Central Nervous System

This lecture covers the following topics: functions of the central nervous system, the brain, the spinal cord, and comparative neuroanatomy and clinical cases.

Divisions

Central nervous system breaks down into the brain and the spinal cord

Embryology

• NS develops via ectoderm: a superficial cellular layer that covers the developing embryo

• The dorsal area creates a flattened stripe of cells along the anteroposterior axis of the embryo – neural plate. Ectodermal cells move inside the growing embryo, and the neural plate folds. The groove finally folds into a tube. This tube closes dorsally and is referred to as the neural tube. The space inside fills with fluid. 

  • The anterior region develops into the brain

  • The posterior region develops into the spinal cord

• Some ectodermal cells organize into 3 longitudinal stripes on both sides of the neural tube and are known as the neural crest. This develops into ganglia and nerves of the PNS.

• 3 major early vesicles = forebrain/procencephalon, midbrain/mesencephalon, and hindbrain/rhombencephalon

  • Forebrain develops into the cerebrum and diencephalon 

  • Hindbrain develops into the pons, cerebellum, and medulla oblongata

Meninges

• CNS is covered by 3 membranes – meninges. 

  • Outermost membrane with collagen fibers, making it strong and tough – dura mater

  • Middle web-like membrane, completely avascular – arachnoid mater

  • Delicate membrane covering the brain and spinal cord – pia mater

• Epidural, subdural, and subarachnoid spaces are often referred to for medical purposes

Cerebrospinal fluid (CSF)

• Exists between layers of meninges in the subarachnoid space (arachnoid trabeculae found here as well – region between outer arachnoid mater and inner pia mater), canals, and cavities inside of the brain 

• Provides protective cushioning function for CNS

• Nutritive and helps maintain stable ion concentrations

• Clear liquid 

• CSF Tap – invasive, difficult procedure that samples the CSF to help with diagnoses of certain diseases or cancers

  • May be needed for animals with sepsis (blood infection) to see if the CNS is septic too

  • Possibly can’t reach outer layers and make itself known 

• Secreted by the choroid plexus

Ventricle system of the brain

• Ventricles of the brain are interconnected cavities within cerebral hemispheres and the brain stem 

• Continues with the central canal of the spinal cord

• Filled with CSF

  • Replenished 3-4 times daily via bodily movement, cilia beating it through, or ventricle holes that allow it to pass through due to pressure 

• Includes the lateral, third, fourth ventricles, and the cerebral aqueduct

Blood brain barrier (BBB)

• Functional barrier that separates the capillaries (ions, molecules, and other particles easily pass through these structures) from the brain to protect the CNS

• Capillary walls have no fenestrations and are covered by glial cells (astrocytes)

  • Pericytes can contract to hold the capillary together; specialized cells that wrap around the tiny blood vessels in the brain and act as crucial gatekeepers to maintain the strength and health of the BBB

  • The tight junction is composed of special proteins, and the walls are very tight together; microscopic, welded seals between neighboring cells that block unwanted substances from passing through

• Prevents many drugs, proteins, ions, and other molecules from free movement between blood and the brain

  • Can pose a challenge if trying to fight infections or diseases in the brain

Brain divisions

• cerebrum

• cerebellum

• diencephalon

• brainstem

  • midbrain

  • pons

  • medulla oblongata

Cerebrum

• largest part of the brain; splits into the left and right hemispheres via the longitudinal fissure

• Further divides into 4 lobes: occipital, parietal, temporal, and frontal

• Performs higher-order functions: interprets impulses, initiates voluntary movement, processes speech and communication in animals, stores information as memory, is involved in reasoning and emotion, and is the seat of intelligence and personality – overall works to integrate memories from elsewhere into the brain 

Cerebellum

• second largest part of the brain located caudal to the cerebrum 

• Arbor vitae (grey and white matter) is visible here – the outer layer of grey matter is referred to as the cerebral cortex

• Responsible for coordinated movement, balance, posture, and complex reflexes

• When undeveloped or diseased, there is incoordination or a lack of fine motor control

• Contains more neurons than the cerebrum 

  • Difficult mechanisms = more work = surplus of neurons (makes up 80% of the body’s entire neuron count)

