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Neurotransmission involves the release of neurotransmitters from the pre-synaptic terminal to act on the post-synaptic terminal. Neurotransmitters are packaged into vesicles and docked on the pre-synaptic terminal, where they undergo fusion exocytosis and endocytosis to open calcium channels.
Synapses allow neurons to pass signals to and from each other.

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There are three main neurotransmitters found within the brain. These are ACh, glutamate, and GABA.
ACh
Acetylcholine is used by all cholinergic neurons and metabotropic and ionotropic receptors. It is fast acting in the neuromuscular juntion, functioning as a neuromodular in the CNS. It is found in different areas of the brain and has different functions dependent on this. ACh is synthesised in the pre-synaptic terminal.
Glutamate
Glutamate is the main excitatory neurotransmitter in the CNS. It is responsible for leaning, memory, and higher cognitive functions. It is synthesised in neurons by local precursors and relies on glial cells. There are no enzymes that can degrade it, so it must be removed from the extracellular space through transporters.
GABA
GABA is the primary inhibitory neurotransmitter and is released from one-third of synapses. It is packaged into vesicles and released into the synapse, where it is quickly removed.
All neurotransmitters are synthesised at the pre-synaptic terminal membrane of the neuron.

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The limbic system is mainly responsible for stress, but is also involved in eating, aggression, memory, and emotion. It consists of the amygdala, hippocampus, septal nucleus, and cingulate gyrus. it compares sensory information and motivational signals, sores this information, and prepares it in an integrative way. The limbic system is the primary area that deals with stress, and immediately responds via the autonomic nervous system in response to perceiving a threat.

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The amygdala organises emotions and is involved in social behaviour. It can distinguish emotional faces, induce fear, and initiate rage and anxiety through electrical stimulation. It receives inputs from the visual, auditory, and tactile systems. Inputs can also come from the prefrontal cortex (logic), thalamus (sensory information transfer), hippocampus (sends information to the amygdala from recollection), and the brainstem (regulates sympathetic activity and signals to the amygdala).
Amygdala → striatum (basal ganglia) → brainstem → motor response (fight or flight)

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The hypothalamus activates the SNS → the adrenal medulla increases epinephrine → the pancreas is activated to increase glucagon to replace ATP. The anterior pituitary gland increases ACTH to release cortisol, which stimulates neurons. The posterior pituitary gland increases vasopressin. The SNS accelerates or represses metabolic function in normal conditions. Stress can alter plasticity in the limbic system, impair explicit learning in the hippocampus, and enhance implicit fear conditioning by amygdaloid plasticity. Oxidant are also expressed in a stress response. Excitatory junction currents also increase significantly during stress responses.

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Noradrenaline does not know when to shut off, and therefore circulates and activates neurons longer than necessary. it is excitatory within the brain, and can kill cells if not protected against. Cortisol activating glucocorticoids affects the amygdala and hippocampus, and it requires a concious effort to decrease the effects and establish normal metabolic equilibrium.
Catecholamines increase the SNS response: blood moves faster to the brain, heart, and muscles, senses become keener, memory sharpens, and pain sensitivity decreases.
Glucocorticoids receptors increase calcium conductance → prolonged opening of calcium-dependent potassium channels → prolonging afterhyperpolarisations → decreased neuron excitability

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The olfactory is affected during stress responses. Certain smells can decrease the effects of chronic stress, by increasing dendrite length and branching.

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The SNS has the following effects throughout the body:
- increase HR
- constricts blood vessels
-  dilates airways
- decreases motility of digestive tract
- relaxes gall bladder and urinary tract
- dilates pupils
- causes glycogenolysis in the liver
- causes lipolysis in adipose tissue
- increase brain activity
- exocrine glands: pancreas decreases secretion, sweat glands increase secretion
- endocrine glands: pancreas decreases insulin, noradrenaline and adrenaline released

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All internal organs are under the control of the autonomic nervous system (ANS). The ANS is responsible for functions that occur outside the realm of consciousness, such as digestion, circulation, and sweating.
The structure of the ANS can be seen in the image below.

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The PNS has the following effects:
- decrease HR
- dilation of blood vessels in genitals only
- constricts airways
- increases motility of digestive tract
- contraction of gall bladder
- contraction of bladder
- constriction of pupils
- exocrine glands: increased secretion of salivary glands and pancreas
- endocrine glands: increased output of glucagon and insulin