Why is a rat stiff? - briefly
A rat may exhibit rigidity because post‑mortem rigor mortis sets in, or because severe stress, injury, or neurological disease causes sustained muscle contraction.
Why is a rat stiff? - in detail
A rat becomes rigid when its muscles enter a state of involuntary contraction that cannot be relaxed. The most common mechanism is post‑mortem stiffening, which begins within minutes after death as calcium ions accumulate in muscle fibers, causing cross‑bridge formation that locks the sarcomere. This rigor mortis progresses from the neck and forelimbs to the hind limbs and tail, reaching maximum tension in 12–24 hours before enzymatic degradation gradually restores flexibility.
Other physiological conditions can produce similar rigidity while the animal is alive:
- Neuromuscular disorders – damage to the spinal cord, peripheral nerves, or motor neurons leads to loss of inhibitory signals, resulting in sustained muscle tone.
- Severe hypothermia – low body temperature slows enzymatic activity, increasing calcium retention in the sarcoplasmic reticulum and producing a cold‑induced contracture.
- Electrolyte imbalance – hyperkalemia or hypocalcemia disrupts the normal excitation‑contraction coupling, causing uncontrolled contraction.
- Toxin exposure – agents such as strychnine block inhibitory glycine receptors in the spinal cord, producing generalized rigidity and convulsive spasms.
- Traumatic injury – fractures or severe soft‑tissue damage can trigger reflexive muscle guarding, making the animal appear stiff.
Diagnostic evaluation should include:
- Observation of the onset and distribution of rigidity.
- Measurement of body temperature to assess hypothermia.
- Blood chemistry for electrolyte and toxin levels.
- Neurological examination to locate lesions in the central or peripheral nervous system.
- Post‑mortem examination if death has occurred, focusing on the progression of rigor and the presence of muscle or nerve pathology.
Understanding the underlying cause directs appropriate intervention, whether it is rewarming for hypothermia, antidotes for toxins, correction of electrolyte disturbances, or supportive care for neurological impairment.