Understanding the Horse’s Brain: Anatomy, Function & Related Diseases

The equine brain is a complex organ responsible for processing sensory information, controlling movement, and managing emotions. Unlike humans, whose brains are heavily developed for logical reasoning and problem-solving, horses rely on instinct and sensory perception. Their brains are wired for survival, constantly assessing threats and reacting quickly to potential dangers.

Weighing approximately 1.5 pounds (700 grams), the brain represents only about 0.1% of the horse’s total body weight. Key structures of the equine brain include the forebrain, midbrain, and hindbrain.

The cerebrum, the largest part of the brain, is responsible for decision-making and memory. The midbrain acts as a relay station between the forebrain and the rest of the body. The hindbrain, the most primitive part of the brain, controls involuntary functions such as breathing, heart rate, and digestion.

Understanding the anatomy of the horse’s brain provides valuable insights into equine behavior, learning patterns, and overall well-being. By exploring the intricate anatomy of the horse’s brain, we gain a deeper appreciation for how these animals think and behave.

The Horse’s Brain

The horse’s brain is a complex structure that controls all the body systems, from intentional movement to uncontrolled mechanisms like the heartbeat.

At its core, the brain is a network of neurons, specialized cells that produce and carry electrical signals. Neurons work by producing an electric current in response to a stimulus. The electric current travels down axons, the branching terminal ends of the cell, which transmit the electrical signal to other axons, muscles, or glands. ^[1]

At the end of an axon, the axon terminals interact with other neurons to continue the signal, or with a tissue to produce a response. ^[1] For example, an axon terminal stimulating a muscle may cause that muscle to contract.

From its position in the horse’s head, the brain integrates information it receives from the body, makes decisions on necessary changes, and outputs a signal to implement those changes. There are three basic areas of the brain that process information: ^[2]

• Forebrain: The forebrain is primarily responsible for conscious decision making, perception of the surrounding world, and intentional movement. ^[2]
• Midbrain: The midbrain acts as a relay station, passing signals from the forebrain to the rest of the body. ^[2] It also has roles in auditory and visual processing.
• Hindbrain: The hindbrain is responsible for basic body functions that sustain life, such as heart rate, breathing, and digestion. ^[2]

Illustration: Dr. Ana Mesa, PhD

Within each brain section, there are several component parts that have their own major roles. These components are also susceptible to disease, resulting in neurologic clinical signs (symptoms). Evaluating the type of neurologic signs that a horse displays allows veterinarians to determine the location of the neurologic injury, and in some cases make a diagnosis.

The Forebrain

There are two main portions to the forebrain: the telencephalon and the diencephalon. ^[3]

The telencephalon is responsible for “higher-order” thinking, such as decision making and complex thought. This part of the brain is much larger in species with a high degree of intelligence, such as humans. The main component of the telencephalon is the cerebral cortex, a convoluted structure with several indentations (sulci) that covers most of the brain’s surface. ^[3]

The diencephalon sits below the telencephalon and is responsible for motor control, sensation, and some involuntary actions. Its main components are the thalamus, hypothalamus, pineal gland and pituitary gland. ^[3]

Cerebral Cortex

The cerebral cortex plays a major role in the horse’s day-to-day life, allowing them to move voluntarily, make decisions, and interpret their environment. There are several subsections to the cortex, each with their own dedicated function: ^[1]

• Olfactory bulbs: These are rounded structures at the very front of the brain that interpret smells and odors entering the nasal cavity
• Frontal lobe: A large section of the cortex near the front of the brain. Responsible for movement and planning the associated actions required
• Parietal lobe: The section of cortex at the top of the brain, behind the frontal lobe. Responsible for perceiving sensation
• Temporal lobes: Sections on either side of the brain, near the ears, that process auditory input
• Occipital lobe: The section at the back of the brain, near the cerebellum, that processes visual information from the eyes

The two halves of the cerebral cortex are connected by the corpus callosum. The corpus callosum is a group of nerve fibers that allow communication between the left and right sides of the brain.

