The Nervous System: An Overview

Overview of the Nervous System

• Neurobiology is the study of the nervous system.
• The endocrine system and the nervous system work together to maintain internal coordination.
• The nervous system coordinates in three steps via sense organs and nerve endings:
  • Receiving information about the body and environment changes and sending messages to the spinal cord and brain.
  • Processing information, relating it to past experiences, and deciding on a response.
  • Issuing commands to muscle and gland cells to carry out the response.
• The nervous system consists of the central nervous system (CNS) and peripheral nervous system (PNS), PNS has additinal subdivisions.

Central and Peripheral Nervous Systems

• The central nervous system (CNS) includes the brain and spinal cord, enclosed and protected by the cranium and vertebral column.
• The peripheral nervous system (PNS) includes all nervous system components outside the brain and spinal cord, composed of nerves and ganglia.
  • A nerve is a bundle of nerve fibers (axons) wrapped in connective tissue.
  • A ganglion is a knotlike swelling in a nerve where neuron cell bodies concentrate.
• The PNS has sensory (afferent) and motor divisions.
  • The sensory division carries sensory signals from receptors to the CNS, informing it of stimuli.
    • The somatic sensory division carries signals from skin, muscles, bones, and joints.
    • The visceral sensory division carries signals from thoracic and abdominal viscera.
  • The motor (efferent) division carries signals from the CNS to gland and muscle cells (effectors), which respond to stimuli.
    • The somatic motor division sends signals to skeletal muscles for voluntary and involuntary contractions (somatic reflexes).
    • The visceral motor division (autonomic nervous system) sends signals to glands, cardiac muscle, and smooth muscle, controlling visceral reflexes without voluntary control.
• The autonomic nervous system contains the sympathetic and parasympathetic divisions.
  • The sympathetic division arouses the body for action by increasing heartbeat and respiratory airflow and inhibiting digestion.
  • The parasympathetic division has a calming effect, slowing heartbeat and stimulating digestion.

Properties of Neurons

• Nerve cells, or neurons, have three key physiological properties for communication:
  • Excitability: Responding to stimuli.
    • All cells are excitable
    • Neurons have developed excitability to the highest degree
  • Conductivity: Producing electrical signals in response to stimuli.
  • Secretion: Secreting a neurotransmitter to stimulate the next cell when an electrical signal reaches a nerve fiber's end.
• Neurons are divided into sensory, interneurons, and motor classes based on nervous system functions.
  • Sensory (afferent) neurons detect stimuli and send information to the CNS, beginning in almost every organ and ending in the CNS.
    • Some receptors are neurons (touch and smell), but others aren't and communicate with sensory neurons (taste and hearing)
  • Interneurons (association neurons) are within the CNS, receiving signals from other neurons to carry out integrative functions and "make decisions" about responses.
    • About 90% of neurons in the human body are interneurons that interconnect sensory and motor pathways in the CNS.
  • Motor (efferent) neurons send signals to muscle and gland cells.
    • Most lead to muscle cells, carrying signals away from the CNS.

Neuron Structure

• A motor neuron of the spinal cord represents a neuron's structure.
• The soma, neurosoma, or cell body, is the control center containing a centrally located nucleus with a large nucleolus.
  • The cytoplasm contains mitochondria, lysosomes, a Golgi complex, inclusions, rough ER, and a cytoskeleton.
  • The cytoskeleton includes microtubules and neurofibrils, compartmentalizing the rough ER into Nissl bodies.
    • Nissl bodies are unique to neurons for identification.
  • Mature neurons don't divide but can live long lives. Stem cells in the CNS can divide and develop into new neurons.
  • Cytoplasmic inclusions include glycogen granules, lipid droplets, melanin, and lipofuscin.
    • Lipofuscin forms when lysosomes digest worn-out organelles and accumulates with age, pushing the nucleus aside, known as "wear-and-tear granules."
• The soma extends a few thick processes that branch into numerous dendrites, which are the main sites for receiving signals from other neurons.
  • Dendrite number varies from one to thousands, providing precise pathways for signal reception and processing.
• The axon (nerve fiber) originates from the axon hillock on the soma.
  • The axon, cylindrical and mostly unbranched, may have axon collaterals.
  • Specialized for rapid nerve signal conduction, the axon's cytoplasm is axoplasm, and its membrane is the axolemma.
    • A neuron has no more than one axon; some have none.
  • Somas range from 5 to 135 µm in diameter, and axons range from 1 to 20 µm in diameter and a few millimeters to over a meter in length.
  • The distal end of an axon usually has a terminal arborization with fine branches.
    • Each branch ends in a synaptic knob (terminal button), forming a junction (synapse) with the next cell and containing synaptic vesicles full of neurotransmitter.
    • Autonomic neurons have varicosities (beads) on their axons.
• Neurons vary in structure and are classified by the number of processes from the soma.
  • Multipolar neurons have one axon and multiple dendrites and are the most common.
  • Bipolar neurons have one axon and one dendrite, such as olfactory, retina, and ear sensory neurons.
  • Unipolar neurons have a single process from the soma and carry sensory signals to the spinal cord.
    • Also called pseudounipolar because they start as bipolar neurons in embryos, but their processes fuse as the neuron matures.
    • The process branches like a T, with a peripheral fiber bringing signals from a sensation source and a central fiber continuing to the spinal cord.
    • Dendrites are short receptive endings and the rest of the fiber is the axon due to myelin and action potential production.
  • Anaxonic neurons have multiple dendrites but no axon, communicate through dendrites, don't produce action potentials, and are found in the brain, retina, and adrenal medulla.

