Neuroanatomy: Spinal Nerves Overview

Spinal Nerves

• Peripheral Nervous System (PNS) connects the CNS to the body, conducting impulses.
• Comprises 31 pairs of nerves:
  • 8 cervical nerves
  • 12 thoracic nerves
  • 5 lumbar nerves
  • 5 sacral nerves

Voluntary & Involuntary Actions

• Voluntary (somatic): Actions like picking up objects.
• Involuntary (autonomic): Includes bodily functions such as heart rate and digestion.
• Sympathetic Nervous System: Responsible for 'fight or flight' response, increasing blood flow to muscles, heart rate, and blood pressure.
• Parasympathetic Nervous System: Promotes 'rest and digest' functions, directing blood flow to digestion and maintaining lower heart rates.

Plexuses

• Cervical plexus: Nerve connections to head, neck, and shoulder.
• Brachial plexus: Supplies skin and musculature of upper limbs.
• Lumbar plexus: Supplies the back, abdomen, and lower limbs.
• Sacral plexus: Innervates pelvis, buttocks, and lower legs.

Cervical Nerves

• 7 cervical vertebrae but 8 cervical nerves (C1-C8); C8 emerges below C7.
• Cervical plexus (C1-C4) consists of:
  • Motor branches: Innervate neck and diaphragm muscles.
    • Phrenic nerve (C3-C5): Crucial for diaphragm innervation.
  • Sensory branches: Provide sensation to neck, upper thorax, scalp, and ear.

Brachial Plexus

• Comprises roots (C5-T1), trunks, divisions, cords, and branches.
• Superior trunk: C5-C6; Middle trunk: C7; Inferior trunk: C8-T1.
• Major branches include:
  • Musculocutaneous Nerve: Innervates biceps and brachialis.
  • Axillary Nerve: Innervates deltoid and teres minor.
  • Median Nerve: Innervates flexor muscles and sensation to lateral palm.
  • Radial Nerve: Innervates triceps and sensory to posterior arm.
  • Ulnar Nerve: Innervates hand muscles and provides sensation to medial fingers.

Injuries

• Upper brachial plexus injury (Erb's Palsy): Affects C5-C6, impairing shoulder and arm movements.
• Klumpke Palsy: Involves T1 root, causing hand muscle paralysis and sensory loss on medial arm.

Thoracic Nerves

• 12 spinal nerves (T1-T12) control upper chest, mid-back, and abdomen.
• Anterior division: Intercostal nerves for rib control.
• Posterior division: Medial branches innervate upper back skin.

Lumbar Nerves

• 5 spinal nerves (L1-L5) form the lumbar plexus.
• Contains nerves like:
  • Iliohypogastric and Ilioinguinal: Innervate abdominal muscles and skin in the pelvic regions.
  • Genitofemoral: Innervates the cremasteric muscle and skin of the thigh.
  • Femoral: Supplies quadriceps and provides sensation to the anterior thigh.

Sacral Nerves

• 5 pairs of nerves, including the sciatic nerve, which is crucial for lower limb movement and sensation.
• Major branches include:
  • Superior Gluteal Nerve: Innervates gluteus minimus and medius.
  • Sciatic Nerve: Tibial portion innervates posterior thigh muscles, while the common fibular portion innervates lateral and anterior leg muscles.

Cervical Nerves

• Greater Auricular nerve (C2, C3): Provides sensation to the external ear and parotid gland.
• Transverse Cervical nerve (C2, C3): Responsible for sensation to the anterior neck.
• Lesser Occipital nerve (C2): Supplies sensation to the posterosuperior scalp.
• Supraclavicular nerves (C3, C4): Convey sensation over the supraclavicular fossa and the upper thoracic region, including the sternoclavicular joint.

Brachial Plexus

• Consists of roots (C5-T1), trunks, divisions, cords, and branches, supplying the skin and musculature of the upper limb.
• Roots converge to form three trunks: Superior (C5-C6), Middle (C7), and Inferior (C8-T1).
• Trunks divide into anterior and posterior divisions within the posterior triangle of the neck, passing through the axilla.

Brachial Plexus Cords

• Three cords named for their position relative to the axillary artery:
  • Lateral cord: Formed by anterior divisions of the superior and middle trunks.
  • Posterior cord: Formed by posterior divisions of all three trunks.
  • Medial cord: Formed by the anterior division of the inferior trunk.

Major Branches of Brachial Plexus

• Musculocutaneous Nerve (C5, C6, C7): Innervates biceps brachii and brachialis; sensory to lateral anterior forearm and a portion of the posterior forearm.
• Axillary Nerve (C5, C6): Innervates deltoid and teres minor; sensory to the inferior region of the deltoid ("regimental badge").
• Median Nerve (C6, C7, C8, T1): Supplies flexor muscles of the forearm and sensation to the lateral palm and lateral 3.5 fingers.
• Radial Nerve (C5, C6, C7, C8, T1): Innervates triceps and extensors of the wrist and fingers; sensory to the posterior arm and forearm.
• Ulnar Nerve (C8, T1): Innervates hand muscles; sensory to anterior and posterior surfaces of medial 1.5 fingers.

Brachial Plexus Injuries

• Erb's Palsy (Upper Brachial Plexus Injury): Affects C5-C6; leads to shoulder and arm movement impairments. Typical presentation includes "waiter's tip" position.
• Klumpke's Palsy (Lower Brachial Plexus Injury): Affects T1; causes hand muscle paralysis and sensory loss in the medial arm.

Thoracic Nerves

• Comprised of 12 spinal nerves (T1-T12) controlling the upper chest, mid-back, and abdomen.
• Anterior division forms intercostal nerves for rib control, and the posterior division innervates the upper back skin.

Functions of Thoracic Nerves

• T1-T5: Affect upper chest, mid-back, and abdominal muscles; control rib cage, lungs, diaphragm.
• T6-T12: Affect abdominal and back muscles; assist with balance, posture, coughing, and expulsion from airways.

Lumbar Nerves

• Formed by five spinal nerves (L1-L5) with branches innervating abdominal muscles and skin in pelvic regions.
• Lumbar Plexus: Includes important nerves, such as Iliohypogastric (L1), Genitofemoral, and Femoral nerves which innervate specific muscles.

Sciatic Nerve

• Comprised of Tibial (L4, L5, S1, S2, S3) and Common Fibular portions; innervates the posterior thigh muscles and provides sensory innervation to the foot.
• Sciatica refers to pain associated with dysfunction of the sciatic nerve.

Posterior Femoral Cutaneous and Pudendal Nerves

• Posterior Femoral Cutaneous (S1, S2, S3): Provides sensory functions to the posterior thigh and leg and perineal skin.
• Pudendal Nerve (S2, S3, S4): Innervates perineal skeletal muscles, external urethral and anal sphincters, and provides sensation to genitalia and perineum.