Spinal cord

• Spinal cord

  • 42-45cm Long cylindrical structure extending from upper border of C1 down through vertebral column till lower border of L1
  • Pathway connecting CNS and PNS
  • Occupied upper 2/3 of vertebral canal
  • Enclosed by 3 meninges.
  • Superior border: medulla
  • Inferior border: conus medullaris

  
• Spinal cord is surrounded by 3 meninges

  • Dura mater: outermost; tough, loose membrane of CT. Extend from foramen magnum to S2.

  Spaces: Epidural space: btw dura and vertebral periosteum. Contains internal vertebral venous plexus + CT and fat

  Subdural space: potential space between dura and arachnoid. Contains thin film of fluid.

  
• Spinal cord is surrounded by 3 meninges

  • Arachnoid matter: thin, web-like middle layer, located between dura and pia matter.
  • Spaces: subarachnoid space; located between arachnoid matter and pia matter. Filled with cerebrospinal fluid and contains blood vessels. Provide cushion for spinal cord.
  • Subarachnoid space expands at level of conus medullaris forming lumbar cistern- site of lumbar puncture.

  
• Pia matter

  • Innermost layer
  • Thin, delicate membrane that closely adheres to surface of spinal cord following its contours.
  • Special features:

  Denticulate ligaments: starting from foramen magnum to T12, pia shows 21 pairs of lateral projections: denticulate ligaments that anchor spinal cord to dura providing stability, holding it in place.

  Filum terminale: extension of pia that continues from conus medullaris to coccyx anchoring spinal cord longitudinally.

  
• Spinal cord enlargements

  • Spinal cord had two notable enlargements that correspond to increased neural input and output necessary for control of linbs.
  • Cervical enlargement: extends from C4 to T1. Supply nerves of upper limb. Due to increase motor and sensory neurons to accommodate innervation of upper limb.
  • Lumbar enlargement; extends from L2 to S3. Supply nerves of lower limb

  
• Gray matter of spinal cord

  • H or butterfly shaped area in center of cord
  • Consists of neuronal cell bodies, dendrites, unmyelinated axons and glial cell.
  • In center of gray matter is central canal with CSF lined with ependymal cells
  • The gray matter from left. + right sides are joined by a gray commisure

  
• Gray matter of spinal cord

  H shaped:

  • Dorsal (posterior) horn: involved in processing sensory info. Contains cell bodies of 2 order sensory neurons, interneurons and terminal afferent (sensory) fibers.
  • Ventral (anterior) horn: contains cell bodies of motor neurons that sends axons through ventral roots to innervate skeletal muscles. Involved in motor control.
  • Lateral horn: seen in thoracic and upper lumbar segments (T1-L2). Contains neurons of SNS, these sends axons to innervate viceral organs.

  
• Variations on internal structure of spinal cord at different levels

  • Shape on transverse section changes at different levels
  • Relatively amount of gray and white matter also changes
  • Amount of gray matter seen is correlated to mass of tissue supplied: increase as we go down into spinal cord
  • As we go down spinal cord amount of white matter decrease; decrease amount of ascending and descending fibers.

  
• Functions of spinal cord

  • Sensory processing: neurons in dorsal horn receive and process sensory info from body
  • Neurons in ventral horn are responsible for sending motor commands to skeletal muscles: controlling voluntary movement
  • Autonomic function: neurons in lateral horn (T1-L2) manage sympathetic activities
  • Reflex arch
  • Integration and coordination: interneurons in gray matter help coordinate complex motor patterns and integrate sensory input with motor inputs contributing to smooth + coordinated movement.
• Ventral horn nuclei

  • Contain motor neuron that innervate skeletal muscle and control voluntary movement.
  • Subdivided into:

  Median group: dorsomedial + ventromedial nuclei→ innervates axial musculature of neck and trunk.

  Central group: phrenic (diaphragm)+ spinal accessory (trapezius and sternocleidomastoideus) + lumbosacral nuclei ( external sphincter, muscle of erection, ejaculation).

  Lateral group: dorsolateral, ventrolateral, retrodorsal nuclei→ found in cervical and lumbosacral enlargement, supply distal limbs.

  
• Nuclei in dorsal gray horn

  Primarily processes sensory information.

  From inwards to outwards:

  • Posterior marginal nucleus: T+ fastbpain (part of anterior spinothalamic tract)
  • Substancia gelatinosa (lamina II):processes pain and temperature
  • Nucleus proprios (lamina III + IV): processes touch and pressure + proprioceptive info
  • Dorsal nucleus of Clarke: in T1-L2: proprioception of lower limbs to cerebellum.

