PNS AND CNS

• Spinal Cord


  Major communication link between the brain and the PNS (spinal nerves). Participates in the integration of incoming information and produces responses through reflex mechanisms
• Spinal Cord


  • Gives rise to 31 pairs of spinal nerves
  • Cervical and lumbosacral enlargements give rise to the spinal nerves of the limbs
  • Nerves from the end of the spinal cord form the cauda equina
  • Shorter than the vertebral column
• Meninges of the Spinal Cord

  • Dura mater
  • Arachnoid mater
  • Pia mater
• Epidural space


  Between the periosteum of the vertebral canal and the dura mater
• Subarachnoid space


  Between the arachnoid mater and the pia mater
• The spinal cord consists of peripheral white matter and central gray matter
• White matter


  • Organized into columns (funiculi), which are subdivided into tracts (fasciculi or pathways), which consist of ascending and descending axons
• Gray matter


  • Divided into horns
  • Dorsal horns contain sensory axons that synapse with interneurons
  • Ventral horns contain the neuron cell bodies of somatic motor neurons
  • Lateral horns contain the neuron cell bodies of autonomic motor neurons
• The gray and white commissures connect each half of the spinal cord
• Spinal nerves


  • Arise from the spinal cord in numerous rootlets
  • The rootlets combine to form a ventral root and a dorsal root
• Dorsal root


  Contains sensory axons
• Ventral root


  Has motor axons
• Spinal nerves


  Have sensory and motor axons
• Reflexes


  • Stereotypic, unconscious, involuntary responses to stimuli
  • Maintain homeostasis
• Types of reflexes


  • Somatic reflexes
  • Autonomic reflexes
• Somatic reflexes


  Mediated through the somatic motor nervous system and includes responses that remove the body from painful stimuli, keep the body from suddenly falling, and cause movement because of external forces
• Autonomic reflexes


  Mediated through the ANS and are responsible for maintaining variables within their normal ranges, such as blood pressure, blood carbon dioxide levels, and water intake
• Reflex arc


  The functional unit of the nervous system
• Reflex arc


  1. Sensory receptors respond to stimuli and produce action potentials in sensory neurons

  2. Sensory neurons propagate action potentials to the CNS

  3. Interneurons in the CNS synapse with sensory neurons and with motor neurons

  4. Motor neurons carry action potentials from the CNS to effector organs

  5. Effector organs, such as muscles or glands, respond to the action potential
• Convergent and divergent pathways interact with reflexes
• Reflexes are integrated within the brain and spinal cord
• Higher brain centers can suppress or exaggerate reflexes
• Stretch reflex


  Muscle spindles detect the stretch of skeletal muscles and cause the muscle to shorten reflexively
• Golgi tendon reflex


  Golgi tendon organs respond to increased tension within tendons and cause skeletal muscles to relax
• Withdrawal reflex


  Activation of pain receptors causes contraction of muscles and the removal of some part of the body from a painful stimulus
• Structure of nerves


  • Individual axons are surrounded by the endoneurium
  • Groups of axons (fascicles) are bound together by the perineurium
  • Fascicles form the nerve and are held together by the epineurium
• Spinal nerves


  • There are 31 pairs of spinal nerves
  • Eight cervical
  • Twelve thoracic
  • Five lumbar
  • Five sacral
  • One coccygeal
  • Spinal nerves have specific cutaneous distributions called dermatomes
• Spinal nerves and plexuses

  1. Spinal nerves branch to form rami

  2. Dorsal rami supply the muscles and skin near the middle of the back

  3. Ventral rami in the thoracic region form intercostal nerves, which supply the thorax and upper abdomen

  4. Cervical, lumbar, sacral and coccygeal ventral rami join to form plexuses

  5. Communicating rami supply sympathetic nerves
• Major spinal nerve plexuses

  • Cervical (C1-C4)
  • Brachial (C5-T1)
  • Lumbar (L1-L4)
  • Sacral (L4-S4)
  • Coccygeal (S5-coccygeal)
• Axons from different levels of the spinal cord intermingle within plexuses and give rise to nerves that have axons from more than one level of the spinal cord
• Nerves arising from plexuses are distributed to skeletal muscles throughout the body
• Nerves arising from plexuses also supply the skin
• Brain


  • Contained in the cranial cavity
  • Is the control center for many of the body's functions
• Parts of the brain

  • Brainstem
  • Cerebellum
  • Diencephalon
  • Cerebrum
• Brainstem


  • Consists of the medulla oblongata, pons, and midbrain
  • Connects the spinal cord and cerebellum to the remainder of the brain
  • Ten of the 12 pairs of cranial nerves arise from it
  • Damage to small areas can cause death, whereas damage to relatively large areas of the cerebrum or cerebellum do not
• Medulla oblongata


  • Continuous with the spinal cord and contains ascending and descending tracts
  • Medullary nuclei regulate the heart, blood vessels, breathing, swallowing, vomiting, coughing, sneezing, hiccupping, balance and coordination
  • The pyramids are tracts controlling voluntary muscle movement
• Pons


  • Superior to the medulla
  • Ascending and descending tracts pass through the pons
  • Connects the cerebrum and the cerebellum
  • Pontine nuclei regulate breathing, swallowing, balance, chewing, and salivation
• Midbrain


  • Superior to the pons
  • Corpora quadrigemina consist of four colliculi, two inferior involved in hearing and two superior in visual reflexes
  • Substantia nigra and the red nucleus help regulate body movements
  • Cerebral peduncles are the major descending motor pathway
• Reticular formation


  • Consists of nuclei scattered throughout the brainstem
  • Regulates cyclic motor functions, such as breathing, walking, and chewing
  • Reticular activating system maintains consciousness and regulates the sleep-wake cycle
• Cerebellum


  • Gray matter forms the cortex and nuclei
  • White matter forms the arbor vitae
  • Connects the cerebellum to the rest of the CNS
  • Flocculonodular lobe controls balance and eye movements
  • Vermis and medial part of the lateral hemispheres control posture, locomotion, and fine motor coordination
  • Lateral hemispheres are involved with the planning, practice, and learning of complex movements