Dept. Cell and Neurobiology and Neurology SENSORY PATHWAYS Thomas H. McNeill, PhD Dept. Cell and Neurobiology and Neurology February 23, 2017

LEARNING OBJECTIVES FOR SENSORY PATHWAYS List the general principles of organization of all sensory pathways. Nam e the major pathways that convey sensations from the neck, trunk, limbs and face, and know the sensory modalities conveyed by each. Trace the origin and course of the sensory pathways with emphasis on the level of their decussation. Learn the somatotopic organization within sensory pathways. Identify the connections that mediate corneal reflex and jaw jerk reflex.

Principles of Organization for All Sensory Pathways Sensory pathways that carry information from the neck, trunk and limbs are separated from those that carry sensory information from the face. Sensory pathways for discriminative touch, two point discrimination, vibration and proprioception are separated from those that carry pain, temperature and crude touch. All somatosensory pathways are composed of a three neuron chain that begins at the sensory receptor and ends in the primary sensory cortex.

Principles of Organization (con’t) Sensory pathways that carry information from the neck, trunk and limbs are separated from those that carry sensory information from the face. Somatosensory information carried by individual spinal nerves from the skin form a dermatome pattern on the body wall. Somatosensory information from the face is carried by one of the three subdivisions of the trigeminal nerve (CNV). dermatome map CNV innervation pattern of face

All somatosensory information from skeletal muscle, joints and skin is carried back to the spinal cord via the dorsal and ventral rami of spinal nerves and enters the dorsal horn via a dorsal root

Dorsal Column-Medial Lemniscus (DC-ML) pathway 2. Sensory pathways from the body wall for discriminative touch, two point discrimination, vibration and proprioception are separated from those that carry pain, temperature and crude touch in the spinal cord. Two Major Somatosensory Pathways Dorsal Column-Medial Lemniscus (DC-ML) pathway Fine touch, proprioception and vibration SpinoThalamic Tract (STT) Pain: (including referred pain from the internal organs via sympathetics) Temperature: hot and cold Crude touch

***PRINCIPLES OF ORGANIZATION: 3. General sensory pathways are organized as a chain of 3 neurons from the receptor to the sensory cortex. Sensations from the left side Right primary sensory cortex ***PRINCIPLES OF ORGANIZATION: Neuron 1: is in the PNS in the dorsal root ganglion (DRG) or sensory ganglion of a cranial nerve Neuron 2 is in the CNS: spinal cord/brain stem Axons of neuron 2 cross the midline , i.e, decussate, conveying the sensations to the side contralateral to their origin. Neuron 3 is in a sensory nucleus in the thalamus Decussation/crossing Thalamus DRG DC/ML STT Decussation/crossing Decussation

Summary of 3 Neuron Chain for Sensory Pathways Summary Table of 3 Neuron Chains that form the Sensory Pathways Summary of 3 Neuron Chain for Sensory Pathways   Neuron 1 Neuron 1 Axons Neuron 2 Pathway of Neuron 2 Neuron 3 DC/ML DRG Fas. Gracilis Fas. Cuneatus Nuc. Gracilis Nuc. Cuneatus ML VPL (thalamus) STT N/A Nuc. proprius TL Trigeminal ganglion Main Sensory Nuc of V VPM (thalamus) TTT Spinal tract of V Spinal Nuc. of V

Primary sensory cortex Pathway Tracing Primary sensory cortex Thalamus

Dorsal Column-Medial Lemniscus (DC-ML) pathway Receptors for discriminative touch, vibration, proprioception from neck, upper limb, trunk and lower limb Thalamus Neuron 3 VPL nuc. Thalamus Midbrain Pons Medial Lemniscus Medulla F. Cuneatus F. Gracilis Neuron 2 Neuron 1 DRG Dorsal roots C1-T5 Dorsal column Dorsal roots T6-S2 DRG

DC-ML PATHWAY – axons of the dorsal column ascend ipsilateral to neuron 2 in the nuc. Gracilis and Cuneatus. Axons of neuron 2 cross contralateral to the ML in the brainstem Thalamus medulla Neuron 3 VPL nu. Thalamus Midbrain Pons Medial Lemniscus Nuc.G Medulla Nuc.C Neuron 1 Neuron 2 DRG Dorsal roots C1-T5 Dorsal column Dorsal roots T6-S2 Neuron 2 DRG

