2. the somatosensory system
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THE SOMATOSENSORY
SYSTEM
Ahmed A. E. Eljack 1
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Objectives:
By the end of this session you should be able to:
■ List the different types of sensory receptors.
■ List the different types of nerve fibers and their properties.
■ Discuss the characteristics of different stimuli.
■ Discuss the role of somatosensory cortex in perception of
sensations.
■ Discuss the different modalities of sensation.
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Sensory Receptors:
■ They are specialized neural structures.
■ Types of sensory receptors:
– Mechanoreceptors.
– Thermoreceptors.
– Nociceptors.
– Electromagnetic receptors.
– Chemoreceptors.
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Types of Nerve Fibers:
■ Either myelinated or unmyelinated.
■ Classified numerically into: Ia, Ib, II, III, and
IV (the only unmyelinated nerve fiber).
■ Classified alphabetically into: Aα, Aβ, Aδ,
and C (the only unmyelinated nerve fiber).
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Characteristics of Stimuli:
■ Modality: refers to the type of stimuli.
■ Intensity.
■ Duration.
■ Location.
■ Transduction (important!).
■ Receptive field.
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Spatial and Temporal Summation:
■ In spatial summation, the increase in signal
strength is transmitted using a larger
number of fibers.
■ In temporal summation, the increase in
signal strength is transmitted by increasing
the frequency of nerve impulses in each
fiber.
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Receptor Potential and Transduction:
■ The receptor potential is the change in the membraneelectrical potential of the receptor when a stimulus excites thereceptor.
■ Transduction is initiated by:
– Mechanical deformation (opens ion channels in thereceptor’s membrane).
– Chemicals (opens ion channels).
– Change in the temperature of the receptor (changes thepermeability of the membrane).
– Electromagnetic radiation (changes the membranecharacteristics).
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Receptor Potential and Transduction:
■ Sensory receptors adapt, they can be
divided into”
– Slow adapting receptors (like
nociceptors).
– Rapidly adapting receptors (like
pacinian corpuscles).
■ Adaptation occurs in different ways.
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The Somatosensory Cortex:
■ Lies in the anterior parietal lobe (postcentral gyrus).
■ Divided into somatosensory area I (more important
and usually discussed) and somatosensory area II.
■ It shows somatotopic organization with some parts
represented in larger areas.
■ Amorphosynthesis is losing the ability to recognize
complex objects (in case of sensory association
area damage).
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Important Laws Regarding somatic Sensations:
■ Law of Projection: sensations are projected
to the receptors.
■ Labeled line principle: the type of sensation
felt when a nerve fiber is stimulated is
determined by the point in the nervous
system to which the fiber leads.
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MODALITIES OF SENSATIONS
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Tactile Sensations:
■ Include touch, pressure, and vibration. They are mechanical sensations.
■ Two-point discrimination.
■ Transmitted through the anterolateral and dorsal column systems
■ Tactile receptors are:
– Free nerve endings.
– Meissner’s corpuscle.
– Expanded tip tactile receptors.
– Hair end-organ (hair follicle).
– Ruffini’s endings.
– Pacinian corpuscle.
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Proprioception:
■ Mechanical sensation of the position.
■ Conscious proprioception
– Mediated mainly by free nerve endings.
– Transmitted by the dorsal column-medial lemniscus
system
■ Nonconscious proprioception
– Mediated mainly by muscle spindles and Golgi tendon
organs.
– Transmitted by the anterior and posterior
spinocerebellar tracts.
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Proprioception:
■ Static proprioception: The perception of
orientation of different body parts.
■ Dynamic proprioception: Rate of movement.
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Pain:
■ Detects damage to the tissues (protective mechanism).
■ Stimuli include:
– Mechanical stimuli.
– Chemical stimuli..
– Thermal stimuli
■ Mediated by free nerve endings and transmitted through theanterolateral system. The main neurotransmitter is substanceP (glutamate is also involved).
■ Divided into:
– Sharp pain (through type Aδ nerve fibers).
– Chronic pain (through type C nerve fibers)
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Pain:
■ The severity depends mainly on the rate of destruction.
■ Thalamus plays an important role.
■ The analgesia system suppress the pain input (through
serotonin and enkephalin). It is composed mainly of:
– The periaqueductal gray matter and periventricular
areas in the midbrain.
– The raphe magnus nucleus and the nucleus reticularis
paragigantocelluraris.
– Pain inhibitory complex in the dorsal horn of the spinal
cord.
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Ahmed A. E. Eljack 20
Siegel, A., & Sapru, H. N. (2015). Essential
Neuroscience. Lippincott Williams &
Wilkins. p257
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Pain:
■ Clinical association:
– Referred pain.
– Visceral pain
– Phantom limb.
– Hyperalgesia.
– Thalamic pain syndrome.
– Neuralgia.
– Headache.
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Thermal Sensations
■ Divided into:
– Cold sensation (free nerve endings transmitted
in type C nerve fibers).
– Warmth sensation (Myelinated nerve endings
transmitted in type Aδ nerve fibers).
■ Cold receptors are more than warmth receptors (3
to 10 times).
■ Mediated through the same pathways of pain.
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Further Reading:
■ Barret, K.E, Barman, S.M, Boitano, S, Brooks, H.L. Ganong's
Review of Medical Physiology. (24th ed.). : McGraw-Hill; 2012.
■ Costanzo, L.S. Physiology. (5th
ed.). Philadelphia: Saunders; 2014
■ Hall, J.E. Guyton and Hall Textbook of Medical
Physiology. (12th ed.). Philadelphia: Saunders; 2011.
■ Eljack, A. A. E. Eljack’s Lecture Notes in Neuroscience.
Khartoum; 2015
■ Siegel, A, Sapru, H.N. Essential Neuroscience. (3rd
ed.). Philadelphia: Lippincott Williams & Wilkins; 2015.
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