Spinal Cord, Ascending Tracts & Sensation Flashcards
What type of sensory receptors exist and what do they sense?
Meissner corpuscle: discriminative touch
Pacinian corpuscle: deep pressure + vibration mechanoreceptors (distortion)
Ruffini ending: touch, sheer stress/forces
Merkel cells/discs: light, sustained touch
Free nerve endings: pain + temperature
Muscle spindles: muscle stretch receptors
Where do sensory receptors exist?
Located at multiple levels within tissues e.g. epidermis, dermis etc. but they overlap so the same patch of skin can detect different stimuli
What sensory receptors are rapidly adapting?
Free nerve endings (temperature)
Meissner corpuscle
Pacinian corpuscle
What sensory receptors are slowly adapting?
Free nerve endings (pain)
Merkel cells/disc
Ruffini ending
What does rapidly adapting mean?
That the receptor will quickly detect relative changes
Where are the sensory cortexes?
Within the left and right parietal lobes
Where is the primary somatosensory cortex?
Post-central gyrus posterior to the central sulcus
What input does the primary somatosensory cortex receive?
Contralateral sensory input from the body e.g. taste
What does the superior parietal lobe do?
Integration of sensory inputs, sensory memory and perception of contralateral self/world
Define somatotopy.
The point-for-point correspondence of an area of the body to a specific point on the CNS
What is the internal capsule?
Dense collection of white matter (myelinated axons) projection fibres that fan out becoming the corona radiata that is somatotopically organized carrying sensory (ascending) and motor (descending) white matter tracts to and from the cortex
What will a stroke/tumor in the internal capsule present like?
Widespread contralateral motor and/or sensory symptoms
What are the structures that make up the internal capsule?
Anterior limb: contains connections between dorsal nucleus of thalamus, prefrontal cortex, pontine nuclei + prefrontal cortex
Genu + posterior limb: contain motor fibres in corticospinal + corticobulbar tract as well as sensory input from VPM/VPL to the primary somatosensory cortex
Retrolenticular: contains lateral (visual) + medial (auditory) geniculate nuclei travelling from thalamus to respective cortices
What is included in a typical sensory tract?
1st order neurons: ascend spinal cord ipsilaterally to nuclei in lower medulla
2nd order neurons: decussate in lower medulla + ascend to synapse in the thalamus via lemniscus tract
3rd order neurons: ascend to cortex via internal capsule
What is the thalamus?
The 2 thalami (L/R) represent an organised collection of subcortical relay nuclei
What are the 2 main nuclei of the thalamus for somatosensory input?
- Ventral posterior medial nucleus (VPM): sensory input from body i.e. limbs/trunk
- Ventral posterior lateral nucleus (VPL): sensory input of face (CNV)/most of head
What does the thalamic ventroposterior complex receive input from?
Dorsal column pathway (medial lemniscus), spinothalamic tract (spinal lemniscus) + trigeminal lemnisci projecting to primary somatosensory cortex with additional projection to secondary somatosensory area via projection fibres of white matter pathways internal capsule + corona radiata
How are spinal cord axial sections viewed?
As if looking at them from above which goes AGAINST normal standard for viewing axial CT/MRI scan images
The sensory dorsal and motor ventral horn are organised into zones called __ ___.
Rexed Lamina
What is contained within Rexed Lamina?
The areas where ascending (sensory) or descending (motor) fibres may synapse onto other neurons thus cell bodies are found in these regions
What are some examples of Rexed Lamina zones? What do they do?
Sensory dorsal horn:
I - pain, temperature + touch
II - neurons mediating pain transmission
Motor ventral horn:
VIII + IX: lower motor neurons
What groups can the white matter be organized into?
Funiculi (bundles of > 1 similar tract):
- Dorsal
- Lateral
- Ventral
What will a typical cross section of the cervical region of the spinal cord look like? Why?
Large grey horns given the large population of neurons required to innervate upper limbs
What will a typical cross section of the thoracic region of the spinal cord look like? Why?
Small grey horns as segmental thoracic innervation requires relatively few neurons
What will a typical cross section of the lumbar region of the spinal cord look like? Why?
Large grey horns given the large population of neurons required to innervate lower limbs
What will a typical cross section of the sacral region of the spinal cord look like? Why?
Small white matter tracts given the very few white matter tracts at this low level
What are the 3 major ascending sensory pathways?
- Dorsal column
- Spinothalamic tract
- Spinocerebellar tract
What comprises the dorsal column pathway? What does it sense?
