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Dmitry Usoskin led a study using single-cell RNA sequencing to classify mouse sensory neurons. Analysis of 622 individual neurons distinguished eleven principal groups with different molecular and functional properties. Measuring each cell separately preserved differences that a pooled RNA sample would average together.^[ref]

The groups included neurons associated with mechanical sensation, body-position information, temperature, itch and damaging stimuli. Molecular clusters provide candidates for distinct cell types; their interpretation becomes stronger when combined with localization and functional experiments. The paper appeared online in 2014 and in the 2015 journal volume.

A dorsal root is the bundle of sensory axons entering the posterior side of the spinal cord at a spinal level. The associated dorsal root ganglion contains the soma of many of these sensory neurons. Their peripheral processes receive information from the body, while their central processes enter through the dorsal root.^[ref]

The dorsal root joins the ventral root to form a mixed spinal nerve. This organization separates sensory entry from motor exit close to the spinal cord while allowing both directions to share a peripheral route. The dorsal root ganglion houses cell bodies; it is not normally a chain of sensory-to-sensory synapses.

A dorsal root ganglion is a collection of sensory neuron cell bodies on a dorsal root associated with a spinal nerve. These neurons carry information from peripheral tissues toward the spinal cord, including signals relevant to touch, temperature, pain and proprioception.

The sensory axon typically branches into a peripheral process and a central process; the ganglion is not simply a relay in which every incoming signal must cross a synapse onto another neuron. Its neurons also comprise multiple functional populations. Single-cell RNA sequencing of mouse dorsal root ganglion neurons separated molecularly distinct sensory types without assigning them beforehand solely from a few selected markers.^[ref] Anatomical location, molecular profile and stimulus response provide complementary descriptions of a sensory neuron.

An intrafusal muscle fibre is a specialized skeletal muscle fibre within a muscle spindle. Its mechanical state is monitored by endings of sensory neurons, contributing to proprioception, and motor input adjusts that state. The ordinary force-producing extrafusal muscle fibres lie outside the muscle spindle. Intrafusal contraction therefore helps regulate a sensory apparatus as well as changing local tension.

In a cat soleus preparation, researchers repetitively stimulated a single motor fibre supplying a muscle spindle, then imposed a ramp stretch while recording a primary sensory ending. Prior motor stimulation produced an extra burst of impulses at the start of the stretch. A subsequent stretch abolished this after-effect, whereas without that stretch it could persist for more than a minute.^[ref]

Two stretches with the same imposed movement can thus produce different sensory signals because the intrafusal muscle fibres have different recent activation histories. The authors proposed persistent interactions between actin and myosin as a mechanism; that proposal should be distinguished from the directly recorded impulses.^[ref]

A muscle spindle is a specialized stretch-sensitive receptor within a skeletal muscle. It contains small intrafusal muscle fibres with sensory endings that report changes in their mechanical state. This supplies proprioception: information about the body's own position and movement, even without seeing the limb.

The receptor is adjustable. Motor input to the intrafusal muscle fibres changes tension within the spindle and can alter the response of its sensory neurons. In a cat soleus experiment, researchers electrically stimulated an individual motor fibre supplying a muscle spindle, then applied a controlled ramp stretch while recording the sensory output. The prior stimulation changed the subsequent burst of impulses.^[ref]

The same imposed movement can therefore produce different sensory signals depending on the spindle's previous activation. A muscle spindle is an actively regulated sensor, rather than a passive ruler reading muscle length independently of neural state.

Oxytocin is a nine-amino acid peptide hormone synthesized by neurons in the hypothalamus and released into the circulation from the posterior pituitary. It promotes contraction of uterine smooth muscle during childbirth and contraction of cells around mammary secretory units, ejecting milk. Milk ejection is distinct from producing the milk.^[ref]

Childbirth illustrates positive feedback: stretching of the cervix stimulates a neural pathway that increases oxytocin release; stronger uterine contractions can increase the stretch until delivery removes the initiating stimulus. During suckling, sensory input similarly couples an external event to hormone release and milk ejection. These circuits connect sensory neurons, endocrine signaling and mechanical action.^[ref]

A sensory neuron carries information about a stimulus toward the central nervous system. Its input can concern the external world or the body's own state. For example, stretching a muscle spindle changes the activity of a sensory neuron; the resulting action potentials help the spinal cord regulate muscle activity through a reflex.^[ref]

Many sensory neurons supplying the body have their soma in a dorsal root ganglion and a branching axon with peripheral and central processes. Thus the soma is not simply an obligatory relay between an input dendrite and an output axon. Other sensory pathways use different arrangements; a sensory function does not specify one universal neuron shape.^[ref]