Nervous System Organization and Exam Preparation

The nervous system divides into two main branches: the central nervous system (CNS), consisting of the brain and spinal cord, and the peripheral nervous system (PNS), which includes all nerves extending from the CNS. The autonomic nervous system (ANS), a division of the PNS, controls involuntary functions like heart rate and digestion.

For Exam 1, focus on multiple choice and diagram-related questions covering neurotransmitters and their actions. Key topics include characterizing neurotransmitters, understanding synaptic connections, and recognizing various drug delivery systems. Knowledge of autoimmune disorders, their symptoms, and treatments will also be essential.

Study Tip: Create tables comparing different neurotransmitters and their receptors to help organize this information—the exam will likely ask you to match neurotransmitters with their specific actions and receptors.

Understanding pharmacokinetics (how the body affects drugs) and pharmacodynamics (how drugs affect the body) forms the foundation of neuropharmacology. Remember that drug absorption determines entry into the bloodstream, distribution delivers drugs to target tissues, while metabolism and elimination remove drugs from the system.

Neuropharmacology Fundamentals

Neuropharmacology studies drugs affecting the nervous system, influencing everything from mood to motor function. The concepts of drug efficacy and potency are crucial: potency refers to the strength of drug-target binding, while efficacy describes the biological effect produced when the drug binds.

Three important drug actions to remember:

• Agonists activate receptors to produce effects
• Antagonists block receptor activation without producing effects themselves
• Inverse agonists produce effects opposite to agonists

The neuron structure is specialized for communication. The cell body (gray matter) contains organelles for protein synthesis, while axons (white matter) conduct electrical impulses to other neurons. Dendrites receive signals from other neurons, forming the "dendritic tree" that processes incoming information.

Cytoskeletal components provide structural support for neurons. Microtubules (made of tubulin tau) are essential for transport within neurons, and their dysfunction is linked to Alzheimer's disease. Intermediate filaments (neurofilaments) and actin filaments create the structural scaffold for neuronal function.

Remember: Dendritic pruning—the elimination of unused neural connections—is a critical process in brain development that shapes neural circuitry based on experience and use.

Glial Cells and the Blood-Brain Barrier

Glial cells, once thought to be merely supporting cells, play crucial roles in nervous system function. Five types of glial cells perform distinct functions:

1. Astrocytes (20% of glial cells) guide neuronal development, maintain the blood-brain barrier (BBB), and manage neurotransmitter levels. Their dysfunction is linked to Alzheimer's disease.

2. Oligodendrocytes produce myelin in the CNS to enable rapid impulse conduction. Multiple sclerosis results from autoimmune destruction of these cells and their myelin sheaths.

3. Schwann cells are the PNS equivalent of oligodendrocytes, wrapping around axons to form myelin sheaths through concentric layering of their plasma membranes.

4. Microglia function as immune cells in the brain, performing phagocytosis of pathogens. They can harbor HIV undetected and are implicated in Alzheimer's disease.

5. Endothelial cells form the lumen of the BBB, allowing only highly lipophilic molecules to enter the brain.

The blood-brain barrier (BBB) consists of endothelial cells joined by tight junctions that restrict molecule passage into the brain. This protective feature presents challenges for drug development, as medications must be designed to cross this barrier to reach brain targets.

Clinical Connection: Many neurological disorders involve glial cell dysfunction—understanding these cells is essential for developing treatments for conditions like multiple sclerosis and Alzheimer's disease.

Synaptic Transmission and Neurotransmitter Identification

Synapses are specialized junctions where neurons communicate. Different types include axodendritic (axon to dendrite), axo-axonic (axon to axon), and axosomatic (axon to cell body) connections. At these junctions, electrical signals convert to chemical signals (neurotransmitters), which then trigger electrical or chemical responses in the target cell.

Scientists identify neurotransmitters through four key criteria:

1. Localization in presynaptic vesicles (visualized through immunohistochemical staining)
2. Calcium-dependent release (verified through microdialysis and HPLC)
3. Synaptic mimicry (exogenous application produces effects similar to endogenous release)
4. Synaptic pharmacology (effects blocked by specific receptor antagonists)

Neurotransmitters fall into six main categories:

1. Peptides (substance P, opioids)
2. Amino acids (GABA, glutamate)
3. Lipid-derived molecules (endocannabinoids)
4. Diffusible gases (nitric oxide)
5. Monoamines (dopamine, serotonin, norepinephrine)
6. Nucleosides (ATP, GTP, AMP)

Exam Focus: Be able to match neurotransmitters with their receptor types, locations in the body, actions, and antagonists. For example, GABA binds to GABA-A receptors in the limbic system, causing chloride ion influx that hyperpolarizes cells and decreases action potential likelihood.

Autonomic Nervous System Anatomy

The autonomic nervous system (ANS) controls involuntary functions through its sympathetic $"fight-or-flight"$ and parasympathetic $"rest-and-digest"$ divisions. Key structures in the ANS include:

• Limbic cortical areas and the amygdala: Initiate autonomic responses to emotion and pain
• Hypothalamus: The main coordinating center with direct connections to the pituitary and peripheral neurons
• Brainstem: Receives input from and sends output to target organs

The ANS innervates every organ in the body and functions independently of consciousness, maintaining homeostasis by adjusting blood pressure, heart rate, and body temperature in response to environmental cues.

In the sympathetic division, preganglionic fibers originate in the thoracolumbar region of the spinal cord, with short preganglionic fibers and long postganglionic fibers. All preganglionic fibers are cholinergic (release acetylcholine).

The parasympathetic division has cell bodies in the brainstem and sacral spinal cord, with long preganglionic fibers and short postganglionic fibers. Like the sympathetic division, all preganglionic fibers are cholinergic.

