How exactly does behaviour work and what happens in the body at that moment? 

Behaviour and stimuli 

An animal continuously receives information, both from the environment and from its own body. It needs this information to survive and to reproduce, the two main goals in an animal’s life. The information that the animal, a horse in this case, receives are called stimuli. A horse’s behaviour is determined by internal stimuli (its own motivation) and external stimuli. Internal stimuli are the motivation for the horse to respond or not to respond to an external stimulus. You could describe an external stimulus as a change in the environment that you can perceive with one of your senses. An example of a stimulus and subsequent behaviour is: a horse in nature is thirsty (internal stimulus), it sees a pool of water (external stimulus) and starts drinking (behaviour). 

Processing of stimuli: the nervous system and endocrine system 

What exactly happens when a stimulus enters the body? The body has two systems for this: the nervous system and the endocrine system. The nervous system consists of the central nervous system (brain and spinal cord), peripheral nervous system (nerves) and autonomic nervous system (regulates the function of the internal organs). The entire system plays a coordinating role in all actions in the body, including the processing of stimuli. To illustrate how this stimulus processing works, let’s look at an example of a horse fleeing. 

The horse is quietly grazing. Its senses then perceive a change in the environment, with its ears it hears a rustling sound (= external stimulus) and with its eyes it sees movement in the bushes (= external stimulus). These stimuli are converted into an electrical message by the senses. This is then converted into the release of neurotransmitters, which are chemicals that contain messages. A sensory nerve cell transfers the message from the sensory cells to the central nervous system. This is processed in the brain and it is determined whether and what action should be taken. The horse wants to survive (= motivation) and its instinct is to flee at the sight of possible danger. So the brain has to make sure that the body goes into flight mode. Via the motor neuron cells, the brain sends a signal to those muscles that are necessary for the horse to run away. The moment the horse is at a safe distance, its motivation to flee will decrease. After all, it is no longer in immediate danger. Because its motivation decreases, the behaviour will also decrease, and it will continue with the next behaviour that motivates it, for example eating. 

The hormone system works in a similar way, only it works with hormones instead of neurotransmitters. Hormones are also signal substances, just like neurotransmitters, and are involved in a variety of bodily processes. The difference is that hormones are transported through the bloodstream and not through nerve cells. This type of transport is slower than through the nervous system, and it can therefore take longer for the body to respond to hormones (ranging from a few seconds to several days). There are a number of glands in the body that produce hormones. Some glands control their own production, but the majority is controlled by the pituitary gland (part of the brain). Let’s now continue with the example of the fleeing horse. The moment the brain activates the flight response, a signal is not only sent to the motor neurons, but also to the pituitary gland. The pituitary gland then directs the adrenal cortex (another gland) to produce the hormone adrenaline. Adrenaline provides a number of functions, including accelerating the breathing and heart rate, releasing energy, reducing blood flow to the intestines (digestion is not a priority) and increasing alertness. These are all functions that will increase the chance of survival at that time. If the flight behaviour decreases, the production of adrenaline will also decrease. 

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So in short: The horse receives external stimuli. This information is passed on to the central nervous system. Here the information is processed and it is determined whether to respond to the external stimulus. If this motivation (internal stimulus) is there, the body will be directed to take the correct action(s). These actions are reflected in the behaviour of the horse. 

How an animal responds to a certain stimulus, in other words what behaviour it shows, has to do with its genes and experiences. We humans will react very differently to a rustling bush. We perceive the same stimuli as the horse (hearing and seeing the rustling), but our brains are programmed differently, so that other actions will be taken. We would rather walk towards the bush to investigate where that sound is coming from. And if it turns out to be a danger, we would rather fight to survive.