Organization of Human Nervous System

Functional Organization of Human Nervous System

  • This lecture discusses the functional and structural organization of the human nervous system, including the central and peripheral nervous systems, and clinical applications.

Reading Material

  • Neuroscience: Fundamentals for Rehabilitation.
    • 5th Edition: Chapter 2, 17
    • 6th Edition: Chapter 2, 18 (Appendix 18.1-18.2)

Outline

  • Describe functional organization of human nervous system
  • Describe anatomical structures of central and peripheral nervous systems
  • Review anatomy of peripheral nervous system
  • Discuss clinical applications

Functional Organization of Movements

  • The functional organization of movement involves several key structures:
    • Cerebral Cortex (Motor Areas): Responsible for skilled movements and the initiation and selection of motor programs.
    • Thalamus: Acts as a relay station for motor information.
    • Basal Ganglia: Involved in motor control and coordination.
    • Cerebellum: Plays a critical role in motor coordination and error correction.
    • Brainstem (RS, VS, RbS): Manages posture and eye movements.
    • Spinal Cord: Facilitates muscle movement and contraction, spinal reflexes, and locomotion.
    • Sensory Receptors: Provide sensory input that influences motor actions.

Neuroanatomy

  • Structural (system) level
    • Central nervous system
    • Peripheral nervous system
  • Regions
    • Peripheral, spinal, brainstem,
    • cerebellar, and cerebral regions
  • Functional divisions
    • Somatic system
      • Somatic sensory
      • Somatic motor
    • Autonomic system

Neural Tissue Organization

  • Central Nervous System (CNS):
    • Gray Matter Organization:
      • Neural Cortex: Gray matter on the surface of the brain.
      • Nuclei: Collections of neuron cell bodies in the interior of the CNS.
      • Centers: Collections of neuron cell bodies in the CNS with specific processing functions. Higher centers are the most complex.
    • White Matter Organization:
      • Tracts: Bundles of CNS axons sharing a common origin, destination, and function.
      • Columns: Several tracts forming an anatomically distinct mass.
      • Pathways: Columns and tracts connecting the brain with other organs and systems, including sensory (ascending) and motor (descending) pathways.
  • Peripheral Nervous System (PNS):
    • Gray Matter: Ganglia (collections of neuron cell bodies in the PNS).
    • White Matter: Nerves (bundles of axons in the PNS).

Organization of Peripheral Nervous System (PNS)

  • The PNS is divided into sensory (afferent) and motor (efferent) divisions.
    • Sensory (Afferent) Division:
      • Somatic Sensory: General senses (touch, pain, pressure, vibration, temperature, and proprioception) in skin, body wall, and limbs; special senses (hearing, equilibrium, and vision).
      • Visceral Sensory: General senses (stretch, pain, temperature, chemical changes, and irritation in viscera; nausea and hunger); special senses (taste and smell).
    • Motor (Efferent) Division:
      • Somatic Motor (Voluntary Motor): General motor innervation of all skeletal muscles.
      • Visceral Motor (Involuntary Motor): General motor innervation of smooth muscle, cardiac muscle, and glands; equivalent to the autonomic nervous system (ANS).

Neural Organization of PNS

  • Spinal Region
    • Spinal nerve (31 pairs)
  • Peripheral Region
    • Rami
      • Doral ramus
      • Ventral ramus
    • Plexus
    • Peripheral nerves

Spinal Nerve

  • Each spinal nerve connects to the spinal cord via a ventral (anterior) root and a dorsal (posterior) root.
    • Dorsal Root: Contains sensory fibers.
    • Dorsal Root Ganglion: Contains afferent cell bodies.
    • Ventral Root: Contains motor fibers arising from the anterior gray column.
  • The dorsal and ventral roots merge to form a spinal nerve in the intervertebral foramen.
  • Outside the foramen, the spinal nerve re-branches into rami (dorsal and ventral rami).

