The Limbic System (The Pituitary Gland, Amygdala, Thalamus, HypoThalamus, Locus Coerulus, Cingulate Gyrus, and Hippocampus), affects Emotionally driven Behaviors, Anxiety, Fear and Aggression; Altruistic Care of the young; Basic Religious tendencies; Olfaction (Smell), Taste, and Sexuality; Short-Term Memory Storage (into Long-Term Memory); Visual Attention, and Attention Span. (View Image)
These Sub-Cortical areas subserves many reflex phenomenon, including the Association of Sensory imput with specific Mood states (Sexual Pleasure, Migraine, Displeasure). These facts form the basis for considering the Emotional aspect of Pain.
Controversially, and oversimplifying a little, this serves as the final common pathway for pleasure in the Brain. Neuronal Dopamine release is usually triggered by Adaptive environmental stimuli.
The Axons and Dendrites of MesoLimbic Dopaminergic Neurons innervate the higher Cortical regions of the Brain. They thereby help mediate the Genetically adaptive "EnCephalization of Emotion".
Entirely non-coincidentally, the realization of our most emotionally charged types of intentional object tends to promote the inclusive fitness of our Genes. Crudely, we like most what's good for them.
Dopaminergic Neurons die off at a rate of over 10% per decade. Their death ensures that Senescence is marked by a decline in Drive, Libido, Pleasure and the Intensity of experience itself.
Even in one's youth, the fullest and most beautiful scope for expression of the Dopaminergic pleasure cells is continually frustrated by inhibitory feedback. This derives both from their own Pre-Synaptic Autoreceptors and the processes of other, often less benign, Neurons that Synapse upon them.
The functional regions of the Brain which subserve physical agony, the "Pain Centres", and the mainly Limbic Substrates of Emotion, appear in phyloGenetic terms to be remarkably constant in the Vertebrate line.
The Neural Pathways involving Serotonin, the Periaquaductal Gray Matter, Bradykinin, Dynorphin, ATP Receptors, the major Opioid families, Substance P, etc., all existed long before Hominids walked the Earth.
The Serotonin producing SubGenual PreFrontal Cortex