BMS2-4 Tracts of the Spinal Cord PDF

Neural Pathways in the Spinal Cord ==== Tracts==== Dr A Aktar Neural Systems of the Spinal Cord • 3 major neural systems • Use neurons in the grey matter & tracts (or fasciculi) of axons in the grey matter • Have components found in all levels of the CNS Tracts Tracts (Pathways) of the Spinal...

Neural Pathways in the Spinal Cord ==== Tracts==== Dr A Aktar Neural Systems of the Spinal Cord • 3 major neural systems • Use neurons in the grey matter & tracts (or fasciculi) of axons in the grey matter • Have components found in all levels of the CNS Tracts Tracts (Pathways) of the Spinal Cord Ascending tracts: Sensory Deliver information to brain Fibers enter cord via dorsal roots of spinal nerves, or derived from intrinsic spinal neurones Descending tracts: Motor Deliver information to periphery Propriospinal tracts: Localized within cord Both ascending & descending Intersegmental coordination Ascending & Descending Pathways Have Well Defined Locations in Spinal White Matter • Fibers having similar connections typically band together • Funiculus: Small bundle of axons (nerve fibres), enclosed by the perineurium • A small nerve may consist of a single funiculus • A larger nerve will have several funiculi collected together into larger bundles known as fasciculi • Fasciculi are bound together in a common membrane, the epineurium Tracts of the Spinal Cord • Posterior funiculus: Ascending tracts only • Anterior funiculus: Predominantly descending tracts • Lateral funiculus: Both ascending & descending tracts Naming the Tracts of the Spinal Cord • Tract name beginning w/”spino” : A sensory tract delivering information FROM the spinal cord • Tract name ends w/ “spinal”: A motor tract that delivers information TO the spinal cord Examples: • Spinocerebellar: A sensory tract delivering information from the spinal cord to the cerebellum • Vestibulospinal: A motor tract that delivers information from vestibular apparatus to the spinal cord Sensory Tracts – General Organization • Posterior (dorsal) column tract • Spinothalamic tract (Anterolateral system) • Spinocerebellar tract • First-order neuron: In the periphery. Innervates a sensory receptor Cell body is in dorsal root ganglion Carries info to spinal cord in dorsal root of a spinal nerve • Second-order neuron: An interneuron Cell body may be in spinal cord or brainstem Axon crosses midline & is carried up in a tract • Third-order neuron: Cell body in thalamus Axon projects to primary somatosensory cortex Sensory Tracts – General Organization • 3 neurons to process sensory info • 2nd order neuron cell body ipsilateral to 1st Spinothalamic Tract • A.k.a. Anterolateral System • Pain, temperature, crude touch • Axons ascend or descend 1-2 segments • 2nd neuron in dorsal horn grey matter • Axons cross in ventral white commissure • • Coalesce to form spinothalamic tract in ventral part of lateral & anterior funiculus (*) Course thru entire length of spinal cord • 3rd neuron in VPL nucleus in thalamus • Primary somatosensory cortex in postcentral gyrus Spinothalamic Tract Anterior Spinothalmic Crude touch, pressure Lateral Spinothalamic Pain, temperature, tickle, itch, sexual sensations • Axons ascend or descend 1-2 segments • 2nd neuron in dorsal horn grey matter • Axons cross in ventral white commissure • Coalesce to form spinothalamic tract in ventral part of lateral funiculus • Course thru entire length of spinal cord Lesions of the Spinothalamic Tract Clinical Correlate • Pain & temperature information crosses very soon after it enters spinal cord, therefore a lesion in this tract in the brain stem or spinal cord usually manifests with contralateral loss of pain & temperature • Analgesia begins 1-2 segments below the lesion & includes everything below that level Dorsal Column - Medial Lemniscal Pathway • A.k.a. Dorsal (Posterior) Column • Discriminative touch, joint position sense (kinesthetic