Autonomic Nervous System
The central nervous system (CNS) consists of the brain and spinal cord. It plays a central role in most functions of the body. The CNS communicates with the rest of the body via the peripheral nervous system (PNS). The PNS has two parts, the somatic PNS and visceral PNS. The somatic PNS innervates skin, joints and muscles under voluntary control. The CNS communicates directly with skeletal muscle through a somatic motor fiber. The visceral PNS, also known as the Autonomic Nervous System (ANS), innervates internal organs, blood, vessels, and glands. The lower motor neurons lie outside the CNS. ANS ultimately deals with involuntary control.
The ANS is divided into two parts, a sympathetic division and parasympathetic division. The two divisions often work against each other. The sympathetic division is known as the “fight or flight” response. An increase in sympathetic activity will likely increase one’s heart rate, breathing rate, sweating, and regulate digestion and immune response. The idea is that the body uses energy toward functions that are necessary to perform in “fight or flight” scenarios. For example, sympathetic activity can be induced by stress, exercise, and exciting stimulus. One of the reasons why I am more prone to getting sick during finals week in school is stress. My immune response is suppressed, making me more vulnerable to illness.
On the other hand, the parasympathetic division deals with processes like digestion, growth, immune response, and energy storage. Therefore, when one isn’t faced by a frightful or highly stimulating environment, the body uses its energy to perform upkeep on the body. The parasympathetic division performs many regulatory functions in the body, such as reducing heart rate and breathing rate. It also stimulates other parts of the body that the sympathetic nervous system suppressed, like digestion. The parasympathetic division can be thought of as an overall rest process, equally necessary to survival as the sympathetic nervous division.
The sympathetic and parasympathetic divisions both receive input from the CNS. Preganglionic neurons leave the CNS and communicate with post ganglionic neurons in the PNS. Sympathetic ganglions use norepinephrine (NE) as their neurotransmitter, while parasympathetic ganglion use Acytel Choline (ACh). The preganglionic fibers of the sympathetic nervous system are short stemming from the middle third of the spinal cord. On the other hand, the preganglionic fibers of the parasympathetic system are long and come from the brain stem or sacral region of the spinal cord. The harmonic balance between sympathetic and parasympathetic activity is remarkable. It is an efficient and clever way of performing and regulating bodily functions.
Hypothalamus is a small portion of the brain located below thalamus along the walls of third ventricle. It contains a cluster of small nuclei with multiple crucial functions, including:
- maintaining homeostasis
- releasing hormones
- maintaining light-dark cycle
- controlling attention
- regulating emotional responses
Hypothalamus is a vital part of the chemical control of the brain and rest of the body. It is highly interconnected with other parts of the CNS, particularly the brainstem and its reticular formation from which it receives most of its inputs. It also has an important function to link the nervous system to the endocrine system via the pituitary gland which in turn is linked to various endocrine glands and organs.
Pituitary gland is divided into two parts: posterior and anterior. Hypothalamus links to posterior pituitary via magnocellular neurosecretory cells which project their axons down the stalk of pituitary and into the posterior lobe. Oxytocin and vasopressin are neurohormones synthesized in the hypothalamus and sent down the axons to the posterior pituitary where they are released into the bloodstream.
Unlike the posterior lobe, which is a part of the brain, anterior lobe is an actual gland. Hypothalamus reaches the axon projections of parvocellular neurosecretory cells down to the capillary bed at the floor of the third ventricle where they release hypophysiotropic hormones to the blood. The blood vessels run down the pituitary stalk and reach the frontal lobe. This network of blood vessels is called hypothalamo-pituitary portal circulation. The secreted hormones cause either a stimulatory or inhibitory response to synthesize and secrete hormones in the anterior pituitary where multiple hormones are secreted to the bloodstream.
Hypothalamus is a great example on how a small region of the brain can be interconnected to multiple other brain regions and from there communicate with the cells of the entire body, acting sort of like a “supergland”.
Diffuse modulatory systems
Diffuse modulatory systems can be thought of as collections of core neurons that use a certain neurotransmitter to control a certain aspect of internal commands within the brain. The book used a great analogue for this phenomenon; modulatory systems are similar to volume, treble, and bass controls that change the vibe of a song, but not the song itself. The set of neurons of each system, called the core, has several thousand neurons. Most of the neurons travel from the brain stem of the central core of the brain. The systems are called “diffuse” because information from one axon diffuses to over 100000 postsynaptic neurons. They are able to do this because the neurotransmitters are released into the extracellular fluid, instead of the synaptic cleft, where they can reach more neurons.
The locus couruleus is a small modulatory system in the pons that releases norepinephrine. The circuitry of the locus couruleus is very vast, one of its 12000 neurons can reach over 250000 synapses. In addition, a neuron in the locus couruleus can have one of its axons in the cerebellar cortex and another in the cerebral cortex. The core neurons play a part in attention, arousal, learning and memory, pain, mood, anxiety, as well as regulating sleep-cycles. Instead of being fully responsible for these aspects, though, the neurons act in strengthening or lessening these different states. Studies have shown that the neurons of the locus coeruleus are activated when something new or exciting is happening in the person’s environment, and are not so active when the person is at rest.
The nine raphe nuclei located in the midline of the brain stem contain a core set of serotonin-containing neurons. Each nucleus sends information to different parts of the CNS. Raphe neurons are modulate the same kind of signals as the locus coeruleus neurons, but especially sleep-wake cycles. Both raphe and locus coereleus neurons form the ascending reticular activation system, which works to “wake up” the forebrain. Serotonin also controls mood and emotional behavior.
Two modulatory systems in the midbrain release dopamine. One is the substantia nigra. The axons project to the striatum and control voluntary motor movements. The substantia nigra therefore works to create motor responses to stimuli in the environment. Degeneration of neurons in this modulatory system leads to motor disorders like Parkinson’s disease. Another dopaminergic modulatory system in the midbrain is in the ventral tegmental area. The axons of this system project to the frontal cortex and limbic system. Studies have shown that this modulation reinforces adaptive behaviors by releasing dopamine.
Acetylcholine is the neurotransmitter at two major modulatory systems; the basal forebrain complex and the pontomesencephalotegmental complex. Not much is known about the neurons in the basal forebrain complex, but research has shown that they are the earliest cells to die during Alzheimer’s disease. The cells in the pontomesencephalotegmental complex project to the dorsal thalamus where it affects the excitability of sensory relay nuclei.
Psychoactive drugs create mind altering effects by interfering with synaptic transmissions in the modulatory systems. Lysergic acid diethylamide (LSD) and the active ingredient in the psilocybe mushroom are very similar to serotonin. Therefore, it is an agonist at the serotonin receptors in the raphe nuclei. LSD causes hallucinations and a general “dream-like” state of mind. While the exact nature of LSD’s effect on the raphe nuclei is unknown, research has suggested that the hallucinations are a result of creating connections between nuclei that are not formed naturally during the release of serotonin. CNS stimulants cocaine and amphetamine interfere with connections at the dopaminergic and noradrenergic systems. These stimulants increase alertness and self-confidence as well as a general sense of euphoria. They “copy” the effects of the sympathetic division. In addition to these effects, cocaine and amphetamine cause users to become dependent on their use because of the increased dopamine transmission in mesocorticolimbic dopamine system.