Neuron 3 VPL nu. Neuron 1 Neuron 2 SENSORY PATHWAYS EXHIBIT SOMATOTOPY AT ALL LEVELS Somatotopy = Localization of different body parts in sensory pathways. Thalamus Neuron 3 VPL nu. Thalamus Midbrain Pons Medial Lemniscus Medulla Neuron 1 Neuron 2 DRG Dorsal roots C1-T5 Dorsal column Dorsal roots T6-S2 L T A DRG

Somatotopy of the Sensory Axons from the VPL is Maintained in the Posterior Limb of the Internal Capsule and Cortex X Horizontal section at the level of the internal capsule Coronal section of primary sensory cortex

Sensory information remains somatotopically organized in the primary sensory cortex (SSI) (Broadman’s areas 3,1,2) Sensory information related to intensity is sent to somatosensory area II (SSll) in the wall of the Sylvian fissure Clinical note: Different parts of the sensory cortex receive blood supply from different cerebral arteries. This is important for understanding the clinical symptoms of stroke. sensory homunculus.

Multimodal association cortex in the Inferior parietal lobule Information from the primary sensory cortex projects to the general sensory association cortex in the superior parietal lobule (Broadman’s area 5 & 7). This association cortex is responsible interpreting the information for object identification without the aid of vision: Stereognosis – the ability to identify an object by the sense of touch Graphesthesia - the ability to recognize writing on the skin Postcentral gyrus General sensory association cortex: - Superior parietal lobule Multimodal association cortex in the Inferior parietal lobule

Loss of discriminative touch, two-point discrimination, tactile localization, with retention of “crude” touch, pain and temp. carried by STT Loss of vibration sense Loss of proprioception with sensory ataxia, loss of balance and ataxic gait, and a positive Romberg sign Abnormal sensations: Paresthesias/Dysthesias (tingling, numbeness, tightness etc.) Astereognosis and agraphesthesia with DC/ML lesion or a cortex lesion alone Pathology or lesions of the DC-ML pathway results in specific sensory deficits in the effected dermatomes

LOCATING THE LESION Lesions/pathology at Neuron 1 or in the spinal cord results in ipsilateral loss of sensations. Lesions/pathology of the pathway after the crossing over results in contralateral loss of sensations. Thalamus Neuron 3 VPL nu. Midbrain Pons Medial Lemniscus Medulla Neuron 1 Neuron 2 DRG Dorsal roots C1-T5 Dorsal column Dorsal roots T6-S2 DRG

SPINOTHALAMIC TRACT (STT) Pain: (including referred pain from the internal organs via sympathetics) Temperature: hot and cold Crude touch

Spinothalamic Tract (STT) Axons of the from the DRG enter the dorsal horn and synapse on Neuron 2 of the chain (nucleus proprius) Nucleus Proprius DRG Neuron 3 VPL nuc. Thalamus STT STT Receptors for crude touch, pain and temperature from neck, upper limb, trunk and lower limbs. Neuron 1 Dorsal roots C1-T5 DRG STT Dorsal roots T6-S2 Neuron 2 DRG Dorsal horn (Nucleus Proprius)

The course of pain and temperature fibers in the spinal cord differ from the DC/ML pathway in two ways STT Nu. Proprius White commissure Axons of the nucleus proprius in the dorsal horn cross in the white commissure anterior to the central canal. Crossing occurs obliquely through 1-2 spinal segments above the entry of the dorsal roots.

Axons of the STT ascend contralateral in the spinal cord and in the brain stem Axons of the DC/ML ascend ipsilateral in the spinal cord and in the brain stem midline Neuron 3 DRG VPL nuc. Thalamus contralateral STT STT Neuron 1 Dorsal roots C1-T5 STT DRG STT Dorsal roots T6-S2 Neuron 2 DRG Dorsal horn (Nucleus Proprius)

Axons of STT synapse with Neuron 3 in the VPL Axons of VPL neurons enter the dorsal part of the posterior limb of the internal capsule and synapse in the primary sensory cortex along with axons from the DC/ML pathway. VPL Nu VPL Internal capsule