or conscious proprioceptive), vibration, pressure sensation • Fasciculus Cuneatus: Upper limb, T5 • Fasciculus Gracilis: Lower limb, T6 Found at all levels of cord Closest to midline • F. cuneatus + gracilis form dorsal column • 2nd neuron in lower part of medulla Dorsal Column - Medial Lemniscal Pathway • 2nd neuron in lower part of medulla  Nucleus cuneatus & nucleus gracilis • Fibers cross midline & ascend as medial tract (medial lemniscus) in brain stem • Terminate in VPL nucleus of thalamus • Fibers project to primary somatosensory area of postcentral gyrus in most anterior part of parietal lobe (thalamicocortical fibers) Lesions of the Dorsal Column Clinical Correlate • Result in loss of joint position sensation, vibratory & pressure sensations & 2-point discrimination • Astereognosis: Loss of ability to identify an object by touching with hands (by its shape, size, consistency etc.) • Dorsal column medial lemniscal lesions typically evaluated by testing vibratory sense : 128 Hz tuning fork Spinocerebellar Tracts • Tactile & unconscious proprioceptive input from muscle spindles & golgi tendon organs (GTO) to cerebellum • Info is used to monitor & modulate movements GTO: A proprioceptive sensory receptor in muscles that senses changes in muscle tension. Posterior Spinocerebellar Tract • Tactile, pressure & proprioceptive from muscle spindles & GTO • Lower extremities & lower trunk • Travel in posterior column • Synapse on Clarke’s nucleus  Rounded collection, large cells, medial surface of base of posterior horn, from T1 to L2  Relay nucleus • Axons to ipsilateral lateral funiculus  posterior spinocerebellar tract • Projects to ipsilateral cerebellum • Some collateral fibers end in nucleus gracilis • Afferents caudal to L2 ascend in fasciculus gracilis & synapse on Clarke’s nucleus Posterior Spinocerebellar Tract • q Posterior Spinocerebellar Tract Clarke’s Nucleus  Rounded collection, large cells, medial surface of base of posterior horn, from T1 to L2  Relay nucleus Cuneocerebellar Tract Same as posterior spinocerebellar tract EXCEPT: • Afferents entering in cervical & upper thoracic segments • Travel in fasciculus cuneatus • Synapse on external (lateral) cuneate nucleus in medulla • Axons form cuneocerebellar tract Cuneocerebellar Tract Fasciculus Cuneatus Anterior Spinocerebellar Tract • From lower extremities • More complex input • GTO, cutaneous receptors, spinal interneurons, fibers of descending tracts  attempted movement + proprioceptive • Synapse on spinal border cells (projection neurons) on lateral surface of lumbar anterior horn T12 – L5 • Fibers cross midline at cord level • Ultimately end at ipsilateral side Anterior & Posterior Spinocerebellar Tracts Cross Sectional View Showing Locations of Major Ascending Tracts Lesions of the Spinocerebellar Tracts Clinical Correlate • Rarely affected in isolation  Clinically detectable deficts can seldom be attributed to spinocerebellar damage • Spinocerbellar atrophies: Family of inherited diseases. Other cord areas always affected. E.g.: Friedreich’s Ataxia Other Ascending Pathways Spinotectal tract • Afferent info for spinovisual reflexes • Terminate by synapsing on neurons in superior colliculus Spinoreticular tract • Afferents influencing levels of consciousness • Terminate on neurons in reticular formation Spinoolivary tract • Balance & unconsious proprioceptive afferent info to cerebellum • 3rd order neurons in inferior olivary nuclei in medulla • Axons cross & enter cerebellum Sensory Tracts - Organization Somatotopic: Ascending tracts are arranged according to the site of origin Medial-lateral rule: Sensory neurons that enter a low level of the spinal cord are more medial within spinal cord Sensory neurons that enter at a higher level are more lateral within spinal cord Motor System • Voluntary innervation of skeletal muscle • Basic neural circuit: 1 upper motor neuron (UMN) 1 lower motor neuron (LMN) • Initiating a voluntary contraction: A LMN must be innervated by a UMN • UMN: • LMN: Cell bodies in brain stem & cerebral cortex Cell bodies in ventral horn of spinal cord + Cranial nerve nuclei in brain stem • LMN (in spinal cord) axons exit spinal cord in a ventral root join & course with a spinal nerve  reach & synapse on a neuromuscular junction in skeletal muscle • LMN in brain stem – axons exit in cranial nerves Motor Neuron Groups in Spinal Cord Anterior Grey Column : Alpha Efferents Axons leave cord in ant. roots of spinal nerves Alpha (α) motor neurons : Large, multipolar LMN of brainstem & spinal cord Inervate extrafusal muscle fibers of skeletal muscle (1) Medial: -Present in most segments -Innervates skeletal muscles of neck, trunk, intercostals, abdominals (2) Central: -Present in some cervical & lumbosacral segments -In C 3-5 segment, some axons innervate diaphragm -In C 1- 6, some axons innervate SCM m. & trapezius m. - In L2- S1, axons coalesce on (3) Lateral: -Present in cervical & lumbosacral segments -Innervates skeletal muscles of extremities  phrenic nucleus  accessory nucleus  lumbosacral nucleus Corticospinal Tract • Voluntary innervation of skeletal muscle • Primary motor cortex: Precentral gyrus of frontal lobe • Corticospinal fibers descend thru brain stem in ventral portion of mid brain pons & medulla Corticospinal Tract • In lower medulla 80-90% of fibers cross (at the decussation of the pyramids) • Continue in contralateral part of spinal cord as the lateral corticospinal tract • Descends full length of cord in lateral part of white matter • As it descends, axons leave and enter the grey matter of the ventral horn to synapse on a LMN Corticospinal Tract • In lower medulla 80-90% of fibers cross at the decussation of the pyramids Corticospinal Tract Reflex Innervation of Skeletal Muscle • Involuntary motor responses to a sensory stimulus • Requires a minimum of 2 neurons: (1) a sensory neuron that initiates a motor response from (2) lower motor neurons • Reflexes range from simple 2-neuron circuits to more complex reflexes that use multiple interneurons in addition to the sensory and motor neurons Deep Muscle Stretch (Myotatic) Reflex • A.k.a Deep tendon reflex (DTR) - simplest of the reflexes • Involves only 1 synapse btwn 1 sensory & 1 motor neuron • Testing integrity of DTRs in the limbs provides important clinical information when assessing different types of motor weaknesses & dysfunction • DTR present in most skeletal muscles & provides the important physiological mechanism of regulating & maintaining proper muscle tone throughout the skeletal muscular system • Motor component is the alpha lower motor neuron (LMN) in ventral horn of spinal cord • The LMN results in contraction of the extra-fusal muscle fibers & the muscle jerk. Deep Muscle Stretch (Myotatic) Reflex The Muscle Spindle • The sensory receptor involved in initiation of the deep muscle stretch reflex • Embedded in skeletal muscles • Arranged in parallel with muscle fibers • Responds to stretching of muscle & change in length & force & rate of change in length of the extrafusal muscle fibers • The muscle spindle initiates a sensory stimulus using the la sensory neuron • Central processes of la neurons enter spinal cord via the dorsal root of spinal nerves & synapse directly on alpha lower motor neuron in the ventral horn • la fibers also provide reciprocal inhibition, which inhibits alpha neurons of antagonist muscles Deep Muscle Stretch (Myotatic) Reflex The Muscle Spindle Upper Motor Neuron Control of Deep Muscle Stretch Reflex • UMN systems, like the corticospinal tract, play a role in regulating muscle tone & have a net inhibitory control over muscle stretch reflexes • An UMN lesion results in hyperactive reflexes (spasticity) Gamma Motor Neurons Intrafusal muscle • A strip of modified skeletal muscle fibers within muscle spindles • Innervated by gamma neurons