Lecture 5 - Sensory Pathways

Question 1

Primary Sensory Neurons

Answer 1

Originate from peripheral receptors such as Merkel’s receptors; enter spinal cord (or brain) via dorsal roots of spinal (cranial) nerves; cell bodies of these neurons are located in the dorsal root ganglia; synapse in spinal cord with secondary neurons

Question 2

Secondary Neurons

Answer 2

Originate in spinal cord gray matter; travel through spinal cord in one of the myelinated columns; make up tracts in spinal cord and brainstem; Decussate; synapse in thalamus with tertiary neurons

Question 3

Tertiary Neurons

Answer 3

Travel from thalamus to primary sensory cortex; travel through internal capsule (myelinated pathway between thalamus and some of the basal nuclei); terminate in somatosensory cortex

Question 4

What ascending pathways are responsible for conscious perception?

Answer 4

Spinothalamic system (lateral and anterior tracts) and medial lemniscal system

Question 5

What ascending pathways are responsible for unconscious perception?

Answer 5

Spinocerebellar, spino-olivary, spinotectal, and spinoreticular

Question 6

Lateral Spinothalamic Tract

Answer 6

Carries pain and temperature; primary fibers ascend/descend 1-2 spinal cord segments before synapsing with secondary fibers; travels in lateral column of spinal cord; secondary axons decussate through anterior gray and white commissures

Question 7

Trigeminothalamic Tract

Answer 7

Pain and temperature from face and teeth; join secondary fibers of lateral spinothalamic tract in brainstem

Question 8

In the lateral spinothalamic tract, where do secondary fiber collaterals project to and what do they do?

Answer 8

Project to reticular formation and stimulate wakefulness and consciousness

Question 9

In the lateral spinothalamic tract, where do secondary fibers project to and what do they synapse with?

Answer 9

Ventral posterolateral (VPL) nucleus of thalamus and they also synapse with tertiary fibers in VPL

Question 10

In the lateral spinothalamic tract, where do tertiary fibers (corticopetal fibers) synapse and what do they form?

Answer 10

Synapse in postcentral gyrus (somatic sensory areas 1-3) and form part of internal capsule

Question 11

Anterior Spinothalamic Tract

Answer 11

Carries light touch (crude touch), pressure, tickle, and itch; primary neurons may ascend 8-10 spinal cord segments before synapsing with secondary neurons; secondary neurons decussate in anterior gray or white commissures and they ascend to synapse with tertiary fibers in VPL nucleus of thalamus; tertiary fibers ascend through internal capsule to primary sensory cortex

Question 12

Dorsal Column/Medial Lemniscal Pathway Primary Fibers

Answer 12

Carries sensations for two-point sensation (fine touch), pressure, and vibration; “two-point discrimination” touch refers to the ability to distinguish two separate points as close as 2 mm apart; primary fibers ascend entire length of spinal cord and synapse with secondary neurons in medulla (Fasciculus gracilis and Fasciculus cuneatus)

Question 13

Medial Portions of 2 Dorsal Columns

Answer 13

Axons from the lower limbs travel through here; each of these areas of the dorsal columns (1 per column) is called fasciculus gracilis; fibers of the fasciculus gracilis synapse in nucleus gracilis and convey sensations from below midthoracic level

Question 14

Lateral Portions of 2 Dorsal Columns

Answer 14

Axons from the upper limbs travel through here; each of these areas of the dorsal columns (one per column) is called fasciculus cuneatus; fibers of fasciculus cuneatus synapse in nucleus cuneatus and convey sensations from above midthoracic level; also conveys proprioceptive sensation from arms to cerebellum

Question 15

Dorsal Column/Medial Lemniscal Pathway Secondary and Tertiary Fibers

Answer 15

Secondary fibers decussate; ascend to synapse in VPL of thalamus; tertiary fibers ascend through internal capsule to primary sensory cortex

Question 16

Which Brodmann’s Areas make up the primary somatosensory area?

Answer 16

Areas 1-3

Question 17

Which Brodmann’s Areas make up the somatosensory association area?

Answer 17

Areas 5-7

Question 18

What does widespread bilateral excision of somatosensory area 1 cause?