  
• Nucleus in lateral horn

  • Present in thoracic and upper lumbar segment.
  • Contains neuron involved in autonomic function
  • T1-L2 segments.
  • Intermediolateral cell column: preglanglionic sympathetic neurons. Sends axons to sympathetic ganglia which innervates various organs.
  • Interomediomedial nucleus: in S2-4 segments. Contains preglanglionic parasympathetic neurons. Sends axons to pelvic ganglia which innervate lower colon, bladder and genital organs

  
• • Anterior median fissure: deep groove running along ventral midline of spinal cord. Divided spinal cord into left + right halves
  • Posterior median sulcus: shallow groove running along posterior midline.
  • Anterolateral and posterolateral sulci: these are grooved located on lateral sides of spinal cord. Anterolateral sulci is the entry point for ventral toot fibers and posterolateral sulci are entry points for dorsal root fibers.

  
• • White matter of spinal cord composed of myelinated axons which are nerve fibers covered by fatty substance myelin.
  • Myelin acts as insulator, increasing the speed of electrical impulses traveling along the axon.

  
• Organisation of white matter within spinal cord

  • White matter surrounds gray matter, which is located centrally in spinal cord in H-shape
  • White matter is organised into funiculi (column), which are large bundled of axons ascending and descending in spinal cord.
  • There are 3 main funiculi on each side of spinal cord: Dorsal (posterior), lateral and ventral (anterior) funiculi

  
• Function of white matter of spinal cord

  • Signal transmission: primary function of white matter is transmit nerve signals. Ascending tracts carry sensory information from PNS to brain while descending tracts carry motor commands from brain to PNS.
  • Communication pathways: white matter facilitates communication between different segments in spinal cord and between spinal cord and brain

  
• Dorsal (posterior) funiculus

  • Located between posteriomedial sulcud snd dorsal horn of gray matter
  • Carry only ascending tracts,
  • Primary tracts:

  Fasciculus grascilis: sensory info from lower body(fine touch, vibration + proprioception)

  Fasciculus Cuneatus: sensory info from upper body ( above T6).

  Dorsal funiculus is mainly involved in transmission of proprioceptive and tactile information to brain

  
• Lateral funiculus

  • Situated between dorsal and ventral horns of gray matter
  • Carry both sensory and motor tracts.

  Ascending tracts;

  1. Dorsal spinocerebellar: enteroception
  2. Lateral spinothalamic (Pain + Temperature)
  3. Ventral spinocerebellar: enteroception
  4. Spinoolivary: propioception (inf olives to cerebellum)

  Descending tracts:

  1. Lateral corticospinal (85% of fibers decussating in pyramids): voluntary control of skeletal muscles
  2. Rubrospinal (flexors of lower body)
• Ventral column

  • Lies between anterior median fissure and ventral horns of spinal cord
  • Both sensory and motor tract

  Ascending tracts:

  1. Anterior spinothalamic ( crude touch + Pressure)

  Descending tracts:

  1. Anterior corticospinal (15% fibers that do not decussate in pyramids: voluntary skeletal m control)
  2. Vestibulospinal: extensors: posture and balance
  3. Medullary reticulospinal: flexors
  4. Pontine reticulospinal: extensors
  5. Tectospinal: head, neck, eye movements.

  
• Lissauer tract

  • Allow Pain and Temperature Fibers to Ascend or Descend 1-2 segments before entering the gray matter of the dorsal horn.
  • receptor picks up pain /temperature →carries it via the sensory neuron →Just before it enters the posterior grey horn →moves up one or two levels→ synapse on the cell body in the posterior grey horn→ Cross over→ Ascends upwards through the spinothalamic tract

  
• Rexed lamina and associated nuclei

  
• Spinal nerves

  • essential part of peripheral nerve system
  • Brings afferent sensory information and sends efferent motor commands
  • There are 31 pairs of spinal nerves each corresponding to an specific segment in spinal cord
  • 8 pairs of cervical nerves: C1-7
  • 12 pairs of thoracic nerves: T1-12
  • 5 pairs of lumbar nerves: L1-5
  • 5 pairs of sacral nerves: S1-5
  • 1-3 pairs of coccygeal nerves: Co

  
• • Each spinal nerve emerges from spinal cord and exist through intervertebral foramen bellow their corresponding vertebra
  • Except for cervical spinal nerves that emerge from vertebral canal above their corresponding vertebra due to the fact that C1 nerve emerge between occipital bone and atlas (C1 segment) and so C8 nerve emerges between C7 cervical vertebral and L1

  
• Structural anatomy of spinal nerves

  • Each spinal nerve of formed by union of 2 roots

  Dorsal root: afferent sensory fibers that carry info from periphery into spinal cord. Their cell bodies are in DRG.

  Ventral root: efferent motor fibers: carry motor commands to skeletal muscles and glands. Have their cell bodies in ventral gray horn.

