the spinal cord & the nerves

• Spinal cord


  • About 100 million neurons and even more neuroglia compose it
  • Part of the central nervous system that extends from the brain
  • Contains neural circuits that control some of your most rapid reactions to environmental changes
• If you pick up something hot, the grasping muscles may relax and you may drop the hot object even before you are consciously aware of the extreme heat or pain
• Spinal cord reflex


  A quick, automatic response to certain kinds of stimuli that involves neurons only in the spinal nerves and spinal cord
• The gray matter of the spinal cord is a site for integration (summing) of excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs)

  
• The white matter of the spinal cord contains a dozen major sensory and motor tracts, which function as the "highways" along which sensory input travels to the brain and motor output travels from the brain to skeletal muscles and other effectors

  
• The spinal cord is continuous with the brain and together they make up the central nervous system (CNS)

  
• Protective structures of the spinal cord

  • Hard bony skull and vertebral column
  • Meninges (3 membranes between bony encasement and nervous tissue)
  • Cerebrospinal fluid in subarachnoid space between arachnoid mater and pia mater
• Vertebral column


  Vertebral foramina of all vertebrae form the vertebral canal that surrounds the spinal cord

  
• Meningeal layers


  • Dura mater (outer)
  • Arachnoid mater (middle)
  • Pia mater (inner)
• Dura mater


  Thick, strong layer of dense irregular connective tissue
• Arachnoid mater


  Thin, avascular covering comprised of cells and thin, loosely arranged collagen and elastic fibers
• Pia mater


  Thin transparent connective tissue layer that adheres to the surface of the spinal cord
• Denticulate ligaments


  Triangular-shaped membranous extensions of the pia mater that suspend the spinal cord in the middle of its dural sheath
• Subarachnoid space


  Space between the arachnoid mater and pia mater that contains shock-absorbing cerebrospinal fluid
• Epidural space


  Space between the dura mater and the wall of the vertebral canal, containing a cushion of fat and connective tissue
• The spinal cord extends from the medulla oblongata, the inferior part of the brain, to the superior border of the second lumbar vertebra

  
• In newborn infants, the spinal cord extends to the third or fourth lumbar vertebra
• Spinal cord enlargements


  • Cervical enlargement (C4 to T1)
  • Lumbar enlargement (T9 to T12)

  
• Conus medullaris


  Tapering, conical structure at the inferior end of the spinal cord

  
• Filum terminale


  Extension of the pia mater that extends inferiorly, fuses with the arachnoid mater and dura mater, and anchors the spinal cord to the coccyx

  
• Spinal nerves


  • 31 pairs of spinal nerves that emerge at regular intervals from intervertebral foramina
  • Each pair of spinal nerves is said to arise from a spinal segment

  
• Types of spinal nerves

  • 8 pairs of cervical nerves (C1-C8)
  • 12 pairs of thoracic nerves (T1-T12)
  • 5 pairs of lumbar nerves (L1-L5)
  • 5 pairs of sacral nerves (S1-S5)
  • 1 pair of coccygeal nerves (Co1)

  
• Posterior (dorsal) root


  Contains only sensory axons that conduct nerve impulses from sensory receptors into the central nervous system

  
• Posterior (dorsal) root ganglion

  Swelling on the posterior root that contains the cell bodies of sensory neurons

  
• Anterior (ventral) root


  Contains axons of motor neurons that conduct nerve impulses from the CNS to effectors (muscles and glands)

  
• Because the spinal cord is shorter than the vertebral column, nerves that arise from the lumbar, sacral, and coccygeal regions of the spinal cord descend through the vertebral canal before exiting through the intervertebral foramina

  
• This collection of descending nerve roots is called the cauda equina (horse's tail)

  
• Conus medullaris


  Cone-shaped structure at the inferior end of the spinal cord, ending at the level of the intervertebral disc between L1 and L2

  
• Cauda equina


  The roots of the lower spinal nerves (lumbar, sacral, and coccygeal) that angle inferiorly alongside the filum terminale in the vertebral canal, resembling a horse's tail

  
• Spinal segment


  The portion of the spinal cord associated with a pair of spinal nerves

  
• The spinal cord is partially divided into right and left sides by the anterior median fissure and posterior median sulcus

  
• Gray commissure


  The crossbar of the H-shaped gray matter in the spinal cord, containing the central canal

  
• Central canal


  Small space in the center of the gray commissure, extending the length of the spinal cord and filled with cerebrospinal fluid
• Anterior gray horn


  Contains somatic motor nuclei, clusters of cell bodies of somatic motor neurons that provide nerve impulses for contraction of skeletal muscles
• Lateral gray horn


  Contains autonomic motor nuclei, clusters of cell bodies of autonomic motor neurons that regulate the activity of cardiac muscle, smooth muscle, and glands
• Posterior gray horn


  Contains axons of incoming sensory neurons and cell bodies and axons of interneurons
• Anterior white column


  One of the three broad areas the white matter is divided into on each side of the spinal cord

  
• Lateral white column


  One of the three broad areas the white matter is divided into on each side of the spinal cord

  
• Posterior white column


  One of the three broad areas the white matter is divided into on each side of the spinal cord

  
• Ascending tract


  Sensory tracts consisting of axons that conduct nerve impulses toward the brain