Chemical Defenses

Explain what a gradient is
Indicate the direction of energy or material flow under different conditions
Predict the permeability of membranes under different conditions
Compare and contrast the ways that molecules move across membranes.
Explain the relationship between diffusion and concentration gradients.
Explain how processes of passive transport work including:
- Osmosis
- Diffusion
Explain how mechanisms of active transport work including:
- Sodium-Potassium Pump
Explain how larger objects/molecules cross membranes including:
- Exocytosis
- Endocytosis
Predict when each of these transport mechanisms might be in use

Describe the structure of a neuron.
Describe the forces that maintain the resting potential in a neuron.
Understand what is meant by electrochemical gradient
Compare and contrast a graded potential, the threshold potential, and an action potential.
Diagram and describe the events of an action potential.
Diagram and describe how chemical and electrical synapses work.
Explain the function of neurotransmitters.
Explain how nerve actions result in paralysis and convulsions
Predict which drugs or conditions would result in paralysis or convulsions
Propose hypotheses for the appearance of paralysis or convulsions under various conditions

Overview of topics: Basic Neuro-anatomy, Action Potential, Cellular Secretion, Drug Effects, Calabar bean, Structure of nerves, How nerves work, Cellular secretion, and Applications of Cell Transport Mechanisms.

Parts of a Neuron:
- Cell Body: Contains the nucleus, mitochondria, and other organelles. Acts as the control center for the neuron.
- Dendrites: Short and branched extensions that receive information toward the cell body; involved in signal transmission.
- Axon: Also known as axon or nerve fiber; conducts nerve impulses away from the cell body.
- Synaptic Terminals: ends of the axon that connect to other cells, forming synapses.
- Myelin Sheath: Fatty layer that covers segments of the axon, increasing the speed of nerve impulse transmission.
- Nodes of Ranvier: Gaps in the myelin sheath where potassium and sodium channels are concentrated.

Neurons are classified into three categories:
- Sensory Neurons: Carry information to the central nervous system (CNS) from the body.
- Interneurons: Connect neurons within the CNS; process information within the CNS.
- Motor Neurons: Transmit signals from the CNS to effectors such as muscles or glands.

A neuron maintains a resting potential, which is the electrical charge difference across its membrane when not conducting impulses. This resting potential is crucial for preparing the neuron to transmit an action potential.
The electrochemical gradient refers to the combined effect of concentration gradients and electrical gradients across the neuron's membrane.
Forces maintaining the resting potential include:
- Sodium-Potassium Pumps utilizing ATP to transport 3 Na⁺ ions out of the cell and bring 2 K⁺ ions into the cell, thus creating a negative interior charge.

An action potential is a rapid, temporary change in the electrical membrane potential that travels along the axon. It is triggered by various stimuli, such as touch or chemical signals from other neurons.

Depolarization: Na⁺ channels open; Na⁺ flows into the cell causing the interior to become less negative.
Repolarization: Following the depolarization phase, K⁺ channels open, allowing K⁺ to exit the neuron and re-establish the negative interior charge.
Hyperpolarization: Membrane potential temporarily dips below resting potential before returning to resting level.

Graded potentials are changes in membrane potential that vary in magnitude depending on the strength of the stimulus and decay with distance.
If the graded potential is strong enough and reaches the threshold potential, an action potential is initiated.

In myelinated axons, action potentials jump from one Node of Ranvier to the next, a process called saltatory conduction, significantly increasing speed compared to unmyelinated axons.

Communication between neurons occurs at synapses, which consist of a presynaptic neuron, a synaptic cleft, and a postsynaptic cell (which could be another neuron, a muscle cell, or a gland).
When an action potential arrives at the synaptic terminal, it opens voltage-gated calcium channels leading to:
- Calcium influx that triggers vesicles containing neurotransmitters to fuse with the presynaptic membrane.
- Release of neurotransmitters into the synaptic cleft.

Neurotransmitters bind to receptors on the postsynaptic cell membrane, leading to the opening of ion channels and altering the likelihood of a subsequent action potential in the postsynaptic cell:
- Excitatory neurotransmitters: increase the likelihood of generating an action potential.
- Inhibitory neurotransmitters: decrease the likelihood of generating an action potential.

The postsynaptic neuron integrates all incoming signals, both excitatory and inhibitory, to determine whether to fire an action potential based on the net effect of all stimuli received.

Secretory Pathway:
- Involves proteins synthesized in the rough endoplasmic reticulum, processed in the Golgi apparatus, and transported out via vesicles which fuse with the plasma membrane to release substances outside the cell.

Physostigmine: Inhibits acetylcholinesterase leading to prolonged action of neurotransmitters, affecting signal transmission and potentially leading to convulsions or paralysis.
Atropine: Antidote for certain poisons that block neurotransmitters, particularly effective in cases of poisoning from belladonna.

Various animals have evolved toxins as defenses:
- Pufferfish (Tetrodotoxin): Blocks sodium channels, leading to paralysis and possibly death when ingested.
- Poison Arrow Frogs: Utilize potent skin toxins to deter predators.
- Curare: A neuromuscular blocker used historically in hunting; can cause paralysis.

Understanding these mechanisms and their applications is crucial for studying the nervous system and developing treatments for neurological disorders.