Resting Membrane Potential

Resting Membrane Potential

• Cells maintain a difference in electrical charge across their membrane, with the outside positive and the inside negative.
• Negatively charged proteins inside the cell can't leave.
• Potassium ions (K+) inside the cell balance out the negative charge.
• Potassium leak channels allow K+ to leak out slowly.
• Voltage-gated potassium channels have one gate, causing slower opening/closing.
• High intracellular K+ concentration and low extracellular K+ concentration.
• Sodium leak channels are present, but sodium (Na+) is more concentrated outside the cell and leaks in faster.
• Voltage-gated sodium channels have two gates, allowing quicker opening/closing.
• More potassium leak channels than sodium leak channels.
• The electrical gradient attracts potassium back into the cell, but passive leakage also plays a factor.
• Cells are more permeable to potassium (25 times more permeable) than sodium.
• Electrochemical gradient: considers both charge and concentration of ions.
• Concentration gradient: considers only the concentration difference of ions across the membrane.
• The Sodium-Potassium pump maintains ion gradients (3 Na+ out for 2 K+ in).

Ion Channels

• Leakage (non-gated) channels: Always open.
• Voltage-gated channels: Part of the protein changes shape to open/close in response to changes in membrane potential.
• Ligand-gated (chemically-gated) channels: Part of the protein changes shape to open when a ligand or substrate binds.

Membrane Potential Changes

• Used as communication signals within cells.
• Cause two types of signals:
  • Graded potentials: Incoming signals operating over short distances, starting at dendrites; can be excitatory or inhibitory.
  • Action potentials: Long-distance signals along axons, starting at the axon hillock.

Graded Potentials

• Can be hyperpolarizing or depolarizing.
• Are decremental (decrease in amplitude as they travel).
• Vary in magnitude, depending on the strength of the stimulus.
• Short-distance communication.
• Can be summated (added together).
• Can trigger an action potential.

Action Potentials

• Generated by graded potentials that reach threshold (approximately -55 mV).
• Always depolarizing; no summation.
• All-or-none phenomenon: either happens completely or not at all.
• Always excitatory.
• Not decremental: maintain amplitude as they travel.
• Strength coded by frequency (rate coding), not amplitude.

Refractory Periods

• Periods where another action potential is harder to produce.
• Absolute refractory period: the neuron cannot produce another action potential, activation gates are still open.
• Relative refractory period: the neuron can fire another action potential, but it needs a larger stimulus, inactivation gates have closed, but voltage is further from threshold.

Action Potential Propagation

• The movement of an action potential along an axon.
• Positive feedback involving voltage-gated channels is vital for propagation.
• At the axon terminal, voltage-gated calcium channels open, triggering neurotransmitter release.
• Hyperpolarization can make it harder to generate an action potential.

Events of an Action Potential

• Voltage-gated Na+ channels open, increasing Na+ permeability. Depolarization occurs.
• Voltage-gated K+ channels open, increasing K+ permeability.
• Hyperpolarization occurs.

Events of Action Potential at a Synapse

• Neurotransmitter (e.g., ACh) released at axon terminal, binds receptors on dendrites.
• Receptor opening causes Na+ influx, generating graded potentials.
• Magnitude of graded potential depends on neurotransmitter binding.
• Sufficient graded potential triggers action potential at axon hillock.
• Axon potential propagation occurs.
• At the axon terminal, voltage-gated Ca2+ channels open, stimulating neurotransmitter release.

Postsynaptic Potentials

• Synonymous with graded potentials.
• Excitatory postsynaptic potential (EPSP): Depolarizing graded potential.
• Inhibitory postsynaptic potential (IPSP): Hyperpolarizing graded potential.

Synaptic Summation

• Spatial summation: Multiple graded potentials from different locations are added together at the trigger zone.
• Temporal summation: Multiple graded potentials from the same location arrive in rapid succession.

Synapses

• Different neurons store different neurotransmitters.
• Cholinergic neurons: Contain acetylcholine (ACh).
• Adrenergic neurons: Store norepinephrine (NE) and epinephrine.
• GABA: Main inhibitory neurotransmitter in the CNS; involved in memory, learning, and anxiety; opens Cl- channels.
• Glutamate: Main excitatory neurotransmitter in the CNS; involved in learning, memory, and cognition.
• Dopamine: Important in motor control, reward and motivation.
• Norepinephrine: Excitatory neurotransmitter, plays a role in learning and behavior control.
• Serotonin: Regulates mood, sleep, appetite, and body weight.

Autonomic Nervous System

• Controls involuntary functions (organs, glands, smooth muscle).
• Sympathetic: Pre-ganglionic (cholinergic, short), post-ganglionic (adrenergic, NE). Generally, excitatory.
• Parasympathetic: Pre-ganglionic (cholinergic, long), post-ganglionic (cholinergic, muscarinic receptors). Generally, inhibitory.
• Ganglia are connected to autonomic nervous system pathways..

Endocrine System

• Controls growth, development, metabolism, and reproduction through hormones.
• Hormones: Chemical messengers transported through the blood.
• Endocrine glands: Secrete hormones.
• Exocrine glands: Have ducts.

Hormone Classes

• Peptide hormones: Made of amino acids; fast-acting; bind to receptors on cell surface..
• Steroid hormones: Made of cholesterol; slow-acting; bind to intracellular receptors.

Target Cell Specificity

• Hormones act only on cells with specific receptors.

Hormone Release Control

• Stimulated by:
  • Tropic hormones: Hormones that stimulate the release of other hormones.
  • Humoral factors: Substances in the blood (e.g., glucose, calcium).
  • Nervous system: E.g., the sympathetic nervous system releasing epinephrine.

Pituitary Gland

• Connected to the hypothalamus.
• Posterior pituitary: Neural tissue; secretes antidiuretic hormone (ADH) and oxytocin.
• Anterior pituitary: Glandular tissue; secretes tropic hormones (e.g., growth hormone).