Pyramidal & Extrapyramidal Pathways Flashcards
pyramidal tract
the largest descending pathway in humans, is composed of corticonuclear (bulbar) and corticospinal tracts.
The corticonuclear tract
is destined to end at motor nuclei of the cranial nerves, which are located in the different parts of the brainstem Corticonuclear fibres branch away from the pyramidal tract as they descend through the brainstem to synapse on their target motor nuclei. Most of these corticonuclear fibres decussate (cross-over) before terminating on these nuclei
the corticospinal tract
travels down to the spinal cord. All axons of the motor neurones which give rise to the pyramidal tract originate mostly from the motor cortex and descend through the cerebral white matter in the corona radiata, then converging as they descend through the posterior limb of the internal capsule . From the internal capsule, the pyramidal tract descends through the crus cerebri of the midbrain, then through the base of the pons, the pyramids of the medulla, down to anterior (ventral) and lateral funiculi of the spinal cord to ultimately synapse with the α- and γ- motoneurons in the anterior horn of the spinal cord, but also in sensory parts of posterior grey horn.
sensory ascending pathways
all travel through the posterior limb of internal capsule and later through the corona radiata en route to the somatosensory cortex)
after all the corticonuclear tract have left the pyramidal tract
the corticospinal tract proceeds through the pyramids of the medulla oblongata. .
the lateral corticospinal tract
ust caudal to the pyramids, 80-90% of the fibres of the corticospinal tract decussate (cross-over) in the anterior median fissure of the medulla (= pyramidal decussation) and continue to descend in the contralateral lateral funiculus of the spinal cord as the lateral corticospinal tract
the anterior corticospinal tract
The remaining 10-20% descend ipsilaterally in the anterior column as the anterior corticospinal tract, and decussate eventually at various levels of the spinal cord. The corticospinal fibres synapse with α-motoneurons, γ- motoneurons, and interneurons in the anterior horn of the spinal cord.
The internal capsule
is white matter containing both ascending and descending axons:
The internal capsule - anterior limb
the anterior limb separates the caudate nucleus and the putamen,
The internal capsule - posterior limb
the posterior limb separates the thalamus from the lenticular nuclei (putamen + pallidus). The corticospinal and corticonuclear tracts are located in the posterior limb of the internal capsule.
The corticonuclear tract synapses with the:
- vertical gaze centre (midbrain reticular formation & pretectal area)
- horizontal gaze centre (paramedian pontine reticular formation (PPRF)
- motor cranial nerve nuclei: CN V for muscles of mastication, CN VII for muscles of facial expression, CN IX for muscles of the pharynx, CN X for muscles of the larynx, CN XI for sternocleidomastoid and trapezius muscles, CN XII for muscles of the tongue.
Postural control (reflex action)
is complex and involves the regulation of activities of different agonist and antagonist muscles for maintaining a body position, which requires multilevel organizational units (spinal and supraspinal [brainstem]). At both levels, it is largely automated (i.e. functions subconsciously = involuntary = reflex). Its maintenance requires all sorts of sensory inputs needed to analyse the position of the body and its various parts in space, as well as its orientation and direction of its movement.
postural control sensory input
- vestibular (non-auditory input from the inner ears that provides information on head movement)
- visual (e.g. closing your eyes increases body sway due to the lack of visual feedback)
- proprioceptive afferent systems:
~ Conscious proprioception via dorsal-column-medial-lemniscal pathway (from musculoskeletal system)
~ Unconscious proprioception via spino- & cuneocerebellar pathways (from musculoskeletal system)
spinal stretch (proprioceptive) reflexes
are innate (genetically determined) and are stereotyped (show predictable pattern with no flexibility for variations or adaptability). They are very fast as they are only controlled by spinal cord (= no involvement of supraspinal centres). Their main function is to protect muscles from injury (e.g. overstretching) and to stabilize joints.
for maintaining posture
the supraspinal (brainstem) reflex centres utilize and integrate all sorts of sensory information they get through the cranial nerves, i.e. vestibular and visual information but also proprioceptive (via the cerebellum that receives such proprioceptive sensory information via the spinal cord [e.g. posterior spinocerebellar and cuneocerebellar tracts] from the periphery)
supraspinal reflexes
- are important in providing postural adjustments and in maintaining body balance and muscle tone in response to visual, vestibular, and proprioceptive signals.
- act on numerous muscle groups at the same time.
- supraspinal rflxs that allow static and dynamic postural reactions are mainly maintained by extrapyramidal system, primarily the reticulospinal and vestibulospinal tracts.
spinal rflxes - 2
reflexes are constantly complemented in their actions by supraspinal reflexes.
supraspinal reflexes adaptivity
- are goal-oriented and adaptive, i.e. they can be trained and have “unlimited flexibility” to learn.
RUBROSPINAL TRACT
is located in the tegmentum of the midbrain and is a motor nucleus. . The colour is due to deposits of iron.
- Fibres decussate in the midbrain & descend near the lateral corticospinal tract through lateral column of the spinal cord, and project to interneurons at the level of the cervical cord.
RUBROSPINAL TRACT
function
The rubrospinal tract controls flexor muscles of upper limbs (i.e. it controls tone of flexor muscles, which are important for grasping) unilaterally (not bilaterally). Therefore, it is a flexor-biased tract. It complements the function of the lateral corticospinal tract. It receives afferent input from the motor cortex.
TECTOSPINAL TRACT
The tectospinal tract originates in the superior colliculi (bypasses LGN), crosses over at midbrain tegmentum and descends to the anterior funiculus (column) of the spinal cord to terminate on motoneurons in the cervical spinal cord that innervate the neck muscles that are important for head movement.
