Spinal Nerves, Plexuses, and Reflexes: Study Notes for Anatomy & Physiology

Spinal Nerves and Spinal Nerve Plexuses

Overview of Spinal Nerves

Spinal nerves are essential components of the peripheral nervous system, connecting the spinal cord to various body regions. Each spinal nerve is a mixed nerve, carrying both sensory and motor fibers.

• Mixed Nerves: All 31 pairs of spinal nerves contain both sensory and motor fibers.

• Naming Convention: The first 7 cervical spinal nerves (C1–C7) emerge above their corresponding vertebrae; the rest emerge below.

• Spinal Nerve C8: Emerges between the 7th cervical and 1st thoracic vertebra.

• Identification: On spinal cord models, nerves are labeled by letter and number (e.g., T6 for thoracic nerve 6).

Spinal Nerve Plexuses

Plexuses are networks of intersecting nerves that serve specific regions of the body. The main spinal nerve plexuses are:

• Cervical Plexus: Roots C1–C5

• Brachial Plexus: Roots C5–T1

• Lumbar Plexus: Roots T12–L4

• Sacral Plexus: Roots L4–S5

Students should be able to identify the location of these plexuses on anatomical models.

Functions of Key Spinal Nerves

Each plexus gives rise to specific nerves with distinct functions:

• From Cervical Plexus:

  • Lesser Occipital: Sensation from back of head and neck

  • Greater Auricular: Sensation around ear and sideburn area

  • Transverse Cutaneous: Sensation lateral and front of neck

  • Supraclavicular: Skin over the clavicle

  • Phrenic: Activates diaphragm (important for breathing)

• From Brachial Plexus:

  • Axillary: Shoulder muscles and skin

  • Musculocutaneous: Upper arm flexors (e.g., biceps); skin sensation on lateral forearm

  • Radial: Muscles on back of arm (triceps and wrist extensors); skin sensation on back of arm and hand

  • Median: Flexors of forearm and hand; sensation of anterior forearm and palm, thumb, index finger

  • Ulnar: Flexors of forearm and hand; sensation of anterior forearm and pinky and half of ring finger

• Thoracic Plexus: There is no thoracic plexus. Thoracic spinal nerves T2–T11 form dorsal ventral rami; those going ventral between the ribs are intercostal nerves.

• From Lumbar Plexus:

  • Serve anterior/lateral/medial aspect of the leg

  • Femoral Nerve: Two main branches:

    • Anterior Femoral Cutaneous: Activates quadriceps and skin over anteromedial femoral aspect

    • Saphenous Nerve: Sensation to lower knee, calf, ankle, and medial foot

  • Lateral Femoral Cutaneous: Skin sensation on lateral thigh

  • Obturator Nerve: Sensation of upper medial thigh; activates adductor muscles

• From Sacral Plexus:

  • Serve posterior aspect of the legs

  • Sciatic Nerve: Largest nerve; at popliteal region divides into:

    • Tibial Nerve: Runs in calf region

    • Common Fibular Nerve: Runs on lateral aspect of lower leg

  • Important for activating hamstrings and lower leg muscles; sensation along back of leg and foot

Dermatomes

Dermatomes are strips of skin served by specific spinal nerves. Damage to a spinal nerve will result in loss of sensation in its corresponding dermatome.

• Clinical Application: Mapping dermatomes helps diagnose nerve injuries.

Reflex Arc and Reflexes

Definition and Components of a Reflex Arc

A reflex is an automatic, rapid response to a stimulus, typically without conscious thought. Reflexes are distinct from voluntary reactions, which require conscious processing.

• Components of a Reflex Arc:

  1. Receptor: Detects the stimulus

  2. Sensory Neuron: Transmits the signal to the CNS

  3. Integration Center (Interneurons): Processes the information

  4. Motor Neuron: Carries the response signal

  5. Effector: Muscle or gland that carries out the response

Types of Reflexes

Reflexes can be classified based on their function and complexity:

• Somatic Reflexes: Involve skeletal muscles

• Visceral Reflexes: Involve smooth muscle, cardiac muscle, or glands

• Innate vs. Acquired Reflexes: Innate are present at birth; acquired are learned

• Cranial vs. Spinal Reflexes: Cranial reflexes are mediated by cranial nerves; spinal reflexes by spinal nerves

• Monosynaptic vs. Polysynaptic Reflexes: Monosynaptic involve one synapse; polysynaptic involve multiple synapses

Spinal Reflexes

Spinal reflexes are mediated by the spinal cord and are crucial for rapid responses to stimuli.

• Stretch Reflex: Maintains muscle tone and posture (e.g., knee-jerk reflex)

• Tendon Reflex: Prevents muscle damage from excessive tension

• Flexor Reflex: Withdrawal from painful stimuli

• Crossed Extensor Reflex: Balances the body during withdrawal

Visceral Reflexes

• Pupillary Reflex: Controls pupil diameter in response to light

• Consensual Reflex: Both pupils constrict when light is shone in one eye

Cranial Reflexes

• Gag Reflex: Prevents choking

• Corneal Touch Reflex: Protects the eye from foreign objects

Superficial Cord Reflexes

• Plantar Reflex: Normal response is toe flexion

• Babinski's Reflex: Extension of the big toe and fanning of other toes (normal in infants, abnormal in adults)

Table: Major Spinal Nerve Plexuses and Key Nerves

Plexus	Roots	Key Nerves	Main Functions
Cervical	C1–C5	Lesser Occipital, Greater Auricular, Phrenic	Sensation to head/neck, diaphragm activation
Brachial	C5–T1	Axillary, Musculocutaneous, Radial, Median, Ulnar	Motor/sensory to shoulder, arm, forearm, hand
Lumbar	T12–L4	Femoral, Lateral Femoral Cutaneous, Obturator	Motor/sensory to anterior/lateral/medial leg
Sacral	L4–S5	Sciatic (Tibial, Common Fibular)	Motor/sensory to posterior leg, foot

Example: Knee-Jerk Reflex (Patellar Reflex)

• Stimulus: Tap to patellar tendon

• Receptor: Muscle spindle in quadriceps

• Response: Quadriceps contract, leg extends

• Type: Monosynaptic, spinal, somatic reflex

Key Formula: Nerve Conduction Velocity

Nerve conduction velocity is a measure of how fast an electrical impulse travels along a nerve.

• Formula:

$\text{Velocity} = \frac{\text{Distance}}{\text{Time}}$

Additional info: Academic context and definitions have been expanded for clarity and completeness. Table entries and some nerve functions have been inferred and grouped for study purposes.