Neuroanatomy 4 INSTRUCTOR.pdf

BMS Neuroanatomy Lecture 4 Spinal Cord Motor Pathways (In-Person Class) Presented By: Dr. K. Lumsden; [email protected] (Toronto Campus) Dr. M. Doroudi; [email protected] Boucher Campus) Major Motor Tracts of the Spinal Cord v The motor tracts of the spinal cord relay the motor instruction either fr...

BMS Neuroanatomy Lecture 4 Spinal Cord Motor Pathways (In-Person Class) Presented By: Dr. K. Lumsden; [email protected] (Toronto Campus) Dr. M. Doroudi; [email protected] Boucher Campus) Major Motor Tracts of the Spinal Cord v The motor tracts of the spinal cord relay the motor instruction either from motor areas of the frontal lobe of the cerebral cortex or some of the nuclei of the brainstem to the spinal cord. Two classifications: v The pyramidal tract conveys impulses to control the skeletal muscles of the limbs and trunk and includes the lateral and ventral corticospinal tracts. v The extrapyramidal tracts conduct signals that regulate those body movements that are subconscious or postural in nature. v Rubrospinal, Tectospinal, Vestibulospinal, and Reticulospinal tracts. v Both pyramidal and extrapyramidal tracts consist of two sets of neurons. The pyramidal tracts' upper motor neurons (UMNs) reside in the cerebral hemispheres' motor cortex. The UMNs of the extrapyramidal tracts are housed within the brainstem and cerebellar cortex. The lower motor neurons (LMNs) of both pyramidal and extrapyramidal tracts are located in the anterior horn of the spinal cord. The axons of LMNs exit the spinal cord as the ventral root of the spinal nerve and target the skeletal muscles. Motor Pathways (in general) 1. Upper motor neurons (UMN): begin in the cortex and terminate in the brainstem or spinal cord The corticobulbar (Corticonuclear) tract terminates in the brainstem: synapse on cranial nerve nuclei (III, IV in the midbrain; V-VIII in pons; IX-XII in the medulla) The corticospinal tract terminates in the spinal cord and synapses with the motor neurons in the ventral horn of the spinal cord. 2. Lower motor neurons (LMN) begin in the brainstem or spinal cord and innervate muscles. Lower motor neuron of cranial nerves: cell body in the motor nuclei of the cranial nerves in the brainstem Lower motor neuron of spinal nerves: cell body in the ventral horn of the spinal cord that form the motor root of the spinal nerve START! Peripheral Nervous System Corticospinal tract: Transmits: Voluntary Motor / Conscious (“GSE”) control of skeletal muscles of the body PRE-CENTRAL GYRUS OF FRONTAL LOBE Pathway: Neuron 1: Upper Motor neuron “UMN” Originates in primary motor cortex of frontal lobe (pre-central gyrus) Descends via large white matter tract within diencephalon Tract decussates at medulla oblongata Travels in antero-lateral white columns of spinal cord Synapses in anterior grey horn of spinal cord Neuron 2: Lower Motor neuron “LMN” Cell body originates in anterior grey horn of spinal cord Exits cord via ventral root of spinal nerve Travels via peripheral spinal nerve to reach target (skeletal muscle) FINISH! 4 Corticospinal Tracts (CST) 1. 2. 3. 4. 5. 6. 7. 8. CST is part of the pyramidal pathways, the UMNs of which reside in the motor cortex of the cerebrum. The descending fibres pass through the internal capsule, the crus cerebri of the midbrain, the basilar portion of the pons, and the pyramids of the medulla. At the junction of the medulla and spinal cord, most of the fibres cross at the decussation of the pyramids These crossed fibers form the lateral corticospinal tract and descend in the lateral column of the spinal cord The fibers that do not cross at the decussation of the pyramids descend in the anterior column of the spinal cord as the anterior corticospinal tract Fibers of the anterior corticospinal tract cross in the spinal cord before synapsing in the anterior horn Both anterior and lateral fibers are responsible for coordinated, precise, and voluntary skeletal muscle movements The CST receives input from the sensory cortex, cerebellum, etc. 5 Motor Pathways Lesions Upper motor neuron lesions (UMNL): corticospinal tract (note: contralateral signs) Spastic muscles (increased muscle tone: No inhibitory influences) Exaggerated or pathological reflexes (i.e. Babinski sign) Paresis (weakness) or paralysis when voluntary movement attempted Lower motor neuron lesions (LMNL): motor root of spinal nerves Flaccid muscles and atrophy (decreased muscle tone: no reflex input) Loss of reflexes Paresis or paralysis when voluntary movement attempted vNote: There are also specific symptoms associated with lesions of cranial nerves in addition to the general symptoms listed above, but these will be covered later in the course with cranial nerves. Clinical Correlate: Lesions of Motor Neurons Upper motor neuron (UMN) Lower motor neuron (LMN) FUNCTION: Control of movement--> determines which LMN will be active, which will turn off Site of lesion/injury CNS: Motor cortex, medulla, anterolateral PNS: Anterior grey horn, Ventral white columns of spinal cord root, spinal/peripheral nerve DAMAGE RESULTS IN: Spastic paralysis (no control) Flaccid paralysis (loss of signal) Muscle tone changes Increased (abnormal muscle tightness due to prolonged contraction) Decreased (loss of signal) Reflexes issues Increased: Hyper-reflexia Decreased: Hypo-reflexia Fasciculations (muscle twitching ) Absent Present Atrophy (muscle wasting) Slow progression Delivers signal to “contract” to the muscle Rapid 7 Corticospinal Tracts (CST) Corticospinal Tracts (CST) Major Motor Tracts of the Spinal Cord v The extrapyramidal tracts conduct signals that regulate those body movements that are subconscious or postural in nature. v Rubrospinal, Tectospinal, Vestibulospinal, and Reticulospinal tracts. The EPS serves an essential function in maintaining posture and regulating involuntary motor functions. In particular, the EPS provides: 1. Postural tone adjustment 2. Preparation