🤕 Pain + Nociceptors

Learning Objectives

• Explain why pain is a critical animal welfare issue

• Distinguish between nociception, pain and suffering

• Describe the biological mechanisms underlying pain

• Recognize and categorize different types of pain

• Pain assessment

• Routine procedures cause pain

• Principles of pain mitigation/analgesia

Insights – Human and Animal Suffering

Key Question
 Not: Can they reason? or Can they talk?
 But: Can they suffer? – Jeremy Bentham, 1748–1832

Human Perspective
 Suffering arises when negative affective states combine or interact, often with fear

Application to Animals
 Suffering is likely when repeated poor handling causes pain and animals learn to fear the handler

Inflammatory Airway Diseases – Symptoms

Respiratory Symptoms
 Coughing
 Nasal discharge
 Labored breathing

Breathlessness
 Difficulty breathing
 Fear of oxygen deprivation

Chest Pain
 Inflammation can cause chest pain, tightness, and discomfort

Examples
 Porcine Reproductive and Respiratory Syndrome – pigs
 Infectious bronchitis – chickens
 Kennel cough – dogs

History of Analgesia – Key Points

19th Century
 ~30% of human amputations performed without anesthesia/analgesia (1860s, Pernick, 1985)

Late 20th Century
 Laboratory animals often received no analgesics post-surgery (Philips, 1993)

21st Century
 Many laboratory rodents still do not receive sufficient analgesia

Analgesic Use in Laboratory Animals

Purpose
 Reduce pain and suffering during and after experimental procedures

Analgesic Dosing

Goal
 Provide effective pain relief without causing adverse effects

Recognizing Pain in Veterinary Practice

Veterinarian Concern
 96% of French veterinarians are moderately or extremely concerned about recognizing and alleviating pain

Analgesic Use
 Varies by procedure:
  Orthopedic surgery – 84% receive analgesics
  Castration – only 17% receive analgesics

Barriers to Analgesic Use
 1. Difficulty in recognizing pain
 2. Lack of knowledge about appropriate therapy

Recognizing Pain on the Farm

Challenges
 Farm animals may hide signs of pain due to natural behavior or predator-prey instincts
 Subtle changes in posture, gait, or feeding behavior are often the only indicators

Indicators of Pain
 Behavioral – reduced activity, reluctance to move, vocalizations, abnormal postures
 Physiological – increased heart rate, respiratory rate, or stress hormone levels
 Production – decreased feed intake, growth, or milk yield

Definition and Components of Pain

Pain
 An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage

Components
 1. Nociception – neural detection and processing of potentially harmful stimuli
 2. Emotional experience – the subjective feeling of discomfort or suffering

Expanded Definition of Pain

Pain
 A distressing experience linked to actual or potential tissue damage

Components
 Sensory – detection of harmful stimuli
 Emotional – unpleasant feelings associated with damage
 Cognitive – interpretation and attention to pain
 Social – effects on interactions with others

Pain – Revised Definition

Pain
 An unpleasant sensory and emotional experience associated with actual or potential tissue damage

Pain – Descriptive Terms and Significance

Terms Used to Describe Pain
 Aching, pulling, throbbing, burning, sharp, needle-like, itching, beating

Key Points
 Pain perception serves as a warning mechanism for actual or potential tissue damage
 Being indifferent to pain can be potentially harmful

Animal Pain – Earlier Definition

Physiological and Behavioural Changes
 Pain changes the animal’s physiology and behaviour

Purpose
 To reduce or avoid damage, prevent recurrence, and promote recovery

Animal Emotions – Conceptual Basis

Private Experience
 Emotional states are private – we can only be certain of our own experience

Comparative Evidence
 Shared ancestry, anatomy, and physiology (e.g., nervous system, cortex or equivalent structures)
 Similar responses to similar situations suggest animals can experience emotions

Animal Pain – Analogy Approach

Brain Structures
 All vertebrates have primitive brain areas to process nociceptive information: medulla, thalamus, limbic system, and cortex (or equivalent system)

