Avian Neurology

Question 1

Describe the CNS anatomy of avian species.

What are the 3 embryonic divisions?

How do those differ from mammals?

Why is CSF collection in birds difficult and potentially dangerous?

What are three anatomic differences between the brain of a bird and a mammal?

Answer 1

Central Nervous System

• 3 embryonic divisions of the brain: prosencephalon, mesencephalon, and rhombencephalon
• The latter two are similar to mammals, the prosencephalon has followed a divergent line of evolution
• Paired cerebral hemispheres with deep lateral ventricles. Integrates motor and sensory information as well as memory.
  
  • Outer surface has no neocortex, they are lissencephalic
• Diencephalon made of thalamus and hypothalamus
• Midbrain contains the optic lobe (analogous to superior colliculus in mammals) and caudal colliculus
• Metencephalon = pons and cerebellum, myelencephalon = medulla
• Prominent venous sinuses in the subarachnoid space, so CF collection poses high risk of hemorrhage
• No lateral foramina, instead transfer of water and solute occurs by diffusion through the 4^th ventricle

Question 2

Describe the function of the various part of the avian brain.

What cranial nerves come from the brainstem?

How does the avian cerebellum compare to the mammalian one?

What is the function of the optic lobe? What else is it called? What cranial nerve emerge from this structure?

What is the function of the hypothalamus?

What about the epithalamus?

How does the avian olfactory lobe compare to the mammalian one?

How does the avian telecephalic complex compare to teh mammalian neocortex?

Answer 2

Brainstem

• No obvious pons, but pontine fibers exist
• Cranial nerves V-XII come from the brainstem

Cerebellum

• Similar anatomy and function to mammals

Optic lobe

• Also known as the optic tectum, mesencephalic colliculus, and mesencephalic tectum
• CN III and IV emerge from this lobe
• Coordinates eye and head movements
• Red nucleus also contained within the midbrain

Hypothalamus

• Control of homeostasis, as in mammals

Epithalamus

• Region of the pineal gland involved in reproduction, strongly responsive to light in birds

Olfactory lobe

• Smaller in birds than in mammals
• There is no neocortex. There is a telencephalic complex in birds, a combination of elements in the avian cerebrum for which there is no comparative structure in the mammalian brain. Functionally, the avian telencephalic complex is similar to the mammalian neocortex.

Question 3

When does the spinal cord develop in avian species?

What are three differences between the avian spinal cord and the mammalian one?

Describe the composition of the spinal nerves?

Answer 3

Spinal cord development

• Begins at 16-18 hours of incubation
• Invagination of the neural plate begins rostrally and moves caudally, terminating in the rhomboid sinus
• Neural crest cells migrate and form the dorsal and ventral roots

Spina cord anatomy

• Grey and white matter arranged similarly to mammals
• Spinal cord is same length as the vertebral column
• Spinal nerves pass laterally rather than caudally as in mammals
• Cervical and lumbosacral enlargements of the spinal cord
• The glycogen body is found in the lumbosacral cord and is a collection of periependymal glycogen cells with nests of argentaffin cells. Also present are nerve terminals that may play a role in vascular reflexes and are believed to have neurosecretory roles
• The dorsal column consists of ascending, afferent pathways
  
  • These fibers end in the nucleus gracilis or nucleus cuneatus
  • Fibers from the dorsal ascending bundle sends info from the muscle receptors of the wings to the cerebellum ipsilaterally (CP)
  • Fibers from the ventrolateral ascending bundle sends info from the pelvic limbs contralaterally, decussates again at the rostral cerebellar peduncle (CP)
  • Dorsolateral fasiculus sends info on pain, temp, and tactile
  • Prospinal system, vague non-localized pain
• Descending pathways are not as well known
  
  • Lateral reticular, visceral motor function
  • Rubrospinal.l tract, flexor tone
  • Corticospinal tract, UMN input to the cervical region only
  • Vestibulospinal tract – extensor tone
  • Reticulospinal tract – alters somatic and visceral motor tone
  • Tectospinal tract – coordinates reflex movement between eyes and neck/upper body
• Spinal nerves
  
  • Dorsal and ventral roots
  • All spinal nerves are mixed sensory and motor once leaving the spinal canal
  • 4 types of fibers: Somatic and visceral afferents, somatic and visceral efferents
  • Numbering of spinal nerves depends on the number of vertebrae in a given species

Question 4

List the twelve cranial nerves of avian species.

Describe how they can be assessed clinically (if possible) and what signs would be present if they were damaged.