Carries information from Meissner’s, Merkel’s, Pacinian, Ruffini + Joint proprioceptors that detect discriminative touch, vibration + conscious proprioception
Comprised of medial fasciculus gracilis (below T6 i.e. lower limb/trunk) + lateral fasciculus cuneatus (above T6 i.e. upper limb/trunk)
What comprises the spinothalamic tract? What does it sense?
Carries information from free nerve endings that detect pain (nociception), temperature + simple touch
Divided into anterior (ASTT) + lateral (LSTT) parts that merge and become the spinal lemniscus
What comprises the spinocerebellar tract? What does it sense?
Carries information from muscle spindles (dorsal) + Golgi tendon organs (ventral) that detect unconscious proprioception i.e. monitor muscle length, speed of contraction + tension
4 separate tracts
Describe the dorsal column pathway.
- 1st order neurons pass into dorsal cord + ascend ipsilaterally to lower medulla
- 1st order neurons synapse with 2nd order neurons in lower medulla (nucleus gracilis + cuneatus respectively)
- 2nd order neurons decussate + ascend to thalamus
- 2nd order neurons ascend to VPL of thalamus + synapse with 3rd order neurons
- 3rd order neurons travel to primary sensory cortex
Describe the dorsal column pathway of the face.
- 1st order trigeminal nerve sensory neurons enter pons + synapse in CN V nucleus
- 2nd order neurons ascend in trigeminal lemniscus of brainstem to VPM of thalamus decussating at different levels
- 3rd order neurons pass to primary sensory cortex
What can occur if there is damage to the contralateral thalamus?
Loss of sensation on contralateral face and body
The size of a lesion will determine the _____________.
Amount and type of sensory loss or change
How is the CN V sensory nucleus arranged?
It is large and split into different sensory modalities such as proprioception, touch, pain + temperature
How can the dorsal column be damaged?
Compression
Infarction
Infection (E.G. tertiary syphilis causes demyelination + destruction)
B12 deficiency
What occurs after damage to the dorsal column?
Ipsilateral symptoms following spinal cord damage below the level of the lesion
Symptoms:
- Pseudoathetosis (writhing of digits, hands + feet)
- Sensory ataxia (leads to +ve Romberg sign + stamping gait)
When considering nervous system damage, what questions must you ask yourself?
- What senses would be lost?
- What side would they be lost on?
- How might a patient present?
Describe the spinothalamic tract.
- 1st order neurons from body ascend 1-2 vertebral levels (in Tract of Lissauer) in dorsal grey matter before synapsing with 2nd order neurons
- 2nd order neurons decussate here across anterior white commissure
- 2nd order neurons ascend in tract to VPL thalamic nucleus + synapse with 2rd order neurons here
- 3rd order neurons travel to primary sensory cortex
What is syringomyelia?
Cavitation/expansion of central canal in cervical region which damages only the decussating neurons of the spinothalamic tract, not the spinothalamic tracts themselves
Lost sensation in both arms and across top of back/chest
What are the causes of syringomyelia?
Idiopathic
Trauma
Developmental CNS disorder (e.g. Chiari malformation)
How is the spinothalamic tract organised?
Somatotopically e.g. into sacral, leg, arm/thorax, head + neck
What is sacral sparing?
An expanding ventral grey matter tumour can knock out all contralateral pain/temperature sensation but may not affect the sacral region
Describe the dorsal spinocerebellar tract.
Muscle spindle:
- Upper limb (above T6) information runs with fasciculus cuneatus of dorsal column then once synapsed in the Pons proceeds with cuneocerebellar tract
- Lower limb (below T6) information enters cord and synapses in Clarke’s dorsal nucleus then ascends in this tract (sits between C8-L3)
What is special about the dorsal spinocerebellar tract in contract to the other tracts?
It is an uncrossed ipsilateral 2-neuron pathway
Describe the ventral spinocerebellar tract.
Golgi tendon organ:
- Upper limb (above T6) information passes through rostral tract but is uncrossed
- Lower limb (below T6) information passes into tract, crosses & re-crosses
How many neurons are in the spinocerebellar tract pathway?
2
How many neurons are in the dorsal column and spinothalamic tract pathways?
3
What symptoms do you see with spinocerebellar tract damage?
Rarely seen in isolation so symptoms normally masked by other motor weakness/paralysis from other tracts
Pure lesions could cause:
- Ataxia/malco-ordination of motor action
- Wide-based gait
What sensory loss would you get if you had a brainstem lesion?
Ipsilateral facial sensory loss
Contralateral body sensory loss
What sensory loss would you get if you had a thalamic lesion?
Contralateral sensory loss of face and body
What conditions affects Clarke’s dorsal nucleus?
Fredrich’s ataxia = spinocerebellar degeneration
