Spinal Nerves and Plexuses

• Peripheral nervous system consists of spinal, cranial, and autonomic nerves.

Spinal Nerves

• Spinal nerves are nerve fibers bundled from the spinal cord.
• There are 31 pairs of spinal nerves.
• Spinal nerves emerge from the vertebral canal through the intervertebral foramina.
• Dorsal roots contain sensory neuron fibers.
• Ventral roots contain motor neuron fibers.
• 2 roots join to form a spinal nerve.
• Spinal nerves include 8 cervical pairs, 12 thoracic pairs, 5 lumbar pairs, 5 sacral pairs, and 1 coccygeal pair.

Plexuses

• Plexuses are groups of nerve fibers emerging from the ventral rami.
• The 4 main plexuses are cervical, brachial, lumbar, and sacral.
• Thoracic spinal nerves do not form plexuses.
• T1-T11 spinal nerves are intercostal nerves.
• T12 spinal nerves are subcostal nerves.
• The cervical plexus involves C1-C5 and serves the head and neck.
• The brachial plexus involves C5-T1 and serves the upper limb.
• The lumbar plexus involves L1-L4 and serves the abdomen and lower limb.
• The sacral plexus involves L4-S4 and serves the pelvis, perineum, and lower limb.
• The cervical plexus is derived from C1, C2, C3, and C4.
• The phrenic nerve (C3-C5) is the main nerve supply of the diaphragm.
• The brachial plexus is derived C5, C6, C7, C8 nerves and 1st thoracic (T1) and supplies the skin and muscles of the upper limb.
• Major nerves of the brachial plexus include musculocutaneous, axillary, median, radial, and ulnar nerves.
• The lumbar plexus is derived from T12-L4 spinal nerves.
• The obturator nerve supplies the muscles and skin on the medial aspect of the thigh and comes from anterior divisions of L2 - L4 nerves.
• The femoral nerve supplies muscles and skin on the anterior aspect of the thigh and comes from posterior divisions of the L2-L4 nerves.
• The sacral plexus is derived from L4, L5, S1, S2, S3 spinal nerves.
• Sciatic nerve is the the largest nerve in the body.
• The Sciatic nerve divides into the tibial and common peroneal nerves.
• It gives off major innervations to the lower extremities

Dermatomes

• C2 dermatome is 1 cm lateral to the occipital protuberance or 3 cm behind the ear.
• C3 dermatome is the supraclavicular fossa (posterior to the clavicle) and at the midclavicular line.
• C4 dermatome is over the acromioclavicular joint.
• C5 dermatome is the lateral side of the antecubital fossa (just proximal to the elbow crease).
• C6 dermatome is the thumb, dorsal surface, proximal phalanx.
• C7 dermatome is the middle finger, dorsal surface, proximal phalanx.
• C8 dermatome is the little finger, dorsal surface, proximal phalanx.
• T1 dermatome is the medial side of the antecubital fossa, just proximal to the medial epicondyle of the humerus.
• T2 dermatome is the apex of the axilla.
• T3 dermatome is the midclavicular line and the third intercostal space (IS).
• T4 dermatome is the fourth IS (nipple line) at the midclavicular line.
• T5 dermatome is the midclavicular line and the fifth IS (midway between T4 and T6).
• T6 dermatome is the midclavicular line and the sixth IS (level of xiphisternum).
• T7 dermatome is the midclavicular line and the seventh IS (midway between T6 and T8).
• T8 dermatome is the midclavicular line and the eighth IS (midway between T6 and TIO).
• T9 dermatome is the midclavicular line and the ninth IS (midway between T8 and T10).
• T10 dermatome is the midclavicular line and the tenth IS (umbilicus).
• T11 dermatome is the midclavicular line and the eleventh IS (midway between T10 and T12).
• T12 dermatome is the midclavicular line and the mid-point of the inguinal ligament.
• L1 dermatome is the midway distance between the key sensory points for Tl2 and L2.
• L2 dermatome is on the anterior-medial thigh at the midpoint drawn connecting the midpoint of the inguinal ligament (T12) and the medial femoral condyle.
• L3 dermatome is the medial femoral condyle above the knee.
• L4 dermatome is the medial malleolus.
• L5 dermatome the dorsum of the foot at the third metatarsal phalangeal joint.
• S1 dermatome is the lateral heel (calcaneus).
• S2 dermatome is the mid-point of the popliteal fossa.
• S3 dermatome is the ischial tuberosity or infragluteal fold.
• S4-5 dermatome is the perianal area less than one cm lateral to the mucocutaneous junction.

