Table 1.
Reflexes used to assess unconsciousness in livestock after stunning
| Present in animals that are 1 | |||||
|---|---|---|---|---|---|
| Reflex | Definition | Conscious | Unconscious | Based on | Remarks |
| Brain stem reflexes | Reflexes that originate from the brain stem | Functional cranial nerves originating from the brain stem | Reflexes may be present in animals that are unconscious, depending on the method of stunning (Gerritzen and Hindle, 2009) Absence of these reflexes though are considered valuable indicators for assessing unconsciousness (von Holleben et al., 2010) Cannot be tested when seizures occur (Blackmore and Delany, 1988) | ||
| Cornea reflex | Involuntary blinking in response to stimulation of the cornea | + (−) | − (+) | Functional cranial nerves V and VII and eye muscles | One of the most commonly used reflexes after stunning In general the last reflex to be lost in anaesthetised animals (Dugdale, 2010) May be present after electrical stunning, but never after effective captive bolt stunning (Roos and Koopmans, 1936; Gregory and Shaw, 2000) |
| Palpebral reflex | Involuntary blinking in response to touching the medial canthus of the eye | + (−) | − (+) | Functional cranial nerves II and III and eye muscles | Disappears earlier than the cornea reflex in anaesthetised animals (Dugdale, 2010) |
| Pupillary light reflex | Narrowing of the pupil in response to light that falls on the retina | + | − | Functional cranial nerves V and VII and eye muscles | Considered of little value during exsanguination, as the blood supply to the retina is restricted during this period (Blackman et al., 1986) Pupillary dilatation is considered a sign of total brain dysfunction (Blackman et al., 1986) May be absent in paralysed, though conscious animals (Blackmore and Delany, 1988) |
| Threat reflex | Involuntary blinking or withdrawal of the head in response to bringing a finger or hand with speed towards the eye of an animal | + | − | Functional cranial nerve VII, eye muscles and integration with motor cortex | Cannot be tested when the eyes are closed |
| Spinal reflexes | Reflexes that originate from the spinal cord | Require a functional spinal cord, but do not necessarily require cerebral coordination | May occur more vigorously when there is lack of inhibition from the brain (e.g. captive bolt stunning; Blackmore and Delany, 1988) | ||
| Pain withdrawal reflex | Withdrawal of the body part that has had a painful stimulus applied to | + (−) | − (+) | In a survey on expert opinion, the pain withdrawal reflex was ranked high, and thus valued highly, as an indicator to assess unconsciousness after all types of stunning (Gerritzen and Hindle, 2009) | |
| Pedal reflex | Withdrawal of the foot in response to pinching (the skin between) the toes of an animal | + (−) | − (+) | Difficult to assess when convulsions occur Not easy to perform in all species. Mainly used in poultry | |
| Righting reflex | Bringing the body into its normal position when taken out of its normal upright position | + (−) | − (+) | Difficult to assess when convulsions occur (Blackmore and Newhook, 1982; Anil, 1991) | |
1
Presence and absence of reflexes are presented as follows:+=present, −=absent, (+)=may be present, (−)=may be absent.