L2-Spinal Cord, Brainstem, Cortex Control of Motor Function II Flashcards
define pyramidal system
these are tracts that pass through the medullary pyramids. Other motor pathways are extrapyramidal.
what are the components of the pyramidal system?
- corticospinal tract
- corticobulbar tract
what are the characteristics of the pyramidal system?
upper motor neurons:
- originate in the motor cortices
- 75-85% decussate in pyramids and form the lateral corticospinal tracts
- remaineder decussate near synapse with lower motor neurons and form anterior corticospinal tracts
- most synapse with association neurons in spinal cord central gray
how are the pyramidal system Upper Motor Neurons classified?
according to where they synapse in the ventral horn
what are the classifications?
- medial activation system: innervate postural and girdle muscles
- lateral activation system: associated with distally located muscles used for fine movements
- nonspecific activating system: facilitate local reflex arcs
what else is the corticospinal tract referred to as?
pyramidal tract
where does the tract originate?
- primary motor cortex
- premotor cortex
- somatosensory area
what is the corticospinal tract pathway?
site of origin –> internal capsule –> medullary pyramids –> X in lower medulla (most fibers) –> lateral columns of spinal cord (lateral corticospinal tract)
- some fibers do not cross but continue down ipsilaterally in ventral corticospinal tract
what are the corticospinal tract divisions?
- lateral corticospinal tract
- anterior corticospinal tract
describe the lateral CT
- made up of corticospinal fibers that have crossed the medulla
- supply all levels of spinal cord
describe the anterior ct
made up of uncrossed corticospinal fibers that cross near level of synapse with LMNs
supply neck and upper limbs
what are the functions of the corticospinal tract divisions?
- adds speed and agility to conscious movements (especially movements of hand)
- provides a high degree of motor control (i.e. movement of individual fingers)
describe the Giant Pyramidal (Betz) cells:
- located in motor cortex
- large cells (60 mue meters)
- large fibers (16 mue meters)
- transmit at 70 m/sec
- make up about 3% (34K) of fibers in the tracts
- send collaterals back to cortex
- synapse directly on LMNs, especially those that innervate forearm and hand muscles
describe other fibers from cortex
- pass into caudate nucleus and putamen
- pass to red nucleus
- pass to reticular substance and vestibular nuclei
- large numbers of fibers pass into pontine nuclei
describe what happens if theres corticospinal tract lesions
- reduced muscle tone
- clumsiness
- weakness
- not complete paralysis
- note: complete paralysis results if both pyramidal and extrapyramidal systems are involved (as is often the case)
describe the Corticobulbar tract
- upper motor neurons of the cranial nerves - innervating the face, head, and neck
- innervates the head
- most fibers terminate in reticular formation near cranial nerve nuclei
describe the association neurons
- leave reticular formation and synapse in creanial nerve nuclei
- synapse with lower motor neurons
describe the corticobulbar tract lesions (CTL)
- lesions are typically unilateral
- results in mild muscle weakness
what happens if the CTL occurs in the hypoglossal nerve?
lesions of cranial nerve 12 results in paralysis of the ipsilateral side = deviation of the tongue towards the damaged side
what happens if the CTL occurs in the facial nerve?
lesions of cranial nerve 7 results in spastic paralysis of the ipsilateral lower 1/4 of the face
describe the extrapyramidal system
- this system includes pathways that contribute to motor control but that are NOT part of the corticospinal system
- includes descending motor tracts that do not pass through medullary pyramids or corticobulbar tracts
what tracts does the extrapyramidal system include?
- rubrospinal tracts
- vestibulospinal tracts
- reticulospinal tracts
- olivospinal tracts
exam: describe the olivospinal tract
originates in medulla in inferior olivary nucleus
involved in reflex movement
recent studies have questioned its existence
what do the extrapyramidal subcortical nuclei include?
- substantia nigra
- caudate
- putamen
- globus pallidus
- thalamus
- red nucleus
- subthalamic nucleus
describe the red nucleus
- fibers from primary motor cortex (corticorubral pathway) and branches from corticospinal tract synapse in magnocellular portion of red nucleus
- large neurons from magnocellular region of red nucleus give rise to rubrospinal tract which decussates in lower brain stem
exam: what is important to know about the magnocellular region?
it has a somatotopic representation of all the muscles of the body
what does stimulation of the red nucleus result in?
- stimulation of flexors
- inhibition of extensors (antigravity muscles)
random info about the red nucleus
- evolutionarily primitive portion of the brain
- in vertebrates without corticospinal tract, it controls movements. in primates, it is less important. is involved in crawling and some arm movement.
