The Spinal Cord and Spinal Nerves

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The Spinal Cord and Spinal Nerves
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Meninges

• Connective tissue coverings that encircle the
  spinal cord and brain and are continuous with one
  another
• Spinal Meningessurround the spinal cord
• Cranial Meningessurround the brain

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Meninges (continued)

• Dura Materthe most superficial of the meninges
• Arachnoid Materthe middle meninx (singular form
  of meninges)
• Pia Materthe innermost meninx adheres to the
  surface of the brain and spinal cord contains
  many blood vessels

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Spaces between the Meninges

• Epidural Spacea space between the dura mater and
  the wall of the vertebral canal contains fat and
  connective tissue
• Subdural Spacea thin space between the dura
  mater and the arachnoid mater contains
  interstitial fluid
• Subarachnoid Spacebetween the arachnoid mater
  and the pia mater contains cerebrospinal fluid

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Spinal Cord

• Located within the vertebral canal of the
  vertebral column
• Protected by bony vertebrae, the meninges, and
  cerebrospinal fluid
• Denticulate ligaments are extensions of the
  innermost of the meninges, the pia mater, which
  hold the spinal cord in place and protect against
  shock and displacement

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Spinal Cord (continued)

• In an adult, the spinal cord extends from the
  medulla oblongata, the lower portion of the
  brain, to about the level of L2
• Its length is about 16-18 inches, and its
  diameter is about ¾ inch
• The spinal cord does not extend the entire length
  of the vertebral column it stops growing at
  about age 4-5 years, but the vertebral column
  continues to lengthen

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Spinal Cord (continued)

• The spinal cord has two enlargements
• Cervical Enlargementextends from C4-T1 nerves
  to and from the upper limbs arise here
• Lumbar Enlargementextends from T9-T12 nerves to
  and from the lower limbs arise here

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Spinal Cord (continued)

• The spinal cord tapers to a point and ends at
  about L1-L2 this portion is called the conus
  medullaris
• Arising from the conus medullaris is an extension
  of pia mater that anchors the spinal cord to the
  coccyx this is called the filum terminale
• The spinal nerves that extend down to the lower
  parts of the body angle downward beyond the conus
  medullaris this area is called the cauda equina

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Internal Anatomy of the Spinal Cord

• The spinal cord is divided into right and left
  sides by a deep, wide groove, called the anterior
  median fissure, and a shallow, narrow slit,
  called the posterior median sulcus
• The gray matter of the cord is shaped like the
  letter H (or a butterfly) and is surrounded by
  white matter
• Gray matter consists primarily of the cell bodies
  and dendrites of neurons, neuroglia, and
  unmyelinated axons white matter consists of
  mainly myelinated axons

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Internal Anatomy of the Spinal Cord (continued)

• The gray commissure is gray matter that forms the
  crossbar of the H
• The central canal is a canal that extends the
  length of the spinal cord and contains
  cerebrospinal fluid it is seen as a hole in the
  center of the gray commissure
• The anterior (ventral) white commissure is a
  bridge of white matter that connects the right
  and left sides of the spinal cord

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Internal Anatomy of the Spinal Cord (continued)

• The gray matter of the spinal cord is subdivided
  into regions called horns this area of the
  spinal cord receives and integrates incoming and
  outgoing information
• The white matter of the spinal cord is subdivided
  into regions called columns this area of the
  spinal cord allows nerve impulses to travel to
  and from the brain

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Internal Anatomy of the Spinal Cord (continued)

• There are three horns and columns on each side of
  the spinal cord
• Anterior (ventral) gray horns
• Posterior (dorsal) gray horns
• Lateral gray horns
• Anterior (ventral) white columns
• Posterior (dorsal) white columns
• Lateral white columns

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Internal Anatomy of the Spinal Cord (continued)

• The horns contain clusters of cell bodies called
  nuclei, which are found only in the CNS
• The columns contain bundles of axons which run up
  and down the spinal cord (to and from the brain)
  called tracts tracts are actually nerves that
  are located in the CNS

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Internal Anatomy of the Spinal Cord (continued)

• Tracts that run up the spinal cord to the brain
  are called ascending tracts these are sensory
  tracts
• Tracts that run down the spinal cord from the
  brain are called descending tracts these are
  motor tracts

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Spinal Nerves

• Spinal nerves are the paths of communication
  between the spinal cord and the nerves that go to
  specific regions of the body
• They are a part of the PNS
• There are 31 pairs of spinal nerves, which are
  named and numbered according to the region and
  level of the vertebral column from which they
  emerge
• The first pair emerges between the atlas (C1) and
  the occipital bone

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Spinal Nerves (continued)

• Cervical nerves8 pairs (C1-C8)
• Thoracic nerves12 pairs (T1-T12)
• Lumbar nerves5 pairs (L1-L5)
• Sacral nerves5 pairs (S1-S5)
• Coccygeal nerves1 pair (Co1)

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Spinal Nerves (continued)

• Two bundles of axons, called roots, connect each
  spinal nerve to the spinal cord
• The dorsal (posterior) root contains only sensory
  axons and conducts impulses to the CNS from
  receptors in the skin, muscles, and internal
  organs
• The ventral (anterior) root contains only motor
  axons and conducts impulses from the CNS to
  effectors

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Spinal Nerves (continued)

• Spinal nerves are mixed nerves because they
  contain both sensory and motor roots
• Each dorsal (posterior) root has a swelling,
  called a dorsal (posterior) root ganglion, which
  contains the cell bodies of sensory neurons

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Connective Tissue Coverings of Nerves

• Epineuriumthe superficial covering of a nerve
  fuses with the dura mater as the nerve passes
  through the intervertebral foramen
• Perineuriumsurrounds groups of axons, called
  fascicles
• Endoneuriumthe covering of an individual axon,
  whether myelinated or unmyelinated

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Branches of Spinal Nerves

• A spinal nerve divides into several branches
  (called rami) after passing through the
  intervertebral foramen
• The posterior (dorsal) ramus serves the deep
  muscles and skin of the dorsal surface of the
  trunk
• The anterior (ventral) ramus serves the muscles
  and structures of the upper and lower limbs and
  the skin of the lateral and ventral surfaces of
  the trunk

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Branches of Spinal Nerves (continued)

• The meningeal branch reenters the vertebral canal
  and supplies the vertebrae and meninges
• The rami communicantes are branches of the spinal
  nerves which are a part of the ANS

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Networks of Spinal Nerves

• The anterior rami of spinal nerves, except for
  thoracic nerves T2-T12, form networks of axons
  called plexi
• The main networks and the nerves they involve are
  as follows
• Cervical plexusC1-C5 supplies the head, neck,
  upper shoulders and chest, and diaphragm
• Brachial plexusC5-C8 and T1 supplies the
  shoulders and upper limbs
• Lumbar plexusL1-L4 supplies the abdominal wall,
  external genitals, and part of the lower limbs
• Sacral plexusL4-L5 and S1-S4 supplies the
  buttocks and lower limbs
• Coccygeal plexusS4-S5 and the coccygeal nerves
  supplies the coccyx region

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Networks of Spinal Nerves (continued)

• The anterior rami of spinal nerves T2-T12 do not
  form plexi
• They become intercostal nerves, which serve the
  thoracic region of the body

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A Reflex Arc

• The most basic conduction pathway through the
  nervous system is a reflex arc
• It consists of 5 components
• Receptor
• Sensory neuron
• CNS integrating center (either in the brain or
  spinal cord)
• Motor neuron
• Effector