Answer 18

Loss of ability to localize the different sensations in the different parts of the body (crude localization still possible), loss of ability to judge critical degrees of pressure against the body, loss of ability to judge weights of objects, loss of ability to judge shapes or forms (astereognosis), loss of ability to judge texture of materials

Pain and temperature sense is still preserved but poorly localized

Question 19

Where does somatosensory area 2 receive signals from?

Answer 19

From brainstem, transmitted upward from both sides of the body; secondarily from somatosensory area 1; other sensory areas of the body, including visual and auditory

Projections from somatosensory area 1 are required for function of area 2 and removal of area 2 has no apparent effect on the response of neurons in area 1

Question 20

Layers 1 and 2 of Somatosensory Cortex (superficial layers)

Answer 20

Receive input signals from lower brain centers

Question 21

Layers 2 and 3 of Somatosensory Cortex

Answer 21

Send information through corpus callosum to opposite hemisphere

Question 22

Layers 5 and 6 of Somatosensory Cortex

Answer 22

Large neurons in layer 5 project to distant areas such as basal nuclei, brain stem, and spinal cord; axons from layer 6 project to thalamus

Question 23

Layer 4 of Somatosensory Cortex

Answer 23

Receives incoming sensory signals

Question 24

Lateral Inhibition

Answer 24

Blocking the lateral spread of excitatory signals, thereby increasing the degree of contrast in the cerebral cortex; occurs at each synaptic level:

Dorsal column nuclei, ventrobasal nuclei of thalamus, and somatosensory cortex

Question 25

Characteristics of Fast Pain

Answer 25

Felt within 0.1 s after stimulus is applied, not felt in most deeper tissues, generally elicited by mechanical and thermal stimuli, typically carried by A-delta fibers; these pain fibers typically terminate in lamina I (lamina marginalis) of the dorsal horns of the spinal cord and they use glutamate as their neurotransmitter

Question 26

Characteristics of Slow Pain

Answer 26

Begins about 1 s after stimulus is applied and increases over several seconds, may be described as aching, slow burning, throbbing, nauseous, and chronic, can be elicited by mechanical, thermal, and chemical stimuli; slow chronic pain is carried by C type fibers which mostly terminate in layers 2 and 3 (substantia gelatinosa) of the dorsal horns of the spinal cord; release both glutamte and substance P (glutamate acts instantaneously but substance P is released more slowly)

Question 27

Anterolateral Tracts

Answer 27

This is where pain travels; primary pain fibers synapse in the dorsal horns of the spinal cord (layers 1, 2, or 3) with secondary fibers, which immediately decussate and make up the anterolateral pathways; fast pain fibers make up the neospinothalamic tracts and slow pain fibers make up the paleospinothalamic tracts; tertiary fibers ascend to the somatosensory cortex

Question 28

Secondary Fibers of Paleospinothalamic Tracts

Answer 28

Terminate throughout the brainstem, although some pass all the way to the thalamus

Question 29

Brown-Sequard Syndrome

Answer 29

Occurs when their is a hemisection of the spinal cord; all motor functions are blocked on the side of the transection in all segments below the level of transection; sensations of pain, heat and cold (spinothalamic pathway) are lost on the opposite side of the body in dermatomes two to six segments below the level of transection; kinesthetic and position sensations, vibration sensation, discrete localization, and two-point discrimination are lost on the side of the transection in all dermatomes below the level of transection

Question 30

Analgesia System

Answer 30

Consists of periaquaductal gray and periventricular regions of brainstem and third ventricle, raphe magnus nucleus and reticular nuclei in medulla, and pain inhibitory complex in dorsal horns of spinal cord

Question 31

What are the three types of receptors that discriminate thermal gradations?

Answer 31

Cold, warmth, and pain

Question 32

Warmth Nerve Endings

Answer 32

Mainly transmitted over C-type fibers; free nerve endings?

Question 33

Cold Receptors

Answer 33

3-10 times as numerous as warm nerve receptors; small type A-delta myelinated endings

Question 34

Referred Pain

Answer 34

Occurs when visceral pain fibers are stimulated and stimulate some of the pain fibers that conduct pain signals from the skin; severe pain results when there is diffuse stimulation of pain nerve endings throughout the viscera (as opposed to highly localized); such diffuse pain can result from ischemia; all visceral pain from thoracic and abdominal cavities is transmitted via type C pain fibers

Question 35

Headaches

Answer 35

Result of pain referred to the surface of the head from deep head structures; note that the brain itself is almost completely insensitive to pain