Sensory and Motor Pathways

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Cross Section of Spinal Cord

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Ascending and Descending Tracts/Pathways

= sensory

= motor

Sensory Nerve Tracts• Each pathway involved with different categories of

sensory information• First half of word indicates “origin”, second half

indicates “destination”

• Dorsal Column System – proprioception, fine-touch,

two point discrimination, pressure

• Anteriolateral System —Spinothalamic Tracts: somatic sensory information from cutaneous receptors to Thalamus

• pain, pressure, crude touch, temperature

• Spinocerebellar--periphery of lateral and dorsal columns– proprioception to cerebellum

Anterolateral System: Spinothalamic Tract

• Anterior portion: crude touch/pressure• Lateral portion: pain and temp• Contralatteral

– Decussates in spinal cord

• Three neuron system– Primary neuron (1st order) : from receptor to posterior horn of spinal

cord• Synapse with interneurons

– Secondary neuron (2nd order): cross to opposite side, enter spinothalamic tract, ascend to thalamus

– Tertiary neuron (3rd order): thalamus to somatic sensory cortex

Spinothalamic -- Anteriolateral Tracts

•Anterior portion: crude touch/pressure•Lateral portion: pain and temp•Contralateral

– Decussates in spinal cord•3 neuron system

– Primary neuron (1st order) : from receptor to posterior horn of spinal cord

• Synapse with interneurons– Secondary neuron (2nd order): cross

to opposite side, enter spinothalamic tract, ascend to thalamus

– Tertiary neuron (3rd order): thalamus to somatic sensory cortex

Dorsal-Column (Medial-Lemniscal) System• Carries sensations of:

fine touch, two-point discrimination, • proprioception, vibration to cerebrum, and cerebellum• Contralatteral

– Decussates in medulla

• 3 neurons– Primary neurons have cell bodies in dorsal root ganglion.

• Axons enter spinal cord and ascend to the medulla oblongata without decussating where they synapse with secondary neurons.

– Secondary neurons in medulla: • axons decussate and ascend to thalamus

– Tertiary neurons: • project to somatic sensory cortex

Dorsal Column•fine touch, two-point discrimination, •Contralatteral

– Decussates in medulla•3 neurons

– Primary neurons have cell bodies in dorsal root ganglion.

• Axons enter spinal cord and ascend to the medulla oblongata without decussating where they synapse with secondary neurons.

– Secondary neurons in medulla: • axons decussate and ascend to thalamus

– Tertiary neurons: • project to somatic sensory cortex

Spinocerebellar System

• Carries proprioceptive information to cerebellum, most of which is unconscious

• Actual movements monitored and compared to cerebral information representing intended movement

• Posterior and anterior spinocerebellar tracts

• Ipsilatteral=“same side”– Does not decussate

Spinocerebellar tracts:• proprioceptive to cerebellum• Posterior and anterior spinocerebellar tracts• Ipsilatteral=“same side”

• Does not decussate

• 2 neurons– Primary neuron from receptor to spinal cord

• Cell body in posterior root ganglion

– Secondary neurons from spinal cord to cerebellar cortex

Descending Pathways• Two functions:

– Somatic and Visceral motor control• Through both excitation and inhibition

– Modification of sensation: • cortex may reduce the conscious perception of

sensations

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Descending Pathways through spinal cord white matter

• Corticospinal (pyramidal)—within dorsal and anterior columns– voluntary movements

• Indirect Pathways– involuntary movement, upright posture,

balance, walking, reflexive movements of head and neck in response to visual and auditory stimuli

The Descending Pathways• Direct pathways

(pyramidal system): maintenance of muscle tone, controlling speed and precision of skilled movements

• Indirect pathways (extrapyramidal system): less precise movements.

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Direct Pathways (pyramidal System) Corticospinal tracts

Skeletal Muscle Control• Motor system:

– maintains posture and balance; moves limbs, trunk, head, eyes; facial expression, speech.

– Reflexes and voluntary movement

• Two neurons: upper and lower – Upper motor neurons:

• Arise in cortex connect to lower motor neurons (sometimes with an interneuron)

– Lower motor neurons: • axons leave the CNS, extend through PNS to skeletal

muscles. • Cell bodies in anterior horns of spinal cord and in cranial

nerve nuclei of brainstem

Direct Pathways• Control muscle tone and conscious fine, skilled movements in

the face and distal limbs• Direct synapse of upper motor neurons of cerebral cortex with

lower motor neurons in brainstem or spinal cord• Contralatteral

– Decussats in medulla (decussation of pyramids)

• Tracts – Corticospinal: direct control of movements below the head

• Lateral = limb muscles• Anterior = axial muscles

– Corticobulbar: direct control of movements in head and neck

Corticospinal Tracts

• Axons of upper motor neurons descend to pyramids of medulla oblongata

• 75-85% decussate and descend in the lateral corticospinal tracts. Supply all levels of body

• Remaining fibers descend decussate near level of synapse with lower neurons. Supply neck; upper limbs

Direct Pathways

•Contralatteral– Decussates in medulla (decussating of pyramids)

• Corticospinal Tracts

– direct control of movements below the head • Lateral column tracts = limb muscles• Anterior column tracts= axial muscles