Sensory Neurons, Receptive Fields, Dermatomes & Referred Pain

First-order somatic sensory neuron

Pseudounipolar neuron that detects a somatic sensory stimulus and carries the information toward the CNS.

Three main components of a first-order somatic sensory neuron

Cell body, peripheral process, and central process.

Cell body of a first-order somatic sensory neuron

Located in a posterior root ganglion lateral to the spinal cord.

Cell bodies of cranial sensory neurons

Located in cranial nerve ganglia in the head and neck.

Peripheral process

Long axon of a first-order sensory neuron that splits into nerve endings containing sensory receptors at one end.

Central process

Process that exits the sensory neuron's cell body and travels through the posterior root to enter the posterior horn of the spinal cord.

Central process of a cranial sensory neuron

Travels toward and enters the brainstem rather than the spinal cord.

First-order somatic sensory neuron pathway

Sensory receptor/peripheral process → cell body in posterior root ganglion → central process → posterior root → posterior horn of spinal cord.

Sensory neuron conduction speed

Determined largely by the diameter of the axon and thickness of its myelin sheath.

Fastest sensory axons

Largest-diameter axons with thick myelin sheaths conduct action potentials fastest.

Information carried by fast sensory axons

Includes proprioceptive information and discriminative and nondiscriminative touch information.

Slowest sensory axons

Smallest-diameter axons with little myelin conduct action potentials slowest.

Information carried by slow sensory axons

Includes pain and temperature stimuli transmitted to the CNS.

Axon diameter and conduction speed

Larger axon diameter is associated with faster action potential conduction.

Myelin thickness and conduction speed

Thicker myelin is associated with faster action potential conduction.

Receptive field

Area served by a sensory neuron.

Large receptive field

Area supplied by more branching from fewer sensory neurons; associated with body regions having less sensation.

Small receptive field

Area supplied by less branching from more sensory neurons; associated with body regions having greater sensation.

Receptive fields and sensory precision

Smaller receptive fields provide greater sensory discrimination than larger receptive fields.

Fingertip receptive fields

Fingertips have small receptive fields and therefore greater sensory discrimination.

Two-point discrimination

Method used to measure the relative size of receptive fields.

Better two-point discrimination

Occurs in areas with smaller receptive fields and greater sensory sensitivity.

Poorer two-point discrimination

Occurs in areas with larger receptive fields and less sensory sensitivity.

Dermatome

Segment of the body defined by the spinal nerve that supplies that region with somatic sensation.

Dermatome map

Map showing the sensory regions supplied by the spinal nerves.

Spinal nerve excluded from the dermatome map

The first cervical spinal nerve (C1) is not represented on the dermatome map.

Clinical use of dermatomes

Used to test the integrity of sensory pathways to different parts of the body.

Referred pain

Pain originating in an internal organ that is perceived as pain in a cutaneous region of the body.

Cause of referred pain

Occurs where spinal nerves carry both somatic and visceral sensory neurons.

Somatic sensation

Sensory information originating from structures such as the skin and musculoskeletal system.

Visceral sensation

Sensory information originating from internal organs.

Receptive field size relationship

Large field = fewer neurons with more branching and less sensation; small field = more neurons with less branching and greater sensation.

Conduction speed relationship

Large diameter + thick myelin = faster; small diameter + little myelin = slower.

Fast vs. slow sensory information

Proprioception and touch generally travel through faster-conducting axons, while pain and temperature travel through slower-conducting axons.

Dermatome vs. receptive field

A dermatome is a region supplied by a particular spinal nerve; a receptive field is the area served by an individual sensory neuron.