Human vs. horse neuroanatomy
Comparative anatomy of the human vs. horse motor and sensory pathways
Variations in the 'wiring' for different lifestyle and survival priorities!
Comparing the neuroanatomy of humans and horses is a fascinating study in how evolution prioritises different survival needs. While the basic 'wiring' is very similar - using upper and lower motor neurons and specialized sensory receptors—the scaling and complexity of these pathways differ significantly.
1. Key Structural Differences
Horses have really long nerves
You thought human nerves were long - a lower limb motor neuron can be well over a metre long to reach from its cell body in, for example, the S2 segment to the intrinsic muscles of the foot. But because horses are much larger, their axons must be incredibly long. A single motor neuron in a draft horse can be over two metres long - in fact the longest nerve in a horse is the recurrent laryngeal nerve that reaches from its nucleus in the brainstem, all the way down the neck to the the aortic arch and then loops back on itself!
To maintain speed of transmission over these distances, horses rely on heavy myelination and large diameter axons to prevent "lag" in their reflexes.
Proprioceptive Loading
Horses have a much higher density of muscle spindles in their neck muscles compared to humans. This provides the brain with constant, high-speed feedback about the position of their heavy head and neck, which acts as a 'balance beam' for the rest of the body.
Cerebellar size
The horse's cerebellum is relatively larger (unlike their overall brain size!) than the human's and is already well developed at birth.
The human cerebellum is almost empty at birth, with the cerebellar stem cells developing over the first few years of infancy and childhood - that's why children 'toddle' and fall over rather a lot!
2. Motor Pathways: Precision vs. Power
The primary difference lies in the corticospinal tract (CST), which governs voluntary movement.
Human Motor Pathways
- Direct Control: Humans have a highly developed corticospinal (pyramidal) tract that connects the motor cortex directly to spinal cord neurons. This allows for conscious voluntary control with incredible dexterity and fine motor skills (like typing or surgery).
- Decussation: About 90% of fibers decussate (cross over), leading to strong contralateral control.
- Decussation allows for a massive amount of 'discussion' between the two hemispheres via the corpus callosum.
- This is vital for complex tasks like walking or galloping, where the left and right sides must be perfectly synchronised but independently adjustable.
- However, decussation is absolutely critical for language and speech. While language is primarily 'lateralised' to the left hemisphere in the majority of people, the physical act of speaking requires massive, high-speed coordination of muscles that are inherently decussated.
- Focus: Significant cortical space is dedicated to the hands, face, and tongue.
Equine Motor Pathways
- Indirect Control: In horses, the cortical spinal tract is much smaller and less influential. Instead, they rely heavily on extrapyramidal pathways (like the rubrospinal and reticulospinal tracts).
- these pathways have many connections with the cerebellum, basal nuclei, pons as well as the cerebrum.
- they also have more connections at the spinal cord level leading to more
- automation: These pathways are better suited for rhythmic, powerful movements like galloping and maintaining posture. Horses are essentially 'hard-wired' for locomotion from birth.
- Brain vs. spine: A horse's spinal cord has more direct reflex arcs at the local spinal levels. It therefore can coordinate complex gaits with less input from the higher brain.
Fig 1: Corticospinal (pyramidal) tract
High levels of voluntary control.
Fig 2: Vestibulospinal (extrapyramidal) tract
High levels of reflex and unconscious reactions.
3. Sensory Pathways: Fine Touch vs. Environmental Awareness
Both species use the
dorsal column-medial lemniscus (fine touch/proprioception) and
spinothalamic(pain/temperature) pathways, but the sensitivity profile is specialised:
