Cranial Nerves and Spinal Cord Anatomy

Cranial Nerves

• CN VI (Abducent): Motor nerve controlling eye movement.
• CN VII (Facial): Both motor and sensory functions; controls facial expressions, salivation, tears, and taste.
• CN VIII (Vestibulocochlear): Sensory nerve responsible for balance and hearing.
• CN IX (Glossopharyngeal): Both motor and sensory functions involved in swallowing, salivation, taste, and tongue movement.
• CN X (Vagus): Both motor and sensory functions, providing sensory input from the GI tract and respiratory tree, motor control to the larynx and pharynx, and parasympathetic motor control to the abdominal and thoracic organs.
• CN XI (Accessory): Motor nerve controlling head movement, works in conjunction with the vagus nerve.
• CN XII (Hypoglossal): Motor nerve responsible for tongue movement.

Spinal Cord

• The spinal cord conducts sensory information and motor instructions between the brain and the body's periphery.
• The gray matter contains neuron cell bodies and extensive synapses.
• In cross-section, the gray matter is located in the medulla and contains the central canal filled with CSF.
• The white matter of the cord comprises the cortex.
• Dorsal nerve roots contain sensory (afferent) fibers.
• Ventral nerve roots contain motor (efferent) fibers.
• Sensory information travels to the brain or other cord parts through the dorsal horns.
• Motor signals go to the spinal nerves via the ventral horns.

Epidural Anesthesia

• Can be achieved via local anesthetics (like those with a "-caine" suffix), that act as sodium channel blockers.
• Anesthesia blocks sensory information to prevent depolarization.
• It is commonly used in large animals but plays a role in managing pain for small animals too.
• Can be administered at the L7 or sacrococcygeal location.
• Is useful for procedures like a rectal or uterine prolapse.

Autonomic Nervous System

• The autonomic nervous system is divided into the sympathetic and parasympathetic nervous systems.
• Sympathetic nerves originate from the thoracolumbar vertebral regions.
• Parasympathetic nerves originate from the brain and sacral vertebral regions.
• Efferent neurons in both systems consist of two neurons.
• The first neuron's cell body is in the brain or spinal cord, projecting to a cluster of neuronal bodies outside the CNS called an autonomic ganglion.
• The first neuron synapses with the second neuron in the ganglion.
• The first neuron is the preganglionic neuron and the second is the postganglionic neuron.

Sympathetic Nervous System

• The sympathetic nervous system causes a fight-or-flight response.
• It promotes bronchodilation for greater gas exchange.
• It causes vasodilation to increase circulation to muscles.
• Vessels in the skin, GI tract, and kidneys constrict, shunting blood to the rest of the body.
• It causes pupil dilation for a wider field of vision, improving vision in low light conditions.

Parasympathetic Nervous System

• The parasympathetic nervous system facilitates a "rest and digest" state.
• The parasympathetic nervous system increases GI tract activity.
• The parasympathetic nervous system reduces heart rate.

Autonomic Neurotransmitters

• The sympathetic system primarily uses norepinephrine.
• Neurons that release norepinephrine are called adrenergic neurons.
• Alpha1-adrenergic receptors, found on blood vessels, cause constriction in the skin, GI tract, and kidneys.
• Beta1-adrenergic receptors enable the sympathetic system to increase heart rate and force of contraction.
• Beta2-adrenergic receptors cause bronchodilation.
• Parasympathetic system neurons secrete acetylcholine.
• Neurons that release acetylcholine are called cholinergic neurons.
• Cholinergic neurons are divided into nicotinic and muscarinic receptors.
• Nicotinic receptors are found in the postganglionic neuron of both sympathetic and parasympathetic systems, as well as between motor neurons and muscles in the somatic system.
• Muscarinic receptors are found on target organs and tissues supplied by the postganglionic neuron of the parasympathetic system.

Reflexes and Reflex Arcs

• Reflexes are rapid, automatic responses to stimuli to protect the body and maintain homeostasis.
• The reflex arc allows for rapid turnaround from sensory input to motor output.
• Muscle reflex stimulation is used to help diagnose and localize spinal cord or peripheral nerve damage.
• Palpebral reflex originates from receptors on the eyelid margins, traveling via CN V to the pons, then via CN VII to the muscles, causing a blink.
• Palpebral reflex, assessed by tapping the medial canthus, indicates anesthetic depth.
• Pupillary light reflex (PLR) includes the retina, CNII neurons in the diencephalon, and CN III motor neurons.
• Shining light into the eye causes the iris of both eyes to constrict.
• PLR does not assess vision; animals with damage to visual brain centers may have normal PLRs.

Clinical Neurology

• Local anesthetics prevent depolarization of sensory nerves.
• Nerve impulses stop where the local anesthetic blocks channels.

Seizures

• Seizures are caused by many factors, including metabolic imbalances (hypoglycemia), hepatoencephalopathy, toxin ingestion, brain tumors, or juvenile epilepsy.
• Evaluation begins with a neurologic exam and blood work.
• Treatment starts if the cause is found.
• Anticonvulsant use is considered if the cause is not identifiable.
• Serious seizure considerations include: Seizures lasting >5 minutes, more than once a month, or occurring in clusters.
• Seizures with violent or prolonged postictal periods can result in vocalization, urination, and defecation during the event.

Intervertebral Disc Disease (IVDD)

• IVDD is when the disc herniates into the spinal canal, impinging on the spinal cord.
• Herniation can be acute or chronic.
• Acute herniation causes pain and varying paresis or paralysis.
• Chronic herniation has a slow onset of clinical signs, common in long-backed dog breeds.
• Lesion location is determined by neurological exam and possibly myelogram or MRI.
• The arrangement of the spinal cord means the severity of the lesion is reflected by clinical signs.
• Superficial cord compression leads to loss of proprioception.
• Moderate cord compression results in loss of proprioception and motor function.
• Severe cord compression results in loss of proprioception, motor function, and deep pain.
• Treatment involves anti-inflammatories (steroids), strict rest, or decompressive surgery.

Head Trauma

• Head trauma may cause changes in postural reactions and reflexes.
• PLR and pupil size help determine head trauma in unconscious patients.
• Opisthotonus (forelimb extensor rigidity, head and neck in extension) indicates severe head trauma and herniation of the caudal cerebrum under the tentorium cerebelli.