Neurophysiology/sensory physiology
Lect. Dr. Zahid M. kadhim
SENSORY PHYSIOLOGY
OBJECTIVES
After studying this lecture, you should be able to:
 Name the types of touch and pressure receptors found in the
skin.
 Describe the receptors that mediate the sensations of pain and
temperature.
 Define receptor potential.
 Explain the differences between pain and nociception, first and
second pain, acute and chronic pain, hyperalgesia and
allodynia.
 Describe and explain visceral and referred pain.
 Compare the pathway that mediates sensory input from touch,
proprioceptive, and vibratory senses to that mediating
information from nociceptors and thermoreceptors.
 Describe processes involved in modulation of transmission in
pain pathways.
Sensory physiology
The afferent division of the peripheral nervous system transmits
information detected by sensory receptors that respond to specific
types of stimuli from the periphery to the central nervous system
(CNS). Whereas some of these receptors detect stimuli from the skin
like touch and pain (called cutaneous receptors), others, called
visceral receptors, detect stimuli that arise within the body like
baroreceptor and chemoreceptors. Following table (1) show the
classification of sensory receptors:-
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Neurophysiology/sensory physiology
Lect. Dr. Zahid M. kadhim
Type of sensation
A- Mechanoreceptors
Receptor
I. Free nerve ending
II. Merkel's discs
III.
Ruffini's endings
IV. Meissner's corpuscles
V. Hair end-organs
VI. Pacinian corpuscles
Muscle receptors
I. Muscle spindles
II. Golgi tendon receptors
Hearing
Sound receptors of cochlea
Equilibrium
Vestibular receptors
Arterial pressure
Baroreceptors of carotid sinuses and aorta.
B- Thermoreceptors
I. Cold receptors
II.
Warm receptors
C- Nociceptors
Free nerve endings
DElectromagnetic
Rods Cones
receptors
E- Chemoreceptors
I. Taste-Receptors of taste buds
II. Smell-Receptors
of
olfactory
epithelium
III. Arterial oxygen-Receptors of aortic and
carotid bodies
IV.
Osmolality- supraoptic nuclei
V. Blood CO2-Receptors in medulla and in
aortic and carotid bodies
VI. Blood glucose, amino acids, fatty
acids-Receptors in hypothalamus
table (1) showing the classification of sensory receptors.
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Lect. Dr. Zahid M. kadhim
Receptor Physiology
Sensory receptors are specialized structures that detect a specific
form of energy in the external environment. Each of the principal
types of sensation that we can experience like pain, touch, sight,
sound, and so forth-is called a modality of sensation. Each receptor
is sensitive and respond to one modality ex. nociceptors respond
only to painful stimuli and will not be stimulated by pressure, but if
pressure become so intense and causes damage to the tissue, it will
activate the pain receptors and perceived as painful stimulus.
The particular form of energy to which a receptor is most sensitive is
called its adequate stimulus. The adequate stimulus for the rods and
cones in the eye, for example, is light (an example of electromagnetic
energy).
Receptor potentials
When a small amount of pressure is applied to a sensory receptor
like Pacinian corpuscle, a non-propagated depolarizing potential
resembling an excitatory postsynaptic potential (EPSP) is recorded in
the receptor. This is called the receptor potential. This potential
results from converting some form of energy like mechanical or
thermal energy into an electrical response (change in membrane
potential), the magnitude of which is proportional to the intensity of
the stimulus. As the pressure is increased, the magnitude of the
receptor potential is increased. Thus, the responses are described as
graded potentials rather than all-or-none as is the case for an action
potential.
The intensity of sensation is determined by the amplitude of the
stimulus applied to the receptor. As a greater pressure is applied to
the skin, the receptor potential in the mechanoreceptor increases,
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and the frequency of the action potentials in a single axon is also
increased, activation of receptors with higher threshold, because of
overlap and interdigitation of one receptive unit with another,
receptors of other units are also stimulated, and consequently more
units fire.
Duration and adaptation
If a stimulus of constant strength is maintained on a sensory
receptor, some receptor types continue to respond to the stimulus
as long as its applied while others adapt, that is mean the frequency
of the action potentials in their sensory nerve declines over time.
This phenomenon is known as receptor adaptation or
desensitization. The degree to which adaptation occurs varies from
one sense to another. Receptors can be classified into rapidly
adapting receptors like olfactory receptors and Pacinian corpuscles
and slowly adapting receptors like muscle spindles and nociceptors.
