Spinal Cord Maryann Martone 1/20/2011

Functions of Spinal Cord Final common pathway for the somatomotor system Conveys somatosensory information from the body Autonomic neurons Reflexes Central pattern generators

Cervical (C) - Neck (C1-8) Thoracic (T) - Chest (T1-12) Lumbar (L) - Back (L1-5) Sacral (S) - Pelvis (S1-5) Coxygeal Spinal cord segments, nerves and vertebrae Vertebra Spinal segment Spinal root

Caudal End conus medullaris –(Spinal cord terminates around vertebra L1) filum terminale (composed of pia mater): anchors spinal cord cauda equina

Meninges Dura transitions to epineurium at spinal nerve

Sensory and Motor Roots Dorsal: sensory Ventral: somatic motor + visceral motor

Alar vs Basal Plates

Internal structure Section of human spinal cord (C8) myelin stain Posterior/Dorsal Anterior/Ventral) Anterior/ Ventral Horn Posterior/ Dorsal Horn Posterior Funiculus (white matter) Anterior Column Lateral Column/ Funiculus Anterior median fissure Posterior median sulcus Central canal Anterior white commissure White matter external; gray matter internal

Levels of the Spinal Cord Cervical Sacral Upper Thoracic Lumbar The ratio of white matter to gray matter increases from caudal to rostral

Principles of Cord Organization 1) Longitudinal Arrangement 1) Longitudinal Arrangement Fibers (White Matter) White Column / Fasciculus Fibers (White Matter) White Column / Fasciculus Cell Groups (Gray Matter) Gray Column / Nucleus Cell Groups (Gray Matter) Gray Column / Nucleus 2) Transverse Arrangement Afferent & Efferent Fibers Afferent & Efferent Fibers Crossing (Commissural and Decussating) Fibers Crossing (Commissural and Decussating) Fibers 3) Somatotopical Arrangement

Rexed’s Laminae Lissauer tract Lamina I: posteromarginal nucleus Lamina II: Lamina II: substantia gelatinosa Lamina III, IV: nucleus proprius Lamina V, VI Lamina VII: intermediate gray Clarke’s column (Nucleus dorsalis; C8-L2, sensory relay) intermediolateral cell column -sympathetic T1-L2, parasympathetic in sacral cord) Lamina VIII Lamina IX: Lamina IX: anterior horn motor neurons Lamina X: gray commissure

Somatic Motor Neurons Multipolar Transmitter: acetylcholine Output: One motor neuron to several muscle cells = one motor unit  -motor neurons: muscle fiber innervation  -motor neurons:muscle spindle innervation Input from – dorsal root ganglion cells spinal interneurons long tracts from brain Integrate Somatotopic Map proximal to distal muscle: medial to lateral in spinal cord Flexor-extensor muscle: dorsal to ventral in spinal cord

Autonomic neurons Sympathetic and parasympathetic preganglionic neurons are segregated in the spinal cord –Parasympathetic= cervical and sacral –Sympathetic = thoracic and lumbar Intermediate horn

Dorsal root ganglion neurons Pseudomonopolar Cells have specialized endings for each modality Encapsulated –(touch, proprioception) Unencapsulated (free nerve endings) –Pain and temperature Differ in size of axon, cell body, myelination, transmitter

Human Nervous System: –31 pairs of spinal nerves Divided into cervical, thoracic, lumbar and sacral levels –Spinal nerves impose segmentation; actual cord consists of columns of cells –Innervation of body is segmented (dermatomes) Dermatomes The region of skin innervated by a single dorsal root ganglion

Mono- vs. polysynaptic reflexes Monosynaptic reflex –Sensory neuron synapses directly on a motor neuron Polysynaptic reflex –At least one interneuron between sensory afferent and motor efferent –Longer delay between stimulus and response –Length of delay is proportional to the number of synapses involved. –more complicated responses than monosynaptic reflexes as the interneurons can control several muscle groups. Copyright 2004 Pearson Education, Inc., publishing as Benjamin Cummings

