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neuro > Tracts > Flashcards

Tracts Flashcards

1
Q

what are the 5 properties of primary afferent neurones?

A
  • modality
  • threshold
  • adaptation rate
  • conduction velocity (of axons)
  • site and extent of peripheral termination (receptive field)
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2
Q

define modality

A

principle type of adequate stimulus that is transduced into an electrical signal by a primary afferent neuron

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3
Q

define threshold

A

threshold potential is the critical level to which a membrane potential must be depolarized to initiate an action potential

low threshold units: respond to low intensity stimuli (non-painful and non-damaging)

high threshold: respond to high, but not normally low intensity stimuli. painful but subject to modulation e.g. thermal, chemical and polymodal nociceptors

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4
Q

define adaptation

A

slow vs rapid adaptation
rapidly acting dynamic response only responds to changing stimulus whereas slow is continuous information to CNS

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5
Q

define conduction velocity

A

axons from skin:
- Aa (proprioceptors of skeletal muscle)
- Ab (mechanoreceptors of skin)
- Ao (pain, temperature)
- C (temperature, pain, itch)

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6
Q

define receptive field

A

the Rf of an afferent neuron is the region that when stimulated an adequate stimulus causes a response in that neuron

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7
Q

where do sensory neurones travel to?

A

travel to the dorsal root to enter gray matter of the spinal cord

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8
Q

describe segmental organisation of the spinal cord in terms of sensory pathway

A
  • most peripheral nerves communicate with CNS through spinal cord
  • area of skin innervated by left and right dorsal roots of a single segment is a dermatome
  • arrangement of paired dorsal and ventral roots is repeated 31 times
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9
Q

describe sensory input to the spinal cord?

A

spinal cord consists of grey (cell bodies and sensory afferent terminals) and white matter (ascending and descending tracts)
- gray matter - dorsal and ventral horns and 10 distinct laminae of rexed

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10
Q

what is the ascending somatosensory pathway?

A

made up of the:
- dorsal column medial lemiscal pathway
- anterolateral system e.g. spinothalamic tract

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11
Q

what is the DCML pathway?

A

the major route by which touch and proprioceptive information ascends to the cerebral cortex

sensory pathway of the CNS system

as well as:
- pressure
- vibration
- two-point discrimination
from the skin and joints

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12
Q

describe the 2 parts of the DMCL pathway

A

consists of 2 parts.
1. The dorsal (post) column which runs from spinal cord to medulla
2. Medial lemniscus which runs as a continuation of the dorsal column, from medulla -> cortex

in the cortex the DCML pathway projects onto the primary somatosensory cortex of the postcentral gyrus.

here sensation location is ‘mapped’ using a somatotopic arrangement, represented as a homunculus.

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13
Q

what is the dorsal column formed by?

A

2 large fasciculi (bundles of nerve fibres) running through the posterior spinal cord:
- fasciculus gracilis
- fasciculus cuneatus

these fasciculi gather sensory info from body’s periphery via skin and joint receptors, + send this info to superior cerebral structures

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14
Q

Name 3 of the most important conscious cutaneous receptors for the DCML system?

A
  • ruffini endings (bulbous corpuscles): detect tension deep in the skin and connective tissue or fascia
  • merkel nerve endings (merkel discs): detect sustained pressure
  • lamellar corpuscles (Pacinian corpuscles): detect rapid vibrations (200-300Hz)
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15
Q

describe the DCML pathway from the start? (long!)

A
  • when an AP is generated by a mechanoreceptor in the tissue, the impulse travels along the peripheral axons of the 1st order neuron
  • the 1st order neuron is pseudounipolar in shape with its body in the dorsal root ganglion
  • the input travels along its axon, through the dorsal root and into the posterior horn, via medial dorsal root entry zone
  • once in spinal cord, these axons give off small collaterals in order to facilitate spinal reflexes or send part of the sensory input through spinothalamic tracts
  • majority, however, will leave the dorsal horn gray matter without synapsing and enter the dorsal funiculus in order to constitute either the fasciculus gracilis or fasciculus cuneatus
  • together these fasciculi ascend the spinal cord to reach lower part of the medulla oblongata.
  • in the medulla, axons in the gracile fasciculus synapse with gracile nucleus and cuneate fasciculus synpase w neurones in the cuneate nucleus.
  • neurones in the 2 medullary nuclei are second order neurones.
  • their axons cross over to the other side of the medulla and are named internal arcuate fibers. crossing is known as = sensory decussation.
  • internal arcuate fibers eventually form medial lemniscus.
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16
Q

what information does the gracilis fasciculus carry?

A
  • tactile and proprioceptive information from the lower half of the body - T7 to the first coccygeal nerve.
  • lies medial to the fasciulus cuneatus within post half of posterior spinal cord
  • lower = medial spatial relationship
17
Q

does the dorsal column fibers of the fasciculus gracilis ascend and cross over?

A

no, they do not cross - they only ascend until they reach the gracilis nucleus
- here they synapse w 2nd order neurones (internal arcuate fibers) and run anteromedially and then cross the midline of the MIDBRAIN - to the contralateral side
- after decussation - the inernal arcuate fibers form the medial lemniscus which ascends towards the ventral posterolateral nucleus of the thalamus and then projects onwards to the primary somatosensory cortex.

18
Q

does the gracilis fasciculus or the fasciclus cuneatus run most medially?

A
  • gracilis fasciculus
19
Q

what information does the fasciculus cuneatus carry?

