Vertebrate Peripheral Sensory Neurons

Vertebrate peripheral sensory neurons are generally described by their sensory ending anatomy. They may or may not be myelinated. Parameters sensed are heat (T above ambient body temperature), cold, pain, touch, pressure, strain (muscle tension), and muscle length. Chemoreceptors (for taste, odor, pH, pO2. etc.), photoreceptors (rods and cones), and hearing receptors (hair cells) are generally considered to be part of the large receptor arrays having direct connection with the vertebrate brain.

Peripheral receptors have a common plan. They all are of pseudo-unipolar design. The sensory ending is where the transduction of the physical quantity sensed causes membrane depolarization to take place. Depolarization leads to the generation of nerve spikes that propagate down the axon to synaptic connections in the spinal cord. Sensory neuron spikes generally originate in the axon near the sensory ending.

Sensory endings differ widely in design, ranging from naked dendrites on pain sensors (nociceptors) to complex, single structures such as Pacinian corpuscles (pc, rate of change of pressure), Meissner's corpuscles (flutter), Ruffini corpuscles (steady skin indentation), Krause's corpuscles, Merkel's disks (steady skin indentation). Muscle spindles (muscle length) and Golgi tendon organs (GTO, muscle force) have simple, encapsulated, endings in intimate contact with the tissues in which they sense strain. Figure 1.1-2 shows some of the receptor endings in situ.

- Hairy skin

Epidermal-dermal junction Merkel's receptor

Meissner's corpuscle

Bare nerve ending

Hair receptor

- Hairy skin

Subpapillary Plexus Skin

Stratum corneum pidermis

Dermis

Subpapillary plexus i

FIGURE 1.1-2 Sensory receptors found in the hairy and hairless (glabrous) skin of primates. Not specifically labeled are the smooth nerve endings that respond to heat, cold, and pain (nociceptors). (From Kandel, E.R. et al., 1991. Principles of Neural Science, 3rd. ed., Appleton & Lange, Norwalk, CT. With permission from the McGraw-Hill Companies.)

Stratum corneum pidermis

Dermis

Subpapillary plexus i

FIGURE 1.1-2 Sensory receptors found in the hairy and hairless (glabrous) skin of primates. Not specifically labeled are the smooth nerve endings that respond to heat, cold, and pain (nociceptors). (From Kandel, E.R. et al., 1991. Principles of Neural Science, 3rd. ed., Appleton & Lange, Norwalk, CT. With permission from the McGraw-Hill Companies.)

All peripheral sensors exhibit some kind of proportional plus derivative response to their stimulus. That is, if a step of stimulus is applied, the receptor responds first by firing at a high initial rate, and then its spike frequency output slows down to a lower, steady-state value. For receptors such as the PC, the steady-state frequency

Pacinian Corpuscles Flow Diagram
FIGURE 1.1-3 Schematic cross section of a Pacinian corpuscle. The onionlike wrappings around the naked sensory ending cause this sensor to fire bursts for increasing or decreasing pressure in the tissues around it. The bulblike ending is about 1.5 mm in length.

is zero for an applied step of pressure. When the step of pressure is removed, the PC again fires a burst. As Figure 1.1-3 shows, the PC is a single, tapered nerve ending surrounded by layers of lamellae like the layers of an onion. In the steady state, internal pressure is uniform around the nerve ending, causing no stimulation. When a step of pressure is applied to the tissues surrounding the PC, it is generally asymmetrical. Forces are transferred through the lamellae into the viscous fluid directly surrounding the neuron tip, causing shear forces to distort one side of the tip, which in turn opens Na+ channels and depolarizes the tip, leading to a burst of spikes. Within hundreds of milliseconds, the pressure around the tip equilibrates, removing the shear force that opened the ion channels and the spikes cease. When the stimulus pressure is removed, the process repeats itself; there is a redistribution of external forces on the lamellae of the PC causing a transient distortion of the tip and another burst of spikes. A slower decrease in the overall response of a PC may be caused by neural accommodation. Accommodation may involve the mechanically sensitive ion channels on the tip losing their sensitivity to repeated stimuli, or a temporary exhaustion of Na+ ions in the volume surrounding the tip.

A vertebrate, peripheral sensory neuron has a soma connected by a short neurite to the axon, the soma lies in the dorsal root ganglia outside the segments of the spinal cord. The axon enters through a dorsal root nerve trunk into the spinal cord, where it synapses with an appropriate, ascending, sensory projection interneuron. Some peripheral sensory neuron axons are myelinated (the PC, sharp pain, spindles, hair receptors, etc.); others are not (slow pain fibers, some warm and cold fibers, etc.). It is not the purpose here to describe the details of sensory neuron endings and their transduction processes; the interested reader who wishes to pursue these details should see chapters 24 through 27 in Kandel et al. (1991) and chapters 46 and 47 in Guyton (1991).

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