SENSORY PHYSIOLOGY OBJECTIVES After studying this lecture, … · 2016-02-24 · Neurophysiology/sensory physiology Lect. Dr. Zahid M. kadhim 1 SENSORY PHYSIOLOGY OBJECTIVES After

Neurophysiology/sensory physiology Lect. Dr. Zahid M. kadhim

1

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:-


2

Type of sensation Receptor

A- Mechanoreceptors

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

D- Electromagnetic receptors

Rods Cones

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.


3

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,


4

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).


5

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


6

stimulus 2. Touch sensations requiring transmission of fine gradations of

intensity 3. Phasic sensations, such as vibratory sensations 4. Sensations that signal movement against the skin 5. Position sensations from the joints 6. 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,


7

where they synapse in the Gracilus and Cuneate nuclei. The second-

order 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 post-

central 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


8

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.


9

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.


10

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 second-

order 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


11

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.


12

figure (1) shows the somatosensory pathways


13

figure (2) shows the sensory areas of the brain

figure (3) shows somatotropic organization of the spinal cord


14

figure (4) somatotropic organization of somatosensory cortex

figure (5) modulation of pain signals


15

Figure (6) endogenous analgesia system


16

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.


17

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.


18

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.

Documents

SENSORY PHYSIOLOGY OBJECTIVES After studying this lecture, … · 2016-02-24 · Neurophysiology/sensory physiology Lect. Dr. Zahid M. kadhim 1 SENSORY PHYSIOLOGY OBJECTIVES After