of predisposing tonic attitudes for involuntary movements 3. Performing movements that make voluntary movements more natural and correct 4. Control of automatic modifications of tone and movements 5. Control of the reflexes that accompany the responses to affective and attentive situations (reactions) 6. Control of the movements that are originally voluntary but then become automatic through exercise and learning (e.g., in writing) 7. Inhibition of involuntary movements (hyperkinesias), which are particularly evident in extrapyramidal diseases. Extrapyramidal Tracts/Pathways Rubrospinal Tract: Originates from the red nucleus of the midbrain Crosses the midline as it exits from nuclei in the caudal midbrain and descends through brainstem Travels in the ventrolateral white column of the spinal cord. Fibers terminate by synapsing in on lower motor neurons within grey matter of anterior grey horn Involved in mediation of involuntary movement = muscle tone, posture and skeletal muscle control. Extrapyramidal Tracts/Pathways Rubrospinal Tract: The importance of the tract lies in the maintenance of muscle tone and in the regulation of rudimentary motor skills that are refined by corticospinal control With the corticospinal tract, the rubrospinal tract controls hand and finger movements in addition to flexor muscles. Older than CST: used in "four-limb" walking of animals Also in babies: crawling before walking could indicate myelination of RS before CST Flex arm ("Swinging") when walking is automatic: have to stop it consciously Since it is involved in posture, it is not surprising that this tract receives input from the cerebellum. In addition, it receives input from the cerebral cortex and branches of the corticospinal tracts. Rubrospinal Tract: ORIGIN Red Nucleus of midbrain DECUSSATION Midbrain TERMINATION Anterior grey horn of spinal cord FUNCTION Regulation of flexor-extensor tone Tectospinal Tract 1. This tract begins at the tectum of the midbrain. Note that this is where sensory fibers carried by the spinotectal tract terminated. 2. The fibers cross soon after their origin and descend through the brainstem to the anterior column of the spinal cord 3. Terminate in the upper cervical segments of the spinal cord. 4. The tract functions to control movements of the head in response to visual stimuli (similar tract receives info from cochlear (hearing) nuclei and then starts from the inferior colliculi and terminates in the upper cervical segments of the spinal cord for auditory reflexes) Tectospinal Tract: ORIGIN Superior colliculus of midbrain DECUSSATION Midbrain TERMINATION Anterior grey horn of spinal cord cervical region only FUNCTION Postural movements of head in response to visual and auditory stimuli Vestibulospinal Tracts The vestibulospinal tracts (VST) are essential pathways by which higher centers of the brain maintain the balance of the body. Their activity depends upon the position of the head and neck, aided by balance receptors in the inner ear as well as sensory information from the cerebellum. There are two vestibulospinal tracts; the lateral vestibulospinal tract and the medial vestibulospinal tract. Both are responsible for antigravity muscle tone in response to the head being tilted to one side and are indirectly influenced by the cerebellum and the labyrinthine system of inner ear. Lateral Vestibulospinal Tract 1. This tract begins at the lateral vestibular nucleus (in the medulla; CN VIII) 2. The fibres do not cross and descend to the anterior column of the spinal cord. 3. The function of this tract is to regulate muscle tone in response to movements of the head, and hence it plays a role in: balance and equilibrium. 4. The tract receives input from: the inner ear via the vestibulocochlear nerve, and also the cerebellum. Lateral Vestibulospinal Tract: ORIGIN Lateral vestibular nucleus of medulla DECUSSATION Remains ipsilateral (does not cross) TERMINATION Anterior grey horn of spinal cord FUNCTION Postural stabilization Reticulospinal Tract 1. The pontine reticulospinal tract begins from the reticular formation of the pons 2. The medullary reticulospinal tract begins from the reticular formation of the medulla 3. The pontine tract tends to remain uncrossed, while the medullary tract has both crossed and uncrossed fibers 4. The pontine tract descends in the anterior column of the spinal cord, while the medullary tract descends in lateral column 5. Both tracts influence reflexes and voluntary movements (allow for inhibition of antagonist muscle when agonist is contracted), and carry autonomic information from hypothalamus to lateral horn of T1 to L2 and S2,S3,S4 segments of the spinal cord (Sources of sympathetic and parasympathetic N.S respectively) 6. Input for these tracts comes from the cerebral cortex and cerebellum, but as the tracts also carry autonomic fibers, they also receive input from the hypothalamus. Pontine RST; controlling axial and extensor motor neurons, e.g enabling extension of the legs to maintain postural support ; Medullary RST; Inhibits extensor motor neurons to enable modulation of the stretch reflex Reticulospinal Tract: ORIGIN Reticular formation of pons and medulla DECUSSATION Pontine remains ipsilateral (does not cross) Medullary has both crossed and uncrossed TERMINATION Anterior grey horn of spinal cord FUNCTION Muscle tone, balance, postural changes Miscellaneous: Pain Modulation Spinomesencephalic tract carries pain information to the midbrain, synapsing in the periaqueductal gray region around the cerebral aqueduct Stimulation of those PAG neurons by the spinomesencephalic tract sends impulses down to the dorsal horn of the spinal cord (via the connection with the reticular formation nuclei). Here they block the release of Substance P that is necessary to transmit signals from the first to second-order neurons of the lateral spinothalamic tract. This, therefore, blocks the transmission of pain.