Transmission of sensory information to the spinal cord
When a specific stimuli activate its own receptor, receptor potential
will be generated, this receptor potential have to be transmitted
through peripheral sensory nerve to the spinal cord where it will
relay it to the specific areas of the cerebral cortex. According to the
speed of conduction, various types of nerve fibers exist (table 2).
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Lect. Dr. Zahid M. kadhim
Some information need to be transmitted to or from the central
nervous system extremely rapidly; otherwise, the information would
be useless. An example of this is the sensory signals that apprise the
brain of the momentary positions of the legs at each fraction of a
second during running. At the other extreme, some types of sensory
information, such as that depicting prolonged, aching pain, do not
need to be transmitted rapidly, so slowly conducting fibers is
sufficient.
Somatosensory pathways
The sensation evoked by impulses generated in a sensory receptor
depends in part on the specific part of the brain they ultimately
activate. The ascending pathways from sensory receptors to the
cortex are different for the various sensations.
Dorsal Column-Medial Lemniscal System
1. Touch sensations requiring a high degree of localization of the
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2.
3.
4.
5.
6.
Lect. Dr. Zahid M. kadhim
stimulus
Touch sensations requiring transmission of fine gradations of
intensity
Phasic sensations, such as vibratory sensations
Sensations that signal movement against the skin
Position sensations from the joints
Pressure sensations related to fine degrees of judgment of
pressure intensity.
Anterolateral System
1. Pain
2. Thermal sensations, including both warmth and cold sensations
3. Crude touch and pressure sensations capable only of crude
localizing ability on the surface of the body
4. Tickle and itch sensations
5. Sexual sensations
The afferent neuron that transmits information from the periphery
to the CNS is called the first-order neuron. A single first-order
neuron may diverge within the CNS and communicate with several
interneurons. In addition, interneurons may receive converging input
from several first-order neurons. Some of these interneurons
transmit the information to the thalamus, the major relay nucleus for
sensory input; such interneurons are examples of second-order
neurons. In the thalamus, these second-order neurons form
synapses with third-order neurons that transmit information to the
cerebral cortex, where sensory perception occurs. Different sensory
pathways travel through different areas of the thalamus and cortex.
Dorsal column medial leminscal pathway
Fibers ascend ipsilaterally in the dorsal columns of the spinal cord to
the medulla after sending collateral fibers to the dorsal horn cells,
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where they synapse in the Gracilus and Cuneate nuclei. The secondorder neurons from these nuclei cross the midline and ascend in the
medial lemniscus to end in the specific sensory relay nuclei of the
thalamus. This ascending system is called the dorsal column or
medial lemniscal system . The fibers within the dorsal column
pathway are joined in the brain stem by fibers mediating sensation
from the head via the main sensory and mesencephalic nuclei of the
trigeminal nerve.
Somatotopic organization
Within the dorsal columns, fibers arising from different levels of the
cord are somatotopically organized. Specifically, fibers from the
sacral cord are positioned most medially and those from the cervical
cord are positioned most laterally. This arrangement continues in the
medulla.
Somatotopic organization continues through the thalamus and
cortex. Thalamic neurons carrying sensory information project in a
highly specific way to the primary somatosensory cortex in the postcentral gyrus of the parietal lobe. The arrangement of projections to
this region is such that the parts of the body are represented in order
along the post-central gyrus, with the legs on top and the head at the
foot of the gyrus. The size of the cortical receiving area for impulses
from a particular part of the body is proportional to the use of the
part. The cortical areas for sensation from the trunk and back are
small, whereas very large areas are concerned with impulses from
the hand and the parts of the mouth concerned with speech.
In addition to the primary somatosensory cortex, there are two other
cortical regions that contribute to the integration of sensory
information. The sensory association area is located in the parietal
cortex and the secondary somatosensory cortex is located in the
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wall of the sylvian fissure that separates the temporal from the
frontal and parietal lobes. These regions receive input from the
primary somatosensory cortex.
Ventrolateral spinothalamic tract
Fibers from nociceptors and thermoreceptors synapse on neurons in
the dorsal horn of the spinal cord. The axons from these dorsal horn
neurons cross the midline and ascend in the ventrolateral quadrant
of the spinal cord, where they form the ventrolateral spinothalamic
pathway. Fibers within this tract synapse in the thalamic nuclei
where third order neuron transmit information to the
somatosensory cortex.
PAIN
is an unpleasant sensory and emotional experience associated with
actual or potential tissue damage. This is to be distinguished from
the term nociception which is defined as the unconscious activity
induced by a harmful stimulus applied to sense receptors.
