Nociceptors are pain receptors that. Mechanoreceptors respond when tissue is deformed by mechanical forces. Photoreceptors respond to light. Chemoreceptors respond to chemicals. Thermoreceptors respond to changes in temperature. 

Pacinian corpuscles are found in deeper in the dermis rather than in the dermal papillae. Merkel cells are found in the epidermal-dermal border and are responsible for perception of light and touch. Carcinoma is a type of tumor. Meissner's corpuscles are the mechanoreceptors that are found in the dermal papillae, and are very sensitive to light touches.

When there is tissue damage, substance P plays a very important role in the sensation of pain through stimulating primary afferent neurons in pain perception pathways. Enkephalin is a neurotransmitter that inhibits the neurons that transmit pain impulses to the brain. Serotonin is a monoamine neurotransmitter and dopamine is a catecholamine/phenethylamine neurotransmitter. 

Opioid neuroactive peptides include enkephalins, endorphins, and dynorphin. These bind to opiate receptors in the brain, which also bind opiate drugs such as heroin and morphine. These are known to play a role in response to injury stress and pain perception and are released during exercise. Non-opioid neuroactive peptides do not bind opiates and their function is not well understood. Amino acids include glutamic acid and GABA. Biogenic amines include monoamines and catecholamines, which include norepinephrine, epinephrine, dopamine, and serotonin. 

Glutamic acid is an excitatory neurotransmitter. During a cerebrovascular accident, cells that have died due to oxygen deprivation release large quantities of glutamic acid. This causes surrounding cells to become overexcited and eventually die. This process is called excitotoxicity. 

The substantia nigra is composed of primarily dopamine. When the substantia nigra is effected by a disease, a patient can present with Parkinson-like symptoms. Most common symptom is resting tremors.

Acetylcholine utilizes both muscarinic and nicotinic receptors. The nicotinic receptors are ionotropic and are primarily excitatory. Whereas, the muscarinic receptors are metabotropic and can be either excitatory or inhibitory. All receptors that bind acetylcholine are referred to as cholinergic.

Physiologically, when the PAG is stimulated serotonin is produced. Serotonin will increase the pain threshold (ascending tract) and inhibit pain receptors (descending tract). 

There are a finite number of receptors for a neurotransmitter in a neuron. When neurotransmitter molecules are released, they will bind these receptors. Since the number of neurotransmitters is greater than the number of neurotransmitter receptors, there will be a saturation of receptors with neurotransmitter. This is the term 'saturability.'

The affinity of a neurotransmitter for its receptor is governed by its association and dissociation constants. The association constant describes how readily it associates with the receptor. Therefore, a higher association constant means more ready association with the receptor, and higher affinity. On the other hand, the dissociation constant describes how readily it dissociates from the receptor. Therefore, a higher dissociation constant means more ready dissociation from the receptor, and lower affinity. Therefore, a high association constant and low dissociation constant means the neurotransmitter readily binds but does not readily unbind from the receptor, meaning it has a high affinity.