Ramus/Rami

  • Rami are mixed nerves containing both sensory and motor fibers.
    • Dorsal Rami: Supply somatic motor and sensory fibers to smaller nerves innervating the muscles and skin of the posterior surface of the head, neck, and trunk.
    • Ventral Rami: Innervate the muscles and skin of the limbs and the lateral and anterior trunk.
    • Sympathetic Rami: Ventral rami communicate with a chain of sympathetic nerves.

Plexus

  • A nerve plexus is a network of nerves formed by ventral rami (except T2-T11).
  • Fibers from several different rami join together to form individual nerves through criss-crossing and redistribution.
  • Spinal nerves T2-T11 directly form intercostal nerves.
  • Plexuses primarily innervate the limbs and include:
    • Cervical (C1-C4)
    • Brachial (C5-T1)
    • Lumbar (L1-L4)
    • Sacral (part L4 – S4)

Cervical Plexus

  • Originates from ventral rami of nerves C1, C2, C3, C4 (+C5).
    • Carries efferent neurons to many muscles of the neck.
    • Carries afferent neurons from skin of the neck, shoulder, and scalp near the ear.
    • The phrenic nerve (C3-C5) is the sole motor supply of the diaphragm.

Brachial Plexus

  • Arises primarily from C5-T1
    • Carries efferent neurons to muscles of the shoulder, arm, forearm, and hand.
    • Carries afferent neurons from skin of the shoulder, arm, forearm, and hand.
  • Main nerves:
    • Musculocutaneous
    • Median
    • Ulnar
    • Axillary
    • Radial

Lumbar Plexus

  • Originates from ventral rami of nerves L1, L2, L3, L4 (+T12, +L5).
    • Carries efferent neurons to muscles of the anterior pelvis, anterior and medial thigh.
    • Carries afferent neurons from skin of the anterior pelvis, anterior and medial thigh, and medial leg.
  • Main Nerves:
    • Femoral nerve
    • Obturator nerve
    • Saphenous nerve

Sacral Plexus

  • Originates from ventral rami of nerves (+L4), L5, S1, S2, S3, S4, (+S5, Cx1).
    • Carries efferent neurons to muscles of the posterior pelvis, posterior thigh, anterior and posterior leg, and foot.
    • Carries afferent neurons from skin of the posterior pelvis, posterior thigh, anterior and posterior leg, and foot.
  • Main nerves:
    • Sciatic nerve
    • Tibial nerve
    • Common fibular nerve
    • Pudendal nerve

Distributions of PNS

  • Refer to Appendix 18.1-18.2 in the 6th edition of the textbook.

Musculocutaneous Nerve

  • From the lateral cord of the brachial plexus.
  • Motor: Innervates the Coracobrachialis, Biceps brachii (long and short heads), and Brachialis muscles.
  • Sensory: Provides cutaneous innervation to the lateral cutaneous nerve of the forearm.

Median Nerve

  • From the lateral and medial cords of the brachial plexus.
  • Motor: Innervates the Pronator teres, Flexor carpi radialis, Palmaris longus, Flexor digitorum superficialis, Flexor digitorum profundus (radial side only), Flexor pollicis longus, Pronator quadratus, Abductor pollicis brevis, Flexor pollicis brevis (superficial head), Opponens pollicis, and the first and second lumbricals.
  • Sensory: Provides sensory innervation.

Ulnar Nerve

  • From the medial cord of the brachial plexus.
  • Motor: Innervates the Flexor carpi ulnaris, Flexor digitorum profundus (ulnar side only), Palmaris brevis, Abductor digiti minimi, Opponens digiti minimi, Flexor digiti minimi brevis, Palmar and dorsal interossei (all), Third and fourth lumbricals and Adductor pollicis.
  • Sensory: Provides sensory innervation.

