Arousal is essential to regulating consciousness, attention, alertness and information processing. It is crucial for motivating certain behaviours including mobility, the pursuit of nutrition and the fight-or-flight response.
Differences in arousal levels are also thought to explain personality differences such as extraversion and introversion. Most arousal measures are either peripheral autonomic measures such as skin conductance or heart rate and central EEG recordings.
What Part of the Brain Controls Arousal?
When arousal is high, we’re more likely to take risks and feel motivated to perform. Arousal also affects our emotions, making us more likely to be aggressive or to experience fear or sadness. The brain is an important control center, regulating arousal levels and ensuring that our actions match our current emotions.
The forebrain consists of the cerebral hemispheres and, underneath them, the brain stem with its thalamus and hypothalamus. The thalamus is often referred to as the “switchboard” of the central nervous system, relaying signals from your senses and bringing them to your cerebellum for interpretation. The hypothalamus is a key hormone control centre, influencing your sex drive, autonomic function, hunger, thirst and sleep.
The limbic lobes are deep in the middle portion of your brain, encompassing the temporal lobes (near the ears on each side), parietal lobes (over the front of your head) and hippocampus (where short-term memory is stored). These parts of your brain are responsible for processing sensory inputs and giving them meaning, as well as controlling your body’s autonomic functions, perceiving pain, storing memories and understanding what you hear and see.
The Reticular Activating System
The reticular activating system, which is located in the brainstem, shapes arousal and consciousness by controlling what incoming sensory signals reach our conscious mind. It also plays a critical role in states of awareness like wakefulness and sleep. The reticular activating system is the origin of ascending pathways, which send messages to the thalamus and cerebral cortex that help to regulate arousal and the level of consciousness. The reticular activating system is important for things like pain modulation, as it contains nerve fibers that act in the spinal cord to block transmission of some painful signals to the brain.
A finely-tuned interaction between the central nervous system and the autonomic nervous system directly controls peripheral functioning, and is a key element of innate, reflexive responses. For example, if you were to walk through the woods and see a rattlesnake on the ground, you would feel alarmed (physiological arousal) but it is your past experience and knowledge of poisonous snakes and dangerous predators that provides the cognition of what to do next (cognition).
All of this work is controlled by a number of neurotransmitter systems. These include glutamatergic, cholinergic and aminergic pathways that have projections to the brainstem reticular formation from areas in the hypothalamus, basal forebrain and thalamocortical regions. There are also descending pathways, which terminate in the ventrolateral preoptic area and release GABA reuptake inhibitors that inhibit wakefulness and arousal.
The Hypothalamus
The hypothalamus is a part of the brain beneath the thalamus and above the pituitary gland. It is a complex region that contains several nuclei with different functions. It is a major regulator of homeostasis, the body’s automatic processes that maintain the body in a stable state. When different parts of the body and the environment send signals that something is out of balance, the hypothalamus sends a series of messages that trigger the appropriate physical response.
Some of the regions in the hypothalamus are involved with sleep, appetite and body temperature. For example, the suprachiasmatic nucleus receives sensory impulses from the retinal ganglion cells and synchronizes body functions with periods of light and dark, including releasing the sleep inducing hormone melatonin. The lateral nucleus of the hypothalamus is involved with hunger and satiety. It contains neurons that produce a neurotransmitter called orexin, which acts as a natural appetite suppressant.
The medial preoptic area, which is involved in sexual desire and arousal, has been the focus of intensive research. When researchers stimulated this brain region in animals, they found that their responses were influenced by the level of arousal. They also found that stimulating this area of the brain led to a decrease in activity in the dorsomedial prefrontal cortex, which is involved in moral reasoning and social judgment.
The Amygdala
The amygdala is an important part of the limbic system within the brain. It is key in processing emotions like fear and pleasure. It also stores memories relating to these emotional experiences. The amygdala has connections with many other areas of the brain, allowing it to connect information from systems that process “higher” cognitive responses (like decision making and motor movements) with those that control “lower” physiological functions such as breathing, touch, and sensitivity.
One of the most famous roles of the amygdala is controlling fear. It’s known that Pavlovian conditioning can cause the amygdala to learn to associate a neutral stimulus with an aversive unconditioned stimulus. For example, if you play a tone for mice right before you administer an uncomfortable foot shock, the mice will begin to freeze when they hear that tone. This shows that the amygdala has learned to associate the tone with a foot shock, which is considered a conditioned fear response. Mice with amygdala lesions lose their ability to acquire and express this type of conditioned fear.
The amygdala is also involved in recognizing dangerous and potentially life-threatening situations. When you are in a dangerous situation, your amygdala sends emergency signals to other areas of the brain to make you react quickly. This is why sudden loud noises can make you jump or flinch.
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