Chapter 8 - General Principles of Sensory Processing, Touch, and Pain

Understand the concepts of adequate stimulus and labeled lines

Adequate stimulus: The type of stimulus for which a given sensory organ is particularly adapted

E.g. Light is the adequate stimulus for the eye

Labeled lines: Particular neurons are dedicated to signaling distinctive sensory experiences

Different axon pathways signal different sensations (e.g. sound, smell, touch)

Allows the brain to recognize different sensory modalities

Describe sensory transduction and sensory threshold

Sensory transduction: The process of converting stimulus energy into electrical signals in receptor cells

Receptor cells are specialized to detect and transduce particular types of energy

Receptor potential: Initial electrical change in receptor cell membrane in response to stimulus

Threshold: Minimum stimulus intensity needed to generate an action potential

Explain sensory adaptation, and contrast tonic versus phasic sensory receptors

Sensory adaptation: Progressive loss of response when stimulation is maintained

Tonic receptors: Show little decrease in action potential frequency with continued stimulation

Phasic receptors: Rapidly decrease action potential frequency when stimulus is maintained

Adaptation emphasizes change in stimuli rather than constant stimuli

Describe the levels of sensory processing, from sensory receptor to cortex, in mammals

Information flows from receptors to spinal cord/brainstem to thalamus to cerebral cortex

Each level accomplishes basic aspects of processing

Most senses relay through thalamus before reaching cortex

Primary sensory cortex processes information and connects with secondary sensory areas

Explain the concept of a receptive field, with examples from the sense of touch

Receptive field: Region of space in which a stimulus alters a neuron's firing rate

Somatosensory receptive fields often have:

Excitatory center with inhibitory surround or

Inhibitory center with excitatory surround

Receptive fields differ in size, shape, and quality of stimulation that activates them

Cortical maps represent body surface, with more sensitive areas having larger representations

E.g. Hands/fingers have larger cortical areas than less sensitive body parts

Name the four main types of touch receptors, and compare their receptive fields

Pacinian corpuscles:

Fast-adapting, respond to vibrations >200 Hz

Large receptive fields

Located in hypodermis

Meissner corpuscles:

Fast-adapting

Small receptive fields

Detect localized movement and texture

Numerous in fingertips, tongue, lips

Merkel discs:

Slow-adapting

Small receptive fields with inhibitory surrounds

Provide high spatial resolution

Respond to isolated points and edges

Ruffini endings:

Slow-adapting

Large receptive fields

Detect skin stretching

Compare the four categories of sensory axons by size, conduction velocity, and which sensory information each conveys

Proprioception (body sense):

Axon type: Aα

Diameter: 13-20 μm

Conduction speed: 80-120 m/s

Touch (Pacinian, Ruffini, Merkel, Meissner):

Axon type: Aβ

Diameter: 6-12 μm

Conduction speed: 35-75 m/s

Pain, temperature (myelinated):

Axon type: Aδ

Diameter: 1-5 μm

Conduction speed: 5-30 m/s

Temperature, pain, itch (unmyelinated):

Axon type: C

Diameter: 0.2-1.5 μm

Conduction speed: <1 m/s

Trace the neural pathway for touch from receptors in the skin to the brainstem, thalamus, and cortex

Touch receptors in skin

Axons through dorsal roots to dorsal column of spinal cord

Ascend to dorsal column nuclei in medulla

Cross midline to opposite side

Ascend to thalamus

Project to primary somatosensory cortex (S1)

Describe evidence of plasticity in somatosensory cortex, in monkeys and people

Monkeys:

Finger removal: adjacent fingers' representations expand

Increased stimulation: enlarged representation of trained fingers

Humans:

Musicians: enlarged representation of left fingers in string players

Foot artists: larger representation of toes

Hand loss: face and upper arm representations expand into hand area

Hand transplant: cortex reorganizes to receive input from new hand

What detects pain?

Nociceptors - free nerve endings in the skin

What are the specialized receptor proteins nociceptors use to detect stimuli?

TRP channels

What are the two types of fibers that transmit pain signals?

Fast Aδ fibers (sharp, immediate pain) and Slow C fibers (dull, lasting pain)

Describe the pain pathway in the spinothalamic system.

Nociceptors synapse on neurons in the dorsal horn of the spinal cord, spinal neurons cross midline and ascend contralateral side, axons terminate in the thalamus, information integrated in the cingulate cortex

TRP channels for temperature and itch:

TRPV1 - responds to heat and capsaicin, on C fibers

TRPM3 - detects higher temperatures, on Aδ fibers

TRPM8 - responds to cool temperatures and menthol, on C fibers

Other TRP channels detect various spices/chemicals

Itch uses separate pathways with TRP channels and C fibers

Neuropathic pain

Phantom limb pain - pain felt in missing limb

Caused by changes in spinal cord and cortex after injury

Microglial cells make dorsal horn neurons hyperexcitable

Chronic pain can cause gray matter shrinkage in prefrontal cortex

Migraines involve spreading cortical depression and trigeminal nerve activation

Pharmacological approach to control chronic pain

Includes cannabis, opiates, and anti-inflammatory drugs

Stimulation approach to control chronic pain

Involves electrical stimulation of periaqueductal gray and acupuncture

Surgical approach to control chronic pain

Involves cutting pain pathways, but has limited effectiveness

Psychological approach to control chronic pain

Includes cognitive strategies and hypnosis

Placebo effects

Inert treatments that provide pain relief

Work by activating endogenous opioid systems

Enhanced by:

Verbal suggestions of efficacy

Medical setting/white coat

Perceived cost/value of treatment

Expectation of relief contributes to effectiveness

Some acupuncture effects may be placebo-related