Somatic sensation

7/29/2019 Somatic sensation

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Somatic sensation & Pain

Prof. Yasmeiny Yazir

dr. Nuraiza Meutia

Dept. Fisiologi

FK USU


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From Sensation to Perception

Survival depends upon sensation andperception

Sensation is the awareness of changes in

the internal and external environment Perception is the conscious interpretation

of those stimuli


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Organization of the Somatosensory

System

Input comes from exteroceptors,proprioceptors, and interoceptors

The three main levels of neuralintegration in the somatosensorysystem are:

Receptor level the sensor receptors

Circuit level ascending pathways

Perceptual level neuronal circuits inthe cerebral cortex


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Pathway for

somatic sensation


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Processing at the Receptor Level

The receptor must have specificity for thestimulus energy

The receptors receptive field must be

stimulated Stimulus energy must be converted into a

graded potential

A generator potential in the associatedsensory neuron must reach threshold


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Simple Receptors: Unencapsulated

Table 13.1.1


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Simple Receptors: Encapsulated

Table 13.1.2


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Table 13.1.3


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Table 13.1.4


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Processing at the Circuit Level Chains of three neurons (first-, second-, and third-

order) conduct sensory impulses upward to thebrain

First-order neurons soma reside in dorsal root or

cranial ganglia, and conduct impulses from the skinto the spinal cord or brain stem

Second-order neurons soma reside in thedorsal horn of the spinal cord or medullary

nuclei and transmit impulses to thethalamus or cerebellum

Third-order neurons located in thethalamus and conduct impulses to the

somatosensory cortex of the cerebrum


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Processing at the Perceptual Level

The thalamus projects fibers to:

The somatosensory cortex

Sensory association areas

First one modality is sent, then thoseconsidering more than one

The result is an internal, conscious imageof the stimulus


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PAIN

Pain is an unpleasant sensory and emotional

experience associated with actual or potential tissue

damage or described in terms of such damage.

(Pain, Suppl 3, 1986) Function : warning that something is wrong

Cause :

Physiologic / acute pain Pathologic :

1. inflammatory pain

2. neuropathic pain


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Neuropathic pain arises from disordered, ectopic

nerve signals. It is burning or shocklike pain.

Classic cases are post-stroke pain and tumor

invasion of the brachial plexus.


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Receptors & Pathway

Receptors : naked nerve endings

Categories of pain receptors :

1. Mechanical nociceptors (cutting, crushing,

pinching)2. Thermal nociceptors (temp.extremes)

3. Polymodal nociceptors (irritating chemicals)

Fibers :

1. Myelinated A (2-5 m)

2. Unmyelinated C (0.4-1.2 m)


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Pathway

Pain receptors transmit

stimuli through sensory

nerves into the dorsal horn.

These impulses synapse in

the dorsal horn, cross thecord, and ascend by either the

neospinothalamic tract (fast

pain) or the

paleospinothalamic tract(slow/dull pain).


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The neospinothalamic tract

ascends to the thalamus

(pain sensation) andproceeds further to the

cortex (precision and

discrimination).

The paleospinothalamic

tract ascends and branchesinto the brain stem (pons

and medulla) and limbic

system.


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Function of the Reticular Formation, Thalamus, and

Cerebral Cortex in the Appreciation of Pain

Pain impulses entering the brain stem reticularformation, the thalamus, and other lower braincenters cause conscious perception of pain.

This does not mean that the cerebral cortex hasnothing to do with normal pain appreciation;

However, it is believed that the cortex plays anespecially important role in interpreting painquality, even though pain perception might beprincipally the function of lower centers.


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Characteristic of Pain

Fast Pain Slow Pain

Occurs on stimulation ofmechanical & thermalnociceptors

Occurs on stimulation ofpolymodal nociceptors

Carried by myelinated A-delta

fibers

Carried by unmyelinated C

fibersProduces sharp, pricklingsensation

Produces dull, aching, burningsensation

Easily localized Poorly localized

Occurs first Occurs second; persist forlonger time; more unpleasant.


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Deep Pain

The deep structures is relative deficiencyof A fibers.

Poorly localized, nauseating, and

frequently associated with sweating &changes in blood pressure.


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Muscle spasm & Rigidity

Visceral pain can initiates reflexcontraction of nearby skeletal muscle.Usually in the abdominal wall rigid.

Muscle Spasm as a Cause of Pain.

This pain probably results from the directeffect of muscle spasm in stimulating

mechanosensitive pain receptors, or effect of muscle spasm to compress the

blood vessels and cause ischemia.


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Referred Pain

Pain that is present in an area removed/distantfrom its point of origin.

The area expressing the referred pain issupplied by the same spinal segment as theactual pain site.


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Pain Suppression(Analgesia) System

in the Brain and Spinal Cord

Several transmitter substances are involved in

the analgesia system; especially are enkephalinand serotonin.

Many nerve fibers derived from theperiventricular nuclei and from the

periaqueductal gray area secrete enkephalin attheir endings.

Thus, the endings of many fibers in the raphemagnus nucleus release enkephalin .

Fibers originating in this area send signals to thedorsal horns of the spinal cord to secreteserotonin at their endings.


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The serotonin causes localcord neurons to secreteenkephalin as well.

The enkephalin is believed tocause bothpresynapticand

postsynaptic inhibition ofincoming type C and type Adpain fibers where theysynapse in the dorsal horns.

Thus, the analgesia systemcan block pain

Somatic sensation

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