Hummingbirds are amazing little creatures. Their ability to hover mid-air and fly backwards sets them apart from other birds. But what about their anatomy – do hummingbirds have a spinal cord?
The quick answer is yes, hummingbirds do have a spinal cord. Like all vertebrate animals, hummingbirds have a spine that runs from their head down to their tail. This spine is made up of small bones called vertebrae that are connected together. The spinal cord runs through the hollow center of these vertebrae, allowing signals to travel between the brain and the rest of the body.
The spinal cord is a major part of the central nervous system in all vertebrates, including birds. It carries motor signals from the brain down to the muscles to initiate movement. It also transports sensory information from the skin, joints, and muscles back up to the brain. The spinal cord allows for rapid communication between the brain and body which is essential for the complex movements that hummingbirds perform.
Anatomy of a hummingbird spinal cord
Hummingbirds have vertebrae that are fused into a solid structure called the notarium or the fused thoracic vertebrae. Unlike mammals that have distinct cervical, thoracic, lumbar, sacral, and caudal sections, a hummingbird’s vertebral column consists primarily of the thoracic vertebrae. There are between 11-17 thoracic vertebrae in most hummingbird species.
The hummingbird’s spinal cord runs through the canal formed by the hollow thoracic vertebrae. It extends from the brain stem all the way to the end of the vertebral column, connecting to nerves that serve the tail feathers and surrounding musculature.
Though fused, each vertebra has a ventral neural arch that protects the spinal cord as it descends through the column. Intervertebral discs separate each of the thoracic vertebrae, allowing flexibility and shock absorption.
Like other birds, hummingbirds do not have the same complex spinal curvatures (lordosis and kyphosis) found in mammalian spinal cords. Their spinal cord is relatively straight. The thoracic vertebrae are tightly connected to the sternum or breastbone by short ribs. This gives added protection and stability to the spinal cord within.
Segments of the hummingbird spinal cord
The hummingbird spinal cord can be divided into segments, each supplying a different region of the body:
- Cervical spinal cord – Connects to nerves of the head and neck
- Thoracic spinal cord – Connects to nerves of the torso and wings
- Lumbar spinal cord – Connects to nerves of the legs and feet
- Sacral spinal cord – Connects to nerves of the tail
The thoracic spinal cord makes up the majority of the length of a hummingbird’s spinal cord. This region innervates the wing muscles that allow them to beat their wings up to 80 times per second!
The cervical spinal cord connects to the nerves that control the neck, head, and bill movement. This allows precision control during the rapid head and bill maneuvers that hummingbirds use to drink nectar while hovering.
The lumbar and sacral sections make up a short portion of the spinal cord extending past the wings to connect to the nerves of the legs and tail. The strong control of these regions allows for abrupt stops, backward flight, and precision hovering.
Internal structure
If we look inside the spinal cord, it contains:
- Grey matter – Made up of neuron cell bodies and short nerve fibers
- White matter – Made up of longer nerve fibers called axons
- Central canal – A hollow channel in the center containing cerebrospinal fluid
The grey matter is located more internally, shaped like a butterfly in cross section. The white matter surrounds it completely.
The neuron cell bodies in the grey matter receive sensory input from the body and also send motor commands down the spinal nerves. The white matter allows rapid conduction of signals to and from the brain through longer nerve fibers.
The central canal runs the length of the hummingbird’s spinal cord as a continuous hollow tube. It is filled with cerebrospinal fluid which circulates nutrients and chemicals.
Spinal cord function
The key functions of the hummingbird spinal cord include:
- Connecting the brain to the peripheral nervous system
- Providing a two-way communication pathway via nerves
- Allowing reflex arcs that produce quick, automatic reactions
- Coordinating basic patterns of movement through central pattern generators
- Acting as a minor conduction center, making some decisions without brain input
The spinal cord acts as the main information super-highway between the brain and the rest of the body. It relays commands from the brain’s motor cortex down motor nerves to initiate muscle contractions. It also sends sensory information from the body up sensory nerves to the brain’s sensory cortex.
Reflex arcs allow quick protective reactions without needing input from the brain. For example, if a hummingbird touches a hot object, sensory nerves send a signal into the spinal grey matter which can immediately activate withdrawal reflexes and fly away before the message even reaches the brain.
The spinal cord also houses central pattern generators that coordinate complex muscle movement patterns. This allows a certain degree of walking, flying or other rhythmic motions without constant input from the brain.
