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Year 1 Semester 2 > Neural pathways > Flashcards

Neural pathways Flashcards

1
Q

Explain the 3 types of sensory neuron in the somatosensory system

A
  • Primary afferent is a first order neuron and terminates in spinal cord of brain stem
  • The second order neuron projects to the thalamus
  • The third order neuron projects to the brain
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2
Q

Through what routes does sensory info enter the spinal cord?

A

Dorsal routes of the spinal nerves

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3
Q

What systems relay sensory info to the brain from the spinal cord?

A
  • Dorsal column-medial lemniscal system

- Anterolateral system

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3
Q

What systems relay sensory info to the brain from the spinal cord?

A
  • Dorsal column-medial lemniscal system

- Anterolateral system

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4
Q

What are some features of dorsal column-medial lemniscal pathways?

A
  • Large myelinated fibres
  • 30-110m/s
  • Discrete types of mechanoreceptive sensation
  • High degree of spatial orientation of the nerve fibres with respect to their origin
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5
Q

What are some features of anterolateral pathways?

A
  • Smaller fibres
  • Up to 40m/s
  • Broad spectrum of sensory modalities
  • Less spacial orientation
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6
Q

What information is carried via dorsal column-medial lemniscal system?

A
  • Fine touch
  • Vibration
  • Conscious proprioception
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7
Q

What information is carried via anterolateral system?

A
  • Pain
  • Temperature
  • Course touch
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7
Q

What information is carried via anterolateral system?

A
  • Pain
  • Temperature
  • Course touch
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8
Q

Describe the dorsal column-medial lemniscal system

A
  • Enter spinal cord vis dorsal spinal root
  • Continue up to the medulla in the dorsal columns of spinal cord
  • Synapse there
  • Cross to opposite side in medulla
  • Up through brainstem to thalamus via medial lemnisucus
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9
Q

Describe the anterolateral system

A
  • Enter spinal cord via dorsal spinal root
  • Synapse in dorsal horns of spinal grey matter
  • Cross to opposite side of cord
  • Ascend through anterior and lateral white columns of cord
  • Terminate at all levels of lower brain stem and in thalamus
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10
Q

What does the gracile fasciculus carry?

A

Neurons that enter from the lower body

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11
Q

What does the cuneate fasciculus carry?

A

Neurons that enter from the upper body

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12
Q

What spinal level does gracile takeover from cuneate?

A

about T6

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13
Q

Which is more medial, gracile or cuneate?

A

Gracile

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14
Q

What are the steps of the dorsal column-medial lemniscus pathway?

A
  • First order axons enter the dorsal grey horn of spinal cord via dorsal roots
  • Pass to either cuneate or gracile fasciculus (above T6 enter fasciculus cuneatus, below T6 enter fasciculus gracilis)
  • Synapse with second order neuron in either nucleus cuneatus or nucleus gracilis
  • Fibres then decussate via internal arcuate fibres to enter the medial lemniscus
  • Fibres of medial lemniscus ascend and synapse with third-order neurons of the ventral posterolateral (VPL) nucleus of the thalamus
  • The third order neurons leave the VPL passing through the internal capsule to terminate in the primary somatosensory cortex
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15
Q

Where is the somatosensory cortex located?

A

In a strip posterior to the post central sulcus of the brain

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16
Q

What is the difference between area I and II of the somatosensory cortex?

A

Area I has a high degree of localisation of different body parts (primarily upper limb)

Area II has poor localisation

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17
Q

What are the characteristics of transmission in the anterolateral pathway?

A

Same as dorsal column-medial lemniscal system except:

  • The velocities of transmission are only 1/3-1/2 those in the dorsal column-medial lemniscus system ie 8-40m/s
  • The degree of spacial localization of signals is poor
  • The gradations of intensities are far less accurate, with most sensations being recognised in 10-20 gradations of strength rather than as many as 100 gradations for the dorsal column system
  • The ability to transmit rapidly changing or rapidly repetitive signals is poor
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18
Q

What are the steps of the anterolateral spinothalamic pathway?

A

-First order axons enter the dorsal grey horn of the spinal cord via dorsal roots
-Enter Lissauer’s tract to ascend or descend 1-2 spinal cord levels
-Synapse in either lamina 1, 2 (substantia gelatinosa) or 5 of the dorsal grey horn
-Decussate via the anterior white commissure to enter the contralateral anterolateral spinothalamic tract - it can take 2-3 segments to reach the contralateral side
Ascend to the ventral posterolateral (VPL) nucleus of the thalamus where they synapse with the third order neurons
-The third order neurons leave the VPL passing through the internal capsule to terminate in the primary somatosensory cortex

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19
Q

Describe fast pain

A
  • Felt within 0.1s after a pain stimulus is applied
  • AKA sharp pain, pricking pain, acute pain, electric pain
  • Fast-sharp pain is not felt in most deep tissues of the body
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20
Q

Describe slow pain

A
  • Begins 1s or more after pain stimulus applied, then increases slowly over s or min
  • AKA slow burning pain, aching pain, throbbing pain, nausseous pain, chronic pain
  • Usually associated with tissue destruction
  • Can lead to prolonged, almost unbearable suffering
  • Slow pain can occur both in the skin and in almost any deep tissue or organ
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21
Q

What are the 3 types of stimuli for pain receptors?