Visual Tip: Picture the sympathetic trunk running alongside the spinal column, containing paired ganglia where preganglionic neurons synapse with postganglionic neurons. This structure allows nerve signals to travel to spinal nerves above and below their origin point.

Autonomic Nervous System Function and Neurotransmitters

The ANS uses two primary neurotransmitters: acetylcholine (ACh) and norepinephrine (NE). Cholinergic neurons include all preganglionic neurons (both sympathetic and parasympathetic), all parasympathetic postganglionic neurons, and sympathetic postganglionic neurons that innervate sweat glands. Adrenergic neurons include all other sympathetic postganglionic neurons.

ACh binds to two types of receptors:

• Nicotinic receptors (found on all ganglionic neurons and adrenal medulla cells) - always produce excitatory effects
• Muscarinic receptors (found on parasympathetic target organs and sweat glands) - usually excitatory except on the heart, where they're inhibitory

NE binds to adrenergic receptors:

• Alpha receptors (α1 and α2) - found on most sympathetic target organs except the heart and bronchioles
• Beta receptors (β1, β2, and β3) - β1 receptors are found on the heart and kidneys; β2 on bronchioles and coronary arteries; β3 on adipose tissue and the urinary bladder

The sympathetic division typically produces "fight-or-flight" responses: increased heart rate and blood pressure, pupillary dilation, bronchodilation, decreased digestive activity, and increased sweating. The parasympathetic division produces opposite effects: decreased heart rate, increased digestive activity, pupillary constriction, and bronchial constriction.

Clinical Application: Beta blockers (like metoprolol) treat high blood pressure by blocking beta receptors, preventing norepinephrine binding and reducing heart rate. Beta agonists (like albuterol) treat asthma by stimulating beta-2 receptors, causing bronchodilation.

Autonomic Disorders and Drug Delivery Systems

Several disorders affect the autonomic nervous system, disrupting its normal regulation of involuntary functions:

• Dopamine beta-hydroxylase deficiency: A rare genetic disorder preventing conversion of dopamine to norepinephrine, causing hypotension and fainting
• Familial dysautonomia: Affects both sympathetic and parasympathetic systems due to ELP1 gene mutations, causing difficulty swallowing, poor breathing control, and temperature regulation problems
• Pure autonomic failure: Associated with Lewy Bodies restricting norepinephrine production, causing hypotension and bladder control issues
• Acute pan dysautonomia: Possibly related to Guillain-Barré syndrome, severely affecting both sympathetic and parasympathetic functions
• Diabetic autonomic neuropathy: Affects over 30% of diabetics, with multiple factors contributing to nerve damage

For drug delivery to the brain, several technologies show promise:

• Nanoparticles can deliver drugs across the blood-brain barrier using transcytosis
• Viral-mediated gene therapy uses vectors like adeno-associated viruses to deliver functional genes
• Focused ultrasound targets specific brain areas for drug delivery without invasive procedures
• Mannitol temporarily opens the blood-brain barrier by creating osmotic gradients

Real-World Example: Zolgensma, an adeno-associated virus vector therapy for spinal muscular atrophy, delivers a functional copy of the SMN1 gene through a one-time infusion into the brain. While effective, it costs $2.1 million and is limited to children under two years old.

Case Study: Familial Dysautonomia and the Autonomic Nervous System

Familial dysautonomia (FD) is a rare genetic disorder affecting the development and survival of neurons in the autonomic nervous system. The case study follows Natalia, who discovered she and her husband are carriers for this condition during genetic testing for IVF.

FD causes a variety of strange symptoms related to autonomic dysfunction:

• Constipation and incontinence
• Inability to feel pain
• Blood pressure fluctuations
• Inability to produce tears
• Temperature regulation problems
• Vision adaptation difficulties

The autonomic nervous system (ANS) maintains physiological homeostasis based on internal and external environmental changes, functioning independently of consciousness. It regulates involuntary functions like pupillary dilation, sneezing, sweating, heart rate, body temperature monitoring, and food movement through the digestive system.

FD patients have difficulty moving from dimly lit to bright rooms because their ANS cannot properly regulate pupillary constriction in response to light changes. This illustrates how the ANS continuously adjusts bodily functions to maintain homeostasis in changing environments.

Clinical Insight: People with FD may also experience difficulty breathing, irregular heart rate, hypotonia (reduced muscle tone), vomiting, and swallowing difficulties—all functions normally regulated by the properly functioning ANS.

Autonomic Nervous System Organization

The ANS is divided into sympathetic $"fight-or-flight"$ and parasympathetic $"rest-and-digest"$ branches, each producing opposite effects on target organs:

Sympathetic effects:

• Increased heart rate and blood pressure
• Pupillary dilation
• Decreased saliva production
• Increased blood flow to skeletal muscles
• Decreased blood flow to digestive organs
• Bronchodilation
• Decreased peristalsis
• Increased sweating

Parasympathetic effects:

• Decreased heart rate and blood pressure
• Pupillary constriction
• Increased saliva production
• Decreased blood flow to skeletal muscles
• Increased blood flow to digestive organs
• Bronchoconstriction
• Increased peristalsis
• Decreased sweating

Anatomically, the sympathetic division is called "thoracolumbar" because its nerves originate in the thoracic and lumbar regions of the spinal cord. The parasympathetic division is "craniosacral" because its nerves originate in the cranial and sacral portions of the CNS.

Everyday Example: When you're nervous about public speaking, your mouth gets dry because sympathetic activation decreases saliva production. Similarly, sweaty palms during a job interview result from sympathetic-driven increases in sweat gland activity—both part of your body's "fight-or-flight" response to stress.