Sensory receptors and ascending pathways

A graded potential is a local change whose size depends on the stimulus strength, and it fades as it spreads. An action potential is all-or-none and is regenerated along the axon without fading.

• It is all-or-none and travels without fading:
• Correct: Its size varies with the stimulus and it fades with distance:
• It occurs only in muscle cells:

The thalamus is the relay and filter for sensory input to the cortex. The cerebellum coordinates movement, and the hypothalamus controls autonomic and endocrine functions.

• Correct: Thalamus:
• Cerebellum:
• Hypothalamus:

The primary somatosensory cortex lies in the postcentral gyrus, just behind the central sulcus. The precentral gyrus holds the primary motor cortex, and the occipital lobe holds the primary visual cortex.

• In the precentral gyrus of the frontal lobe:
• In the occipital lobe:
• Correct: In the postcentral gyrus of the parietal lobe:

The first-order neuron runs from the fingertip up the same side of the cord to the medulla. The second-order neuron crosses there and rises in the medial lemniscus to the thalamus. The third-order neuron reaches the somatosensory cortex of the opposite hemisphere.

• Correct order: 1. A lamellated corpuscle in the fingertip depolarizes, firing action potentials 2. The first-order axon passes its cell body in a cervical dorsal root ganglion and enters the cord 3. The axon climbs, without synapsing, in the left fasciculus cuneatus 4. It synapses in the nucleus cuneatus of the medulla oblongata 5. The second-order axon crosses the midline and rises in the right medial lemniscus 6. A third-order neuron in the right thalamus projects to the right postcentral gyrus

The dorsal column pathway climbs on the same side and crosses only in the medulla, so the cut removes vibration and position from the left. Pain and temperature axons cross within a segment or two of entering, so the left spinothalamic tract carries signals from the right, and those are lost on the right.

• All sensation lost on the left side only: This would need both pathways to run on the left, but the spinothalamic axons in the left cord serve the right side of the body.
• All sensation lost on the right side only: This would need both pathways to have crossed below T10, but dorsal column axons do not cross until the medulla.
• Pain and temperature lost on the left; vibration and position lost on the right side: This reverses the two pathways. Pain and temperature cross early, so the left tract serves the right side.
• Correct: Vibration and position lost on the left; pain and temperature lost on the right: Correct. Dorsal column loss is on the same side; spinothalamic loss is on the opposite side.

The anesthetic leaves transduction intact but stops conduction along the axon, so nociception at the ending never becomes a perception.

• Receptor potentials in the nociceptor endings in the tooth: no change. The drilling still opens channels in the endings, which lie beyond the blocked stretch of nerve, so receptor potentials still form.
• Action potentials reaching the brainstem from the tooth: down. Action potentials need voltage-gated sodium channels to regenerate along the axon. Where the channels are blocked, conduction fails.
• Pain perceived from the tooth: down. Without signals reaching the brainstem, thalamus and cortex, no pain is perceived, even though the tissue is being stimulated.

Visceral pain axons from the heart enter the cord at about T1 to T4, the same segments that receive skin axons from the chest and inner arm. They converge on shared second-order neurons, and the brain assigns the pain to the skin that line usually reports.

• Nerves from the heart run down the arm: No nerves run from the heart into the arm; the arm pain is referred, not local.
• Blood backs up into the arm and stretches its vessels: A heart attack does not stretch the arm’s vessels, and his arm is healthy.
• Correct: Heart and arm pain axons converge on the same dorsal horn neurons: Correct. This is referred pain, explained by convergence in the dorsal horn.
• The phrenic nerve carries heart pain up to the C3 to C5 dermatomes of the shoulder: The phrenic nerve carries sensation from the diaphragm and causes shoulder-tip pain, not inner arm pain from the heart.

Second-order pain and temperature axons cross the midline in front of the central canal, within a segment or two of entry. A central cavity cuts these crossing axons from both sides at the levels it spans, while the dorsal columns, off to the back, are spared.

• The cavity has destroyed both dorsal columns along the back of the cord: If the dorsal columns were destroyed, fine touch and vibration would be lost, but they are normal.
• The cavity presses on the dorsal root ganglia: The dorsal root ganglia lie outside the cord in the intervertebral foramina; a cavity inside the cord cannot reach them.
• Nociceptors in the arms have died: Nociceptors in the skin are not affected by damage inside the spinal cord.
• Correct: Crossing spinothalamic axons pass just in front of the central canal: Correct. A central cord cavity cuts the crossing spinothalamic axons.

The first-order neuron synapses in the dorsal horn, and the second-order axon crosses in front of the central canal within a segment or two, then climbs on the opposite side.

• In the medulla oblongata, at the same level where the dorsal column axons cross: The dorsal column pathway crosses in the medulla; the spinothalamic tract has already crossed by then.
• In the thalamus: The thalamus is where the second-order neuron ends, not where it crosses.
• Correct: In the spinal cord, within a segment or two of where the signal entered: Correct. Spinothalamic axons cross in the spinal cord close to their level of entry.
• In the internal capsule: The internal capsule carries third-order axons from the thalamus to the cortex; no crossing happens there.

They sit in an internal organ, so they are interoceptors, and they respond to stretch, a physical deformation, so they are mechanoreceptors.

• Exteroceptors and chemoreceptors: Exteroceptors respond to stimuli from outside the body, and stretch is not a chemical stimulus.
• Proprioceptors and thermoreceptors: Proprioceptors are in muscles, tendons and joints, and stretch is not temperature.
• Interoceptors and nociceptors: They are interoceptors, but ordinary filling is not a damaging stimulus, so they are not nociceptors.
• Correct: Interoceptors and mechanoreceptors: Correct. Internal location, stretch stimulus: interoceptor and mechanoreceptor.

By the time sensory pathways reach the thalamus, every one of them, from the body and from the face, has crossed. The right thalamus therefore relays all sensation from the left side of the body and face.

• Right side of the body only, all sensations: The right thalamus serves the left side, because all pathways have crossed below it.
• Left face only: The face pathway and the body pathways both pass through the thalamus, so the body would be affected too.
• Fine touch on the right, pain on the left: A split pattern like this arises in the spinal cord, where the two pathways cross at different levels, not in the thalamus.
• Correct: Left side of the body and face, all sensations: Correct. Above the medulla all pathways have crossed, so the loss is on the whole opposite side.