Nervous System and Neurons

• The nervous system includes the central nervous system (CNS) and the peripheral nervous system (PNS).
• The CNS includes the brain and spinal cord and controls higher functions.
• The PNS includes nerves and ganglia outside the CNS; it connects the CNS to the rest of the body.

Neurons

• Neurons are the functional units of the nervous system, responsible for transmitting information.
• Neurons consist of the soma (cell body), dendrites, axon, and axon terminals.
• The soma houses the nucleus and other organelles.
• Dendrites receive signals from other neurons and transmit them to the soma.
• The axon transmits signals from the soma to other neurons or effector cells; it is myelinated to facilitate saltatory conduction.
• Axon terminals release neurotransmitters into the synapse.

Neuronal Electrophysiology

• It is based on ion movement across the cell membrane, creating electrical potential differences.
• Membrane potential is the electrical potential difference across the cell membrane.
• Depolarization is a decrease in membrane potential, making it less negative (more positive).
• Repolarization is the return of the membrane potential to the resting potential after depolarization.
• Hyperpolarization increases the negativity of the membrane potential beyond the resting potential.
• Ion channels are proteins in the cell membrane that allow selective ion passage; they can be voltage-dependent, chemical, or mechanical gated channels and are crucial for generating and propagating electrical signals in neurons.
• Chemical gated channels are activated by chemicals.
• Voltage gated channels respond to changes in the membrane potential and have activation and inactivation gates that open and close at different membrane potential levels.
• Mechanical gated channels open in response to a change in the membrane due to pressure.

Glossary of Terms

• Neuron: a fundamental cell responsible for transmitting information through electrical and chemical signals.
• Membrane Potential: Electrical potential difference across the cell membrane.
• Depolarization: Decrease in membrane potential (making it less negative).
• Repolarization: Return of the membrane potential to the resting potential after depolarization.
• Hyperpolarization: Increase in the negativity of the membrane potential beyond the resting potential.
• Ion Channels: Proteins in the cell membrane that allow the selective passage of ions.
• Graded Potential: Small variation in membrane potential that propagates over a short distance and decreases with distance.
• Action Potential: "All or nothing" electrical signal that propagates long distances along the axon.
• Absolute Refractory Period: Period during which it is impossible to generate a new action potential.
• Relative Refractory Period: Period during which a stronger than normal stimulus is required to generate a new action potential.
• Saltatory Conduction: Rapid propagation of the action potential in myelinated axons, jumping between nodes of Ranvier.
• Synapse: Junction between two neurons or between a neuron and an effector cell, where the signal is transmitted.
• Neurotransmitter: A chemical released by a neuron at a synapse to transmit a signal to another cell.
• EPSP (Excitatory Postsynaptic Potential): Depolarization of the postsynaptic membrane that increases the probability of an action potential occurring.
• IPSP (Inhibitory Postsynaptic Potential): Hyperpolarization of the postsynaptic membrane that decreases the probability of an action potential occurring.
• Receptor: Protein in the cell membrane that binds to a neurotransmitter and triggers a response in the cell.
• Myelin: Insulating sheath that covers some axons, accelerating the speed of conduction.

Electrical Signals

• Graded Potentials respond to stimuli, are produced in receptors and cell bodies, and can be depolarizing or hyperpolarizing.
• Graded potential intensity depends on the strength of the stimulus and travels short distances, losing strength as it travels.
• Action Potentials respond to graded potentials that reach or exceed a threshold (all-or-none phenomenon).
• Action potentials occur at the axon hillock, maintains constant intensity, and only occurs if the stimulus reaches a minimum threshold. Once that threshold is reached, the action potential occurs in full, regardless of additional stimulus strength and involves Na+ channels with "activation" and "inactivation gating."
• Absolute Refractory Period is the period during which it is impossible to generate a new action potential due to the inactivation of Na+ channels; it ensures a one-way travel of the action potential.
• Relative Refractory Period requires a stronger stimulus to generate an action potential because the Na+ inactivation gate is restored and K+ channels are closing.
• Saltatory conduction is rapid propagation of an action potential in myelinated axons, jumping between nodes, where myelination and the nodes contribute to the speed and efficiency of transmission, and the intensity of the stimulus determines the frequency of action potentials.
• Subthreshold Graded Potential starts above the threshold but loses strength as it moves through the cell. At its trigger zone, it would be below the threshold, preventing an action potential and resulting in hyperpolarization or depolarization.
• Suprathreshold Graded Potential receives a strong stimulus and creates a graded potential that goes above the threshold, resulting in the production of an action potential.

