Study topic · about 8 minutes

Touch, pain and temperature: the sensory pathways

Signals from your skin, muscles and joints reach the brain through a relay of three neurons. Fine touch, vibration and position sense travel up the dorsal columns of the spinal cord and cross sides in the medulla. Pain and temperature cross soon after entering the spinal cord and travel up the spinothalamic tract. Both pathways stop in the thalamus and end in the primary somatosensory cortex, which holds a map of the body.

See the sensory strip in 3D

Sensors in the skin, muscles and joints

Your body feels the world through several kinds of sensors, or receptors, each tuned to one kind of stimulus. Each receptor is built around the ending of a sensory neuron. The cell body of that neuron sits in a dorsal root ganglion, a small swelling on the nerve root just outside the spinal cord.

Pain comes in two waves. Stub your toe and you first feel a sharp, precise pain, carried by thin fibers with a little myelin (A-delta fibers). A dull, aching pain follows, carried by even thinner fibers with no myelin (C fibers).

A relay of three neurons

Every body sensation reaches the cortex through a chain of three neurons, like runners in a relay race:

1. First-order neuron: carries the signal from the receptor into the spinal cord or brainstem. Its cell body sits in a dorsal root ganglion, or for the face in the trigeminal ganglion.
2. Second-order neuron: starts in the spinal cord or the brainstem. Its axon crosses the midline and climbs to the thalamus.
3. Third-order neuron: starts in the ventral posterior nucleus of the thalamus of the thalamus and carries the signal to the primary somatosensory cortex.

Because the second neuron always crosses, each side of the brain feels the opposite side of the body. The thalamus is more than a relay: it sorts the signals by body part and can turn them up or down before they reach the cortex.

The face follows the same plan. Its first-order neurons run in the trigeminal nerve (cranial nerve V) and end in nuclei in the brainstem. The second-order neurons cross and climb to the inner part of the ventral posterior nucleus, called VPM. Signals from the body arrive in its outer part, VPL. From there, both go on to the face and body areas of the sensory strip.

Find the ventral posterior nucleus in 3D

Two pathways up the spinal cord

Inside the spinal cord, the sensations split into two pathways: the dorsal column-medial lemniscus pathway and the spinothalamic pathway, also called the anterolateral system. Where each one crosses is the key fact to learn.

In the dorsal columns, fibers from the legs and lower body run in the fasciculus gracilis, next to the midline. Fibers from the arms and upper body run beside them in the fasciculus cuneatus. After their first synapse in the medulla, the second-order fibers cross and climb as a ribbon called the medial lemniscus.

Pain and temperature fibers take a shortcut. They synapse in the dorsal horn, and the second-order fibers cross within a segment or two, in front of the spinal cord's central canal. They then climb the opposite side of the cord as the spinothalamic tract.

Why a half-cut spinal cord splits touch from pain

The two crossing points explain a famous puzzle. Suppose an injury, such as a stab wound, cuts through the left half of the spinal cord in the middle of the back. Doctors call this Brown-Séquard syndrome. Below the cut, the person loses different things on each side:

Why? The corticospinal tract, which carries movement commands, already crossed in the medulla. So its fibers in the left half of the cord move the left side of the body. The dorsal columns have not crossed yet, so they also carry the left side. But the spinothalamic fibers crossed soon after entering the cord, so the ones in the left half carry pain and temperature from the right side. Pure half cuts are rare; doctors more often see part of this pattern.

A fluid-filled cavity in the middle of the cord, called syringomyelia, first damages the pain fibers where they cross. In the neck, it can take away pain and temperature across both shoulders and arms, like a cape, while touch stays normal.

The sensory cortex and its body map

The primary somatosensory cortex (S1) fills the postcentral gyrus, just behind the central sulcus and the motor strip. Like the motor strip, it holds a map of the opposite side of the body, the sensory homunculus. The legs and feet sit at the top, curling onto the inner surface in the paracentral lobule. Then come the trunk, arm, hand and face, with the tongue and throat lowest.

Space on the map depends on how many receptors a body part has, not on its size. The lips, hands and fingers get huge areas, while the back gets a thin strip. On a fingertip, two pencil points a few millimeters apart feel like two. On your back, they must be several centimeters apart.

Areas just behind it, in the superior parietal lobule, combine touch with position sense. They let you recognize a key in your pocket by feel and know where your arm is without looking. The map also changes with use: in violinists, the area for the fingers of the left hand is larger than usual, an example of neuroplasticity.

See the sensory homunculus in 3D

Pain: a feeling the brain can turn up or down

Why pain feels bad

Pain has two sides. The route through the ventral posterior nucleus to the sensory strip tells you where it hurts and how strong the pain is. Other branches reach the insula and the anterior cingulate cortex, which add how unpleasant it feels and the urge to make it stop. In a classic brain-scan study, hypnosis made the same pain feel more or less unpleasant. Activity changed in the anterior cingulate, while the sensory strip responded the same.

How the brain turns pain down

The brain has its own pain control. Its hub is the periaqueductal gray, a ring of gray matter around the fluid channel of the midbrain. It works through the raphe nuclei and the locus coeruleus, whose fibers run down into the spinal cord. There, serotonin and noradrenaline quiet the cells that pass pain signals up, partly by switching on small neurons that release enkephalins.

Enkephalins and endorphins are the body's own opioids, and opioid drugs such as morphine act on the same receptors (see neurotransmitters). This system helps explain why injured soldiers and athletes sometimes feel little pain until the danger is over.

The spinal cord has a gate as well. In 1965, Ronald Melzack and Patrick Wall proposed the gate control theory: signals from touch fibers switch on inhibitory neurons in the dorsal horn that "close the gate" on pain signals. That is why rubbing a bumped elbow helps. The details have been revised since, but the core idea still holds: pain is adjusted in the spinal cord before it reaches the brain.

Pain in the wrong place

In referred pain, pain from an organ is felt on the skin. Nerves from the organ and from a patch of skin share neurons in the same spinal cord segments, so the brain blames the skin. A heart attack is often felt in the left chest, shoulder and arm, and early appendicitis around the belly button.

After an amputation, most people still feel the missing limb, and many feel pain in it: phantom limb pain. The limb is gone, but its area in the body map is still there, and neighboring areas can take it over. In some people, touching the face is felt in the missing hand. How this rewiring relates to the pain is still debated.

In these study guides

• MCATSensing the environment
• AP Psychology1.6 Sensation
• Intro psychologySensation (sections 5.3 to 5.5)
• Anatomy and physiologyThe somatic nervous system (chapter 14)
• NursingBrain regions in a neuro assessment
• Medical schoolSensory systems and the thalamus

Take it further in 3D

• Moving and feelingThe motor and sensory strips. A short guided lesson in the 3D app.
• The thalamusThe brain's relay station. A short guided lesson in the 3D app.

Sources

1. Anatomy and Physiology 2e, 14.2 Central Processing, OpenStax
2. Introduction to Behavioral Neuroscience, 9.2 Somatosensation in the Central Nervous System, OpenStax
3. Introduction to Behavioral Neuroscience, 9.4 Pain Relief, OpenStax

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