  • In (T1-L2) ventral roots also contains axons of sympathetic NS neurons. These innervates smooth muscles of internal organs. Have their cell bodies in lat horn
  • In S2-4 ventral root have parasympathetic fibers

  
• • Ventral and dorsal root join together to form the spinal nerves, which merge through intervertebral foramen and divide into anterior and posterior rami
  • The proper spinal nerve is very short in length because it only comprises the space of union between the ventral and dorsal roots until its separation between ventral and dorsal rami
  • Because of this, spinal nerves are mixed nerves containing sensory, motor and autonomic fibers

  
• After exiting vertebral canal, spinal nerve divided into two branches

  1. Ventral rami: innervates Anterior and lateral parts of trunk and limbs.

  One feature of ventral rami is that it forms plexuses: Ant rami of cervical spinal nerves (cervical plexus) supplies amterior neck; Lower cervical + upper thoracic (brachial plexus) supply upper limbs; lower lumbar + upper sacral (lumbosacral plexus) supply lower limbs and thoracic anterior rami remaing segmental becoming intercostal nerves.

  1. Posterior rami: innervates muscles + skin of back.

  
• Intercostal nerves: Nerves that arise from ventral rami of thoracic spinal nerves. They run along costal groove on ribs and innervate intercostal muscles and overlying skin
• Other branches of spinal nerves

  • Meningeal branch: re-enter spinal cord to innervate meninges, vestebrae and intervertebral discs
  • Rami communicantes: connects spinal nerves to sympathetic trunk
  • Gray rami comnunicamtes: exist at all level of spinal cord. Carry unmyelinated postganglionic nerve fibers from sympathetic trunk to their target organs
  • White rami communicants: only in T1-L2, preglanglionic myelinated nerve fibers from spinal cord to sympathetic trunk chain

  
• Spinal nerves types of fibers

  • Somatic efferent: originate in ventral gray column: pass through ventral root and innervates skeletal muscles
  • Somatic afferent: carry sensory info from intero/exteroreceptors (skin, joint, muscles) to dorsal horn. Pass through DRG.
  • Viceral Afferent: sensory information from internal organs
  • Viceral efferent: Autonomic nerve system fibers

  Parasympathetic: S2-4: supply pelvis + lower abdominal vicera.

  Sympathetic: T1-L2: join sympathetic trunk and synapse in postganglionic neurons in myocardium, smooth muscles and glamds.
• Spinal cord blood supply

  Received mainly from 3 longitudinal channels
• Anterior spinal artery

  In anteriomedian sulcus
• Posterior spinal arteries

  In posterolateral sulcus
• Segmental spinal arteries


  Branch from various arteries along length of spinal cord (vertebral, intercostal, lumbar and sacral arteries)
• Segmental arteries branches into

  1. Anterior and Posterior Radicular arteries (supply nerve root and spinal cord segments)

  2. Medullary arteries (supplement anterior and posterior spinal arteries providing add blood flow)
• Artery of adamkiewicz (greater radicular artery)

  Largest medullary artery, supplies lower thoracic and upper lumbar segments
• • Aside from these spinal cord has add blood supply from:Vasocorona: A network of small arteries that form an anastomotic ring around the spinal cord, connecting the anterior and posterior spinal arteries and providing collateral circulation.
  • Pial Plexus: Vascular network on the surface of the spinal cord that supplies the outer portion of the spinal cord.
• • Anterior and posterior spinal arteries are branch of vertebral artery which is a branch os subclavian.
  • Anterior spinal artery supplies anterior 2/3 and runs in anteriomedial fissure. Has penetrating and circumferental branches
  • Posterior spinal artery supplies posterior 1/3 billaterally of spinal cord

  
• Blood supply of spinal cord

  
• Venous drainage of spinal cord

  Veins are arranged in the form of 6 longitudinal channels:

  1. TWO MEDIAN LONGITUDINAL - one in the anterior median fissure and the other in the posteromedian sulcus
  2. TWO ANTEROLATERAL - one on either side, posterior to the anterior nerve roots.
  3. TWO POSTEROLATERAL - one on either side, posterior to the posterior nerve roots.

  These are interconnected to form venous vasocorona which ultimately drain into radicular veins that open into vertebral venous plexus.

  
• Spinal ganglia ala dorsal root ganglia

  • found along dorsal root of spinal nerves
  • Situated in intervertebral foramen
  • Each spinal nerves has a DRG

  Structure:

  • The ganglia is encapsulated by CT capsule
  • They contain cell bodies of primary sensory neurons (pseudounipolar neurons) that have a single process that biforcates into 2 branches:

  Peripheral branch: extends to sensory receptors in skin, muscle

  Central branch enters spinal cord transmitting sensory information