Diseases of the Cerebral Cortex

Conditions affecting the cerebral cortex affect the horse’s personality and behavior. It can also affect their coordination and movement by affecting how they interpret the world around them. Common symptoms seen with cerebral cortex disease include: ^[4]

• Hyperexcitability
• Head pressing
• Circling
• Dull demeanor

There are two main diseases affecting the cerebral cortex in horses: hepatic encephalopathy and leukoencephalomalacia. ^[4]

Hepatic encephalopathy is a neurologic disease caused by liver failure. ^[4] The liver has an important role in removing ammonia waste products from the bloodstream after protein digestion. When horses develop liver disease, their damaged liver is unable to clear ammonia from the bloodstream appropriately. Ammonia is directly toxic to neurons in the frontal lobe. ^[4]

Leukoencephalomalacia is a condition caused by ingesting fungal toxins from moldy corn products. ^[4] The fungal toxin, fumonisin, affects the metabolism of certain lipids, resulting in the accumulation of sphingosine. ^[4] Sphingosine is toxic to neurons in the cerebral cortex, particularly in the frontal lobe. ^[4]

Thalamus

The thalamus is the largest component of the diencephalon, located between the cerebral cortex and the midbrain. Several nuclei, bundles of neurons, make up the thalamus. ^[5] Each nucleus has a different function. Some work to relay sensory signals from the body to the brain, while some send motor (movement) signals out to the muscles. ^[5] The thalamus also helps regulate consciousness and alertness. ^[5]

Thalamic disease is rare in horses and other species. ^[5] Occasionally, infectious diseases such as West Nile virus, Eastern Equine Encephalitis virus and Western Equine Encephalitis virus can affect the thalamus. ^[6] Symptoms of thalamic disease include: ^[6]

• Lethargy
• Weakness of the limbs
• Visual deficits

Hypothalamus

The hypothalamus is a structure immediately below the thalamus. Similar to the thalamus, the hypothalamus contains numerous nuclei. ^[7] Its main role is regulating the endocrine system, the hormonal system that triggers critical body functions such as growth, reproduction, water balance, and metabolism. ^[7]

Similar to the thalamus, hypothalamic disease is rare in horses, although infectious diseases can sometimes affect this area. ^[6] Pituitary adenomas, such as which commonly lead to pituitary pars intermedia dysfunction (PPID), can also compress the hypothalamus and cause damage. ^[6] Damage to this structure may affect growth rate, gestation length, and metabolism by interfering with hormone secretion. ^[8]

Pineal Gland

The pineal gland sits below and slightly behind the thalamus and is made up of specialized cells called pinealocytes. ^[9] These cells produce melatonin, which regulates the body’s circadian rhythm, in response to light entering the eyes. ^[9]

For horses, their circadian rhythm affects both their sleep and reproductive cycles, making them seasonal breeders who enter reproductive cycles only when days are long. ^[6] Disease of the pineal gland is extremely rare in horses.

Pituitary Gland

The pituitary gland extends from the base of the brain, just below the hypothalamus. ^[10] There are two main components to the pituitary gland: the neurohypophysis and the adenohypophysis. ^[10]

The neurohypophysis produces the hormones oxytocin, involved in milk letdown and uterine contractions, and antidiuretic hormone (ADH), which regulates blood volume via the kidneys. ^[10]

The adenohypophysis produces several hormones based on signaling from the hypothalamus: ^[11]

• Adrenocorticotropic hormone, which regulates the adrenal gland
• Prolactin, which initiates lactation in pregnant animals
• Luteinizing hormone and follicle stimulating hormone, which have important roles in reproduction
• Growth hormone, which stimulates growth and repair of tissues
• Thyroid stimulating hormone, which regulates metabolism

Diseases of the Pituitary Gland

One component of the adenohypophysis, the pars intermedia, is particularly significant in horses. This section of the adenohypophysis regulates the production of adrenocorticotropic hormone. ^[8]

Enlargement or dysfunction of the pars intermedia leads to pituitary pars intermedia dysfunction (PPID) in horses. This condition, also called Cushing’s, results in reduced hormone production by the pituitary gland due to increased levels of dopamine. ^[8] Treating these horses with pergolide (Prascend®), a drug that suppresses dopamine, allows the pituitary to return to normal function. ^[8]

Fescue toxicosis is another condition affecting the pituitary gland in horses. This condition occurs in pregnant mares, where dopamine-like compounds ingested from fungus-infested fescue grass reduce hormone production by the pituitary gland. ^[8] Due to reduced production of prolactin, the mare’s reproductive system does not receive signals to trigger mammary gland development and lactation. ^[8]