Chemical Influence

• Extracellular potassium (K+) concentration is critical with changes affecting resting potential and excitability.
• Normokalemia is a normal K+ concentration.
• Hyperkalemia is elevated K+ concentration; depolarizes cells.
• Hypokalemia is low K+ concentration; it hyperpolarizes cells.
• Potassium is primarily responsible for the resting membrane potential.
• Hyperkalemia brings the membrane closer to the threshold, allowing a subthreshold stimulus to trigger an action potential.
• Hypokalemia hyperpolarizes the membrane, making the neuron less likely to fire an action potential in response to a stimulus that would normally be above the threshold.
• The synapse is the junction between two neurons where the signal is transmitted and where the axon terminal communicates with its postsynaptic target cell.
• Neurotransmitters are chemical substances released by a neuron at the synapse to transmit a signal to another cell.
• The neurotransmitter is released from the presynaptic terminal.
• Signal termination occurs through enzymatic degradation in the synaptic cleft, reuptake by the presynaptic neuron or glial cells, or diffusion out of the synaptic cleft; a common neurotransmitter in the body is acetylcholine (ACh).
• Amines, including dopamine, norepinephrine, epinephrine, serotonin, and histamine, are synthesized from tyrosine: Tyrosine → DOPA → Dopamine → Norepinephrine (Noradrenaline) → Epinephrine (Adrenaline).
• Postsynaptic receptors include cholinergic (nicotinic and muscarinic) and adrenergic receptors that bind to norepinephrine.

Postsynaptic Response

• EPSP (Excitatory Postsynaptic Potential) is the depolarization of the postsynaptic membrane that increases the probability of an action potential.
• IPSP (Inhibitory Postsynaptic Potential) is the hyperpolarization of the postsynaptic membrane that decreases the probability of an action potential.
• Refractory periods include absolute (impossible to generate another action potential) and relative (a stronger stimulus is needed).
• Saltatory conduction accelerates the speed of propagation of the action potential in myelinated axons, jumping between the nodes.
• In unmyelinated axons, regeneration is immediate.
• In myelinated axons, it takes time to regenerate, and the charge will spread in both directions, but an action potential cannot be regenerated.
• K+ is high inside and low outside the cell.
• Na+ is low inside and high outside the cell.

Other Key Points

• Resting membrane potential: Voltage-gated Na+ channels are closed and Voltage-gated K+ channels open slowly.
• Depolarization: Voltage-gated Na+ channels open.
• Repolarization: Voltage-gated Na+ channels begin returning to normal state and Voltage-gated K+ channels begin closing
• Action potential stimulus intensity determines the # of action potentials produced at the axon hillock.
• Action potential frequency indicates the stimulus strength, with a weak stimulus releasing fewer neurotransmitters and a strong stimulus causing more action potentials and releasing more neurotransmitters.
• Temporal summation occurs when two graded potentials from one presynaptic neuron occur close together in time.
• No summation occurs if two subthreshold graded potentials are far apart in time.
• Summation causes an action potential if two subthreshold potentials arrive at the trigger zone within a short period and summate to initiate an action potential.
• The strength of the Graded potential determines the numbers of action potential that can be produced as long as a stimulus is applied.

Cerebrum

• Memory: information/data is encoded, stored, and retrieved when needed
• Memory is divided into long term memory (memories retained for years, but can be lost) and short term memory (memories retained for shorter times, maybe seconds, days, or months, but needs to be practiced to retain).
• Types of long-term memories: reflexive (implicit memory) is an automatic response of the body and includes reflexive memory and the procedures and learning process of life that does not require conscious attention; declarative (explicit memory) requires effort and conscious attention.
• Disorders of long-term memory: amnesia (loss of memory), anterograde amnesia (memory before injury is retained, but problems occur to remember the present and future), and retrograde amnesia (memory before injury is lost, and only new memories are remembered and saved).
• Alzheimer's is a decline in memory capacity produced by a protein that kills neurons; it is the most common form of dementia and a decline in mental ability.
• Two language centers are foundational on the left cerebrum hemisphere.
• Wernicke's area is for comprehension of vocal, written, and signed language.
• Broca’s area, on the frontal lobe, is the production and articulation center of speech, also programs the motor cortex to move tongue, lips, and speech muscles.
• The primary motor cortex is responsible for moving tongue muscles.