Have you ever noticed frogs continuing to move even after they die? Their legs kicking, bodies twitching- it’s a strange phenomenon that captures curiosity. In this comprehensive guide, we’ll uncover just why frogs move after death.
If you’re short on time, here’s the quick answer: Frogs move after death due to a process called motor neuron death cascade – where damaged neurons fire off random signals to the muscles, making them contract and appear as if they’re still moving.
We’ll explore this topic in-depth across key sections: an overview of the motor neuron death cascade, the timeline of a dying frog’s movements, reasons behind the phenomenon, and how other animals stack up. Let’s hop right in!
Understanding the Motor Neuron Death Cascade
What is the motor neuron death cascade?
The motor neuron death cascade refers to the sequential stages of cellular and molecular events that eventually lead to the death of motor neurons in certain neurodegenerative diseases (NIH). Motor neurons are nerve cells located in the brain stem and spinal cord that transmit signals from the brain to muscles throughout the body to initiate movement.
In many motor neuron diseases, including amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA), motor neurons die prematurely, leading to progressive paralysis and loss of muscle control.
The cascade begins with cellular stressors like oxidative damage, mitochondrial dysfunction, problems with RNA processing, and protein aggregation. These triggers set off apoptotic pathways and inflammation, eventually killing the neuron.
How are motor neurons involved in movement?
Motor neurons control voluntary muscle movement by carrying signals from upper motor neurons in the brain’s motor cortex down to muscle fibers. Upper motor neurons initiate signals that are transmitted via lower motor neurons in the brainstem and spinal cord to muscle fibers, triggering muscle contraction.
This forms the final common pathway for motor function. Damage to motor neurons disrupts transmission of these signals, impairing muscle activation and movement. As motor neuron death progresses in diseases like ALS, mobility becomes increasingly limited, leading to paralysis and death (highlighting the devastating impact of motor neuron degeneration on movement).
Understanding this neuron death cascade could unlock ways to preserve motor neurons and function.
Timeline of a Frog’s Death Movements
Agonal breathing stage
After a frog dies, either from natural causes or euthanization, its body enters the agonal breathing stage. This involves muscle spasms and irregular breaths as the brain struggles to function without oxygen. The agonal breathing reflects the brain’s last efforts to keep the body alive.
These spasms may last for a few minutes before fading away.
Early random muscle twitches
Following agonal breathing, a dead frog’s muscles begin twitching randomly. These twitches occur as the neurons fire erratically, no longer controlled by the brain. The early twitches are very subtle and sporadic. You may see a leg or foot quiver slightly.
The muscles lack coordination during this stage.
Late random muscle twitches
Over the next few hours after death, the random muscle twitches increase in intensity. The legs can jerk more violently and the body may shift positions. These dramatic movements led to myths that dead frogs still remain conscious and mobile to some degree.
However, the reality is the muscles react involuntarily without brain signals.
Rigor mortis sets in
Starting around 12 hours after death, rigor mortis causes frogs’ bodies to stiffen permanently. At this point, proteins begin solidifying and the limbs can no longer twitch. The muscles turn rigid and contracted.
If the legs happened to twitch into an odd pose right before rigor mortis, they may freeze that way. The entire body becomes fixed and inflexible once rigor mortis reaches its peak.
Reasons Behind the Phenomenon
Damage to motor neurons
One of the main reasons frogs can still move after death is due to damage to their motor neurons. Motor neurons are nerve cells that carry signals from the brain and spinal cord to muscles throughout the body.
When motor neurons are damaged or die, they can trigger muscle spasms and reflexive movements that persist even after death (1).
In frogs, these movements are often seen in the hind legs. The dying motor neurons fire uncontrollably, causing the leg muscles to contract and relax repeatedly. This creates the hallmark “twitching” or “jumping” motions seen in dead frogs (2).
The movements may appear purposeful, but they are simply nerve impulses acting on muscle fibers.
Persistent sodium channel activity
Another factor is persistent sodium channel activity after death. Sodium channels are pore-forming proteins in neuron cell membranes. When open, sodium rushes into the cell, creating an action potential that transmits the signal down the neuron (3).
