Autonomic Nervous System, Part II

All Autonomic impulses are afferent, meaning that they travel from the CNS to the PNS. The ANS can be broken down into two major divisions, the Parasympathetic and the Sympathetic divisions. The Parasympathetic division is responsible for all of the feed-and-breed or rest-and-digest functions of the body and is the restorer of homeostasis. The preganglionic axon is long and the post ganglionic axon is short. The Sympathetic division can be seen as the parasympathetic divisions opposite. The Sympathetic division carries out the fight-or-flight responses in our body. In this division, the preganglionic axon is shot and the post ganglionic axon is long. In both divisions, the target cells are smooth muscle, cardiac muscle and glands. Another commonality is that all preganglionic neurons release Ach and all receptors on the post ganglionic neurons are thus nicotinic.

There are two different categories of receptors for the ANS. Cholinergeric, which includes both Nicotinic and Muscarinic receptors and Adrenergic receptors. Nicotinic receptors always cause excitation or activation and perform as both the receptor and channel. They also only bind Ach. Muscarinic receptors can either cause excitation of inhibition and use either a PKA pathway or a PKC pathway to allow chemicals to enter the neuron. Both pathways utilize G-Proteins. Adrenergic receptors also utilize these pathways and are broken down into a1, a2, b1, b2, b3 receptors. The 1's refer to impulses that cause constriction or contraction and the 2's refer to impulses that cause dilation. B3 is the odd man out and its impulse causes heat production. Muscarinic and Adrenergic receptors are found on the cells of terminal targets.

I understood from lecture that the Adrenal gland, a gland that sits on top of the kidneys, is part of the sympathetic nervous system. This gland produces both epinephrine and norepinepherine by diffusing the hormones into the blood stream. This direct link into the blood stream allows for a fast regulation of the hormone throughout the body. That was as much as I was able to retain from lecture, so I researched a little more on the Internet. I learned that adrenal medulla is the central core of the Adrenal gland. The neurons within it (called Chromaffin cells) produce the hormones that are released into the blood stream instead of producing an impulse as all of the other neurons that we have learned about do. These hormones aid in the fight-or-flight response by increasing blood pressure, metabolic rate and or glucose concentration. The neurons in the Adrenal Medulla also produce dopamine, which low-levels of this hormone has been linked to Parkinson's diseases.

Automonic Nervous System

There are two motor neurons in an autonomic motor pathway. The first neuron is called a preganglionic neuron and its cell body is located in the brain or spinal chord. Its axon (very long) leaves the CNS and extends (as part of a spinal or cranial nerve) to an autonomic ganglion. The second neuron is called the post ganglionic neuron and its axon is short. The preganglionic neurons pass along nerve impulses from the CNS to the autonomic ganglia and the postganglionic neurons relay the impulse from the autonomic ganglia to the terminal target, which could be smooth muscle, cardiac muscle or a gland. The neurotransmitter that is released is always excitatory and is always Ach. The #2 neuron always produces an action potential. Wether the terminal target becomes excited depends on its receptors. They are usually cholinergeric and thus are either Nicotinic (excitatory) or muscarinic (inhibitory).

After browsing the internet, I learned that a malfunction in the autonimic system is called autonomic failure and is a result of the imbalance between the sympathetic and parasympathetic divisions of the autonomic nervous system. Signs that there is an imbalance include orthostatic hypotension and postprandial hypotension. These can cause dizzyness or lightheadedness. Elderly people are most likely to suffer from this condition and the drop in blood pressure is usually a result of the onset of a disease (diabetes, stroke) and the medications that are used to treat them. There is no cure for autonomic disorders, however there are medications that can treat the low blood pressure and the uncomfortableness that accompanies the episodes.

The Thalamus and Hypothalamus

All incoming sensory information from periphery is relayed through the thalamus to an appropriate higher processing center. The Thalamus is segregated into several regions each pertaining to its nuclei and associated functions. The Anterior Group is part of the limbic system, the regulator of emotion. The Medial Group integrates sensory information and relays it to the frontal lobes. The Ventral Group is responsible for passing along information from the basal nuclei of the cerebrum and the cerebellum to the somatic motor areas of the cerebral cortex. It also passes along sensory information to the sensory areas of the cerebral cortex. The Posterior Group includes the Pulvinar, and the lateral and medial geniculate nuclei. The function of the Pulvinar is to process sensory information for projection to the cerebral cortex. The lateral geniculate nucleus passes along visual information to the visual cortex and the medial geniculate nuclei passes along auditory information to the auditory cortex. Lastly, the lateral group processes sensory information as well as effects emotional states.

I am particularly intrigued by mood disorders and the effect they have on the brain. I have a relative with Bi-Polar disorder and although I know what the definition is and how it is treated, I did not really understand physiologically how Bi-Polar Disorder interrupts normal brain activity. I learned that through structural imaging studies, scientist have found a decrease in overall brain volume in patients with Bi-Polar Disorder. They believe that this is in part due to the decreased number of neurons and glial cells in layers II and III (are they referring to the meninges?) in the forebrain. They also know that the frontal and temporal lobes, the pre-frontal cortex, basal ganlia and parts of the limbic system are all involved and effected by this disorder. Some scientist also believe that since the cerebral cortex is responsible for some thought process that it may also be involved in the negative thinking that are part of the negative episodes of the disorder. Some websites offered that it was an excess of neurotransmitters that causes the manic episodes and a decrease in neurotransmitters that causes the depressive episodes, while other websites claimed that it was not the amount of neurotransmitters but the "effectiveness of the cell's functioning" that was to blame.