PSYC304: Sensory Processing and Pain Principles

PSYC304: General Principles of Sensory Processing, Touch, and Pain

• The course covers the general principles of sensory processing, including touch and pain.
• The lecture outline covers principles of sensory processing, types of somatosensation, and pain, thermosensation, and itch.
• Learning objectives include differentiating sensation and perception, top-down vs. bottom-up sensory processes, the number and type of senses, how neural information is interpreted, and how the brain codes for sensory information.
• Learning objectives also include understanding sensation diminishment with constant stimulation, receptive fields, plasticity in the brain, pain, different types of nociceptors and thermoceptors, and pain management.

Basic Concepts of Sensation and Perception (part 1)

• Sensation and perception are parts of a continuous process.
• Sensation involves receiving sensory stimulation.
• Perception involves interpreting sensory information.

Basic Concepts of Sensation and Perception (part 2)

• Bottom-up processing involves detecting specific features of stimuli and combining them into more complex forms.
• Top-down processing involves forming perceptual hypotheses, selecting features to check these hypotheses, and recognizing the stimulus itself.

Basic Concepts of Sensation and Perception (part 3)

• Sensory systems receive stimulation through specialized receptor cells.
• Sensory input is transduced into receptor potentials (e.g. PSPs).
• This process is often driven by ionotropic receptors, but sometimes, metabotropic receptors are used.
• Receptor cells deliver neural information to the brain.

The Five Senses

• Sensory systems use different modalities and adequate stimuli to receive signals.
• Mechanical senses detect physical contact or deformation.
• Visual senses involve visible radiant energy.
• Chemical senses involve substances dissolved in air or water.
• Electrical senses detect differences in electrical current density.

Sensory Receptor Organs

• Sensory systems have a restricted range of responsiveness.
• Receptor organs detect energy or substances.
• Sensory systems respond to a range of wavelengths and frequencies.

Intensity Coding

• Intensity coding occurs with different thresholds for the three types of neurons.
• The brain uses range fractionation to determine stimulus intensity, which involves multiple neurons acting in parallel.
• More neurons are recruited if the stimulus increases.

Sensory Adaptation

• Sensory adaptation is the progressive loss of response to maintained stimuli.
• Tonic receptors are slow-adapting, while phasic receptors are fast-adapting.

Receptive Fields

• Each cortical neuron has a receptive field that is stimulated when a stimulus is applied.
• Cortical neurons respond to inputs in a specific location; if stimulation occurs in the surrounding region, they inhibit in response.

Receptors in the Skin

• Skin receptors include free nerve endings, Merkel's discs, Meissner's corpuscles, Ruffini's endings, and Pacinian corpuscles. These all come in both large and small variations based on their function.
• Each receptor is responsible for detecting a specific stimulus such as temperature, vibration, or pressure and delivers this information to the brain.

Example: The Pacinian Corpuscle

• Pacinian corpuscles detect vibration.
• Transduction occurs via stretch receptors, similar to muscle spindles (proprioception).
• Receptor potentials and action potentials carry information for the stimulus.

Receptor Potentials and Action Potentials

• Receptor potentials lead to action potentials when they surpass a certain threshold.

The Four Main Skin Receptors

• The four main skin receptors are Pacinian corpuscles (vibration), Meissner's corpuscles (light touch, fast-adapting), Merkel's discs (fine touch, slow-adapting), and Ruffini's endings (stretch, slow-adapting).

Afferent Axons from Skin Receptors

• Afferent axons carry sensory information to the CNS.
• Different types of neurons carry information about touch, proprioception, pain, temperature, and itch.
• A alpha (Αα), A-beta (Αβ), A-delta (Αδ), and C fibres have different lengths, diameters, and conduction speeds.

Pathways: The Dorsal Column System

• Sensory information from the body travels through the dorsal columns of the spinal cord and makes its way to the brain via distinct pathways.
• Information is relayed through the medulla, thalamus, and travels to the primary somatosensory cortex.

Dermatomes

• Dermatomes are specific body regions where incoming and outgoing nerves meet.
• Corresponding dorsal roots innervate different dermatomes in the body.
• Many dermatomes may overlap, making it possible for nerves connected to one area of the body to also send information from other areas.

Sensory Pathways

• Sensory pathways are segregated ("labelled lines").
• Signals arrive at the spinal cord or brainstem.
• Most signals travel through the thalamus and are processed by the cerebral cortex.

Cortical Organization of Sensory Information

• Information from different body parts is organized somatotopically in the primary and secondary somatosensory cortex.
• Primary somatosensory cortex (S1) is contralaterally organized.
• Secondary somatosensory cortex (S2) has more complex processing and may use bilateral information.

Somatotopic Organization (Sensory Homunculus)

• The sensory homunculus maps body parts to cortical areas.
• The size of the map area reflects the density of sensory receptors in that part.
• More sensitive areas (e.g. hands, face) have a larger cortical representation.

