Ascending Tracts Neuro Anatomy

Gross Appearance

& Stru cture of the Spina

l Cord

 Cylindrical; < 1 inch diameter; occupies 2/3 of vertebral canal  25 cm shorter than vertebral column.  Begins at foramen magnum  terminates at lower border of L1 (adult) / upper border of L3 (children) as tapered conus medullaris.  Surrounded by meninges: Dura, Arachnoid, Pia (forms denticulate ligament and filum terminale)  CSF surrounds SC in the subarachnoid space  At 2 regions, it is enlarged and gives origin to: (1) Cervical region  brachial plexus (cervical enlargement); (2) thoracic & lumbar Region  lumbosacral plexus (lumbar enlargement)  Lumbar & spinal nerves have long roots & descends in a bundle called cauda equine.  Tapers inferiorly into Conus Medullaris, fr. Apex w/c a prolongation of Pia Matter, descends and is attached to post. Surface of coccyx,  Anterior median fissure – deep longitudinal fissure of the cord in midline anteriorly.  Posterior median sulcus – shallow furrow surface on posterior surface of cord.  Composed of inner gray matter and outer white matter  No indication that the SC is segmented GRA Y MA TTE R

 H-shaped pillar w/ anterior & posterior gray columns or horns united by a gray commissure w/ a central canal  Thoracic & Lumbar segment: has small lateral gray column  Histo: mixture of multipolar neurons, neuroglia and BV 1.

Anterior gray horn • Greater in cervical and lumbosacral segments • Alpha efferents – innervate skeletal muscle • Gamma efferents – smaller, innervate intrafusal muscles of neuromuscular spindle

3 basic groups of nerve cells in ANTERIOR GRAY COLUMN Med ial

Present in most segments Segments C3, 4, 5

Centra l (smallest of the 3)

Upper 5 or 6 cervical

Latera l

L2 to S1 segments Cervical & lumbosacral segments

2.

Innervate skeletal muscle of neck and trunk including intercostals and abdominal musculature Innervate the diaphragm (phrenic nerve) Innervate sternocleidomastoid & trapezius (accessory nerve) Lumbosacral nucleus Innervate skeletal muscle of limbs

Posterior gray horn • 4 nerve cell groups

(2 extend the whole cord, 2 only in thoracic & lumbar segments) 3. Lateral gray horn Interomedial group of cells from Form preganglionic sympathetic T1-L2 (L3) fibers Similar cells in S2, 3. 4 segments Form preganglionic parasympathetic fibers **Rexed’s Laminar organization of the Gray Matter (Manter) Lamina I: Most dorsal part of the dorsal horn Lamina II: Corresponds to substantia nigra Lamina I – VI : Comfined to dorsal horn Lamina VII: intermediate gray area, contains nucleus dorsalis and interomediolateral gray column Lamina VIII: on ventral horn, send commissural fibers to opposite side of cord Lamina IX: restricted to ventral horn, most ventral part, contains alpha and gamma motor neurons Lamina X: Surrounds central canal ** Illustration of Rexed’s 10 layers & Table 4-1: Comparison of structural details in diff. Regions of spinal cord  see Last page :) The G ray Co mm is sure and Cent ral Canal  Connects anterior and posterior gray columns  Central canal – at the center of gray commissure, present throughout SC ⇒ Superiorly: continuous w/ central canal of caudal half of medulla oblongata & above this, opens into the 4 th ventricle cavity. ⇒ Inferiorly: in conus medullaris, expands to fusiform terminal ventricle and terminates below w/in the roof of filum terminale. ⇒ Filled w/ CSF and lined w/ ciliated columnar epithelium (ependyma)

WHI TE MA TTE R  Mixture of nerve fibers, neuroglia, and BC  White color is due to hign proportion of myelinated nerve fibers.  Divided into anterior, lateral & posterior white columns or funiculi  More white matter than gray matter at cervical levels because of fibers connecting the entire cord with the brain  Various nerve bundles or tracts pass through the cords THE ASCEN DI NG TRA CTS OF THE SPI NAL CO RD  Nerve fibers of diff sizes and function are segregated into nerve bundles or tracts in the white matter.  Conducts info w/c may not reach consciousness  Info is divided into 2 main groups: 1. Exteroceptive – originates from outside the body (pain, temp, touch)

2. Proprioceptive – originates from inside the body (muscles & Substantia gelatin osa grou p

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• • Nucl eus propri us

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Nucl eus dorsa lis (Clark’ s column) Visceral afferent nucleus

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Apex of posterior gray horn; Golgi type II neurons Receives afferent fibers of pain, touch & temperature from posterior root Receives input fr. descending fibers from supraspinal levels Inputs of pain and temp. sensation are modified by excitatory or inhibitory information fr. other sensory inputs and by info. fr. Cerebral cortex Are large cells anterior to substantia gelatinosa Main bulk of posterior gray horn Receive fibers from posterior white column that is associated w/ sense of position and movement (proprioception), vibration, 2-point discrimination Base of posterior gray horn (C8 to L3 or L4) Cells are large and associated w/ proprioceptive endings (neuromuscular & tendon spindles) Medium sized cells lateral to nucleus dorsalis From T1 to L3 segments Associated w/ receiving visceral afferent info.

