By Dr. Sandra J. Howlett, DC, CST-D, Certified Veterinary Chiropractor
Veterinary Compendium Webinar Content Advisor
Does a horse “drop” his penis when calm and relaxed, or when he is in a state of anxiety, fear, or confusion?
The terms “drop”, “dropping” and “dropped” are often used by owners and practitioners to refer to both the limp and the erect penis. The proper term is tumescence or penile erection – the physiologic process during which the penis becomes engorged with blood, usually in response to sexual arousal but sometimes spontaneously.
Let’s explore this question by looking at the Autonomic Nervous System (ANS) and the interplay of the Sympathetic Nervous System (SNS) and Parasympathetic Nervous System (PSNS).
The ANS plays a critical role in orchestrating and coordinating involuntary bodily functions, thereby maintaining physiological homeostasis.
We often believe that the ANS is an either/or state of being, where the horse is either in a state of rest and digest or in a state of flight or fight. But the ANS is better viewed as a “both/and” state of being – the interplay of the SNS and PSNS can operate independently, simultaneously, as well as cooperatively. This gives the ANS system exquisite flexibility and adaptability.
When we think of neurotransmitters and the ANS, we default to norepinephrine/noradrenaline as the neurotransmitter for the SNS, as it is part of the flight or fight response, helping the body to prepare for action.
When we think of the PSNS, we default to acetylcholine as being the neurotransmitter, as it is the neurotransmitter from parasympathetic postganglionic neurons, as well as being a crucial neurotransmitter in the body – a chemical messenger between nerve cells and other cells (muscle movement, memory, thinking, and learning).
The drawing here shows the neurotransmitters involved in tumescence. Here are brief descriptions for the neurotransmitters not yet described:
GABA: Inhibitory, crucial for regulating anxiety and fear responses
Glutamate: Excitatory, involved in learning, memory, emotional processing, especially in stressful situations
Dopamine: Linked to reward and motivation, also regulating emotional responses; contributes to a sense of well-being and supports parasympathetic dominance
Serotonin: Modulates fear responses and emotional processing; enhances mood, regulates pain pathways, supports parasympathetic activation
Nitric oxide: Facilitates smooth muscle relaxation, vasodilation, enhances blood flow and gastrointestinal function; it is retrograde, which means it is produced in postsynaptic neurons and diffuses back to presynaptic neurons so that it can modulate the release of other neurotransmitters and influence synaptic plasticity
So how does it all work?
It’s actually quite complex.
Using the drawing as reference, there is constant exteroception – defined as the horse’s sense of the external world, involving the perception of stimuli the origin of which is outside the body, such as sights, sounds, smells, tastes, and touch.
And there is constant interoception – defined as the body’s ability to sense and interpret its internal states, so that it knows what is happening inside the body, with recognizing and understanding sensations like hunger, thirst, pain, heart rate, internal state of organs and tissues, as well as emotional responses.
It’s all about self-regulation and responding appropriately to the horse’s own needs, both consciously and non-consciously. It includes physical sensations AND emotions.
So when a male horse is being trained in a learning environment, especially when he is seeking a reward, both exteroception and interoception are at play. (We won’t discuss proprioception here.)
We all know that sometimes mixed signals are given and received back and forth between the teacher and the student (not necessarily always human-to-horse as it may also occur horse-to-horse), so sometimes when these neurotransmitters fire, either inhibiting responses or exciting responses, having cause and effect, this may result in tumescence.
So the horse may be totally relaxed… or possibly in a confused, anxious state… or in a state of various combinations.
When we are offering appropriate bodywork, tumescence often occurs. So when we consider our touch (exteroception) and the body’s response to touch (interoception), and we look at the neurotransmitters involved in the lumbopelvic area, it is easy to interpret this as a relaxed state.
However, since we are dealing with complex neurophysiology, we need to use other measurements and observations to support our interpretation, e.g., feeling the tissue soften and lengthen in our hands, possibly spontaneous pandiculations, hearing increased gut sounds, to name but a few. And we must remember the brain’s involvement as well, no matter where we have placed our hands.
Of course, this is all happening outside of our control and is relying on the horse’s autonomic nervous system functioning smoothly, maintaining physiological homeostasis and coordinating involuntary bodily functions, as it is designed to do.
Here is an interesting observation from a breeding facility: When the stallion is taken from his stall and the handler turns left, exiting the door at that end, the stallion has learned that he is going to be ridden, and he does not “drop”. When the stallion is taken from his stall and the handler turns right, exiting the door at that end, the stallion knows that he is going to perform his duties as a stud, and he “drops” immediately.