Reflex actions can be suppressed or overridden, though it typically requires conscious effort and practice. This is because reflexes are automatic and occur without conscious thought. The ability to suppress a reflex involves higher brain functions, particularly from the cerebral cortex. For instance, it's possible for trained individuals, such as dancers or athletes, to suppress certain reflexes to perform specific movements that would otherwise trigger a reflex action. However, not all reflexes can be easily suppressed. For example, suppressing the blink reflex can be challenging. The process of suppressing a reflex often involves 'retraining' the nervous system through repeated, controlled exposure to the stimulus, allowing the brain to learn to inhibit the automatic response.

Neurotransmitters play a crucial role in transmitting signals across synapses within a reflex arc. When an electrical impulse reaches the end of a neurone (the presynaptic terminal), it triggers the release of neurotransmitters. These chemical messengers are released into the synaptic cleft, the small gap between neurones. The neurotransmitters then bind to specific receptors on the postsynaptic neurone, causing ion channels to open and leading to the generation of a new electrical impulse in the next neurone. In a reflex arc, this process is vital for transmitting signals from sensory neurones to relay neurones and then to motor neurones. The most common neurotransmitter involved in this process is acetylcholine, which plays a key role in transmitting nerve impulses to muscles, enabling reflex actions like muscle contraction.

The speed of nerve impulse transmission in a reflex arc is typically faster than regular nerve transmission. This increased speed is due to several factors. Firstly, reflex arcs often involve myelinated neurones, which allow for faster nerve impulse conduction through saltatory conduction, where the nerve impulse jumps from one node of Ranvier to the next. Additionally, reflex arcs are usually shorter and more direct pathways compared to the longer and more complex pathways used in regular nerve transmissions. In some reflexes, especially monosynaptic ones like the knee-jerk reflex, the absence of interneurons reduces the time needed for synaptic transmission, thereby speeding up the response. This rapid transmission is crucial for reflex actions, as they need to be quick to effectively protect the body from harm.

A monosynaptic reflex arc involves only one synapse between the sensory and motor neurones, making it the simplest and fastest type of reflex arc. The classic example is the knee-jerk reflex, where the sensory neurone directly communicates with the motor neurone, resulting in a very rapid response. In contrast, a polysynaptic reflex arc involves one or more interneurons (relay neurones) between the sensory and motor neurones. This inclusion of interneurons introduces one or more additional synapses, hence the term 'polysynaptic'. The presence of these extra synapses makes the reflex slower than a monosynaptic reflex but allows for more complex responses. Polysynaptic reflexes include the withdrawal reflex, where the body pulls away from a painful stimulus. This complexity allows for integration and processing of information within the reflex arc, leading to a more coordinated response.

Reflex arcs are a fundamental component of the nervous systems of most animals, but they do vary between species in complexity and function. In simpler organisms, like invertebrates, reflex arcs are often basic, serving rudimentary functions necessary for survival, such as withdrawal from harmful stimuli. In more complex animals, including mammals, reflex arcs are more sophisticated and can involve more intricate neural circuits, allowing for a wider range of reflexive responses. This complexity allows for better adaptation to different environments and more nuanced interactions with their surroundings. In humans, reflex arcs are involved not only in simple reflexes like the knee-jerk reflex but also in more complex autonomic functions such as regulating heart rate and digestive processes. The variation in reflex arcs among different species reflects the diversity in anatomical and physiological adaptations across the animal kingdom.