Special Systems for Special Animals

• Special sensory systems like whiskers and the barrel cortex in certain animals are responsible for processing sensory input relevant to their environment.

Cortical Representation

• Cortical representation maps to the body and has flexible receptive fields.
• Representation is based on the use of specific parts.

Neuroplasticity (due to damage)

• The brain adjusts and modifies itself in response to sensory deprivation, injuries, and other alterations to the body.
• The brain can compensate for damage done to sensory input by transferring the responsible function to another area.

Neuroplasticity (due to use)

• Experience (e.g., use/practice) can change the brain.
• The amount of use directly affects the size and/or function of the responsive area in the cortex.

Neuroplasticity (due to cortical damage)

• After cortical damage, constraint-induced therapies can promote recovery.
• Reorganization occurs, as the brain restructures itself to compensate for this damage.

Phantom Limbs

• Phantom limb sensations show large-scale plasticity.
• The brain reorganizes cortical maps to represent lost limbs.
• This leads to the sensation of feelings in the missing limb, despite the fact there is no physical stimulation to the area.

Are Cortical Sensory Neurons Specialized for Their Type of Sensation?

• Cortical sensory neurons are not specialized in terms of the type of sensation, as neurons in different brain regions can detect nearly any input type.
• The specialized processing stems mainly from differing wiring.

The Brain that Kind of Changes Itself

• Brain plasticity and the organization's limited capacity for change.
• Plasticity is less flexible than previously believed.

Attention: Manipulating Sensory Processing

• Attention enhances specific messages, using the attenuator and filtering unattended messages.
• Attentional processing is carried out by the posterior parietal and cingulate cortex, as well as other regions.

"Association" Cortex

• AKA tertiary cortex is multimodal.
• It comprises a large portion of the brain.
• Is often considered sensory and motor.

Synesthesia: Cross-Modal Stimulation

• Cross-modal stimulation is characterized by an involuntary sensory experience involving stimuli that don't usually evoke that particular sensation.
• It's thought to be driven by learning rules and related to evolutionary processes.

How to Test for Synesthesia

• Testing for synesthesia involves determining if a perceived stimulus is real or imagined (malingering) to assess whether the user is experiencing it genuinely.
• Difficult search tests take longer than basic searches, which can reflect the intensity of the feeling.

Unpleasant but Adaptive

• Pain is associated with tissue damage that results in an unpleasant experience.
• Congenital insensitivity to pain is caused by a voltage-gated sodium channel mutation.
• Pain is necessary for the body's well-being, allowing for recovery from damage, inducing learning experiences, and communicating to others.

Pain Serves Many Functions

• Pain induces withdrawal, recovery, learning from experiences, and communicating with others.
• A simple pain reflex is carried out quickly, while conscious pain processing is delayed.

Pain is Multifaceted

• Pain perception, cognitive processing (e.g. no pain, mild, excruciating experience), motivational affective systems (such as feeling tired/fearful), and the sensory discriminative system (e.g., throbbing, and shooting pain).
• The McGill Pain Questionnaire is commonly used to assess different types of pain.

Nociceptors

• Nociceptors are sensitive to temperature and chemicals released by tissue damage.
• These receptors transmit information about pain to the central nervous system.
• Important components like histamine, serotonin, prostaglandins, and leukotrienes act on these receptors to trigger pain signals.

Different Receptors for Different Temperatures

• Nociceptors with components TRPV1 (hot), TRPM3 (cold), and CMR1 (cold).
• These thermoreceptors differ in their sensitivity, response patterns, and receptor types.

Itch: A Complicated Sensation

• Itch is a multifaceted sensation with multiple receptors and pathways.
• Itch receptors do not exclusively respond to pain.
• Itch cells contain histamine receptors in some cells, but some do not.
• Itch signals are involved in pain.

Pathways: The Spinothalamic System

• The spinothalamic system carries pain and temperature information.
• It crosses the midline in the spinal cord before ascending to the thalamus.

Phantom Limb Pain

• Dorsal horn neurons can become hyperexcitable after damage.
• This can result in phantom limb pain.

Pain Isn't a Physical Sensation

• People may experience pain due to social rejection.
• All types of pain activate the anterior cingulate cortex.
• Painkillers can reduce the pain of social rejection.

Pain Management: Drugs

• Analgesia is the absence or reduction of pain, and many different drugs have been developed to address this issue.
• NSAIDs affect COX, prostaglandins.
• Opioids (endogenous and exogenous) act on spinal cord, descending pathways, and higher brain sites.

Pain Management: PAG

• The periaqueductal gray (PAG) in the midbrain is involved in pain modulation.
• Stimulation in the PAG can block incoming pain signals.
• Endogenous mechanisms involve natural pain relief.

Pain Management: Placebo

• Placebos have shown effectiveness in pain management, suggesting psychological factors are influential in pain perception.
• Placebo's effects vary with expectations and contexts, but likely involve endogenous opioids.