Pain is frequently classified as physiologic or acute pain and
pathologic or chronic pain, which includes inflammatory pain and
neuropathic pain. Acute pain typically has a sudden onset and
recedes during the healing process; it can be regarded as “good pain”
as it serves an important protective mechanism. The withdrawal
reflex is an example of the expression of this protective role of pain.
Chronic pain can be considered “bad pain” because it persists long
after recovery from an injury and is often refractory to common
analgesic agents, including non-steroidal anti-inflammatory drugs
(NSAIDs) and opioids. Chronic pain can result from nerve injury
(neuropathic pain) including diabetic neuropathy, toxin-induced
nerve damage, and ischemia.
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Neurophysiology/sensory physiology
Lect. Dr. Zahid M. kadhim
Hyperalgesia and allodynia
Pain is often accompanied by increased sensitivity of nociceptors to
painful stimuli (hyperalgesia and allodynia). Hyperalgesia is an
exaggerated response to a noxious stimulus, and allodynia is a
sensation of pain in response to a normally innocuous stimulus. An
example of the latter is the painful sensation from a warm shower
when the skin is damaged by sunburn.
hyperalgesia and allodynia might be caused by 1- release chemical
mediators like K+, bradykinin and substance p from injured cells
leading to sensitization of the pain receptors.
2- In addition to sensitization of nerve endings by chemical
mediators. The nerve growth factor NGF released by tissue damage
is picked up by nerve terminals and transported retrogradely to cell
bodies in dorsal root ganglia where it can alter gene expression and
increases production of substance P and converts non-nociceptive
neurons to nociceptive neurons.
3- Another change in the spinal cord is due to the activation of
microglia near afferent nerve terminals in the spinal cord. This, in
turn, leads to the release of pro-inflammatory cytokines and
chemokines that modulate pain processing.
Deep or visceral pain
Afferent fibers from visceral structures reach the CNS via
sympathetic and parasympathetic nerves. Their cell bodies are
located in the cranial nerve ganglia (facial, glossopharyngeal, and
vagus nerves); in the thoracic, lumbar and sacral dorsal roots.
The receptors in the walls of the hollow viscera are especially
sensitive to distension, ischemia and inflammation and relatively
insensitive to cutting or burning.
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Neurophysiology/sensory physiology
Lect. Dr. Zahid M. kadhim
visceral pain is diffuse, poorly localizing and often referred to distant
usually superficial structure. it may be accompanied by nausea,
vomiting, change in vital signs and emotional manifestations.
Referred pain
Irritation of a visceral organ frequently produces pain that is felt not
at that site but in a somatic structure that may be some distance
away. Such pain is said to be referred pain.
One of the best-known examples is referral of cardiac pain to the left
arm. Other examples include pain in the tip of the shoulder caused
by irritation of the diaphragm.
When pain is referred, it is usually to a structure that developed from
the same embryonic segment or dermatome as the structure in
which the pain originates. For example, the heart and the arm have
the same segmental origin. The basis for referred pain may be
convergence of somatic and visceral pain fibers on the same secondorder neurons in the dorsal horn that project to the thalamus and
then to the somatosensory cortex. This is called the convergence–
projection theory.
Modulation of pain signals
Signals about sensory information can be modulated as they are
transmitted along sensory pathways; that is, facilitation or
attenuation of signals can result in changes in the final perception of
that information.
Somatic signals of non-painful sources can inhibit signals of pain at
the spinal level gate-control theory. If a non-painful mechanical
stimulus like touch or pressure is applied simultaneously with a
painful stimulus, the collaterals from the Aβ fibers stimulated by
touch will stimulate inhibitory interneuron present in the spinal cord
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to inhibit the second order neuron that transmit pain information
thereby decreasing the transmission of pain signals.
The gate-control theory describes why rubbing a painful area relieves
the pain. It is also the basis for using transcutaneous electrical nerve
stimulation (TENS) to treat pain.
Endogenous analgesia system
Stressful situations can activate an area in the midbrain called the
periaqueductal gray matter. This area communicates to areas in the
medulla called the nucleus raphe magnus and the lateral reticular
formation. Neurons from these areas descend to the spinal cord,
where they block the communication between nociceptive afferent
neurons and second-order neurons.
