Spinal Nerve and Rami Organization

Spinal Nerve Organization

• Motor axons originate from ventral rootlets.
• Ventral rootlets contain motor axons exiting from the spinal cord.
• Ventral root forms from the ventral rootlets.
• Cell bodies of the motor neurons are found in the ventral and lateral horns of the spinal cord.
• Sensory axons enter the spinal cord via the dorsal aspect.
• Dorsal rootlets arise from the dorsal root.
• Dorsal root contains short central axons of sensory neurons.
• Dorsal root ganglion houses the cell bodies of sensory neurons.

Rami Organization

• Spinal nerves form from the convergence of the dorsal and ventral roots.
• Spinal nerves exit the vertebral canal through the intervertebral foramen.
• Rami are distal to the spinal nerve and are formed by branches of the spinal nerves.
• Dorsal rami innervate the deep muscles of the back and the skin of the back.
• Ventral rami split into multiple branches, innervating the ventral and lateral portions of the trunk, upper and lower limbs.
• Ventral rami form nerve plexuses.
• Rami communicantes are associated with the autonomic nervous system.
• Rami communicantes extend between the spinal nerve and sympathetic ganglions.

Topographical Regionalization

• Spinal nerves are organized topographically based on what they innervate.
• Upper limbs enter and exit the inferior cervical region at the cervical enlargement.
• Lower limbs enter and exit the inferior thoracic and superior lumbar region at the lumbar enlargement.

Dermatome and Myotome

• Dermatome: Area of skin supplied with sensory information by a pair of spinal nerves.
• Myotome: Muscles that a particular spinal nerve innervates.
• Motor activity associated with myotomes.

Nerve Plexuses

• Arise from the interweaving of the anterior rami of the spinal nerves on each side of the body.
• Combine several nerves that innervate the same part of the body into one.
• Allow axons of spinal nerves to travel through various pathways (branches) to reach their destination.
• Damage to a single spinal nerve does not completely affect innervation to the region.
• A nerve arising from a plexus usually has axons from more than one spinal nerve and spinal segment.
• Form the named nerves of the body.
• Four main plexuses exist.

Intercostal Nerves

• Thoracic nerves do not form plexuses except a portion of T1 forms part of the brachial plexus.
• Anterior rami of the thoracic nerves travel between adjacent ribs.
• All innervate intercostal muscles.
• T2 provides sensory information from the skin of the axilla and medial surface of the arm.
• T3 to T6 provide sensory information from the anterior and lateral chest wall.
• T7 to T12 innervate abdominal muscles and overlying skin.

Cervicobrachial Plexus Anatomy

• Cervical plexus (C1 to C4).
• Brachial plexus (C5 to T1).
• Phrenic nerve.

Cervical Plexus

• Located deep at the back of the neck, formed primarily by anterior rami of C1 to C4.
• Nerves innervate superficial neck structures, muscles attached to the hyoid bone, the surface of the neck, and lower portions of the head.
• Cutaneous branches (occipital, auricular, cervical nerves).
• Motor branches (ansa cervicalis, muscles of throat, phrenic nerve).

Brachial Plexus

• Located superior to the clavicle.
• More complex than the cervical plexus.
• Five levels of organization:
  • Rami
  • Trunks (superior, middle, inferior)
  • Divisions (anterior, posterior)
  • Cords (lateral, posterior, medial)
  • Terminal branches innervating the upper limb.
• Five main terminal branches arising from the cords supply the nerves of the upper limb.

Five Major Terminal Branches of the Brachial Plexus

• Axillary nerve: arises from the posterior cord, innervates the deltoid, and receives sensory from the superolateral portion of the arm.
• Musculocutaneous nerve: arises from the lateral cord, innervates anterior arm muscles, and receives sensory from the lateral forearm.
• Radial nerve: arises from the posterior cord, innervates posterior arm and forearm muscles, and receives sensory from the posterior arm and lateral three digits.
• Median nerve: arises from medial and lateral cords, innervates most of the anterior forearm and lateral hand, and receives sensory from lateral palmar and dorsal tips of lateral three fingers.
• Ulnar nerve: arises from the medial cord, innervates anteromedial forearm and intrinsic hand muscles and receives sensory from medial one-and-a-half digits (anterior and posteriorly).

Lumbosacral Plexus Anatomy

• Lumbar plexus
• Sacral plexus.
• Nerves innervate the anterior and posterior aspects of the lower limbs, abdomen, pelvis and buttock.

Lumbar Plexus

• Located in the anterior pelvis.
• Formed by anterior rami of spinal nerves L1 to L4.
• Overlaps with sacral plexus.
• Contains nerves innervating the anterior aspect of lower limbs and abdomen, pelvis and buttock.
  • Femoral nerve innervates anterior thigh muscles for flexion and extension of the knee and hip and receives sensory from the anterior and inferomedial thigh and medial aspect of the leg.
  • Obturator nerve innervates medial thigh, and receives sensory from the superomedial thigh.

Sacral Plexus

• Located in the posterior pelvis (inferior to the lumbar pelvis).
• Overlaps with lumbar plexus.
• Formed by anterior rami of spinal nerves L4 to S4.
• Organized into anterior and posterior divisions.

More Reflexes

• Withdrawal (flexor) reflex: protective movement in response to noxious stimulus.
• Crossed-extensor reflex: occurs with the withdrawal reflex in the contralateral limb, ensuring balance.

Reflexes

• Reflex: rapid, pre-programmed, involuntary reaction of muscles or glands to a stimulus.
• Stimulus: sensory input initiates reflex.
• Rapid response, requires few neurons.
• Pre-programmed response.
• Involuntary response.
• Spinal cord integrates many basic reflexes.
• Kinds of reflexes:
  • Basic (unlearned, built-in responses).
  • Conditioned (acquired through practice and learning).

Reflex Arc

• Basic functional unit of the nervous system.
• Smallest, simplest circuit capable of receiving a stimulus and producing a response.
• Action potential never travels to the brain for processing.
• Five parts:
  • Receptor
  • Afferent pathway
  • Integration center
  • Efferent pathway
  • Effector.

Mono- v. Polysynaptic Reflexes

• Monosynaptic reflex has one synapse between receptor and effector (e.g. stretch reflex).
• Polysynaptic reflex has more than one synapse between receptor and effector (e.g. withdrawal reflex); often involves interneurons, can be modified by input from CNS.

Autonomic v. Somatic Reflexes

• Autonomic reflexes involve organs (e.g. digestion, heart rate).
• Somatic reflexes involve muscles (e.g. stretch reflex).

Stretch Reflex

• Monosynaptic (monitoring and regulating muscle length).
• Monitored by stretch receptors called muscle spindles (internal):
• Intrafusal muscle fibers, innervated by γ motor neurons.
• Sensory neurons wrapped around intrafusal fibers.
• Surround by connective tissue capsule.
• External: extrafusal muscle fibers, innervated by α motor neurons.

Golgi Tendon Reflex

• Polysynaptic (prevents excessive muscle contraction in response to increased tension).
• Sensory receptor: Golgi tendon organ, located in tendons.
• Sensory nerve endings found within a tendon.

Additional Reflexes

• Stretch, Golgi tendon, withdrawal, and crossed-extensor reflexes are among common reflexes.

This quiz covers the organization of spinal nerves and their rami, detailing the pathways of motor and sensory axons as they exit and enter the spinal cord. It also describes the roles of various structures such as the dorsal and ventral roots and the specific innervation provided by dorsal and ventral rami. Test your knowledge of spinal anatomy!