Nervous System Part Three Sensory Function

8/4/2019 Nervous System Part Three Sensory Function

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Part 3

Sensory Function of the Nervous

System


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I Sensory pathways

Sensory systems allow us to detect, analyze andrespond to our environment

ascending pathways Carry information from sensory receptors to the

brain

Conscious: reach cerebral cortex

Unconscious: do not reach cerebral cortex Sensations from body reach the opposite side of

the brain


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1. Sensory receptors

A: Free nerve endings (pain, temperature)

B: Pacinian corpuscle (pressure)

C: Meissners corpuscle (touch)

D: Muscle spindle (stretch)

A

B C

D


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Ruffini's endings respond to tension and stretch in the skin


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2. Sensory pathways: 3neurons

1st: enters spinal cord from periphery

2nd: crosses over (decussates), ascends

in spinal cord to thalamus

3rd

: projects to somatosensory cortex


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2.1 Spinothalamic pathway

Carries pain, temperature,

touch and pressure signals

1st neuron enters spinal

cord through dorsal root

2nd neuron crosses over in

spinal cord; ascends to

thalamus

3rd neuron projects from

thalamus to somatosensory

cortex


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spinothalamicpathway


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Spinothalamic Pathway

Small sensory fibres:

Pain, temperature,

some touch

Primary somatosensorycortex (S1)

Thalamus

Medulla

Spinal cord

Spinothalamictract


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Spinothalamic damage

spinothalamic pathway

Leftspinal cord injury

Loss of sense of:TouchPainWarmth/coldin right leg


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2.2 Dorsal column pathway

Carries fine touch, vibrationand conscious proprioceptionsignals

1st neuron enters spinal cord

through dorsal root; ascendsto medulla (brain stem)

2nd neuron crosses over inmedulla; ascends to thalamus

3rd neuron projects tosomatosensory cortex


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Two-Point Discrimination


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dorsalcloumn

pathway


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Dorsal column pathway

Large sensory nerves:

Touch, vibration, two-point

discrimination, proprioception

Primary somatosensorycortex (S1) in parietal

lobe

Thalamus

MedullaMediallemniscus

Spinal cord

Dorsal column

Dorsal columnnuclei


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Dorsal

columndamage

dorsal columnpathway

Leftspinal cord injury

Loss of sense of:touchproprioceptionvibrationin left leg


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Dorsal column damage

Sensory ataxia

Patient staggers; cannotperceive position or

movement of legs

Visual clues help movement


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Central

Pathways


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3.3 Spinocerebellar pathway

Carries unconsciousproprioception signals

Receptors in muscles &

joints

1st neuron: enters spinal

cord through dorsal root

2nd neuron: ascends to

cerebellum No 3rd neuron to cortex,

hence unconscious


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Spinocerebellar tract damage

Cerebellar ataxia

Clumsy movements

Incoordination of the limbs (intentiontremor)

Wide-based, reeling gait (ataxia)

Alcoholic intoxication produces similareffects!


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4. Somatosensory cortex

Located in the postcentral gyrus of thehuman cerebral cortex.

S i l i i f i l


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Spatial orientation of signals.1) Each side of

the cortex

receivessensory

information

exclusivelyfrom the

opposite side of

the body

(the exception:

the same side

of the face).

S ti l i t ti f i l


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Spatial orientation of signals.2)The lips, face

and thumb are

represented by

large areas in the

somatic cortex,

whereas the trunkand lower part of

the body, relatively

small area.

3)The head in the most lateral portion, and the

lower body is presented medially


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II . Pain


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Pain is an unpleasant sensory and emotional

experience associated with actual or

potential tissue damage or described in

terms of such damage

International Association for the Study of Pain


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Why feel pain?

Gives conscious awareness of tissue

damage

Protection:

Remove body from danger

Promote healing by preventing further damage

Avoid noxious stimuli

Elicits behavioural and emotional responses


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free nerve endings in

skin respond to

noxious stimuli

1. Nociceptors

N i


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Nociceptors Nociceptors are special receptors that respond only

to noxious stimuli and generate nerve impulseswhich the brain interprets as "pain".