Somatotopy of the Sensory Axons from the VPL is Maintained in the Posterior Limb of the Internal Capsule and Cortex X Horizontal section at the level of the internal capsule Coronal section of primary sensory cortex

LOCATION OF THE LESION? Lesions/pathology at Neuron 1 or 2 results in ipsilateral loss of sensations. Lesions/pathology in STT after the crossing results in contralateral loss of sensations. Neuron 3 VPL nu. Thalamus STT Neuron 1 Dorsal roots C1-T5 DRG STT Dorsal roots T6-S2 Neuron 2 DRG Dorsal horn

Pathology/lesions of the STT pathway results in specific sensory deficits in the effected dermatomes. Loss of pain sensation or analgesia, or reduced sensation, hypalgesia, Loss of pain and temperature sensations, with retention of all other sensations related to touch. Abnormal sensations: Paresthesias/Dysthesias (spontaneous shooting pain, burning sensation) Clinical Condition of enhanced pain Allodynia -painful sensations evoked by minor stimuli such as touch that would not normally cause pain.

Effects of oblique crossing of the STT pathway on location of the lesion Lesions of the STT pathway after the crossing (as shown at T6) produce loss of sensations in the dermatomes on the contralateral side of the body beginning approx. 1-2 segments below the level of the lesion (T8)

Central cord lesions such as syringomyelia will result in bilateral pain and temp. in the dermatomes innervated by the affected spinal cord segments. Larger lesions may also effect adjacent structures in the dorsal and ventral horns. STT Nu. Proprius White commissure STT

SENSORY PATHWAYS FROM THE FACE

Principles of Organization Somatosensory pathways that carry information from the face and anterior scalp are separated from those that carry sensory information from the neck, trunk and limbs.. Somatosensory pathways for discriminative touch, two point discrimination, vibration and proprioception are separated from those that carry pain, temperature and crude touch. All sensory pathways are composed of a three neuron chain that begins at the sensory receptor and terminates in the primary sensory cortex.

V1 V2 V3 Trigeminal ganglion 1. Somatosensory pathways from the face are carried by the three subdivisions of the trigeminal nerve (CNV) (posterior part of the scalp is innervated by spinal nerves) 2. Sensory cell bodies are located in the trigeminal ganglia Trigeminal ganglion V1 V2 V3 Ophthalmic 3 Maxillary Mandibular

2. Somatosensory pathways for discriminative touch, two point discrimination, vibration and proprioception are separated from those that carry pain, temperature and crude touch Somatosensory Pathways (face) Trigeminal Lemniscus Fine touch, conscious proprioception and vibration Trigeminal-Thalamic Tract (TTT) Pain Temperature: hot and cold Crude touch Mesencephalic Trigeminal Tract proprioception of jaw movement

3. Trigeminal sensory pathways are composed of a three neuron chain that begins at the sensory receptor and ends in the face area of the primary sensory cortex. Neuron 1 – Trigeminal ganglion Neuron 2 – Trigeminal nuclei Neuron 3 – VPM of thalamus Neuron 3 Neuron 2 Neuron 1

Trigeminal Nuclear Complex The trigeminal nuclear complex can be divided into three parts. Each part has its own pathway and carries a specific type of sensory information. Components of the trigeminal nuclear complex are: Chief sensory nucleus of V Processes discriminative touch, vibration, proprioception Spinal nucleus and tract of V Processes pain and temperature Mesencephalic nucleus of V Processes proprioception from the muscles of mastication

SOMATOSENSORY PATHWAY 1: TRIGEMINAL LEMNISCUS (TL) Face area, Primary sensory cortex Processes sensory information for discriminative touch, proprioception, vibration. Axons of the TL pathway cross in the pons and ascend contralateral in the midbrain In the midbrain the TL joins the trigeminothalamic tract (TTT) Sensory pathways for touch, proprioception and vibration from the face are only found in the pons and midbrain VPM Neuron 3 TTT/TL Post. limb. Int. Cap. Neuron 2 TL Chief sensory nuc. V Trigeminal ganglion TTT Neuron 1