A
  • it carries input from the C1 - T6 spinal cord levels
  • responsible for transmitting vibration, conscious proprioception and fine (discriminative) touch sensations from the upper body.
  • it REPRESENTS the lateral portion of the dorsal column
20
Q

where does the fasciculus cuneatus decussate?

A
  • the fibers of fasciculus cuneatus reach the ipsilateral cunetaus nucleus in caudal medulla, where they synapse w second-order neurones
  • these second order-neurones then cross over as internal arcuate fibers and form the medial lemniscus on the contralateral side
21
Q

where do axons from the second-order neurons go after decussation at the level of the medulla?

A
  • travel up the brainstem as the medial lemniscus on the contralateral side
  • the medial lemniscus terminates and sypnases w third-order neurones - located in the ventral posteronucleus (VPLN) of thalamus within each hemisphere
22
Q

after the third order neurones have sypases with medial lemniscus where does it go?

A
  • axons of third-order neurones ascend from VPLN, through the posterior 1/3 of the posterior limb of the internal capsule, and then through corona radiata, to reach the primary somatosensory cortex of the post central gyrus in the parietal lobe.
23
Q

what pathway allows us to have well developed discriminative sensation/touch in the fingers of humans?

A
  • DCML pathway
  • it provides us w the ability to detect fine textures and to determine an unknown object by its shape, using our hands w/o visual or auditory input
24
Q

DCML pathway
- damage below the crossing point will cause symptoms on what side of the body?

A

it will cause symptoms on the same side of the body as the lesion
- if the damage occurs above the level of decussation, then the symptoms will show up on the OPPOSITE side of the body

25
Q

what are the clinical signs of DCML pathway lesion?

A

astereognosis: the patient cannot recognize an object its shape and weight using touch alone
agraphesthesia: the patient cannot recognise, by touch, a no or letter drawn in the palm of the patient’s hand by the examiner
loss of discrimnation: i.e. 1 single touch from 2 point touch
loss of vibration sense: patient cannot state position of body in space, without visual input
positive romberg test: patient cannot maintain a vertical position due to the loss of the sense of floor under the feet

26
Q

what type of diseases target the DCML pathway?

A
  • usually degenerative in nature
  • Tabes dorsalis in tertiary syphilis
  • degeneration of spinal cord seen in B12 vit deficicency
  • however DCML pathway can also be affected by trauma or infarction of the posterior spinal artery = posterior (spinal) cord syndrome
27
Q

in central cord syndrome, which pathway is preserved?

A
  • DCML sensation is preserved
28
Q

the spinothalamic tract is what kind of pathway of the spinal cord?

A

it is an ascending pathway of the spinal cord
- it is a sensory tract that transmits information from the skin to the thalamus in the brain

29
Q

what is spinothalamic tract responsible for?

A

it together with the medial lemniscus, is one of the most important sensory pathways of the nervous system
- it is responsible for the transmission of pain, temperature, and crude touch to the somatosensory region of the thalamus

30
Q

spinothalamic tract is also referred to as the ventrolateral (anterolateral) system. it is composed of what 4 tracts?

A
  • anterior spinothalamic tract
  • lateral spinothalamic tract
  • spinoreticular tract
  • spinotectal tract
31
Q

what are the components of the spinothalamic tract?

A
  • the anterior and lateral spinothalamic tracts
    -> anterior helps localize crude touch and pressure
    -> lateral for painful or temperature sensation
  • the spinoreticular tract
    -> responsible for increasing our level of arousal/alertness in response to pain or temperature
  • synapses in the reticular formation
  • the spinotectal tract
    -> enables us to orient our eyes and move our head toward the relevant stimulus
  • fibres ascend to synpase in the superior colliculi of the midbrain
32
Q

where is sensory information transmitted to once it is in the thalamus?

A
  • it is sent to the postcentral gyrus, or primary sensory cortex. represented as a homunculus.
33
Q

describe the homoculus

A
  • the somatotopic cortical map - represents either motor or sensory distribution along the cerebral cortex of the brain
  • the motor represents body parts and its correspondants along the precentral gyrus of the frontal lobe
  • the sensory homuculus is a representation of the body parts along the post central gyrus of the parietal lobe
  • essentially a map along the cerebral cortex of where each part of the body is processed. the impulses from the body will be sent into the spinal cord and eventually back to the brain to be processed e.g. trigeminal nerve carries face sensations. the thlamus processes all these sensory impulses, then the thalamus will send it to the cerebral cortex.
34
Q

where are the locations of the different body parts in relation to the sensory homunculus?

A

-lips and hands being the most sensitive parts of our body have larger areas representing them

  • the legs and genitalia lie on the medial surface of the brain (the anterior most gyrus of the parietal lobe), and the face, hands, arms etc lie on the lateral surface
  • lesions affecting certain areas of the primary motor and somatosensory cortex -> contralateral spastic paresis and hemihypesthesia
    e.g. stroke involving anterior cerebral artery will lead to:
  • contralateral muscle weakness and sensory loss of the lower limb
35
Q

what arteries supply the postcentral gyrus?

A
  • middle and anterior cerebral artery (arising from internal carotid artery)
  • middle supplies inferior and lateral region of the postentral gyrus
  • ACA perfuses the medial and superior region of the post central gyrus

*medial and superior part of postcentral gyrus corresponds to legs and genitals respectively on the homunculus
*region corresponding to neck, shoulders, and torso is considered the watershed area.

neuro (21 decks)

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