A
  • Mechanical (typically fast pain)
  • Thermal (typically fast pain)
  • Chemical (slow pain)
  • Slow pain can be elicited by all three
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22
Q

What fibres transmit fast-sharp pain?

A

A-delta fibres at ~6-30m/s

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23
Q

What fibres transmit slow-chronic pain?

A

C fibres at ~0.-2m/s

23
Q

What fibres transmit slow-chronic pain?

A

C fibres at ~0.-2m/s

24
Q

What is the other name for the fast-sharp pain pathway?

A

Neospinothalamic tract

25
Q

What is the other name for the slow-chronic pathway?

A

Paleospinothalamic pathway

26
Q

What is referred pain?

A

Pain in a location remote from tissue causing pain

27
Q

What is the probable mechanism of referred pain?

A
  • Branches of visceral pain fibres synapse in spinal cord on same 2nd order neurons that receive signal form skin
  • When visceral pain receptors are stimulated, pain signals from viscera are conducted through some of the same neurons conducting pain signals from skin, so person feels sensation originate in skin
28
Q

How does unconscious proprioception work?

A
  • Spinocerebellar tracts
  • First order axons enter dorsal gery horn via dorsal roots and synapse in clarke’s nucleus/column
  • Pathway then splits into dorsal and ventral
  • Dorsal - second order axons ascend in the ipsilateral dorsal spinocerebellar tract to the cerebellar cortex. Conveys input from trunk and lower limbs
  • Ventral - majority second order axons decussate and ascend in the contralateral ventral spinocerebellar tract then crosses back over to the ipsilateral cerebellum - minority ascend ipsilaterally to the cerebellum. Conveys input from trunk, upper limbs and lower limbs
29
Q

What are upper motor neurons (UMNs)?

A
  • Cell bodies found in the cortex and brainstem nuclei
  • Restricted to the CNS and do not contact muscle
  • Executive function for lower motor neurons (LMNs) and circuits controlling LMNs
30
Q

What are lower motor neurons (LMNS)?

A
  • Cell bodies found in the brainstem and spinal cord
  • Leave the CNS and contact muscle to stimulate
  • Motor function to muscles
31
Q

Describe the motor cortex

A
  • Anterior to the central sulcus
  • Occupies ~1/3 of frontal lobes
  • 3 sub areas - primary motor cortex, premotor area, supplementary motor area
32
Q

Describe the primary motor cortex

A

->1/2 primary motor cortex controls hands and muscles of speech
Excitation of a single UMN usually excites a specific movement - not one specific muscle

33
Q

Describe the premotor area

A
  • Nerve signals give more complex patterns of movement

- External stimuli

34
Q

Describe the supplemental motor area

A
  • Contractions elicited are often bilateral e.g grasping movements of both hands
  • Internal stimuli
  • Usually functions together with premotor area
  • Gives background movement onto which premotor and primary motor cortex add finer control
35
Q

How are specialised areas of motor control identified?

A

Electrical stimulation or observation of loss of motor function with destructive lesions

36
Q

What are some examples of specialised areas of motor control?

A
  • Broca’s area (motor speech area
  • Voluntary eye movement field
  • Head rotation area
  • Hand skills area
37
Q

Describe the brainstem

A
  • A cranial continuation of the spinal cord
  • Brainstem nuclei are involved in motor + sensory functions of face and head regions
  • Also, provides many special control functions: control of respiration, CV system - partial control of GI function - control of many stereotyped body movements, equilibrium, eye movements
  • Also serves as a way station for command signals from higher neural centres
38
Q

What are the basal nuclei?

A

-Caudate nucleus
-Putamen
-Globus palidus
-Subthalamic nucleus
Subsatia nigra

39
Q

What three things are involved in subcortical motor control?

A
  • Basal nuclei
  • Thalamus
  • Cerebellum
40
Q

Describe the corticospinal tract (pyramidal tracts)

A
  • Carries signals direct from cortex to spinal cord
  • Concerned with voluntary, discrete, skilled movements (especially distal parts of the limbs)
  • Originates: 30% primary cortex; 30% premotor/supplementary motor; 40% other areas (e.g somatosensory cortex)
41
Q

What route do the neurons take starting at primary motor cortex?