SOMATOSENSORY PATHWAYS 2: TRIGEMINALTHALAMIC TRACT (TTT) Face area, Primary sensory cortex Processes sensory information pain, temperature and crude touch Axons of the TTT pathway cross in medulla and ascend contralateral in the midbrain next to the medial lemniscus In the midbrain the TTT is joined by the TL Sensory pathways for pain and temperature begin in the medulla VPM Neuron 3 Post. limb. Int. Cap. Trigeminal ganglion Neuron 1 TTT V Spinal tract & nucleus Neuron 2

Comparison Clinical Defciets Associated with Lesions of the TTT and TL Pathways Dorsolateral Lesions (PICA) affecting the spinal tract of V in the medulla will result in a loss of pain and temperature sensations in the ipsilateral half of the face Medial lesions in the lower medulla will effect will spare the face because of remaining input above the lesion site Lesions of the TTT in the lower pons (basilar art.) will lead to loss of sensations in the contralateral half of the face Lesions of the TTT/TL at and above the upper pons (PCA) will lead to a loss of all sensations in the contralateral half of the face Face area, Primary sensory cortex VPM Neuron 3 Post. limb. Int. Cap. TTT/TL TL 4 Neuron 1 3 Trigeminal ganglion TTT 1 2 V Spinal tract & nucleus Neuron 2

Sensory axons from the VPM are located at the genu of the posterior limb of the internal capsule and are somatotopically distributed in the cortex Horizontal section at the level of the internal capsule Coronal section of primary sensory cortex

Pathology/lesions of the trigeminal pathways results in specific sensory deficits in the face Lesions affecting the V nerve will result in a loss of all sensations in the ipsilateral half of the face depending on the branch affected. Lesions affecting the spinal tract of V in the medulla will result in a loss of pain and temperature sensations in the ipsilateral half of the face. This is one part of a syndrome called the Wallenberg or Lateral Medullary Syndrome that we will discuss in the next lecture. Lesions of the TTT at and above the upper pons will lead to a loss of all sensations in the contralateral half of the face. In the pons the TTT lies close to the DC-ML and STT pathways so that a small lesion is likely to affect all three tracts. Involvement of DC-ML , STT and TTT will cause a loss of all sensations in the contralateral half of the neck, trunk, limbs and face. This type of sensory loss of all modalities on one half of the body including the face is referred to as “Hemianesthesia”.

Trigeminal Nerve Reflexes Corneal Blink Reflex – evolutionary reflex used to protect eye from foreign body or bright light Ex. Touching the right cornea Sensory afferents from the right cornea travel in the right ophthalmic nerve (V1) to reach the right spinal or chief sensory nucleus of V. Axons of the trigeminal nuclei project bilaterally to synapse in the motor nuclei of CN VII in lower pons. Touching one cornea activates both VII motor nuclei that supply the orbicularis oculi muscles causing both eyes to close.

Mesencephalic Trigeminal Tract - proprioception of jaw movement Mesencephalic Nuc. V The neurons of the mesencephalic nucleus are the only first-order primary sensory neurons within the CNS Propriocetive sensations from the muscles of mastication and TMJ via mandibular nerve terminate in the mesencephalic nucleus of V Central processes of these cells synapse on the neurons of the motor nucleus of V which innervate the muscles of mastication Motor Nuc. of V Muscles of mastication Mandibular nerve Muscles of mastication

Jaw-Jerk Reflex – used to test the integrity the trigeminal nerve complex Sensory information from neuromuscular spindles in the muscles of mastication responding to stretch, conveyed by the mandibular nerve to the mesencephalic nucleus. This is likely to be a bilateral input (only one side is shown).   Neurons of the mesencephalic nucleus then sends axons bilaterally to the motor nuclei of V. Neurons in in the somatomotor part of CNV sends axons via the mandibular nerve to the muscles of mastication causing the jaw to close.

Summary of 3 Neuron Chain for Sensory Pathways Summary Table of 3 Neuron Chains that form the Sensory Pathways Summary of 3 Neuron Chain for Sensory Pathways   Neuron 1 Neuron 1 Axons Neuron 2 Pathway of Neuron 2 Neuron 3 DC/ML DRG Fas. Gracilis Fas. Cuneatus Nuc. Gracilis Nuc. Cuneatus ML VPL (thalamus) STT N/A Nuc. proprius TL Trigeminal ganglion Main Sensory Nuc of V VPM (thalamus) TTT Spinal tract of V Spinal Nuc. of V