Spinal Cord Pathways Descending motor Ascending sensory Intraspinal Sympathetic and parasympathetic Nomenclature: usually origin to target e.g. corticospinal: from cortex to spinal cord

Descending pathways 1)Medial Pathways (vestibulo-, reticulo-, tectospinal) –Termination in ventromedial gray matter –Axial and proximal innervation 2)Lateral pathways (cortico-, rubrospinal) –Termination in dorsolateral gray matter –Distal muscle innervation 3)Aminergic pathways (coerulo-, raphaespinal) –Diffuse termination, modulatory systems –serotonin, noradrenalin

Medial pathways Vestibulospinal –Balance and posture / vestibular labyrinth Reticulospinal –posture Ipsilateral projection, mostly on propriospinal and interneurons Tectospinal –Contralateral, only cervical, head and eye movement coordination Aminergic pathways Coerulospinal tract-Locus coeruleus Raphaespinal system- Modulation of pain

Lateral pathways Corticospinal tract (CST) –From motor cortex Rubrospinal tract –From red nucleus in midbrain

Corticospinal tract Also called pyramidal tract Arises primarily from primary motor cortex, premotor and supplementary motor cortex Somatosensory cortex also contributes 70-90% of fibers cross in the lower medulla (decussation of pyramids) Crossed = lateral corticospinal tract Uncrossed = anterior/ventral corticospinal tract Synapses with: Alpha and gamma motor neurons Propriospinal neurons Interneurons

pons medulla decussation spinal cord Midbrain Longitudinal pontine fibers Pyramids Internal capsule Crus cerebri

Species differences in mammalian corticospinal tract anatomy change in location of the corticospinal tract from rat to ape. Function: CST is far more important to humans and monkeys than it is to rats, while the rubrospinal tract becomes less significant in humans. Increase in bilateral projections Kuypers, 1977

Three major sensory systems Dorsal Column system –Fine, discriminitive touch –Conscious proprioception Spinocerebellar system –Unconscious proprioception Spinothalamic system=anterolateral system –Pain, temperature, pressure and vibration

Major Ascending Tracts Dorsal columns –Fine, discriminitive touch, conscious proprioception, pressure and vibration sense –Facisculus gracilis vs fasciculus cuneatus –First synapse: relay nuclei in the medulla (nucleus gracilis and nucleus cuneatus), ascend ipsilaterally in the spinal cord Anterolateral system=spinothalamic system –Pain and temperature –Form Lissauer’s tract=posterolateral tract –First synapse: dorsal horn –2 nd order neuron crosses in anterior white commisure: ascend as spinothalamic tract ( also spinoreticular and spinomesencephalic tracts ) Spinocerebellar –Dorsal and ventral: mostly ipsilateral –Unconscious proprioception –(First synapse: Clarke’s nucleus neurons or acessory cuneate nucl.)

Dorsal Columns First synapse: medulla: nucl. gracilis and nucl. cuneatus Output from relay nuclei to thalamus crosses Conscious touch on contralateral side of the body Somatotopically organized: Ascends ipsilaterally Fasciculus gracilis & cuneatus above T7

Dorsal column/medial lemniscus 1. Dorsal Root Ganglion dorsal root - dorsal column 2. Dorsal Column Nuclei (N. gracilis or N.cuneatus) internal arcuate fiber - lemniscal decussation- medial lemniscus 3. Thalamus (VPL) internal capsule -corona radiata 4. Primary sensory cortex (S I)

Anterolateral system (spinothalamic) DRG axons form Lissauer tract Pain and temperature on contralateral side of body Many peptidergic neurons, transmitter: substance P Crosses in spinal cord Thalamus via spinal lemniscus (spinoreticular, spinomesencephalic tracts)

Summary Voluntary motor control Reflexes and spinal circuitry Ascending Descending

Facsciulus Proprius Propriospinal neurons in dorsal horn (lamina III+IV) Intersegmental coordination of various muscles ~30% of white matter derived from propriospinal axons Axons mostly surrounding gray matter

Corticospinal tract=pyramidal tract Projections from large, layer V pyramidal neurons (including Betz cells) Macaca mulatta (brainmaps.org)