Lastly, the spinal grey matter can integrate some information and make decisions about muscle contractions on its own. This local control allows swift reactions and smoother motor control.
Development of the hummingbird spinal cord
The spinal cord develops early in hummingbird embryonic development. Here is the timeline:
- Fertilized egg – After fertilization, rapid cell division begins
- Neural plate – A thickened strip of cells appear along the embryo’s back
- Neural tube – The neural plate rolls inward forming a hollow tube
- Differentiation – The hollow tube forms distinct regions including the spinal cord
In the early hummingbird embryo, a specialized group of cells forms called the neural plate. This strip thickens along the back surface of the embryo.
During neurulation, the edges of the neural plate start folding inward, sinking into the embryo and rolling together. This forms the hollow neural tube, which sinks below the surface.
The anterior end of the neural tube enlarges into brain regions. The rest lengthens down the back to form the spinal cord. It remains open at both ends while the brain and tail vertebrae develop.
Cellular differentiation causes distinct grey and white matter regions to emerge. The spinal cord enlarges and elongates within the vertebral column as the embryo grows.
Evolution of the spinal cord
The spinal cord has evolved over millions of years across vertebrate species:
- Ancient jawless fish – Primitive spinal cord in a notochord
- Sharks – Enlarged spinal cord controlling a tail fin
- Amphibians – Adapted to use limbs for land locomotion
- Reptiles – Spinal cord controlling greater limb coordination
- Birds – Lightweight fused spine to optimize flight
In primitive vertebrates like hagfish, the spinal cord was just a simple collection of neurons running through a flexible notochord.
As vertebrates evolved, the spinal cord enlarged to handle increased sensory input and motor output. Specialized limb circuitry developed as animals adapted to move on land.
In birds, the spinal cord has evolved to be as light as possible, with fused vertebrae for flight. The thoracic regions expanded to handle sophisticated wing control and aerobatic maneuvers.
Hummingbirds have evolved an extremely lightweight, yet highly adapted spinal cord to meet the demands of their unique hovering ability and rapid flight.
Injuries to the hummingbird spinal cord
Hummingbird’s can injure their spinal cord through trauma such as:
- Collisions – Hitting walls, windows, cars, trees, or the ground
- Attacks – Predators such as cats can damage the spine
- Falls – Falling from high nests or feeders onto hard surfaces
The types of spinal injuries seen include:
- Fractures – Breaks in the vertebrae
- Dislocations – Misalignment of the vertebrae
- Contusions – Bruising or hemorrhage in the cord
- Laceration – Partial or complete severing of the cord
Any damage to the spinal cord can be catastrophic since it interrupts critical communication between the brain and body. Even a small amount of swelling or bruising can lead to partial or complete paralysis.
If the spinal cord is completely severed, it can lead to permanent loss of function in the legs, wings, tail, and bodily organs below the injury. Hummingbirds with severe spinal trauma generally have a very poor prognosis.
Interesting facts about the hummingbird spinal cord
Here are some additional interesting facts about the spinal cord in hummingbirds:
- Makes up about 60% of the total nervous system
- Extends through the neck down to the 6th or 7th caudal vertebrae
- Constitutes about 1-2% of their total body weight
- Has enlargements in the cervical/brachial and lumbosacral regions
- Has between 34-45 spinal nerves attached along its length
- Transmits signals at speeds up to 300 mph
- Coordinated over 200 wing beats per second during flight
The hummingbird spinal cord and connected nerves make up over half of its entire nervous system. It is relatively large compared to the tiny bird’s body size.
Enlargements in specific areas help innervate the wings and legs. Spinal nerves branch out in pairs all along the cord to reach every part of the body.
The spinal cord enables blazing fast signaling critical for controlling hovering, aerobatics, and precision flying. It is an anatomical marvel tailored to the hummingbird’s specialized flying abilities.
Conclusion
In summary, hummingbirds do have a spinal cord that runs through their vertebral column allowing communication between the brain and body. It develops early in embryogenesis forming the foundation of the nervous system.
The hummingbird spinal cord has evolved to be extremely lightweight yet highly specialized for rapid flight control. It transmits neural signals at astonishing speeds to coordinate their specialized hovering capability and nimble maneuvers. Losing function of this vital structure is often catastrophic.
So the next time you see a hummingbird hover in midair, remember it is the hyper-efficient spinal cord transmitting the brain’s commands that makes this possible!
