#11 Flashcards
(28 cards)
Dorsal root fibers enter the spinal cord in two different bundles
a medial bundle of
thickly myelinated fibers conveying impulses from large encapsulated somatic receptors,
e.g. muscle spindle (Ia), Golgi tendon organs (Ib), Pacinian corpuscles (Aβ), Meissner’s
corpuscles (Aβ); a lateral bundle of finely (Aδ) or non-myelinated fibers (C), e.g.nociceptors, thermoreceptors. These dorsal root afferents may contribute directly or
indirectly to long tract pathways or serve as the sensory units in segmental reflexes.
Upon entering the spinal cord, fibers may ascend or descend before
giving off collateral branches.
These collaterals may terminate on:
- Propriospinal interneurons:
These are “proper spinal”
neurons aka “local circuit
neurons,” meaning their cell bodies and axons stay entirely within the spinal cord and yoke together sensory and motor neurons in networks called central pattern generators. - Projection neurons: These neurons project away from the spinal cord to higher centers, sending axons that run in tracts.
Most primary afferent fibers also have side branches that end in the ventral horn (column in yellow) – these participate in
intra-segmental reflexes
a reflex pathway
requires a minimum of two component neurons:
(a) an afferent (sensory) neuron located
in a dorsal root or cranial nerve sensory ganglion; and (b) an efferent (motor) neuron
located in the spinal cord ventral horn or brainstem cranial nerve motor nuclei
3 major spinal reflexes
- Myotatic stretch reflex
* Ia afferents from muscle spindles
* Monosynaptic input to agonist lower motor neuron (LMN)
* Polysynaptic input to antagonist LMN
* Causes contraction of a stretched muscle - Golgi tendon reflex
* Ib afferents from Golgi tendon organs (GTOs)
* Polysynaptic input to agonist and antagonist LMNs
* Causes inhibition of an overly contracted muscle
(protective) - Withdrawal reflex
* Ad nociceptor afferents from skin
* Polysynaptic input to agonist and antagonist LMNs
* Crossed extension component
* Causes withdrawal from painful stimuli
Myotatic stretch reflex forms basis for testing
deep tendon reflexes (DTRs). Diminished or absent DTR indicates lesion affects afferent or efferent neurons for that specific spinal cord segment. Exaggerated DTR indicates lesion or disease affects upper motor neuron pathway
The main sensory pathways of the spinal cord are
- Dorsal column system (the “DC” part of the DCML system)
- Spinothalamic system
- Spinocerebellar system
DORSAL COLUMN SYSTEM
Transmits tactile sensations (discriminatory touch, vibration sense, pressure sense), conscious proprioception (joint position, limb and trunk position) and kinesthesia
(movement sense)
The dorsal columns contain fibers from the
same (ipsalateral) side of the body.
Highly refined somatotopic organization of dorsal columns allows information to be
transmitted with
high degree of spatial resolution
Sacral fibers cable together more a, cervical fibers more b
a medially
b laterally
DCML pathway summarized
I. Neuron 1: dorsal root ganglion neuron
with thickly myelinated central process
enters dorsal white column of spinal
cord and, without synapsing, ascends.
Caudal fibers ascend and shift medially and posteriorly as fibers from more rostral segments join the pathway.
These ascending fibers synapse on:
II. Neuron 2: neurons in gracile nucleus (for fibers arising from ~T6 and down) or cuneate nucleus (for fibers arising from ~T5 or above). Neuron 2 sends
axons that curve ventromedially (internal arcuate fibers) to cross the
midline (sensory decussation) and
ascend as the medial lemniscus. Fibers of the medial lemniscus project to:
III. Neuron 3: VPL in the thalamus, which then projects to limb and trunk
areas of first (primary) somatosensory cortex in the postcentral gyrus.
DCML dysfunction
- Lesion of the dorsal column results in ipsilateral (same side as lesion) loss of conscious proprioception, kinesthesia, 2-point discrimination and vibration sense in body segments below the level of the lesion.
- Movements are clumsy, uncertain, and poorly coordinated (sensory ataxia).
- Lesion of the medial lemniscus: same signs as for dorsal column lesions, but deficits are contralateral to lesion
Romberg’s test
This is a simple test of DCML dysfunction. It relies on the fact that without visual clues, balance requires conscious proprioception.
* Ask patient to stand, feet together with arms by the side, eyes open; if patient sways then probably cerebellar problem (note: Romberg’s is NOT a cerebellar test);
* Ask patient to close eyes; if sways or falls, positive Romberg sign suggesting dorsal column damage
SPINOTHALAMIC SYSTEM
Relays pain, temperature and crude touch information
SPINOTHALAMIC SYSTEM runs in the a, hence sometimes called anterolateral system
anterior (ventral) and lateral funiculi,
Consequently, lesions of the spinothalamic tract in the spinal cord results in
loss of pain and temperature (anaesthesia) on the contralateral side of the body below the level of the lesion
Because Ad and C fibers ascend the spinal cord in Lissauer’s tract before entering the dorsal horn, anaesthesia begins 1-2 segments below the level of a lesion, affecting all caudal body areas
Most pain/temperature fibers from the DRG end in
laminae I (posteromarginal
nucleus)
the lateral part of the
anterolateral system carries the a fibers, while the anterior part carries the b fibers.
a pain and
temperature
b crude touch
Positions of ascending sensory tracts in the pons
Note as the basis pontis swells and the medial lemniscus flattens out, it takes a position
medial to the spinothalamic tract
Positions of ascending sensory tracts in the midbrain
In the midbrain, the medial lemniscus and spinothalamic tract come to lie together in the
lateral tegmentum (as they are both running to the same place, the VPL thalamus).
Spinothalamic pathway summarized
I. Neuron 1: DRG neuron with non- or thinly-myelinated central process enters posterior
gray horn via Lissauer’s tract and may ascend one or two segments before
synapsing on:
II. Neuron 2: posteromarginal nucleus,
substantia gelatinosa, or nucleus proprius.
Neuron 2 gives rise to axons that cross the
midline in the ventral white commissure.
These axons then continue up the spinal
cord in the anterolateral white matter as the
spinothalamic tract until they terminate on:
III. Neuron 3: VPL and other nuclei in the
thalamus, which then project to the first
(primary) somatosensory cortex (postcentral
gyrus) as well as other cortical and telencephalic structures
Spinothalamic tract dysfunction
- Lesion of the spinothalamic tract in the spinal cord results in loss of pain and temperature on the contralateral side of the body below the level of the lesion, 1 to 2
segments below the level of the lesion (due to ascent of fibers in Lissauer’s tract). - Lesion of the anterior white commissure results in a bilateral segmental loss of pain and temperature, 1-2 segments below the level of the lesion.
- Note that the somatotopic organization is opposite that of the dorsal columns, with
sacral levels represented in the more lateral part of the tract. This may be useful in localizing the lesion, e.g. in the case of a tumor within the spinal cord, as encroachment on the spinothalamic tract would result in loss of pain and temperature in the upper extremity first (“sacral sparing”)
the medial lemniscus carries a from the b, in addition to its normal modalities as part of the DCML system
a ascending visceral
pain signals
b contralateral spinal cord