A
  • Primary motor cortex
  • Pyramids pf medulla
  • Majority (~85%) decussate in lower medulla
  • Lateral corticospinal tracts
  • Mainly terminate on interneurons

-Those axons that do not decussate (~15%) pass ipsilaterally in ventral corticospinal tracts

42
Q

Describe the lateral corticospinal tract

A
  • Lateral corticospinal tract innervates mainly distal muscle groups of the extremities via direct communication with the LMN in contact with a specific muscle
  • But does influence muscles of the entire limb
  • Influences LMN circuits to integrate sensory and motor actions to achieve the desired movement pattern e.g gait cycle
43
Q

Describe the ventral corticospinal tract

A
  • Postural adjustments to stabilize trunk during limb movements led by LCSp tract
  • Ventral corticospinal tract does not cross at the pyramidal decussation but majority do cross the midline at the relevant spinal cord level
  • Can bilaterally innervate LMN’s controlling the trunk and proximal musculature
44
Q

Describe conscious movements

A
  • Mainly in lateral pathways
  • Principally controlled by the cerebral cortex via 2 corticospinal tracts
  • General control of voluntary movement
  • Mainly associated with control of distal muscles
45
Q

Describe unconscious movement

A
  • Mainly ventromedial pathways
  • Principally controlled in the brain stem
  • Control of posture and rhythmic movements associated with locomotion
  • Control axial and proximal muscles
46
Q

Name and describe the function of the extra pyramidal tracts

A
  • Reticulospinal tracts - posture, locomotion, autonomic function
  • Vestibulospinal tracts - balance and posture
  • Rubrospinal tracts - red nucleus, influenced by cortical, cerebellar and reticular nuclei, excitatory to flexors - mostly upper limb
  • Tectospinal tract - head and neck movements for visual tracking
47
Q

Describe reticular nuclei

A
  • Primitive motor system
  • Also involved in control of breathing and emotional motor systems
  • Pontine and medullary tracts - mainly function antagonistically to each other
48
Q

Describe the pontine reticular system

A

-Pontine reticulospinal tract in the anterior column of the cord
-Excite axial muscles which support the body against gravity
-Have a high degree of natural excitability
When pontine reticular excitatory system is unopposed by medullary reticular system - powerful excitation of antigravity muscles throughout the body - standing up

49
Q

Describe the medullary reticular system

A
  • Nuclei transmit inhibitory signals to same antigravity anterior motor neurons via medullary reticulospinal tract in lateral column of the cord
  • Medullary reticular nuclei receive strong input collaterals from corticospinal tract, rubrospinal tract and cortical motor pathways
  • Some signals from higher areas of brain can disinhibit medullary system when brain wishes to excite pontine system to cause standing
  • Excitation of MRS can inhibit antigravity muscles in certain portions of body to allow performance special motor activities
49
Q

Describe the medullary reticular system

A
  • Nuclei transmit inhibitory signals to same antigravity anterior motor neurons via medullary reticulospinal tract in lateral column of the cord
  • Medullary reticular nuclei receive strong input collaterals from corticospinal tract, rubrospinal tract and cortical motor pathways
  • Some signals from higher areas of brain can disinhibit medullary system when brain wishes to excite pontine system to cause standing
  • Excitation of MRS can inhibit antigravity muscles in certain portions of body to allow performance special motor activities
50
Q

Describe vestibular nuclei

A
  • Input from cerebellum and vestibular apparatus
  • Function in association with pontine reticular nuclei to control antigravity muscles
  • Lateral and medial vestibulospinal tracts in the anterior columns of the spinal cord - withput vestibular support , thepontine reticular system would lose much of its excitation - inhibited by the red nucleus
  • Specific role of vestibular nuclei is to selectively control the excitatory signals to the different antigravity muscles to maintain equilibrium in response to signals from the vestibular apparatus
51
Q

Describe rubrospinal tract

A
  • Action on flexor muscles
  • Runs in lateral white column, can intermix with lateral corticospinal tract
  • A lot of control form other centres - cortex and cerebellum
  • Acts to inhibit extension action of vestibular nucleus
52
Q

What are signs of upper motor neuron lesions?

A
  • Spastic paralysis
  • Increased deep tendon reflexes
  • Increased tone
  • Clonus
53
Q

How do lower motor neuron lesions present?

A
  • Muscle paralysis
  • Reduced motor tone
  • Reduced stretch reflex
  • Fasciculation
  • Atrophy
53
Q

How do lower motor neuron lesions present?

A
  • Muscle paralysis
  • Reduced motor tone
  • Reduced stretch reflex
  • Fasciculation
  • Atrophy
54
Q

Describe decorticate posturing

A
  • Flexion of upper limb - extension of lower limb
  • Lesion above red nucleus - disinhibitation
  • Corticospinal tract interrupted - extra pyramidal tracts become dominant distally
55
Q

Describe decerebrate posture

A

-Extension
_lesion inferior to red nucleus
-Unopposed extension of vestibulospinal and pontine reticulospinal tracts

Year 1 Semester 2 (26 decks)

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