The Midbrain

The midbrain sits between the forebrain and the hindbrain and acts as a relay station, passing signals from the forebrain to the rest of the horse’s body. ^[12] It also has roles in consciousness and sleep. ^[12]

Two of the major cranial nerves, the oculomotor nerve and the trochlear nerve, emerge from the midbrain. ^[12] These nerves coordinate movement of the eyes and eyelids. They also help with constriction or dilation of the pupil. ^[12]

Diseases of the Midbrain

The most significant disease of the midbrain in horses is nigropallidal encephalomalacia. Horses develop this condition after consuming the plants yellow star thistle or Russian knapweed over a prolonged period. ^[4]

These plants contain repin, a compound which may interfere with dopamine release in the midbrain. ^[4] Without adequate dopamine stimulation, horses develop neurologic signs such as: ^[4]

• Continuous chewing motions
• Frequent yawning
• Difficulty picking up food
• Difficulty drinking water

Other diseases, such as infectious diseases, may also affect the midbrain. General symptoms of midbrain disease include: ^[6]

• Coma
• Paralysis or weakness
• Dilated pupils
• Irregular eye movements
• Unequal pupil sizes

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The Hindbrain

The hindbrain has three major structures: the pons, medulla oblongata, and cerebellum. ^[3] These components work together to maintain basic life functions, such as heart rate, breathing, balance, and digestion.

Pons

The pons sits between the midbrain and the medulla oblongata. ^[13] It primarily contains nerve fibers that transmit information from the forebrain to the cerebellum. ^[1] There is also a nucleus of nerve fibers that control the muscles used in chewing. ^[1]

Infectious diseases are the most common conditions affecting the pons in horses. Diseases include: ^[1][4]

• West Nile virus
• Rabies
• Equine protozoal myeloencephalitis

Symptoms of disease in the pons include: ^[6]

• Lethargy
• Ataxia (uncoordinated movement)
• Weakness and paralysis
• Head tilt
• Facial paralysis
• Abnormal eye movements

Medulla Oblongata

The medulla oblongata connects the rest of the brain to the spinal cord. ^[14] It receives signals from sensory nerves in the body, sends them to the brain for interpretation, and outputs motor signals to adjust the body’s functions. ^[14] It also maintains heart rate, respiratory rate, and blood pressure. ^[14]

Disease of the medulla oblongata is rare in horses, but can occur in infectious diseases like rabies. ^[4] Symptoms of medulla oblongata disease include: ^[6]

• Lethargy
• Ataxia (uncoordinated movement)
• Weakness or paralysis
• Abnormal breathing
• Difficulty eating
• Loose or floppy tongue

Cerebellum

The cerebellum is a small, rounded structure at the back of the brain. ^[15] It controls muscle tone, intentional muscle movement, and balance. ^[15]

The cerebellum receives information from the body, which is processed by Purkinje cells in the outer layer of the cerebellum. ^[1] These cells send signals to nuclei in the middle of the cerebellum, which then output signals to motor neurons – neurons responsible for activating movement. ^[1] These motor neurons tense or loosen muscles as necessary to maintain balance or produce a desired movement.

Disease of the Cerebellum

The main disease affecting the cerebellum is cerebellar abiotrophy, a condition where the cerebellum incompletely develops in newborn foals. ^[6] This condition is most common in Arabians. ^[6] Studies suggest that the disease is likely inherited in this breed. ^[6]

The cerebellum is also susceptible to infectious diseases, such as: ^[6]

• Equine protozoal myeloencephalitis
• Strangles
• Equine herpesvirus-1

Symptoms of cerebellar disease include: ^[6]

• Ataxia (uncoordinated movement)
• Exaggerated movements
• Intention tremors: tremors when moving their head to interact with objects, people, or other animals

Frequently Asked Questions

Here are some frequently asked questions about the horse's brain:

Summary

The horse's brain is a complex organ made up of a diverse network of cells, including neurons and glial cells, that work together to control and coordinate all of the body's systems.

• The horse's brain is much larger than a walnut, weighing approximately 700 grams - closer to the size of an average grapefruit - and makes up 0.1% of total body mass
• The brain is divided into three sections: the fore, mid, and hind brain
• Overall, brain disease is rare in horses, but contagious illness and PPID are more common
• Brain disease is usually associated with neurological symptoms including ataxia, changes in vision, circling, head tilt, behavior changes, and paralysis