The spinal cord

From top to bottom the column has 7 cervical, 12 thoracic and 5 lumbar vertebrae, then the sacrum and coccyx. The lumbar region sits just above the sacrum.

• Thoracic:
• Correct: Lumbar:
• Cervical:

The sensory, or afferent, division carries signals in to the CNS. The motor, or efferent, division carries commands out; the parasympathetic division is part of the motor side.

• The motor (efferent) division:
• Correct: The sensory (afferent) division:
• The parasympathetic division:

In the vertebral canal the dura is not attached to the bone, so the epidural space is real and holds fat and veins. The subarachnoid space holds CSF; the subdural space is only potential.

• The subdural space:
• Correct: The epidural space:
• The subarachnoid space:

Pointer 7 marks the broad gray horn reaching toward the front: the ventral (anterior) horn, which holds the cell bodies of motor neurons to skeletal muscle.

• Dorsal horn: The dorsal horn is the narrow gray horn reaching toward the back, at pointer 3.
• Lateral horn: The lateral horn is the small gray bump at the side, at pointer 4.
• Anterior column: The anterior column is white matter at the front beside the midline, at pointer 8, not gray matter.
• Correct: Ventral horn: Correct. The broad front horn of gray matter is the ventral horn.

The lateral horn, the small gray bump at the side (pointer 4), holds sympathetic motor neurons. It is present only in the thoracic and upper lumbar segments, T1 to L2.

• Pointer 3, at every level: Pointer 3 is the dorsal horn, which is sensory and present at every level.
• Pointer 7, only in the enlargements: Pointer 7 is the ventral horn, which holds somatic motor neurons to skeletal muscle; it is largest in the enlargements but present at every level.
• Correct: Pointer 4, only from T1 to L2: Correct. The lateral horn holds sympathetic neurons, from T1 to L2.
• Pointer 11, only in the lumbar region: Pointer 11 is the dorsal root ganglion, which holds sensory cell bodies at every level.

At L4–L5 the vertebral canal holds no spinal cord, only the cauda equina: the lumbar and sacral roots on their way down. Crushing them causes leg weakness, numbness of the saddle area and loss of bladder control, a surgical emergency.

• The conus medullaris: The conus medullaris lies at about L1–L2, above the burst disc.
• The cervical enlargement: The cervical enlargement is in the neck and supplies the upper limbs.
• The lumbar enlargement of the cord: The lumbar enlargement is part of the cord and lies behind the lower thoracic vertebrae, well above L4–L5.
• Correct: The cauda equina: Correct. At L4–L5 the canal holds only the cauda equina.

Dorsal roots carry sensory axons into the cord; ventral roots carry motor axons out. With the middle root and both its neighbors cut, no sensory route from that band reaches the cord, while the motor axons in the intact ventral roots still drive the muscles. Cutting one dorsal root alone would only dull the feeling, because neighboring roots overlap.

• Paralysis of the muscles, with normal sensation: Paralysis with normal sensation would follow cutting the ventral roots, which carry the motor axons.
• Correct: Loss of sensation, with normal muscle strength: Correct. Cutting the dorsal roots removes sensation and spares strength.
• Loss of both sensation and strength: Losing both would need both roots cut, or the nerve beyond where they join.
• No change, because the ventral roots carry the sensory signals: The ventral roots carry motor axons out; sensory signals enter through the dorsal roots.

The dorsal root brings sensory axons into the back of the cord. Their cell bodies sit outside the cord in the dorsal root ganglion, and the neurons are pseudounipolar: one process runs to the body, the other into the cord.

• Correct: It carries sensory axons into the spinal cord: Yes. The dorsal root is the sensory way in.
• Correct: Its ganglion holds the cell bodies of sensory neurons: Yes. The dorsal root ganglion holds the sensory cell bodies.
• It carries motor axons out to skeletal muscles: No. Motor axons leave through the ventral root.
• Correct: The neurons whose cell bodies sit in its ganglion are pseudounipolar: Yes. Dorsal root ganglion neurons are pseudounipolar.
• Its cell bodies lie in the ventral horn: No. Ventral horn cell bodies send their axons out through the ventral root; dorsal root cell bodies sit in the ganglion.

White matter thins going down the cord: fewer ascending axons have joined yet, and most descending axons have already left for their target segments. The thinnest white matter is at the sacral end, and lateral horns are absent below L2.

• Upper cervical: The upper cervical cord has the most white matter, since it carries every ascending and descending axon for the whole body.
• Middle thoracic: Middle thoracic segments have lateral horns, which this section lacks.
• Upper lumbar: Upper lumbar segments (to L2) still have lateral horns and more white matter than the sacral cord.
• Correct: Sacral: Correct. A thin white rim with no lateral horns fits the sacral cord.

Motor neurons to skeletal muscle sit in the ventral (anterior) horn, not the dorsal horn. The dorsal horn is the sensory side, where many incoming sensory axons end.

• The command leaves the motor areas of the cerebral cortex in a descending tract: Right. Motor commands travel down in descending tracts.
• It runs down the white columns of the spinal cord: Right. Tracts run in the white columns.
• Correct: It reaches motor neurons in the dorsal horn of a lumbar segment: This is the error. The motor neurons are in the ventral horn.
• Their axons leave through the ventral root toward the muscle: Right. Motor axons leave through the ventral root.