Inhibitory interneurons in the spinal cord release the endogenous
opiate neurotransmitter enkephalin, which binds to opioid receptors
on the second-order neuron and induces inhibitory postsynaptic
potentials. Enkephalin also binds to opioid receptors on the axon
terminal of the nociceptive afferent neuron, which inhibits the
release of substance P, causing presynaptic inhibition. Both of these
actions suppress signal transmission from the afferent neuron to the
second-order neuron, thereby decreasing the transmission of pain
signals to the brain. These inhibitory interneurons are activated by
descending neurons of the nucleus raphe magnus and lateral
reticular formation.
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figure (1) shows the somatosensory pathways
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figure (2) shows the sensory areas of the brain
figure (3) shows somatotropic organization of the spinal cord
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figure (4) somatotropic organization of somatosensory cortex
figure (5) modulation of pain signals
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Figure (6) endogenous analgesia system
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CLINICAL CORRELATION
Phantom limb pain
Between 50 and 80% of amputees experience phantom sensations,
usually pain, in the region of their amputated limb. Phantom
sensations may also occur after the removal of body parts other than
the limbs, for example, after amputation of the breast, extraction of
a tooth (phantom tooth pain), or removal of an eye (phantom eye
syndrome).
Brown-Séquard Syndrome
A functional hemisection of the spinal cord causes a characteristic
and easily recognized clinical picture that reflects damage to
ascending sensory (dorsal-column pathway, ventrolateral
spinothalamic tract) and descending motor (corticospinal tract)
pathways, which is called the Brown Séquard syndrome .The lesion
to fasciculus gracilus or fasciculus cuneatus leads to ipsilateral loss of
discriminative touch, vibration, and proprioception below the level of
the lesion. The loss of the spinothalamic tract leads to contralateral
loss of pain and temperature sensation beginning one or two
segments below the lesion. Damage to the corticospinal tract
produces weakness and spasticity in certain muscle groups on the
same side of the body.
MULTIPLE CHOICE QUESTIONS
For all questions, select the single best answer.
1- Nociceptors
A- are activated by strong pressure, severe cold, severe heat, and
chemicals.
B. are absent in visceral organs.
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C. are specialized structures located in the skin and joints.
D. are innervated by group II afferents.
E. are involved in acute but not chronic pain.
2. A receptor potential
A. always leads to an action potential.
B. increases in amplitude as a more intense stimulus is applied.
C. is an all-or-none phenomenon.
D. is unchanged when a given stimulus is applied repeatedly over
time.
E. all of the above .
3. Sensory systems code for the following attributes of a stimulus:
A. modality, location, intensity, and duration
B. threshold, receptive field, adaptation, and discrimination
C. touch, taste, hearing, and smell
D. threshold, laterality, sensation, and duration
E. sensitization, discrimination, energy, and projection
4. Which of the following are correctly paired?
A. Neuropathic pain and withdrawal reflex
B. First pain and dull, intense, diffuse, and unpleasant feeling
C. Physiological pain and allodynia
D. Second pain and C fibers
E. Nociceptive pain and nerve damage
5. A 32-year-old female experienced the sudden onset of a severe
cramping pain in the abdominal region. She also became nauseated.
Regarding visceral pain:
A. shows relatively rapid adaptation.
B. is mediated by B fibers in the dorsal roots of the spinal nerves.
C. is poorly localized.
D. resembles “fast pain” produced by noxious stimulation of the skin.
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E. causes relaxation of nearby skeletal muscles.
6. A ventrolateral cordotomy is performed that produces relief of
pain in the right leg. It is effective because it interrupts the
A. left dorsal column.
B. left ventrolateral spinothalamic tract.
C. right ventrolateral spinothalamic tract.
D. right medial lemniscal pathway.
E. a direct projection to the primary somatosensory cortex.
7. Which of the following CNS regions is not correctly paired with a
neurotransmitter or a chemical involved in pain modulation?
A. Periaqueductal gray matter and morphine
A. Nucleus raphé magnus and norepinephrine
C. Spinal dorsal horn and enkephalin
D. Dorsal root ganglion and opioids
E. Spinal dorsal horn and serotonin
8. A 50-year-old woman undergoes a neurological exam that
indicates loss of pain and temperature sensitivity, vibratory sense,
and proprioception in the left leg. These symptoms could be
explained by:A. a tumor on the right medial lemniscal pathway in the sacral spinal
cord.
B. a peripheral neuropathy.
C. a tumor on the left medial lemniscal pathway in the sacral spinal
cord.
D. a tumor affecting the right posterior paracentral gyrus.
E. a large tumor in the right lumbar ventrolateral spinal cord.
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