in the ventral horn • Gamma neurons are coactivated by UMNs at the same time as alpha motor neurons • Gamma neurons ensure that the stretch reflex stays active during contraction of the extrafusal muscle when one would anticipate that the muscle spindle would be unloaded and would go quiet • Thus, gamma neurons innervate the muscle spindle and modulate their sensitivity to stretch during movement • Contraction of the intrafusal muscle fibers activates sensory firing of the la neuron, which then activates the alpha lower motor neurons, resulting in increased muscle tone • This circuitry is referred to as the gamma loop Inverse Myotatic Reflex – a.k.a. Golgi Tendon Reflex • A.k.a. Inverse stretch reflex • Provides opposite effect from the deep muscle stretch reflex • Uses another sensory receptor: Golgi tendon organs (GTO) - imbedded in tendons of skeletal muscles • GTOs monitor degree of tension & force placed on skeletal muscle • Stimulation of GTO  firing of 1b afferent neurons  route thru dorsal roots back into ventral horn  inhibitory inter-neuron  inhibit alpha motor neurons  decrease contraction of the muscle Deep Muscle Stretch (Myotatic) Reflex Commonly Tested Reflexes • Ankle reflex • Knee reflex • Biceps reflex • Triceps reflex Lesions of the Corticospinal Tract Clinical Correlate • The crossing (decussation) of the corticospinal tract has important clinical implications • Lesions above decussation - weakness seen in muscles on contralateral side of body • Lesions below decussation - ipsilateral weakness • Cell bodies of LMNs are ipsilateral to the skeletal muscle their axons innervate • Lesions to any part of a LMN will produce a weakness in an ipsilateral muscle at the same level of the lesion UMN and LMN Lesions – Contralateral vs. Ipsilateral Clinical Correlate: Clasp Knife Response • Characteristic response of an UMN lesion • Resemblance btwn motion of the limb and the sudden closing of a claspknife after sufficient pressure is applied • In upper motor neuron lesions muscle tonus may increase and resistance of muscle to stretch increases. However, if sufficient force is applied, limb resistance suddenly decreases (also call autogenic inhibition) possibly thru the golgi tendon reflex https://www.youtube.com/watch?v=8xxe2WWWoYI https://www.youtube.com/watch?v=re0WKy1zlY0 Other Descending Pathways Reticulospinal Tract • Inhibit or facilitate voluntary movement • From reticular formation (reticular formation is a set of interconnected nuclei that are located throughout brainstem) • Some fibers cross midline – various levels • Destination: Alpha & gamma motor neurons • Gives multiple branches at each level Gamma motor neuron (γ motor neuron): a LMN • • • • • Represents about 30% of fibers going to the muscle Cell bodies located in the anterior grey column Axons smaller than those of alpha motor neurons Conduction slower then alpha motor neurons Do not directly adjust lengthening or shortening of muscles. They keeping muscle spindles taut, thereby allowing the continued firing of alpha neurons • Play a role in adjusting the sensitivity of muscle spindles Other Descending Pathways Tectospinal Tract • Reflex postural movements in response to visual stimuli (spinovisual reflexes) • From superior colliculus in mid brain • Destination: Alpha & gamma motor neurons Other Descending Pathways Rubrospinal Tract • Facilitates activity of flexor muscles • Inhibits activity of extensor muscles (anti-gravity muscles • Origin: Red Nucleus In tegmentum of mid-brain, superior colliculus level Other Descending Pathways Vestibulospinal Tract • Facilitates activity of extensor muscles • Inhibits activity of flexor muscles • Origin: Vestibular nuclei (CN VIII nuclei) • Destination: Alpha & gamma motor neurons Descending Autonomic Fibers • Controlling autonomic systems • Origin: Cortex, hypothalamus, amygdala, reticular formation • Destination: Symphatetic & paraysmpathetic outflow