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Adequate Stimulation

Temperature

Mechanical damage

Chemicals (released fromdamaged tissue)

Bradykinin, serotonin,histamine, K+, acids,acetylcholine, and proteolyticenzymes can excite the

chemical type of pain.

Prostaglandins andsubstance P enhance thesensitivity of pain endings

but do not directly excitethem.

Nociopectors


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Hyperalgesia:

The skin, joints, or muscles that have already beendamaged are unusually sensitive. A light touch to a

damaged area may elicit excruciating pain;

Primary hyperalgesia occurs within the area ofdamaged tissue;

Secondary hyperalgesia occurs within the tissuessurrounding a damaged area.

2 L li i f P i


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2. Localization of Pain

Superficial Somatic Pain arises from skin areas

Deep Somatic Pain arises from muscle, joints,

tendons & fascia

Visceral Pain arises from receptors in visceral organslocalized damage (cutting) intestines causes no pain

diffuse visceral stimulation can be severe

distension of a bile duct from a gallstone

distension of the ureter from a kidney stone


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Most pain sensation is a combination of the two

types of message.

If you prick your finger you first feel a sharp painwhich is conducted by the A fibres,

and this is followed by a dull pain conveyed along C

fibres.

3. Fast and Slow Pain


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Fast pain (acute)

occurs rapidly after stimuli (.1 second) sharp pain like needle puncture or cut

not felt in deeper tissues

larger A nerve fibers Slow pain (chronic)

begins more slowly & increases in intensity

in both superficial and deeper tissues smaller C nerve fibers


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Impulses transmitted to spinal cord by

Myelinated A nerves: fast pain (80 m/s)

Unmyelinated C nerves: slow pain (0.4 m/s)

nociceptor

nociceptor

A nerve C nerve


to reticularformation


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Impulses ascend to somatosensory cortex via:

Spinothalamic pathway (fast pain)

Reticular formation (slow pain)

reticularformation


thalamus

somato-sensory

cortex

4 Vi l i


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4. Visceral pain

Notable features of visceral pain:Often accompanied by strong autonomic and/orsomatic reflexes

Poorly localized;

may be referred

Mostly caused by distension of hollow organs orischemia (localized mechanical trauma may bepainless)


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Afferent innervation of the viscera.

Often anatomical separation nociceptive innervation (insympathetic nerves) from non-nociceptive(predominantly in vagus).

Many visceral afferents are specialized nociceptors, asin other tissues small (Ad and C) fibers involved.

Large numbers of silent/sleeping nociceptors, awakened

by inflammation.Nociceptor sensitization well developed in all visceralnociceptors.


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Convergence theory:


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Convergence theory:

This type of referred pain occurs

because both visceral and

somatic afferents often convergeon the same interneurons in the

pain pathways.

Excitation of the somaticafferent fibers is the more usual

source of afferent discharge,

so we refer the location of

visceral receptor activation to

the somatic source even though

in the case of visceral pain.

The perception is incorrect.

The convergence ofnociceptor input from theviscera and the skin.

Referred pain

5 P i G t Th


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5. Pain Gate TheoryMelzack & Wall (1965)

A gate, where pain impulses can be gated

The synaptic junctions between the peripheral nociceptor

fiber and the dorsal horn cells in the spinal cord are thesites of considerable plasticity.

A gate can stop pain signals arriving at the spinal cord

from being passed to the brain

Reduced pain sensation

Natural pain relief (analgesia)


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descending nervefibers from brain

axons from touchreceptors

axons from nociceptors

THE PAIN GATEopioid-releasinginterneuron

pain pathways


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How does pain gate work?

The gate = spinal cord interneurons thatrelease opioids.

The gate can be activated by:

Simultaneous activity in other sensory (touch)neurons

Descending nerve fibers from brain


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Applications of pain gate

Stimulation of touch fibres for pain relief: TENS (transcutaneous electrical nerve stimulation)

Acupuncture Massage

Release of natural opioids

Hypnosis

Natural childbirth techniques


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Nervous System Part Three Sensory Function