Answer 4

The Avian Neuro Exam

• Mentation
  
  • Examine both the level and content of consciousness ie look for abnormal or inappropriate behavioral responses
• Cranial nerves
• CN I – olfactory – difficult to assess
• CN II – optic
  
  • Vision perception or avoidance of obstacles
  • PLR also involves CN II but can be overridden in birds. Consensual PLR not present due to complete decussation of the optic nerves
• CN III – oculomotor – PLR
• CN IV – trochlear – dorsolateral strabismus indicates dysfunction
• CN V – trigeminal
  
  • Ophthalmic – dysfunction is loss of sensation to face or beak
  • Maxillary – loss of sensation to face, beak, mouth or palate
  • Mandibular – inability to close eye or beak, diminished beak strength
• CN VI – abducens – loss of function of 3^rd eyelid, medial strabismus
• CN VII – facial – facial droop or asymmetry, dec facial sensation
• VIII – vestibulocochlear – difficult to assess hearing, nystagmus, head tilt, circling
• IX – glossopharyngeal – dysphagia, voice loss, decreased gag reflex
• X – Vagus – regurgitation, vocal dysphoria, loss of gag reflex
• XI – accessory - poor neck mobility
• XII - hypoglossal - decreased tone of the tongue

Question 5

Describe the functions of the following ascending spinal tracts:

• Dorsal column
• Dorsolateral ascending bundle
• Ventrolateral ascending bundle
• Dorsolateral fasiculus
• Spinoreticular tract
• Propriospinal system

Describe the functions of the following descending spinal tracts:

• Lateral reticulospinal tract
• rubrospinal tract
• cerebrospinal tract
• vestibulospinal tract
• tectospinal tract

Answer 5

• The dorsal column consists of ascending, afferent pathways
  
  • These fibers end in the nucleus gracilis or nucleus cuneatus
  • Fibers from the dorsal ascending bundle sends info from the muscle receptors of the wings to the cerebellum ipsilaterally (CP)
  • Fibers from the ventrolateral ascending bundle sends info from the pelvic limbs contralaterally, decussates again at the rostral cerebellar peduncle (CP)
  • Dorsolateral fasiculus sends info on pain, temp, and tactile
  • Prospinal system, vague non-localized pain
• Descending pathways are not as well known
  
  • Lateral reticular, visceral motor function
  • Rubrospinal.l tract, flexor tone
  • Corticospinal tract, UMN input to the cervical region only
  • Vestibulospinal tract – extensor tone
  • Reticulospinal tract – alters somatic and visceral motor tone
  • Tectospinal tract – coordinates reflex movement between eyes and neck/upper body

Question 6

What is different about the avian trigeminal nerve? How is it assessed?

Answer 6

Question 7

A recent study described the recognition and assessment of pain-related behaviors in birds.

What are some potential acute and chronic conditions that could lead to pain in birds?

Has self-medicatnig been documented in birds?

How did changes in weight bearing occur with pain affecting limbs?

How did food and water intake change with pain?

How did behavior change with highly innervated areas having pain?

How did behavior change as acute pain became chronic?

Answer 7

Recognition and Assessment of Pain-Related Behaviors in Avian Species: An Integrative Review.
Mikoni NA, Guzman DS, Fausak E, Paul-Murphy J.
Journal of Avian Medicine and Surgery. 2022;36(2):153-172.

Key Points:
- Acute pain stimuli - beak trimming, induced arthritis, feather plucking, buccal injections, comb/wattle trimming, mechanothermal stimuli, wing clamp, pectoral pinprick
- Chronic conditions - keel bone fractures, orthopedic disease, pododermatitis
- Chickens with orthopedic issues self-select feed containing analgesia or anti-inflammatories compared to sound chickens
- Social interactions were affected by pain
- Behavioral indicators of pain in one species cannot be extrapolated to other species with the same pain stimulus - different social dynamics, prey vs predator
– Additional individual unique pain responses - captive raised vs wild, past exposure to noxious stimulus
- Pain affecting limbs: all species had changes in weight bearing, alterations in body posture, general reduced activity
- Both acute and chronic pain caused decreases in food or water intake, decreased weight gain or general weight loss, reduced activity
- Pain stimuli to highly innervated areas (beak, bill) caused alterations in use of the structures, decreased food and water intake, preening, and exploratory or agnostic pecking
- Acute pain caused aversive/avoidance behaviors; chronic unalleviated pain changed to include withdrawn attitude, adversely affected social interactions, and mating behaviors
- Numerical scales can’t be extrapolated, ideally would create specific scales for species and common clinical presentations/procedures