POSTMORTEM REFLEX ACTIVITY
- Lower reflex activity occurs quite often in meat animals and
poultry after exsanguination and loss of consciousness.
- The intrinsic activity of individual muscle fibers is imperceptible
in a whole carcass, so visible movements of large parts of the body
must involve some degree of neural coordination.
- Postmortem loss of excitability in the axons of motor neurons
follows a proximal to distal pattern, is preceded by
hyperexcitability, and the rate of loss of excitability depends on
temperature.
Neural Pathways
In a recently slaughtered commercial meat carcass, motor neurons in
the ventral horn of the spinal cord receive both inhibitory and
excitatory inputs, but the descending and ascending tracts that
link the brain and spinal cord in meat animals are polysynaptic, so
that motor neurons have a high degree of autonomy relative to the
condition in humans.
Meat animals have only a small direct motor cortex in the cerebrum,
only short motor tracts descend the spinal cord, and sensory areas
of the cerebral cortex are dominated by inputs from the lips and
snout while areas corresponding to the limbs and trunk are very
small.
Source of Activity
- Neural activity may originate intrinsically from motor neurons that
have just been released (by cell death or disruption) from their
normal inhibitory control.
- Motor neurons are normally restrained by a variety of inputs, such
as those from Renshaw cells.
- True reflexes such as the myotactic or stretch reflex of muscles
might occur briefly in carcasses.
- Myotatic reflexes normally act via a sensory input from
neuromuscular spindles so that a muscle will contract to resist a
stretching force.
Inhibition of Activity
- Carcasses normally are suspended on an overhead rail, and some
muscles are stretched before exsanguination.
- Because severe stretching occurs in some muscles, an opposite
response may be created by activation of Golgi tendon organs.
- Golgi organs are located on tendons and, following extreme muscle
stretching, they inhibit appropriate motor neurons and cause sudden
muscle relaxation.
- Stunned animals are usually shackled by only one hindlimb, so
reflex jerking by the free hindlimb may originate from a crossed
extensor reflex acting through reflex pathways normally associated
with locomotion or withdrawal from a painful stimulus.
EMG (Electromyography)
Reflex activity during slaughter may be seen by electromyography
during the time that the carcass is handled after stunning, as in the example below, which shows EMG activity just before, during and after the point of exsanguination (at 0 sec).
EMG may reveal isometric muscle contraction in the shackled limb
that is not otherwise easily visible.
Respiratory Reflexes
As the degree of reflex activity diminishes, the animal's
respiratory reflexes become evident.
Most commonly, the animal coughs and moves its head downwards or
ventrally. This may be detected by EMG activity in the ventral
muscles of the throat or by bouncing of the carcass, detected with
a load cell in the shackling chain, as shown in the example below.
In this example, the upper line shows the bouncing of the carcass on a load cell in the shackling chain, while the lower line shows integrated EMG activity of the throat muscle. Thus, every time the head was pulled down by a postmortem reflex, it was detectable by both methods.
- Less commonly, the pig throws its head backwards or dorsally so
that its vertebral column is concave dorsally.
- In lambs, post mortem reflex activity may be minimized by
sequential application of a head stun, a current across the heart
to stop it, and then a current down the spinal cord to depolarize
spinal neurons.
Commercial Importance
Reflex activity after stunning is important in the meat industry
because it may hinder exsanguination, or accelerate muscle
metabolism to cause PSE.
References
