Reflexes

Sensory axons enter through the dorsal root, and their cell bodies sit in the dorsal root ganglion. Motor axons leave through the ventral root.

• The ventral root:
• Correct: The dorsal root:
• The lateral horn:

Proprioceptors sit in muscles, tendons and joint capsules and report the body's own position and movement. Nociceptors report tissue damage, and chemoreceptors report blood chemistry.

• Correct: Muscle length, muscle tension and joint angle:
• Tissue damage in the skin:
• The chemistry of the blood:

The autonomic nervous system drives smooth muscle, cardiac muscle and glands. Skeletal muscle is driven by the somatic nervous system.

• Skeletal muscles only:
• Sensory receptors in the skin:
• Correct: Smooth muscle, cardiac muscle and glands:

The sensory receptor detects the stimulus, the sensory neuron carries the signal into the CNS, the integration center passes it on through one or more synapses, the motor neuron carries it out, and the effector responds.

• Correct order: 1. Sensory receptor 2. Sensory neuron 3. Integration center 4. Motor neuron 5. Effector

Normal voluntary strength means the motor neurons, ventral roots and muscle all work. Lost feeling plus a lost reflex points to the sensory side of the arc: the dorsal root that carries both skin sensation and spindle signals from that region.

• The ventral root of the lumbar segments: If the ventral root were cut, the quadriceps would be weak on voluntary effort too, and sensation would be normal.
• The quadriceps femoris itself: A diseased quadriceps would be weak when he kicks on purpose, and it would not remove feeling from the skin.
• Correct: The dorsal root of the lumbar segments: Correct. The sensory side has failed while the motor side still works.
• The motor neurons in the ventral horn: Damage to the ventral horn motor neurons would weaken voluntary kicking as well and would leave sensation intact.

The tap stretches the calf muscles through their tendon, and the muscle spindles in those muscles detect the stretch. Despite its name, every deep tendon reflex is a stretch reflex.

• Golgi tendon organs in the calcaneal tendon: Golgi tendon organs sense tension from contraction, not a brief stretch, and they start a reflex that relaxes the muscle rather than contracting it.
• Nociceptors in the skin over the tendon: Nociceptors respond to harmful stimuli. A reflex hammer tap is not painful, and nociceptors start the withdrawal reflex, which bends a limb.
• Correct: Muscle spindles in the calf muscles: Correct. Spindles sense the stretch and excite the calf motor neurons directly.
• Tactile corpuscles in the skin of the heel: Touch sensors in the skin report contact, but they are not the sensory receptor for this reflex. It still works when the skin is numb.

The withdrawal reflex is integrated in the spinal cord, so the flexors contract without waiting for the brain. The same nociceptor signals climb the spinothalamic tract, and pain is felt only when they reach the cortex.

• The cortex decided to move the hand, and the feeling of pain came after the decision: No decision is needed. A voluntary movement would take longer than the reflex, and the pain would have to arrive first.
• Correct: Spinal interneurons drove the flexors while the pain signal was still climbing to the cortex: Correct. The reflex and the perception run in parallel, and the reflex finishes first.
• The heat stimulated the muscles of her hand directly, with no nerve signal: Heat does not make skeletal muscle contract. The movement needs nociceptors, sensory neurons, interneurons and motor neurons.
• The cerebellum detected the heat and started the movement: The cerebellum coordinates movement but does not detect heat or start reflexes. The withdrawal reflex runs in the cord.

A visceral reflex sends its output through a chain of two motor neurons, which synapse in a ganglion outside the CNS, to smooth muscle, cardiac muscle or glands. A somatic reflex uses one motor neuron straight to skeletal muscle.

• Correct: It uses two neurons in series, synapsing in a ganglion outside the CNS: Correct. Two neurons and a ganglion on the way.
• It uses one motor neuron that runs all the way from the CNS to a skeletal muscle: That describes the somatic reflex.
• It has no sensory neuron, so the effector responds directly: Both kinds have a sensory receptor and sensory neuron. They differ on the motor side.
• Its motor axons are all heavily myelinated, so it is faster: The second axon of a visceral chain is unmyelinated, so visceral reflexes are slower than somatic ones.

Hyperreflexia with clonus, a Babinski sign and a stiff, weak leg are upper motor neuron signs. The reflex arcs are intact but have lost the descending control that normally damps them.

• In the dorsal roots of the right lumbar segments: A broken sensory side of the arc would weaken or abolish the reflexes, not exaggerate them.
• In the right sciatic nerve: A damaged peripheral nerve breaks the arc and makes the reflexes weaker, with a floppy leg.
• In the ventral horn of the right lumbar cord: Losing ventral horn motor neurons removes the output of the arc, so reflexes fall and the muscle wastes.
• Correct: In the upper motor neuron pathway to the right leg: Correct. The arcs are released from descending control.

Golgi tendon organs sense tension. Their sensory neurons excite inhibitory interneurons that inhibit the motor neurons of the same muscle: the tendon reflex. The biceps' force drops, and the triceps is excited.

• They are excited directly, so the biceps pulls harder: Direct excitation of the same muscle is what muscle spindles do in the stretch reflex. Tendon organs have the opposite effect.
• Correct: They are inhibited through interneurons, so the biceps' force drops: Correct. The tendon reflex damps the tension in the muscle.
• They are unaffected by tendon organ signals: Tendon organ signals do reach the brain, but they also drive the spinal tendon reflex.
• They are excited through interneurons in the opposite arm: Crossing to the opposite limb is a feature of the crossed-extensor reflex, not of the tendon reflex.

Branches of the quadriceps spindle axons excite inhibitory interneurons, which release glycine onto the hamstring motor neurons and stop them firing. This is reciprocal inhibition.

• Correct: Spindle axons excite inhibitory interneurons that silence the hamstring motor neurons: Correct. Reciprocal inhibition through an inhibitory interneuron.
• The hamstrings are stretched too, so their own spindles relax them: Straightening the knee does stretch the hamstrings, but stretching a muscle excites its own motor neurons. It does not relax it.
• Golgi tendon organs in the quadriceps fire first and switch off the hamstring motor neurons: Tendon organs respond to tension, which rises only after the quadriceps contracts, and their main effect is on the quadriceps itself.
• The cortex sends a signal to relax the hamstrings before the tap: The reflex needs no signal from the brain, and it works when the tap comes as a surprise.