Lecture 10 - Spinal Cord and Brainstem Control of Motor Function II Flashcards
Pyramidal System
These are tracts that pass through the medullary pyramids. Other motor pathways are extrapyramidal
Components: •Corticospinal Tract •Corticobulbar Tract
Upper motor neurons
- Originate in the motor cortices •75 –85% Decussate in pyramids and form the lateral corticospinal tracts. •Remainder decussate near synapse with lower motor neurons. And form anterior corticospinal tracts. •Most synapse with association neurons in spinal cord central gray.
- Classified according to where they synapse in the ventral horn: •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
Corticospinal Tract
aka pyramidal tract
Origin of tract:
•Primary motor cortex
•Premotor cortex
•Somatosensory area
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.
Lateral corticospinal tract
- Made up of corticospinal fibers that have crossed in medulla.
- Supply all levels of spinal cord.
Anterior corticospinal tract
- Made up of uncrossed corticospinal fibers that cross near level of synapse with LMNs.
- Supply neck and upper limbs
Corticospinal Tract Functions
- Adds speed and agility to conscious movements:
- Especially movements of hand.
- Provides a high degree of motor control:
- (i.e., movement of individual fingers)
Giant pyramidal (Betz) cells
- Located in motor cortex
- Large cells (60 μm)
- Large fibers (16 μm)
- Transmit at 70 m/sec.
- Make up about 3% of fibers in the tract.
- Send collaterals back to cortex.
- Synapse directly on LMNs, especially those that innervate forearm and hand muscles.
Other fibers from cortex
Not Giant pyramidal
- Pass into caudate nucleus and putamen
- Pass to red nucleus
- Pass to reticular substance and vestibular nuclei
- Large numbers of fibers pass to pontine nuclei
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).
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.
- Association neurons:
- Leave reticular formation and synapse in cranial nerve nuclei.
- Synapse with lower motor neurons.
Corticobulbar Tract Lesions
- Lesions typically unilateral
- Result in mild muscle weakness
- Hypoglossal nerve
- Lesion of cranial nerve XII results in paralysis of the ipsilateral side = deviation of the tongue towards the damaged side
- Facial nerve
- Lesion of cranial nerve VII results in spastic paralysis of the ipsilateral lower ¼ of the face
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.
- Includes:
- Rubrospinal tracts
- Vestibulospinal tracts
- Reticulospinal tracts
- Olivospinal tracts
Olivospinal tracts
Originates in medulla in inferior olivary nucleus Involved in reflex movement Recent studies have called into question its existence
Extrapyramidal subcortical nuclei include
- Substantia nigra
- Caudate
- Putamen
- Globus pallidus
- Thalamus
- Red nucleus
- Subthalamic nucleus
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. •Magnocellular region has a somatotopic representation of all the muscles of the body. •Stimulation of red nucleus results in: •Stimulation of flexors •Inhibition of extensors (antigravity muscles)
- Evolutionarily primitive portion of the brain. •In vertebrates without corticospinal tract, it controls movement. 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
Rubrospinal Tract
- Originates in red nucleus.
- Decussates in midbrain.
- Descends in lateral funiculus (column).
- Function is closely related to cerebellar function.
- Less important in human for motor control
- Responsible for large muscle movement and some fine motor of UE
- Lesions:
- Impairment of distal arm and hand movement.
- Intention tremors (similar to cerebellar lesions)
- May take over for injured corticospinal tract in some animals.
Vestibulospinal Tract
Originates in vestibular nuclei:
•Receives major input from vestibular nerve(CN VIII) about changes in head position
•Goal of maintaining balance Descends in anterior funiculus (column).
Synapses with LMNs to extensor muscles:
•Primarily involved in maintenance of upright posture.
Lateral vestibulospinal tract
- Major part of tract
- Ipsilateral
- To lumbar region of SC
- Upright posture and balance
- Extensors of LE and trunk
Medial vestibulospinal tract
- Projects bilaterally
* Controls LMN of CN XI and projects indirectly to CN VI and III
Reticulospinal Tract
- Originates in various regions of reticular formation.
- Descends in anterior portion of lateral funiculus (column).
- Thought to mediate larger movements of trunk and limbs that do not require balance or fine movements of upper limbs.
- When standing excites axial extensors
- Controls activity of both alpha and gamma motor neurons
- Mediates autonomic functions like circulatory system and breathing
- Can modulate pain information
Pontine reticular nuclei:
- Fibers make up the pontine reticulospinal tract (anterior column).
- Stimulatory effect on both extensors and flexors, but especially extensors (antigravity muscles)
Medullary reticular nuclei:
•Fibers make up the medullary reticulospinal tract (lateral column). •Inhibitory effect on both extensors and flexors, but especially extensors (antigravity muscles)
Vestibular Apparatus Structure/Functions
- The vestibular apparatus is a membranous labyrinth consisting of three semicircular canals and vestibule (also includes cochlea).
- Vestibule is composed of the utricle and saccule.
- Ducts are filled with endolymph and surrounded by perilymph.
- 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.
- 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 hair cells.
Vestibular Apparatus Components
Utricle
Saccule
Semicircular Canals
Utricle
- Macula:
- Located on a horizontal plane
- Plays role in determining orientation of head when head is upright.
Saccule
Macula:
•Located in a vertical plane
•Signals head orientation when person is lying down.
Macula
- The utricle and saccule each contain a macula.
- Each macula is covered by a gelatinous layer:
- Contains 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.
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
Function of hair cell:
- 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.
Semicircular Canals
- When head begins to rotate in any direction, inertia of the fluid in one or more of the semicircular canals remains stationary while semicircular canal rotates with the head.
- Fluid flows 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.
Semicircular canal alignment
When head is bent forward 30°:
- Lateral ducts are horizontal.
- Anterior ducts are in vertical planes projecting forward and 45° outward.
- Posterior ducts are in vertical planes projecting backward and 45° outward.
Ampulla
- Enlargement at one end of each semicircular canal duct
- Filled (along with duct) with endolymph.
- Crista ampullaris:
- Small crest within each ampulla.
- Cupula:
- Loose mass of gelatinous tissue on top of the crista.