• Relative to the size of the organism and its demand for fine motor control

  • Ex. elephants rely on it for control of their trunks; dolphins for echolocation; elephant fish are known to have the largest cerebellums as aquatic organisms

Diencephalon

• deep in the brain

• Contains 3 key regions: epithalamus, thalamus, and hypothalamus 

• Epithalamus contains the pineal gland, which produces melatonin (regulates day and night cycles; affects seasonal behavior – i.e migration or seasonal reproduction for certain organisms) and a habenular nucleus 

• Thalamus: largest region and acts as a relay station for sensory input to the cerebrum

• Hypothalamus: forms the floor of the diencephalon. Most prominent parts = mammillary bodies (which play a role in memory). Consists of nuclei involved in homeostatic mechanisms – i.e regulation of blood pressure, body temperature, satiety, and thirst centers

• Neurosecretory cells of the hypothalamus produce hormones 

• Hypothalamus and thalamus = important parts of the limbic system (memory, motivation, emotional processing)

Pituitary gland

• plays a large role in negative feedback; pea-sized organ at the base of the brain that produces and circulates hormones into the bloodstream, including:

  • Anterior pituitary gland: TSH – thyroid-stimulating hormone that travels to stimulate the thyroid, ACTH – adrenocorticotropic hormone, GH – growth hormone, FSH – follicle-stimulating hormone which aids egg production, LH – luteinizing hormone which plays a role in ovulation, and prolactin which plays a role in milk production 

  • Posterior pituitary gland: ADH – antidiuretic hormone which regulates water balance, blood volume, and blood pressure in the body; oxytocin which plays a role in social bonding and empathy, or giving birth and laying eggs 

Brainstem

• forms the connection between the brain and spinal cord 

• Many cranial nerves originate here

• Responsible for maintenance of basic autonomic functions of the body, including regulation of the heart, respiration, coughing, swallowing, and vomiting

• Operates at a subconscious level

• Consists of 3 main parts: midbrain, pons, medulla oblongata

Midbrain

front of the brainstem, involved in many functions – i.e vision, hearing, motor control, sleep + wakefulness, arousal, temperature regulation, and the origin of most cranial nerves

Pons/”bridge”

• between the midbrain and medulla oblongata

• Rounded bulge on the underside of the brainstem. 

• Relays sensory information between the cerebellum and cerebral cortex, including hearing, balance, taste, and motor reactions

• Contains nuclei for cranial nerves V, VI, VII, and relays information about movement from the frontal cerebral hemispheres to the cerebellum 

• Coordinates eye movement, respiration, urination, and REM sleep 

• Damage to this structure can be fatal; ex. Locked-in syndrome via tumor, stroke, etc. can cause paralysis of bodily movement while the brain remains cognitive. Euthizol inhibits function of motor skills and easily euthanizes patients

Medulla oblongata

• cone-shaped structure at the base of the brainstem which connects the brain to the spinal cord 

Cranial nerves

1. part of PNS; 12 sets of cranial nerves organized by Roman numeral, name, and function (motor, sensory, or both): 

• I Olfactory – sensory 

• II Optic – sensory

• III Oculomotor – motor 

• IV Trochlear – motor

• V Trigeminal – both

• VI Abducens – motor 

• VII Facial – both 

• VIII Vestibulocochlear – sensory

• IX Glossopharyngeal – both 

• X Vagus – both

• XI Spinal – motor

• XII Hypoglossal – motor

Medical cases

• Hydrocephalus – excess CSF in the brain, present at birth in many small dogs (more prone)

  • Narrow ventricles/openings = surplus of CSF → leads to stunted skull growth → pressure accumulates and pushes on the brain, causing neurological issues like seizures, ataxia (falling), stumbling, head-pressing

  • Can also occur in kittens if prenatal exposure to specific infections or medications while inside the mother's womb, or via genetic predisposition

• Intervertebral disc disease (IVDD): more common in short-legged dogs or really large dog breeds; pulp in the spinal cord can become fibrous and hard, causing it to press into the spinal cord and morph its shape as it does so