- relay information from motor cortex to cerebellum = relay center
- has projections to the contralateral spinal cord via rubrospinal tract or ipsilateral inferior olive
describe the rubrospinal tract
- originates in red nucleus
- decussates in midbrain
- descends in lateral funiculus (column)
- function is closely related to cerebellar function
T/F: rubrospinal tract is less important in human for motor control
True
T/F: the rubrospinal tract is responsible for large muscle movement and some fine motor of UE
True
what occurs during rubrospinal tract lesions?
- impairment of distal arm and hand movement
- intention tremors (similar to cerebellar lesions)
- maytake over for injured corticospinal tract in some animals
describe the Vestibulospinal tract (VBT)
- originates in vestibular nuclei: receives major input from vestibular nerve (CN 8) about changes in head position.
- goal is to maintain balance; knowing where you are in 3D
where does the (VBT) descend?
anterior funiculus (column)
what does the VBT synapse with?
LMNs to extensor muscles: primarily involved in maintenance of upright posture
what are the two vestibulospinal tract compartments?
- lateral vestibulospinal tract
- medial vestibulospinal tract
describe the lateral vestibulospinal tract
- major part of tract
- ipsilateral
- to lumbar region of SC
- upright posture and balance**
- extensors of LE and trunk
describe the medial vestibulospinal tract
projects bilaterally
controls LMN of CN 11 and projects indirectly to CN 6 and 3
describe the Reticulospinal Tract
1) originates in various regions of reticular formation
2) descends in anterior portion of lateral funiculus
3) thought to mediate larger movements of trunk and limbs that do not require balance or fine movements of upper limbs
4) when standing excites axial extensors
5) controls activity of bot alpha and gamma motor neurons
6) mediates autonomic functions like circulatory system and breathing
7) can modulate pain information
what are the two reticular nuclei
pontine reticular nuclei
medullary reticular nuclei
describe the pontine reticular nuclei
- fibers make up the pontine reticulospinal tract (anterior column)
- stimulatory effect on both extensors and flexors, but especially extensors (antigravity muscles)
describe the medullary reticular nuclei
- fibers make up the medullary reticulospinal tract (lateral column)
- inhibitory effect on both extensors and flexors, but especially extensors (antigravity muscles)
what are the structures of the Vestibular Apparatus?
- the vestibular app is a membranous labyrinth consisting of three semicircular canals and vestibule (also includes cochlea)
- composed of the utricle and saccule
- ducts are filled with endolymph and surrounded by perilymph
- the utricle and saccule each contain a macula, which contains hair cells.
- each semicircular canal has an enlargement at one end called the ampulla; the ampulla contains hairs
what are the functions of the VA
detects angular (semicircular canals) and linear (utricle and saccule) acceleration of head.
involved in reflex adjustments of head, eyes, and postural muscles
provides a stable visual image and steady posture
what are the components of the VA?
- utricle (located on a horizontal plane; plays role in determining orientation of head when head is upright)
- saccule (located in a vertical plane; signals head orientation when person is lying down)
- semicircular canals
describe the Macula
- the utricle and saccule each contain a macula
- each macula is covered by a gelatinous layer
what does the macula contain?
- large number of embedded small calcium carbonate crystals (statoconia)
- contains thousands of hair cells which project cilia into the gelatinous layer
- the weight of the statoconia bends cilia in the direction of gravitational pull
describe the hair cell
- has 50-70 small cilia (stereocilia)
- has 1 large cilium (kinocilium) off set to one side
- tips of stereocilia are connected together and to kinocilium
what are the functions of hair cells
- bending of stereocilia towards kinocilium opens hundreds of cation channels causing receptor membrane depolarization and excitation
- bending of cilia in opposite direction closes channels and hyperpolarizes receptor membrane
- hair cells are oriented such that bending the head in different directions causes different groups of hair cells to depolarize
in regards to the semicircular canals, what happens when the head begins to rotate in any direction/
the inertia of the fluid in one or more of the semicircular canals remains stationary while semicircular canal rotates with the head
where does fluid flow (in semicircular canals)?
- from the duct and through the ampulla and causes the cupula to bend to one side
- hundreds of hair cells within each cupula detect this bending and send signals via the vestibular nerve
describe the semicircular canal alignment
when head is bent forward 30 degrees:
- lateral ducts are horizontal
- anterior ducts are in vertical planes projecting forward and 45 degrees outward
- posterior ducts are in vertical planes projecting backward and 45 degrees outward
describe the ampulla
- enlargement at one end of each semicircular canal duct
- filled with endolymph
- crista ampullaris: small crest within each ampulla
- cupula: loose mass of gelatinous tissue on top of the crista
