Spinal nerves and plexuses

The dorsal root carries sensory axons in, and its ganglion holds their cell bodies. The ventral root carries motor axons out from the ventral horn.

• The ventral root:
• Correct: The dorsal root:
• Both roots equally:

Dense irregular connective tissue is packed with collagen fibers running in several directions, so it resists pulling from any side. It wraps organs such as nerves and forms the dermis.

• Correct: Dense irregular connective tissue:
• Areolar tissue:
• Adipose tissue:

A nerve is a bundle of many axons with connective tissue around them. A cluster of cell bodies outside the CNS is a ganglion.

• A single long neuron:
• A cluster of neuron cell bodies:
• Correct: A bundle of axons wrapped in connective tissue:

The plexus formed by the ventral rami of C5 to T1, at the level of the shoulder, is the brachial plexus. Its nerves supply the upper limb.

• Cervical plexus: The cervical plexus is higher, formed by C1 to C4. It supplies the neck and the diaphragm, not the arm.
• Correct: Brachial plexus: Correct. C5 to T1 form the brachial plexus.
• Lumbar plexus: The lumbar plexus comes from L1 to L4 in the lower back and supplies the front of the thigh.
• Sacral plexus: The sacral plexus sits in the pelvis, from L4 to S4, and supplies the back of the thigh, the leg and the foot.

The femoral nerve (L2 to L4) leaves the lumbar plexus, passes into the front of the thigh and supplies the quadriceps femoris.

• Correct: Femoral nerve: Correct. This is the femoral nerve.
• Common fibular nerve: The common fibular nerve is a branch of the sciatic nerve behind the knee, far lower than this.
• Sciatic nerve: The sciatic nerve runs down the back of the thigh from the sacral plexus, not across the front.
• Obturator nerve: The obturator nerve also comes from L2 to L4, but on this figure it is the branch just below the pinned one, heading to the medial thigh.

Each bundle is a fascicle, and the perineurium wraps each fascicle. Its cells are joined by tight junctions and control what passes from the blood into the fascicle.

• Epineurium: The epineurium wraps the whole nerve and holds the fascicles together; it is the outer layer, not the sleeve around each bundle.
• Endoneurium: The endoneurium wraps single axons inside a fascicle, not whole bundles.
• Correct: Perineurium: Correct. The perineurium wraps each fascicle.
• Myelin sheath: Myelin wraps single axons and is made by Schwann cells; it is not a connective tissue layer around a bundle.

The radial nerve spirals around the shaft of the humerus and supplies the wrist and finger extensors. Damage there causes wrist drop.

• Median nerve: The median nerve supplies most of the flexors of the wrist and fingers and the thenar muscles. Damage weakens gripping and the thumb, not extension.
• Correct: Radial nerve: Correct. Wrist drop after a mid-shaft humerus fracture points to the radial nerve.
• Ulnar nerve: The ulnar nerve supplies most of the small muscles inside the hand. Damage weakens finger spreading and can curl the ring and little fingers, but wrist extension is spared.
• Musculocutaneous nerve: The musculocutaneous nerve supplies the elbow flexors at the front of the arm, not the wrist extensors.

The common fibular nerve wraps around the neck of the fibula just under the skin and supplies the dorsiflexors at the front of the leg. Damage causes foot drop.

• Femoral nerve: The femoral nerve supplies the quadriceps; damage makes the knee buckle, not the toes drag.
• Tibial nerve: The tibial nerve runs deep in the back of the leg to the plantar flexors, so it is not easily hit at the side of the knee, and its loss would weaken pushing off rather than lifting the foot.
• Obturator nerve: The obturator nerve supplies the adductors of the medial thigh, not the muscles that lift the foot.
• Correct: Common fibular nerve: Correct. A blow at the fibular neck injures the common fibular nerve, causing foot drop.

The navel lies in the T10 dermatome. The thoracic dermatomes run as horizontal bands, T4 at the nipple line and T10 at the navel.

• T4: T4 is the nipple line, marker 5, well above the navel.
• Correct: T10: Correct. The navel is the T10 landmark.
• L1: L1 supplies the groin, marker 7, below the navel.
• T12: T12 lies below the navel, just above the groin.

Each patch of skin is supplied mainly by one spinal nerve and partly by the ones above and below. With only T8 cut, T7 and T9 still carry some signals from the band, so sensation is reduced but not lost.

• Correct: Neighboring dermatomes overlap, so T7 and T9 still supply part of the T8 band: Correct. Overlap from neighboring dermatomes keeps partial sensation.
• The T8 ventral root carries enough sensory axons to take over: The ventral root carries motor axons out; it cannot bring sensory signals in to replace a cut dorsal root.
• The dorsal root ganglion was left intact, so its neurons still send signals into the cord: The ganglion holds the cell bodies, but their central axons run through the cut root to the cord, so the ganglion alone cannot deliver the signal.
• Sensory axons regrow through the cut root within hours: Regrowth, where it happens, takes weeks to months at about a millimeter a day, and cut dorsal roots regrow into the cord poorly.