Sensory Processing (Lesson 30)
What is the standard 3-order neuron chain that transmits somatic sensory information from the body to the cerebral cortex for conscious perception?
1st-order neuron: Extends from a sensory receptor via a peripheral nerve into the spinal cord's dorsal gray horn
2nd-order neuron: Originates in the dorsal gray horn, decussates (crosses over to the contralateral side), and ascends through white matter tracts to the Ascending Reticular Activating System (ARAS) / thalamus
3rd-order neuron: Extends from the ARAS / thalamus to the dedicated somatosensory cortex for conscious perception
What are the recommended diagnostic steps when a primary care veterinarian suspects nervous system disease, and where does the neurological examination fit in this progression?
The diagnostic sequence begins with taking a thorough history, followed by a general physical examination, and then the neurological examination. Advanced procedures—such as radiography (with or without contrast/myelography), CT, MRI, CSF analysis, and tissue biopsy—are performed only after the neurological exam helps localize the lesion
Which cell type in the central nervous system (CNS) is most sensitive to injury?
Neurons are the most sensitive cells in the CNS to injury, followed by oligodendrocytes, astrocytes, microglia, and endothelial cells
What are the three primary types of nociceptors found on free nerve endings, and what stimuli activate each type?
Mechano-nociceptors: Respond to strong pressure, stretch, and mechanical disruption of tissue.
Chemo-nociceptors: The most common type; respond to chemical substances released during cell and inflammation (e.g., prostaglandins, cytokines, TNF, NGF).
Thermo-nociceptors: Respond to extreme cold or extreme heat1.
What are the three main classes of opioid receptors, and what distinct physiological effects are associated with each?
Mu receptor: Mediates the majority of maximum analgesia (spinal and supraspinal), dose-dependent respiratory depression, decreased GI motility, sedation, euphoria, antidiuresis, and nausea/vomiting
Kappa receptor: Mediates moderate analgesia, mild sedation, decreased GI motility, and diuresis (via suppression of ADH release).
Delta receptor: Mediates analgesia and dysphoria, as well as immunomodulation
How do descending motor pathways differ structurally and functionally from ascending sensory pathways?
Descending motor pathways use a 2-neuron chain to execute voluntary/involuntary movements . Upper Motor Neurons (UMNs) have cell bodies in the cerebral cortex and brainstem, running down spinal cord tracts to synapse in ventral gray horns onto Lower Motor Neurons (LMNs) . LMN axons leave via spinal nerves to innervate skeletal muscles . In contrast, ascending sensory pathways use a 3-order neuron chain from peripheral receptors up to the sensory cortex
How does the neurological examination differentiate between a "loss of function" versus a "gain of function" sign? Give examples of each.
Loss of function (-): Arises from the destruction of functional nervous tissue, producing deficits such as paresis, paralysis, loss of sensation, or absent reflexes.
Gain of function (+): Arises from the release or loss of central inhibition or excitation/increased stimulation, resulting in signs like seizures, tremors, hyperreflexia, tetany, or hyperaesthesia
List four distinct microscopic changes that can occur in the neuronal cell body in response to injury, along with their primary underlying causes.
Ischemic cell change ("red neurons"): Caused by acute hypoxic-ischemic injury, excitotoxicity, and oxidative damage.
Central chromatolysis: Caused by axonal injury, degenerative conditions, viral infections, or inherited disorders.
Intraneuronal inclusion bodies: Caused by viral infections (e.g., eosinophilic Negri bodies in Rabies).
Cytoplasmic vacuolation: Caused by transmissible spongiform encephalopathies (TSEs / prion diseases). (Alternative valid answers from lecture: Cell body enlargement in lysosomal storage diseases; Lipofuscin accumulation in aging; Neurofilament accumulation in degenerative disorders)
Compare the structural, neurochemical, and functional differences between A-delta fibers and C fibers.
A-delta fibers: Small, myelinated axons located primarily in superficial skin. They utilize glutamate as their main neurotransmitter (binding to AMPA and NMDA receptors), which is rapidly released and degraded. They transmit fast, sharp, stinging, well-localized "first pain".
C fibers: Small, unmyelinated axons with slower conduction speeds, located mostly in deep skin and internal tissues. They utilize Substance P as their main neurotransmitter (binding to Neurokinin-1 / NK1 receptors), which is slowly released and degraded. They transmit slow, dull, aching, diffuse "second pain".
Why are systemic opioids alone considered insufficient for managing sharp, localized surgical pain?
The analgesic effects of systemic opioids are largely restricted to C-fiber unmyelinated nociceptors (which transmit slow, dull, aching pain). A\delta nociceptors (which transmit fast, sharp, localized pain) are minimally affected by systemic opioids alone, making them inadequate as single agents for invasive surgical procedures
How does the hypothalamic temperature control center respond when core temperature drops below its set point versus when it exceeds its set point?
Below set point: Triggers heat-conserving/generating mechanisms, including cutaneous vasoconstriction, muscle shivering (shivering thermogenesis), non-shivering thermogenesis (brown fat), increased thyroid/metabolic rate, and behavioral changes .
Above set point: Triggers heat-losing mechanisms, including cutaneous vasodilation, sweating or panting, reduced metabolic rate, decreased appetite, and behavioral changes
What is the exact sequence of clinical sign loss during progressive spinal cord compression, and in what order are functions regained during recovery?
