Introduction to Sensory Pathways and Reflexes

Tract

collection of axons with a common origin

Fasciculus

same as tract

Funiculus

two or more fasciculi

Nucleus

collection of cell bodies within the CNS

Ganglion

collection of cell bodies within the PNS

Contralateral

opposite side

Ipsilateral

same side

Fasciculus cuneatus

lateral in the cord

Fasciculus gracilis

medial in the cord

Conscious pathways starts

from a receptor

Conscious pathways end

in the cerebral cortex

Ascending pathways

afferent (general somatic afferent)

Descending pathways

efferent (somatic motor)

Neurons in GSA pathways (conscious)

3 (or 4 for pain/temp and light (gross) touch

First cell body for GSA pathways

dorsal root ganglion

Second cell body for GSA pathways

posterior horn of gray OR brainstem (closed medulla)

Third cell body for conscious pathways

dorsal thalamus - ventral posterior lateral nucleus (VPL))

Third cell body for reflex pathways

ventral posterior medial (VPM)

Last axon synapse location for conscious sensory pathways

Area 3, 1, 2 in the post-central gyrus of the cerebral cortex

Internal capsule

White matter structure - tract

Types of tactile stimuli

Two-point tactile touch, Light (gross) touch, Pressure, Vibration

Two-point tactile touch

The ability to discriminate 1 and 2 points

Proprioception

Sense of where the body is in space

Where does the conscious sensory pathway for 2PTPPV for the lower extremity start?

At the receptor in the lower extremity

Where is the first cell body located in the conscious sensory pathway for 2PTPPV (lower extremity)

In the dorsal root ganglion in the lumbar sacral region.

Through which tract does the conscious sensory pathway for 2PTPPV for the lower extremity move after the first cell body?

Fasciculus gracilis.

Where is the second cell body located in the conscious sensory pathway for 2PTPPV for the lower extremity?

In the nucleus gracilis in the closed medulla.

Where does the conscious sensory pathway for 2PTPPV for the lower extremity cross over?

In the low medulla at the sensory decussation.

What tract does the conscious sensory pathway for 2PTPPV for the lower extremity move through after crossing over?

Medial lemniscus.

Where is the third cell body located in the conscious sensory pathway for 2PTPPV for the lower extremity?

In the VPL of the dorsal thalamus.

Where does the last axon of the conscious sensory pathway for 2PTPPV for the lower extremity move to?

From VPL to area 3, 1, 2.

Lesion below sensory decussation results in

Ipsilateral deficit

Lesion above sensory decussation results in

Contralateral deficit

Lesion in fasciculus gracilis results in

Ipsilateral deficit of conscious 2-point tactile, pressure, vibration and proprioception of the lower extremity

Where does the conscious pathway for 2PTPPV for the upper extremity start?

It starts at the receptor in the upper extremity.

Where is the first cell body located in the conscious sensory pathway for two-point tactile, pressure, vibration, and proprioception for the upper extremity?

In the dorsal root ganglion in the cervical region (C5-T1).

What tract does the first axon for the conscious sensory pathway for 2PTPPV for the upper extremity first move through?

Fasciculus cuneatus.

Where is the second cell body located in the conscious sensory pathway for 2PTPPV for the upper extremity?

In the nucleus cuneatus in the closed medulla.

Where does the conscious sensory pathways for 2PTPPV for the upper extremity cross over?

In the low medulla at the sensory decussation.

What tract does the pathway for conscious 2PTPPV for the upper extremity move through after crossing over?

The medial lemniscus.

Where is the third cell body located in the conscious sensory pathway for 2PTPPV for the upper extremity?

In the VPL of the dorsal thalamus.

Where does the last axon of the conscious sensory pathway for 2PTPPV for the upper extremity move to?

From the VPL to area 3, 1, 2.

What tracts are in the posterior funiculus of white

Fasciculus gracilis and fasciculus cuneatus.

What is the starting point of the conscious sensory pathway for pain and temperature?

Receptor in the lower extremity or upper extremity

Where is the first cell body located in the conscious sensory pathway for pain and temperature?

Dorsal root ganglion

What is the tract of the first axon in the conscious sensory pathway for pain and temperature?

Short axon in Lissauer's tract

Where is the second cell body located in the conscious sensory pathway for pain and temperature?

Posterior horn of gray

Where does the pathway cross over in the conscious sensory pathway for pain and temperature?

Ventral white commissure

In which tract of the spinal cord does the conscious sensory pathway for pain and temperature travel after crossing over?

Lateral spinothalamic tract in the lateral funiculus

Where is the third cell body located in the conscious sensory pathway for pain and temperature?

VPL of the dorsal thalamus

What is the final destination of the last axon in the conscious sensory pathway for pain and temperature?

Area 3, 1, 2

Where does the conscious sensory pathway for light touch start?

At the receptor in the lower extremity or upper extremity.

Where is the first cell body located in the conscious sensory pathway for light touch?

In the dorsal root ganglion.

What is the name of the tract of the short axon in the conscious sensory pathway for light touch?

Lissauer's tract.

Where is the second cell body located in the conscious sensory pathway for light touch?

Posterior horn of gray.

Where does the conscious sensory pathway for light touch cross over?

Ventral white commissure.

