Spinal Cord & Spinal Nerves

Spinal Cord

  • Spinal Cord

    • Acts as a link btwn the brain and the rest of the body

    • Does some things independently from the brain

  • Spinal cord and spinal nerves

    • Provide a pathway for sensory and motor nerve signals

    • Important for spinal reflexes, which provide the quickest reactions to stimuli that are generally below the neck

  • Cranial nerve reflexes mostly involve the head and neck

  • CNS tissue from the foramen magnum to L1; protected by bone, meninges, and CSF

  • 31 pairs of mixed spinal nerves (leading to PNS) escape btwn vertebrae and attach to the cord by paired roots; innervate the entire body except the head

    • Mixed—both sensory and motor


Spinal Cord Regions

  • Named according to their point of issue

    • 8 cervical (C1 - C8)

    • 12 thoracic (T1 - T12)

    • 5 lumbar (L1 - L5)

    • 5 sacral (S1 - S5)

    • 1 coccygeal (C0)

  • Conus medullaris—end of spinal cord

  • Cauda equina—nerves project inferior to the spinal cord

  • Filum terminale—strand of pia mater within the cauda equina that anchors the spinal cord to coccyx


Protection of Spinal Cord

  • Structures surrounding the spinal cord, from innermost to outermost

    • Pia mater

    • Subarachnoid space

    • Arachnoid mater

    • Subdural space

    • Dura mater

    • Epidural space

    • Vertebra




Spinal Cord Cross-Section

  • Gray Matter located toward center (gray commissure connects 2 sides)

  • White matter surrounds gray matter

    • Divisions

      • Posterior funiculus

      • Lateral funiculus

      • Anterior funiculus (White commissure connects 2 sides)

  • Central canal—carries CSF flowing inferiorly

  • Posterior median sulcus—divides posterior cord medially

  • Anterior median fissure—divides anterior cord medially


Spinal Cord Tracts

  • Ascending tracts—mainly relay sensory info up toward brain

  • Descending tracts—mainly relay motor info down brain


Spinal Roots Join to Form Spinal Nerves

  • Dorsal roots—carry sensory (afferent) neurons from spinal nerves to spinal cord

  • Ventral roots—carry motor (efferent) neurons from spinal cord to spinal nerves


Spinal Nerves and Dermatomes

  • Dermatome—a bilateral section of skin innervated by a single spinal nerve (all nerves except C1)

  • The skin can be sectioned into a dermatome map


Spinal Nerves: Rami

  • The short spinal nerves branch into three or four mixed, distal rami

    • Smaller posterior ramus

    • Larger anterior ramus


Nerve Plexuses

  • All anterior rami except T2 - T12 form interlacing nerve networks called plexuses

  • Plexuses are found in the cervical, brachial, lumbar, and sacral region

  • Each resulting branch of a plexus contains fibers from several spinal nerves

  • Fibers travel to the periphery via several different routes

  • Each muscle receives a nerve supply from more than one spinal nerve

  • Damage to one spinal segment cannot completely paralyze a muscle


Neural Reflexes

  • Reflex—an involuntary, rapid, predictable motor (muscle/gland) response to a stimulus

  • Reflexes may

    • Be inborn (intrinsic) or learned (acquired)

    • Involve only peripheral nerves and the spinal cord

    • Involve higher brain centers as well

The Spinal Cord Can Function Independently From The Brain
Reflex Tests

  • Performed clinically to check health of muscles, spinal nerves, and spinal cord regions

  • A reflex response may be normal, hypoactive, or hyperactive

Reflex Arc

  • 5 components of a reflex arc

    • Receptor: site of stimulus

    • Sensory neuron: transmits the afferent impulse to the CNS

    • Integration center: either monosynaptic or polysynaptic region within the CNS

      • Interneurons never leave the brain or spinal cord

    • Motor neuron: conducts efferent impulses from the integration center to an effector

    • Effector: muscle fiber or gland that responds to the efferent impulse


  • afferent stimulus goes through posterior side of spinal cord
Ipsilateral v. Contralateral Reflex Arcs

  • Ipsilateral: the receptor organ is on the same side of the body as the effector organ

    • A hand poked by a pin will pull away from the pin

  • Contralateral: the receptor organ is on the opposite side of the body as the effector organ

    • If the left wrist is grabbed by a stranger, the left hand will pull back from the stranger while the right hand will extend to throw a punch

Monosynaptic v. Polysynaptic Reflexes

  • Monosynaptic—simple arcs w/o interneurons

  • Polysynaptic—more complex reflex arcs w/ more synapses and interneurons; incd delay time