Cranial Nerves

• There are 12 pairs of cranial nerves attached to the brain.
• Cranial nerve I is olfactory.
  • Location: Telencephalon
  • Function: Smell (Sensory)
• Cranial nerve II is optic.
  • Location: Diencephalon
  • Function: Vision (Sensory)
• Cranial nerve III is oculomotor.
  • Location: Midbrain
  • Function: Eye movement, pupil constriction (Motor, Parasympathetic)
• Cranial nerve IV is trochlear.
  • Location: Midbrain
  • Function: Eye movement (Motor)
• Cranial nerve V is trigeminal.
  • Location: Pons
  • Function: Facial sensation, chewing (Mixed: Sensory & Motor)
• Cranial nerve VI is abducens.
  • Location: Pons
  • Function: Eye movement (Motor)
• Cranial nerve VII is facial.
  • Location: Pons
  • Function: Facial expression, taste, salivation (Mixed: Motor & Sensory, Parasympathetic)
• Cranial nerve VIII is vestibulocochlear.
  • Location: Pons Function: Hearing & balance (Sensory)
• Cranial nerve IX is glossopharyngeal.
  • Location: Medulla
  • Function: Taste, swallowing, salivation (Mixed: Motor & Sensory, Parasympathetic)
• Cranial nerve X is vagus.
  • Location: Medulla
• Cranial nerve XI is accessory.
  • Location: Medulla
  • Function: Head & shoulder movement (Motor)
• Cranial nerve XII is hypoglossal.
  • Location: Medulla
  • Function: Tongue movement (Motor)

Classification of Cranial Nerves

• Sensory nerves carry impulses towards brain called afferent fibers.
• Olfactory (I), Optic (II), and Vestibulocochlear (VIII) are sensory.
• Motor nerves carry impulses away from brain called efferent fibers.
• Oculomotor, Trochlear, Abducens, spinal accessory, and hypoglossal are motor nerves.
• Trigeminal, Facial, Glossopharyngeal and Vagus are mixed.
• Trigeminal is the largest cranial nerve.
• Vagus is the longest cranial nerve.
• Olfactory is the shortest cranial nerve.
• Trochlear is the slender/thin cranial nerve.
• Optic is the Thickest in myelin sheath.

Autonomic Nervous System

• Autonomic nervous system includes the sympathetic and parasympathetic divisions.
• Sympathetic division is also known as the thoracolumbar or adrenergic system.
• Sympathetic division stimulates activities mobilized during emergency stress and involves to the fight, fright, and flight responses.
• It includes increased HR and force of contraction and increased blood pressure.
• Epinephrine is the neurotransmitter for the sympathetic division.
• The sympathetic division is from thoracic and first three lumbar spinal nerves.
• Parasympathetic division also known as the craniosacral or cholinergic system.
• Parasympathetic division stimulates activities that conserve energy and restore body resources.
• It involves heart rate reduction and increased digestion and absorption of food.
• Acetylcholine is the neurotransmitter for the parasympathetic division.
• The parasympathetic division is from cranial nerves III, VII, IX, X and sacral segments (S1-S4).

Anatomical Differences

• The sympathetic division originates from T1-T12 and L1-L3(L4), the thoracolumbar division.
• The parasympathetic division originates from CN III, VII, IX, X and S2-S4, the craniosacral division.
• Sympathetic ganglia are located near the spinal cord.
• Parasympathetic ganglia are located near the effector organ.
• Sympathetic preganglionic fibers are shorter and postganglionic fibers are longer.
• Parasympathetic preganglionic fibers are longer and postganglionic fibers are shorter.
• The sympathetic and parasympathetic nervous systems have opposite responses on body tissues and organs.
• Sympathetic response dilates pupils.
  • Parasympathetic response constricts pupils.
• Sympathetic response dilates bronchioles.
  • Parasympathetic response constricts bronchioles and increases secretions.
• Sympathetic response increases the heart rate.
  • Parasympathetic response decreases the heart rate.
• Sympathetic response constricts blood vessels.
  • Parasympathetic response dilates blood vessels.
• Sympathetic response relaxes smooth muscles of the gastrointestinal tract.
  • Parasympathetic response increases peristalsis.
• Sympathetic response relaxes the bladder muscle.
  • Parasympathetic response constricts the bladder.
• Sympathetic response relaxes the uterine muscle.
  • Parasympathetic response increases salivation.