Pain Signaling
 Structures and pathways for sending and receiving pain signals are well known

Pharmacology
 Pharmacological methods for pain mitigation are well studied

Analogy Reasoning
 Using reasonable analogy, denying the existence of similar pain states in animals is considered absurd

Limits of Analogy

Key Point
 Analogy does not prove that all beings experience identical feelings

Application
 Strength of similarities can justify inferences about an individual animal’s subjective state

Reflexes vs. Conscious Pain

Reflexes
 Automatic responses to stimuli

Pain
 Subjective experience, different from reflexes

Key Point
 Assessment of pain is essential because analgesia treats the felt experience, not just reflex responses

Function of Pain & Three Components

Function of Pain
 Protects the body from injury
 Withdrawal reflex acts as a first protective response

Three Components of Pain

1. Nociception (sensory detection) – detects harmful stimuli; not the same as feeling pain

2. Immediate emotional experience – unpleasant feeling when tissue is damaged

3. Long-term emotional impact – chronic worry, distress, changes in mood or behaviour

Tissue Damage – Examples

Goat – broken horn

Dog – bite wound

Ruptured Abscess – Pain Mechanism

Contents
 Middle contains dead cells, bacteria, and debris

Growth
 Area expands, creating tension under the skin

Inflammation
 Surrounding tissues become inflamed

Pain
 Caused by pressure and inflammation

Allergic Reaction – Pain and Discomfort

Causes
 Flea bites, contact allergens, fungal infections, mite infestations, thyroid or hormonal conditions

Symptoms
 Similar to abscess: inflammation, redness, irritation, discomfort

Pain Mechanism
 Inflammation and tissue irritation trigger unpleasant sensations

Tissue Damage – Chemical Mediators of Pain

Mechanism
 Tissue damage releases chemicals that stimulate free nerve endings
 Signals are sent to the spinal cord via the dorsal root ganglion

Cells Involved
 Mast cells, macrophages, neutrophils

Chemical Stimulators
 Histamine, prostaglandins, bradykinin, cytokines

Effect
 Chemical stimulation triggers nociception and contributes to pain perception

Nociceptors – Specialized Sensory Receptors

Function
 Detect extreme pressure, temperature, and noxious chemicals that can damage tissue

Mechanism
 Transform harmful stimuli into electrical signals

Pathway
 Signals travel to the central nervous system (CNS) for processingNoci

Nociceptors – Classification Based on Stimulus

Mechanical Nociceptors
 Respond to intense pressure

Thermal Nociceptors
 Respond to extreme hot or cold temperatures

Mechano-Thermal Nociceptors
 Respond to both mechanical and thermal stimuli

Fiber Type
 Typically myelinated A fibers

Collective Name
 These three types are called A(δ-β)-nociceptors

Nociceptors – Polymodal & Silent

Polymodal Nociceptors
 Respond to noxious mechanical, thermal, and chemical stimuli
 Fiber type: small, unmyelinated C fibers
 Impulse velocity: < 3 m/s

Silent Nociceptors
 Activated by chemical stimuli (inflammatory mediators)
 Respond to mechanical and thermal stimuli only after activation
 Fiber type: small, unmyelinated C fibers
 Impulse velocity: < 3 m/s

Afferent Fibers & Conduction Speed

A-alpha
 Function: proprioception
 Myelination: yes
 Diameter: ~13–20 µm
 Conduction velocity: 80–120 m/s

A-beta
 Function: touch
 Myelination: yes
 Diameter: ~6 µm
 Conduction velocity: 35–90 m/s

A-delta
 Function: nociceptive (mechanical & thermal)
 Myelination: yes
 Diameter: 2–5 µm
 Conduction velocity: 5–40 m/s

C fibers
 Function: nociceptive (mechanical, thermal, chemical)
 Myelination: no
 Diameter: ~0.2–1.5 µm
 Conduction velocity: 0.5–2 m/s