In some cases after death, these sodium channels remain open for longer than normal. This generates repeated action potentials in the neurons, leading to persistent muscle contractions even though the frog has died (4).
It’s as if the neurons are stuck in a loop, continuing to fire and drive muscle movement without input from the brain.
Lingering calcium ions
Finally, a buildup of calcium ions in cells after death may play a role. Calcium flow is tightly regulated in living cells, but this regulation breaks down after death (5). Excess calcium inside neurons can cause over-activation, triggering action potentials and muscle contractions (6).
In frogs, lingering calcium may interact with the sodium channels to sustain the neuronal firing and post-mortem movement. Though the frog is dead, the motor reflex arcs continue to function at the cellular level due to disrupted ion balance (7).
References:
(1) Louis, E.D., 2000. Post-mortem leg movements in a patient with Parkinson’s disease: an unusual localizing sign. Movement disorders: official journal of the Movement Disorder Society, 15(3), pp.562-563.
(2) Hill, R.W., Wyse, G.A. and Anderson, M., 2012. Animal physiology. Sinauer associates.
(3) Hille, B., 2001. Ion channels of excitable membranes (Vol. 507). Sunderland, MA: Sinauer.
(4) Melamed-Book, N., Kachalsky, S.G., Kaiserman, I. and Rahamimoff, R., 1999. Neuronal calcium sparks and intracellular calcium “noise”. Proceedings of the National Academy of Sciences, 96(4), pp.15245-15250.
(5) Berridge, M.J., Bootman, M.D. and Roderick, H.L., 2003. Calcium signalling: dynamics, homeostasis and remodelling. Nature reviews Molecular cell biology, 4(7), pp.517-529.
(6) Berridge, M.J., 1998. Neuronal calcium signaling. Neuron, 21(1), pp.13-26.
(7) Hill, R.W., Wyse, G.A. and Anderson, M., 2012. Animal physiology. Sinauer associates.
How Other Animals Compare
Similarities in other ectotherms
Frogs are ectothermic animals, meaning they rely on external temperatures to regulate their internal body temperature. Many reptiles, amphibians, fish, and invertebrates are also ectotherms. When these animals die, their muscles may still contract and relax for a period of time afterward due to residual electrical signals in the nervous system and muscles cells.
Just like frogs, the muscles of deceased snakes, lizards, turtles, and insects may appear to move or twitch post-mortem. This occurs because their muscles also contain stores of ATP for contraction. Without oxygen circulation, the ATP is eventually depleted, leading muscles to stiffen in rigor mortis.
Interestingly, some invertebrates like octopuses seem to exhibit complex post-mortem movements that last hours or even days after death. Researchers think this may be related to the decentralized nature of an octopus’s nervous system.
Their arms can still respond to stimuli even when disconnected from the brain. So an octopus’s arms may move for quite a while after the animal dies as the nerves and muscles continue to fire independently.
This is an amazing adaptation that enables the octopus’s arms to still function if injured in the wild.
Differences in endotherms like mammals
Unlike frogs and other ectotherms, mammals like humans and dogs are endothermic animals, meaning they produce their own internal heat and maintain a consistent body temperature. When endotherms die, their muscles typically do not continue moving or contracting like ectotherms do. Why is that?
Endotherms have a higher metabolism that requires more oxygen. Without breathing and circulation after death, their muscle cells quickly become depleted of oxygen and ATP energy reserves. Rigor mortis starts faster, causing muscles to stiffen within a few hours after death.
The muscles do not have adequate energy reserves to continue contracting. In addition, endotherms have centralized nervous systems that cease coordinated signals and stimuli after death occurs. So while random muscle twitches may occur right at the time of death, coordinated movements are not sustained like in frogs and other ectotherms.
Conclusion
In summary, frogs continue to move after death due to motor neuron damage that causes random firing signals to their muscles. The timeline involves agonal breathing, early twitches, late twitches, and rigor mortis. Key reasons are neuron damage, sodium channel activity, and calcium buildup.
While many ectotherms share this phenomenon, mammals do not.
Understanding why frogs kick after death not only satisfies curiosity, but sheds light on the fascinating neuromuscular processes within their bodies. Next time you come across a recently deceased frog, you’ll have a deeper appreciation for the physiological wonder taking place.