joints) Anatomi cal Organ izat ion  Ascending pathway consists of 3 neurons First-order neuron Cell body is at Poste rio r Root Gangl ion Peripheral process: connects w/ sensory receptor Central process: @ SC aand connects w/ 2nd order neuron Second-order neuron Gives rise to axon that cross opposite side (decussates) and ascends to a higher level of CNS, synapses w/ 3rd order N. Third-order order neuron Usually in thalamus Gives rise to projection fiber that passes to a sensory region of cerebral cortex

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 Three-neuron chain is most common, but some have fewer neurons.  Many neurons branch & give input to reticular formation  RF activates cerebral cortex, maintaining wakefulness.  Others are in reflex muscular activity Func tion of the Ascending

Tracts

Lateral spinothalamic tract (LST) Anterior spinothalamic tract (AST) Posterior white column (Fasciculus gracilis, Fasciculus cuneatus)

Unconscious info from Muscle, joint, skin, SQ  Spinovisual reflex (pain, thermal, tactile)  Muscle, joints, skin Indirect pathway for  further afferent information

Pain and thermal sensation Light (crude) touch, pressure Discriminative touch(ability to localize accurately the area of body touched) Two point discrimination Info from joint and muscle - movement & position of different parts of the body Vibratory sensations

Anterior spinocerebellar  Cuneocerebellar Spinotectal tract

lateral surface and between inferior olivary & nucleus of spinal tract of trigeminal nerve (Accompanied by AST & spinotectal tract forming Spinal Lemn is cus  Spinal Lemniscus continues to ascend through posterior part of pons (In midbrain, it lies in tegmentum lateral to medial lemniscus)  LST fibers end by synapsing w/ 3rd-order neuron in Ventral Posterolateral Nucleus of Thalamus. 

 3rd-order neuron pass through posterior limb of internal capsule and corona radiate  somasthetic area in postcentral gyrus of cerebral cortex.

Pathway (s imp li fied )



Superior culliculus of midbrain Reticular formation Cerebellum

Pain and Temperatu re Path ways - Lateral Spinothalamic Tract 3  Peripheral receptors: naked or free nerve endings  Pain impulse (also heat & cold travel via these fibers) transmitted by: A delta – fast conducting (6 and 30 m/sec); C fibers – slow conducting (0.5 and 2.0 m/sec)  Pain perception: Influenced by emotional state, past experiences of individual. warns of potential injury, alerts the person to avoid it  2 types of pain: fast and slow Fast Pain Felt about 0.1 sec after stimulation Sharp, acute or pricking pain

 As spinothalamic tract ascends to medulla oblongata (it is near the

Cerebellum

 Spinoreticular Spinoolivary

** Pain fibers are slightly anterior to temperature fibers.

Axon from receptor  PRG (1st order)  posterior gray horn  Substantia gelatinosa (2nd order)  Tract of Lissauer (posterolateral fasciculus)  Crossing  opposite LST (anterolateral fasciculus)  medulla (LST + AST + ST join and form Spinal Lemniscus)  Pons  Midbrain  VPL of thalamus  Post. Limb of internal capsule  Corona radiate  Postcentral gyrus  Motor area OR Sensory association area

Slo w pain Felt 1 sec after stimulation Burning, aching or throbbing (produced when there is tissue destruction) Greater persistence Less clear location Majority: relays, winding up phenomenon Substance P as NT fr C fibers mechanical, thermal & chemical Ex. Abscess or arthritis

Almost confined to skin Accurately localized Directly ascends in VPL then postcentral gyrus Glutamate as NT fr. A delta & C fibers Elicited by mechanical, thermal stimuli Ex. Needle prick **Serotonin, histamine, bradykinin, lactic acid & K ions - excites free nerve endings; release in response to noxious & inflammatory stimuli. ** Prostaglandins & substance P - Lowers pain threshold but can’t stimulate endings.

Pathwa y: Snel l

 Axons  enter SC from PRG  tip of Posterior gray column  Divide into ascending and descending branches  Branches travel 1 or 2 segments of SC  and Forms Posterolateral tract of Lissauer  these fibers of first-order n. Terminate by synapsing w/ cells in post gray column (including cells in substantia gelatinosa)  **Substance P – peptide, NT at this synapses

 2nd-order neuron cross obliquely to the opposite side in anterior gray & white commissuew w/in one spinal segment of SC  ascend in contralateral white column as lateral spinothalamin tract (medial to spinocerebellar tract)  new fibers are added at anteromedial portion as spinothalamic tract ascends.  **At upper cervical: sacral segments are lateral and cervical segments are medial / Fibers carrying 2 of 2 | Page