Wk10 Overview of Central Sensory and Motor pathways

Question 1

Sensory and motor pathways

Answer 1

• Impulses need to travel from different sensory receptors to inform the correct parts of the brain
• Impulses need to travel from relevant parts of the brain to periphery to control movements
• Sensory pathways = Ascending pathways (carry somatic afferents)
• Motor pathways= Descending pathways (carry somatic efferent fibres)
• Nerve fibres carrying specific information (sensory modalities) travel in specific areas within the brain and spinal cord

Question 2

Cerebral cortex

Answer 2

• Cerebral cortex represents the highest centre for neuronal progressing
• Areas for somatic sensation (conscious sensation) and voluntary movements are separated by the central sulcus
• Separates frontal lobe from parietal lobe
• Crudely can divide the brain into a Motor brain
and sensory brain
• gyri (pl.) either side are important for sensory and motor cortical processing

Question 3

Overview of ascending pathway

Answer 3

• Two tracts carrying sensory info from the body:
• Spinothalamic and Dorsal column tracts
• 3 orders of neurons carry sensory information to somatosensory cortex
1) 1st order (primary)
2) 2nd order (secondary)
3) 3rd order (tertiary)

Periphery —> spinal cord/medulla —> thalamus —> somatosensory cortex

Question 4

1st order neurone

Answer 4

• carry sensory information to CNS
• axons enter at spinal level (+/-1)
• vias dorsal root
• cell bodies are in Dorsal root ganglia
• synapse with 2nd order neurons

Question 5

2nd order neurone

Answer 5

• Differ where they start
• this neuron decussates (crosses midline)
• carry sensory information to the thalamus
• where they synapse with 3rd order neurons

Question 6

3rd Order neuron

Answer 6

• cell body in the thalamus
• project to somatosensory cortex
• Where will depend on what region of the body is stimulated

Question 7

Spinothalamic tract

Answer 7

• 1st order enters spinal cord and synapses in dorsal horn
• 2nd order neuron decussates close to central canal
• travels up contralateral side in spinothalamic tract
• synapses with 3rd order in thalamus

Sensory modalities:
• Pain
• Temperature
• crude touch/pressure

Question 8

Dorsal column tract

Answer 8

• 1st order enters spinal cord via dorsal root
• 1st order travels up the dorsal column of the spinal cord
• Reaches medulla synapses with 2nd order neuron
• 2nd order decussates in medulla
• travels up contralateral side to thalamus and synapses
• 3rd order projects onto the sensory cortex

Sensory modalities:
• fine touch
• proprioception
• Two point discrimination

Question 9

Clinical focus- Syringomyelia

Answer 9

• Syringomyelia is a cyst in the central canal of the spinal cord
• Symptoms include:
• bilateral loss of temperature and pain sensation
• with proprioception intact

Question 10

Which ascending tract is affected here and why? Why is proprioception intact?

Answer 10

Spinothalamic tract carrying temperature and pain sensation will be interrupted by pressure.
Dorsal column tract carrying proprioceptive info should be spared.

Question 11

Sensory info from the face

Answer 11

• Sensory information from face
• Trigeminothalamic pathways
• Use cranial nerves (CN V, VII, IX and X) as 1st order neurons
• 2nd order neurons decussate
• travel to thalamus synapse with 3rd
• 3rd order neurons end in somatosensory cortex
• proprioception/ touch/ pain temperature

Question 12

Unconscious sensation

Answer 12

unconscious sensory information doesn’t reach the somatosensory cortex
unconscious proprioception travels in spinocerebellar tracts to reach cerebellum

Question 13

Summary of sensory info

Answer 13

• Sensory info from the right hand side of the body is processed by the left hemisphere
• 2nd order neuron carries the information across the midline
• Dorsal column tract (proprioception and fine touch)is carried on the ipsilateral side of the spinal cord and synapses with the 2nd order neuron in the medulla- decussation occurs and information is processed on the contralateral side.
• Spinothalamic tract (painandtemp) decussates at respective dermatome and information is carried on contralateral side of the CNS
• somatosensory map is not proportional with body surfaces

Question 14

Overview of Motor pathways

Answer 14

Conscious movements are initiated by the primary motor cortex
These commands are relayed downwards though the brainstem to be executed by the spinal cord, through skeletal muscles
The motor pathway consists of neurons that fit into two classes:
Upper and Lower
Upper= originate in the cortex and other higher centres
• stimulate lower motor neurons (LMN)
Lower= originate in ventral horn of the spinal cord or motor nucleus of a cranial nerve
• Directly innervate target tissue and cause contraction

Question 15

Descending tracts

Answer 15

Two general classes: pyramidal and non-pyramidal

Pyramidal are tracts which start in the cortex and decussate in the medulla: Corticospinal tract- body
Corticobulbar tract- face

Extrapyramidal are tracts which start in the Brainstem and do not pass through the medullary pyramids:
Rubrospinal tract
Tectospinal tract
Vestibulospinal tract Reticulospinal tract

Anything involving the motor cortex is conscious
Tracts run downwards in different white matter tracts of the spinal cord

Question 16

Corticospinal tract

Answer 16

Cortex (‘cortico’) > spine (‘spinal’)

involves the primary motor cortex- voluntary/ conscious movement

Most fibres cross the midline (descussate) at the medullary pyramids

Fibres end on contralateral side

Upper motor neuron: Primary Motor cortex> ventral horn Lower motor neuron: Ventral horn> effector

Question 17

Summary of Motor pathways

Answer 17

Voluntary movements are initiated by upper motor neurons with cell bodies in the primary motor cortex
There are two classes of motor neurons involved in motor pathways
corticospinal tract is responsible for the voluntary control of skeletal muscles in the body
Skeletal muscles on the left hand side of the body are controlled by the right hemisphere of the brain- due to decussation of fibres
Subcortical motor centres of the brain include the thalamus and basal ganglia and are important in the control of motor movement

Question 18

Overall summary

Answer 18

• Both the pre and post central gyrus (motor and sensory cortex) have somatotopic organisation
• Sensory pathways are ascending and bringing sensory information from the periphery to somatosensory cortex
• Motor pathways are descending, and initiate impulses from the primary motor cortex to skeletal muscle
• Ascending pathways consist of 3 orders of neurons
• descending pathways consist of Upper and Lower motorneurons
• Both pathways contain fibres that decussate and therefore control/sense the contralateral side of the body.