Order of Loss (earliest to last): Proprioception/Ataxia -> Voluntary motor -> Voluntary urination -> Superficial pain -> Deep pain6.
Order of Recovery (earliest to last): Deep pain –> Superficial pain -> Voluntary urination ->Voluntary motor -> Proprioception/Ataxia
Under what specific condition should deep pain sensation be evaluated, and what constitutes a true positive test result?
Deep pain should ONLY be tested if the patient is completely paralyzed (has no voluntary motor function)
A dog acutely collapses during anesthesia and develops cortical blindness and seizures two days later. Histology reveals laminar cortical necrosis with "red neurons." What is the primary cellular mechanism behind this lesion?
Glutamate-mediated excitotoxicity (and acute oxidative damage / hypoxia-ischemia), which triggers acute acidophilic neuronal necrosis ("red neurons")
Distinguish between hyperalgesia and allodynia.
Hyperalgesia: An exaggerated, heightened pain response to a stimulus that is normally noxious.
Allodynia: A pain response triggered by a stimulus that is normally non-noxious (e.g., gentle touch or light pressure)
How do the CNS and thermoregulatory effects of full mu-opioid agonists vary between dogs versus cats and horses?
Dogs: Experience prominent sedation due to a high density of central opioid receptors, along with panting as a thermoregulatory response. High doses may cause convulsions.
Cats & Horses: Often exhibit CNS excitement and hyperthermia. Horses are particularly sensitive and demonstrate increased locomotor activity when given pure mu-agonists
Explain the neurophysiological changes occurring during the "chill phase" and "crisis phase" of a fever.
Chill phase: Pyrogens (e.g., bacterial toxins, IL-1, IL-6, TNF) increase the hypothalamic set point . Because actual body temperature is lower than
the new set point, the hypothalamus induces heat-conserving mechanisms like shivering until the fever temperature is reached .
Crisis phase: Pyrogen removal returns the hypothalamic set point to normal . Because actual body temperature is now above the set point, the hypothalamus triggers heat-losing mechanisms such as sweating and vasodilation
Which cranial nerves are assessed when evaluating the palpebral reflex, corneal reflex, and menace response?
Palpebral reflex: Tests CN V (Trigeminal — sensory afferent) and CN VII (Facial — motor efferent to blink).
Corneal reflex: Tests CN V (Trigeminal — sensory afferent) and CN VI (Abducens — motor efferent to retract the globe).
Menace response: Tests CN II (Optic — visual afferent) and CN VII (Facial — motor efferent to blink)
Describe the chronological sequence of Wallerian degeneration and repair in the peripheral nervous system (PNS) following nerve transection or severe compression.
Degeneration & Fragmentation (several days): The axon and its surrounding myelin distal to the injury site fragment and die back.
Phagocytosis (few weeks): Macrophages and Schwann cells clear axonal and myelin debris.
Axonal Regeneration: Axonal sprouts emerge from the proximal stub and grow toward the target tissue.
Remyelinization: Schwann cells align and remyelinate the newly grown axon, establishing shorter internodes
Categorize tactile receptors into free nerve endings versus encapsulated nerve endings, and state their functions.
Free Nerve Endings: Include epidermal free nerve endings (crude touch), Merkel's discs (light touch), and the hair root plexus (highly sensitive touch).
Encapsulated Nerve Endings: Include Meissner corpuscles (light touch), Ruffini corpuscles (pressure/skin stretch), and Pacinian corpuscles (deep pressure and vibration)
Why are full mu-opioid agonists strictly contraindicated in patients with head trauma?
Opioid-induced respiratory depression leads to an accumulation of arterial CO2 (PCO2). Elevated PCO2 causes potent cerebral vasodilation, which increases intracranial pressure and can precipitate life-threatening cerebral edema
How do poikilothermic (ectothermic) species control their body temperature compared to homeothermic species, and what is its clinical significance in captivity?
Ectotherms have broader hypothalamic set point ranges (e.g., 35–39 °C in bearded dragons) and rely primarily on behavioral mechanisms (sun-bathing, shade-seeking, burrowing, skin color changes) rather than metabolic physiological responses . In captivity, housing must provide a thermal gradient so the animal can move to regulate its temperature behaviorally
5. A 6-year-old Female Spayed Border Collie presents with generalized ataxia and multiple cranial nerve abnormalities on physical exam. What is the most likely neurolocalization?
Brainstem. The nuclei for Cranial Nerves III through XII reside within the brainstem, making it the localized region that explains both cerebellar/proprioceptive pathway disruption (ataxia) and multiple cranial nerve deficits
What histopathological lesion pattern is considered classically diagnostic for viral encephalitis?
Perivascular cuffing with mononuclear cells (lymphocytes, plasma cells, and macrophages surrounding cerebral blood vessels)
What is the difference between a dermatome and an autonomous zone in clinical neurology?
Dermatome: An area of skin innervated by sensory fibers from a single specific spinal nerve.
Autonomous Zone: A specific area of skin innervated exclusively by a single plexus nerve (e.g., radial nerve) without overlap from adjacent nerves
What properties make Buprenorphine difficult to reverse with Naloxone, and what long-acting formulation is FDA-approved for cats?
Buprenorphine is a high-affinity partial mu-agonist that binds very tightly to and dissociates extremely slowly from the mu-receptor. Consequently, reversing its effects or treating overdose-induced respiratory depression requires unusually high doses of Naloxone920. Simbadol is an FDA-approved subcutaneous formulation providing 24-hour analgesia in cats