INTS 9: Motor and Sensory Pathways Flashcards

1
Q

Where do the main descending motor pathways originate from?

A
  • they originate from the cerebral cortex and the brainstem
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2
Q

Where do the principal descending motor pathways act on?

A
  • they act via lower motor neurons on skeletal muscles
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3
Q

Which motor pathways originate from the cerebral cortex?

Via where?

A
  • corticospinal tracts: via spinal cord
  • corticobulbar tracts: via brainstem
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4
Q

Which motor pathways originate from the brainstem?

A
  • vestibulospinal tracts
  • reticulospinal tracts
  • rubrospinal tracts
  • tectospinal tracts
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5
Q

Give a brief description of what somatosensory pathways do

A
  • they carry information from the periphery (skin, joints) into the CNS
  • relaying information relating to
  • fine touch
  • pressure
  • vibration
  • crude touch
  • pain
  • temperature
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6
Q

Which two motor pathways originate from the primary motor cortex?

A
  • corticospinal pathway
  • corticobulbar pathway
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7
Q

What is the neuron from the primary motor cortex to the spinal cord (or brainstem) known as?

A
  • upper motor neuron
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8
Q

What is the neuron from the spinal cord (or brainstem) to the muscle known as?

A
  • lower motor neuron
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9
Q

What does it mean that the body is represented somatotopically in the primary motor cortex?

A
  • a specific region of the body can be mapped to a specific region in the nervous system
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10
Q

What does this diagram show?

A
  • they are projections of the corticospinal (a) and corticobulbar (b) tracts from the primary motor cortex through the internal capsule
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11
Q

Describe how the corticospinal fibres are distributed throughout the brain

How do they form the lateral corticospinal and anterior corticospinal tract?

A
  • see image below
  • 85-90% of corticospinal fibres cross in the medulla (the decussation of the pyramids) to form the lateral corticospinal tract
  • the remaining 10-15% form the anterior corticospinal tract
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12
Q

What movements do the lateral corticospinal tract and anterior corticospinal tract control?

A
  • lateral corticospinal tract:
  • skilled voluntary movements
  • anterior corticospinal tract:
  • postural adjustments following limb movements
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13
Q

Describe the pathway and function of the vestibulospinal tract

A
  • the vestibulospinal tract conveys signals from the inner ear (vestibular apparatus) to the musculature
  • it controls movements of the neck muscles and postural adjustments by facilitating extensor muscles and inhibiting flexor muscles
  • image shows the posterior view showing origin of vestibulospinal tracts from pons/medulla
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14
Q

Describe the pathway and function of the rubrospinal tract

A
  • fibres originate from the red nucleus in the midbrain
  • they travel through the brainstem and synapse on lower motor neurons innervating the flexor muscles of the upper limbs
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15
Q

Describe the pathway and function of the vestibulospinal tract

A
  • fibres originate from the reticular formation of the pons and medulla
  • they control the orientation of the body towards or away from stimuli
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16
Q

Describe the pathway and function of the tectospinal tract

A
  • fibres originate from the superior colliculus of the midbrain
  • controls neck musculature in response to visual stimuli
  • orientates the head during eye movements
17
Q

Familiarise yourself with the transverse (horizontal) section through half a spinal cord

Recall functions of the major motor pathways shown

A
18
Q

Which neurons are somatosensory pathways generally composed of?

A
  • generally composed of three neurons:
  • primary
  • secondary
  • tertiary
19
Q

Describe the pathways of the primary, secondary and tertiary neurons of the touch and proprioceptive (the dorsal column) somatosensory pathways

A
  • the primary neurons of the touch and proprioceptive pathway (the dorsal column pathway) enter the spinal cord
  • they ascend on the same side (ipsilaterally) to the medulla where they synapse onto secondary neurons
  • the cell bodies of the secondary neurons are located in the gracile and cuneate nuclei
  • the ascending tracts from these nuclei cross to the opposite (contralateral) side and ascend as the medial lemniscal pathway to the thalamus
  • there, they synapse onto tertiary neurons
  • these project to the relevant area of the primary somatosensory cortex
20
Q

Describe the pathways of the primary, secondary and tertiary neurons of the pain and temperature (the spinothalamic tract) somatosensory pathways

A
  • the primary neurons enter the spinal cord and synapse onto secondary neurons
  • they cross the midline and ascend contralaterally to the thalamus, where they synapse onto tertiary neurons
  • these project to the relevant area of the primary somatosensory cortex
21
Q

Describe the dorsal column pathway

A
  • one of the principal pathway systems
  • the receptors for touch and proprioception are mechanoreceptors
  • they transduce (convert) mechanical stimuli into electrical signsals
  • they are the modified terminals of the peripheral axons of primary sensory neurons
  • cell bodies are in the peripheral nervous system
  • dorsal root ganglia for the body
  • trigeminal ganglia for the head
  • the axons are fast conducting and innervate receptors located in the muscles (spindles) and tendons (Golgi tendon organs)
  • the intensity of the stimulus is coded by the frequency of firing of neuron
  • the amplitude of action potential does not change
  • firing frequency is related to log: 10 fold stimulus increase, then firing frequency doubles
  • somatotopic organisation is maintained through the entire pathway from dermatomes to cortex
  • decussation of the pathway for information coming from the body and head occurs in the brainstem
  • the difference between adjacent inputs is enhanced by lateral inhibition
22
Q

Describe the spinothalamic pathway

A
  • the receptors for pain and temperature are polymodal (triggered by mechanical, thermal or chemical stimulus) free nerve endings
  • they are slowly adapting and have high thresholds.
  • cell bodies are in the peripheral nervous system
  • dorsal root ganglia for the body
  • or trigeminal ganglia for the head
  • receptors are innervated by 2 axon types:
  • larger myelinated fibres: which are faster conducting and produces sharp pain
  • smaller unmyelinated fibres: which are slower conducting and produce dull aching pain.
  • the receptive fields (see below) are usually large and the intensity of the stimulus is coded by frequency of firing.
  • somatotopic organisation is maintained through entire pathway from dermatomes to cortex
  • decussation (crossing) of the pathway for information coming from the body occurs in the spinal cord.
  • decussation of the pathway for information coming from the head occurs in the brainstem.
  • cortical representation of pain is not well understood.
  • localisation and intensity of stimulus only may register there
23
Q

What is the receptive field?

A
  • a region of skin innervated by a single sensory neuron
  • the firing rate of the neuron is altered by a stimulus in the receptive field
  • there is a varying degree of overlap in receptive fields of sensory neurons
  • the size of the receptive fields coupled with the degree of overlap influences the degree to which the CNS localises stimuli
24
Q

What is lateral inhibition?

A
  • precision of localisation of a stimulus may be enhanced with lateral inhibition
  • the central portion of a receptive field has an excitatory region surrounded by an inhibitory regions
  • this creates contrast in sensation allowing sensory precision