In which funiculus does the conscious sensory pathway for light touch travel?

In the ventral/anterior funiculus as the ventral spinothalamic tract.

Where is the third cell body located in the conscious sensory pathway for light touch?

In the VPL of the dorsal thalamus.

Where does the last axon of the conscious sensory pathway for light touch move to?

From the VPL to area 3, 1, 2.

What is the sensory decussation responsible for?

Conscious two-point tactile, pressure, vibratory, and proprioception of upper and lower extremities.

What does the ventral white commissure transmit?

Conscious pain, temperature, and conscious light (gross, superficial) touch.

What does the anterolateral system/spinothalamic pathway involve?

Pain and temperature pathway and light gross touch.

What does the medial lemniscal pathway or dorsal column pathway involve?

Two-point tactile, vibratory, pressure, and proprioception.

Reflex pathways endpoint

End anywhere other than the thalamus (midbrain or medulla).

Where does the reflex sensory pathway for pain and temperature start?

At the receptor in the lower extremity or upper extremity.

Where is the first cell body located in the reflex sensory pathway for pain and temperature?

Dorsal root ganglion.

Where is the second cell body located in the reflex sensory pathway for pain and temperature?

Posterior horn of gray.

Where does the reflex sensory pathway for pain and temperature cross over?

Ventral white commissure.

In which tract does the reflex sensory pathway for pain and temperature travel?

Spinotectal tract.

Where does the reflex sensory pathway for pain and temperature end?

In the tectum of the dorsal part of the midbrain.

Where does the reflex sensory pathway for 2PTPPV for the lower extremity start?

At the receptor in the lower extremity or upper extremity.

Where is the first cell body located in the reflex sensory pathway for the lower extremity?

In the dorsal root ganglion in the lumbar sacral portion of the cord.

Where is the second cell body located in the reflex sensory pathway for the lower extremity?

In Clarke's nucleus in the cord.

Does the reflex sensory pathway for the lower extremity cross over?

No, it does not cross.

In which tract does the reflex sensory pathway for the lower extremity travel?

In the dorsal cerebellar tract.

How does the reflex sensory pathway exit the cerebellum?

Via the inferior cerebellar peduncle.

Reflex sensory pathway for upper extremity

Starts at the receptor in the lower extremity or upper extremity, first cell body is in the dorsal root ganglion in the cervical cord, travels fasciculus cuneatus, second cell body is Accessory cuneate nucleus in the medulla, no crossing, travels in the Cuneocerebellar tract, exits the cerebellum via the inferior cerebellar peduncle towards the cerebellar.

Cerebellum modulation

Modulates voluntary motor activity.

Source of proprioceptive information

Comes from muscle spindles in the muscles regarding information about change in length, and from Pacinian and free nerve endings in joints to determine if they are flexed and extended.

Reflex sensory pathway for spino-olivary tract

Starts at receptors of sensory systems that give information about muscles, travels in the spino-olivary tract, travels to inferior olive of medulla (lateral), information travels to cerebellum to modulate motor activity by the sensory input.

Reflex sensory pathway for spino-reticular tract

Starts at all general receptors, travels via the spinoreticular tract, information travels to the reticular activating system involved in pain transmission and emotional response to pain.

Pain transmission

Transmitted by myelinated axons (A delta) to somatosensory cortex and unmyelinated axons (C fibers) to the reticular formation to the thalamus and to the limbic brain, which do not go to the cortex.

Reticular formation

Found in the core of the brainstem, has two components: sensory (reticular activating system) and motor, transmits information to the thalamus and cortex responsible for the awakened state.

Reticular Activating System (RAS)

Used for sending information from general sensation receptors to the reticular formation.

RAS and consciousness

Reticular formation transmits information from the reticular activating system to the cortex and thalamus; compression can result in unconsciousness.

Principles of Sensory Systems

Includes parallel pathways (more than one pathway), relay nuclei (change message, sharpen it or inhibit a message), topographical representation (hand takes up more information and neighborhoods are preserved), and most sensory pathways cross (except reflex pathways for two-point tactile touch).

Topographical organization in sensory cortex

Relates to the anterior and posterior vascular system and stroke.

Dorsal root ganglion

First cell body location for sensory pathways in both lower and upper extremities.

Clarke's nucleus

Second cell body location in the reflex sensory pathway for the lower extremity.

Accessory cuneate nucleus

Second cell body location in the reflex sensory pathway for the upper extremity.

Cuneocerebellar tract

Pathway traveled by sensory information from the upper extremity to the cerebellum.

Dorsal cerebellar tract

Pathway traveled by sensory information from the lower extremity to the cerebellum.

Inferior cerebellar peduncle

Exit point for sensory information traveling from the cerebellum.

Spinothalamic tract

Part of the anterolateral system affected by diseases impacting white matter of the spine.

Ventral white commissure

Pathway for pain and temperature and light gross touch.

Emotional response to pain

Involves the reticular activating system in pain transmission.

Myelinated axons (A delta)

Transmit sharp pain to the somatosensory cortex.

Unmyelinated axons (C fibers)

Transmit dull pain to the reticular formation and limbic brain.

Lower extremities

Represented medially in the sensory homunculus.

Upper extremities

Represented laterally in the sensory homunculus.