Radial/Axillary Nerves

  • From the posterior cord of the brachial plexus.
  • Motor (Radial Nerve): Innervates the Long head of triceps, Lateral head of triceps, Medial head of triceps, Brachioradialis, Extensor carpi radialis longus, Extensor carpi radialis brevis, Anconeus, Supinator, Abductor pollicis longus, Extensor pollicis brevis, Extensor pollicis longus, Extensor indicis, Extensor digitorum, Extensor digiti minimi, and Extensor carpi ulnaris.
  • Motor (Axillary Nerve): Innervates the Teres minor and Deltoid muscles.
  • Sensory (Axillary Nerve): Superior lateral cutaneous nerve of arm.
  • Sensory (Radial Nerve): Inferior lateral cutaneous nerve of arm, Posterior cutaneous nerve of arm, Posterior cutaneous nerve of forearm, and Superficial radial nerve.

Femoral Nerve

  • From the lumbar plexus (L2-L4).
  • Motor: Psoas major, Iliacus, Pectineus, Sartorius, Rectus femoris, Vastus lateralis, Vastus medialis, and Vastus intermedius muscles.
  • Sensory: Anterior cutaneous branches and Saphenous nerve (pure sensory).

Common Fibular Nerve

  • From the Sciatic nerve (L4-S2).
  • Motor: Biceps femoris short head.
  • Sensory: Lateral sural cutaneous nerve.
  • Divides into:
    • Superficial fibular nerve
      • Motor: Fibularis Longus, Fibularis brevis
      • Sensory: Superficial fibular nerve
    • Deep fibular nerve
      • Motor: Tibialis anterior, Extensor digitorum longus, Extensor hallucis longus, Fibularis tertius, Extensor hallucis and digitorum brevis
      • Sensory: Deep fibular nerve

Obturator Nerve

  • From the lumbar plexus (L2-L4).
  • Motor: Obturator externus, Adductor longus, Adductor brevis, Gracilis, and Adductor magnus (anterior part) muscles.
  • Sensory: Provides sensory innervation.

Tibial Nerve

  • From the sacral plexus (L4-S3).
  • Motor: Plantaris, Gastrocnemius, Popliteus, Soleus, Flexor hallucis longus, Flexor digitorum longus, Tibialis posterior.
  • Motor (Medial plantar nerve): Abductor hallucis, Flexor digitorum brevis, Flexor hallucis brevis, First lumbrical, Adductor hallucis
  • Motor (Lateral plantar nerve): Quadratus plantae, Abductor digiti minimi, Flexor digiti minimi brevis, Three lateral lumbricals, Plantar interossei (all), Dorsal interossei (all)
  • Sensory: Medial calcaneal nerve, Medial sural cutaneous and sural nerves.

PNS: Sensory Input and Motor Output

  • Somatic peripheral nerves are typically mixed, including sensory, autonomic, and motor axons.
  • Sensory (afferent) signals are detected by sensory receptors and carried by nerve fibers of the PNS to the CNS.
  • Motor (efferent) signals are carried away from the CNS, innervating muscles and glands.
  • Divided according to the region they serve:
    • Somatic body region
    • Visceral body region
  • Results in four main subdivisions:
    • Somatic sensory
    • Somatic motor
    • Visceral sensory
    • Visceral motor (autonomic nervous system)

Spinal Nerve Dermatomes

  • A dermatome is an area of skin supplied by the sensory fibers of a given spinal nerve.
  • Note the presence of cutaneous nerve overlap where adjacent spinal nerves innervate overlapping regions of skin.

Spinal Region vs. Peripheral Region Lesions

  • Spinal Region Lesion: Affects myotomal and/or dermatomal distributions.
    • Dermatome: A region of skin supplied by sensory fibers of a given spinal nerve.
    • Myotome: A region of skeletal muscle innervated by motor fibers of a given spinal nerve.
  • Peripheral Lesion: Affects peripheral nerve distribution.
  • Example:
    • Cut C6 nerve root: V mm strength, Anterior paralyzed, other spinal regions assist
    • Cut Median nerve: no finger flexion, lose sensation, severe muscle weakness = paralyzed, distal to injury & Function B from multiple spinal Segments