribute to the sensations of pressure are the type 1 and 2
mechanoreceptors, which are able to respond to steady pressure stimulus as they
are slowly adapting.
3. Pain:
- Free nerve endings
4. Temperature
- Free nerve endings.
Several types of encapsulated mechanoreceptors will mediate the sensations of touch,
pressure, and vibration.
Other touch sensations, as well as itch/tickle sensations are detected by free nerve endings.
The tactile receptors in the skin or subcutaneous layer include corpuscles of touch, hair root
plexuses, type 1/2 cutaneous mechanoreceptors, lamellated corpuscles, and free nerve
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