Axonal Transport

• Axonal transport moves proteins, organelles, and materials along an axon, to and from the soma.
• Anterograde transport moves materials away from the soma, and retrograde transport moves materials up the axon toward the soma.
• Materials move along microtubules of the cytoskeleton using kinesin for anterograde and dynein for retrograde transport which act like myosin heads of muscle to crawl along the microtubules.
• There are two types of axonal transport: fast and slow.

Speed of Axonal Transport

• Fast axonal transport occurs at 20 to 400 mm/day and can be anterograde or retrograde.
  • Fast anterograde transport moves mitochondria, synaptic vesicles, organelles, axolemma components, calcium ions, and enzymes towards the axon's distal end.
  • Fast retrograde transport returns used synaptic vesicles to the soma and informs the soma of the axon terminals' conditions.
    • Pathogens like tetanus toxin and viruses can invade the nervous system by entering axon tips and traveling to the soma via fast retrograde transport.
• Slow axonal transport is anterograde and occurs at 0.5 to 10 mm/day.
  • It moves enzymes and cytoskeletal components down axons, renews worn-out axoplasmic components in mature neurons, and supplies new axoplasm for development or regeneration.
    • Damaged nerve fibers regenerate at a speed governed by slow axonal transport.

Supportive Cells (Neuroglia)

• Neurons are outnumbered 10 to 1 by neuroglia or glial cells, which bind neurons and form a supportive framework.
• There are six types of neuroglia, each with unique functions; four are in the CNS.
• Oligodendrocytes resemble octopuses with up to 15 armlike processes that spiral around nerve fibers, wrapping them in a myelin sheath that speeds up nerve conduction in the CNS.
• Ependymal cells resemble cuboidal epithelium lining brain and spinal cord internal cavities without a basement membrane and have rootlike processes into underlying tissues.
  • Ependymal cells produce cerebrospinal fluid (CSF), which bathes the CNS and fills internal cavities, and have cilia that circulate CSF.
• Microglia are small macrophages from monocytes that wander through the CNS, probing for cellular debris or other problems.
  • They perform check-ups on brain tissue, phagocytizing dead tissue, microorganisms, and foreign matter and concentrate in damaged areas, indicating injury.
• Astrocytes are the most abundant glia in the CNS, making up over 90% of the tissue in some brain areas, and are named for their starlike shape.
  • They form a supportive framework for nervous tissue.
  • Astrocytes have perivascular feet that contact blood capillaries, stimulating them to form the blood-brain barrier.
  • They monitor neuronal activity and regulate blood flow to meet neuronal demand for oxygen and nutrients.
  • Astrocytes convert blood glucose to lactate and supply it to neurons.
  • They secrete nerve growth factors that regulate nerve development.
  • Astrocytes communicate electrically with neurons and influence synaptic signaling.
  • They regulate tissue fluid composition, absorbing neurotransmitters and potassium ions.
  • Astrocytes form hardened scar tissue when neurons are damaged, a process called astrocytosis or sclerosis.
• Schwann cells, or neurilemmocytes, envelope nerve fibers of the PNS.
  • They wind repeatedly around a nerve fiber, producing a myelin sheath and assist in regenerating damaged fibers.
• Satellite cells surround somas in ganglia of the PNS; they provide electrical insulation around the soma and regulate the chemical environment.

Myelin Sheath

• The myelin sheath is a spiral layer of insulation around a nerve fiber, composed of the plasma membrane of glial cells (20% protein and 80% lipids).
• Myelination is the production of the myelin sheath, beginning in the fourteenth week of fetal development, proceeding rapidly in infancy, and completing in late adolescence.
  • Children under 2 years old shouldn't be put on low-fat diets as it may interfere with myelination.
• In the PNS, a Schwann cell spirals around a single nerve fiber, laying down up to 100 membrane layers.
  • The Schwann cell spirals outward, ending with a thick outermost coil called the neurilemma, containing the Schwann cell's cytoplasm, nucelus, and body.
  • External to the neurilemma is a basal lamina and a fibrous connective tissue sleeve called the endoneurium.
• In the CNS, each oligodendrocyte myelinates several nerve fibers in its vicinity and pushes newer myelin layers under older ones.
  • Nerve Fibers of the CNS have no neurilemma or endoneurium.
• In both the PNS and CNS, many Schwann cells or oligodendrocytes cover a single nerve fiber.
  • The myelin sheath is segmented, and the gaps between segments are the nodes of Ranvier, while the myelin-covered segments are called internodes (0.2 to 1.0 mm long).
  • The short nerve fiber section between the axon hillock and the first glial cell is the initial segment.
    • The axon hillock plus initial segment make up the trigger zone.

Unmyelinated Fibers

• Many nerve fibers in the CNS and PNS are unmyelinated, but in the PNS, even unmyelinated fibers are enveloped by Schwann cells.
  • One Schwann cell harbors 1 to 12 small unmyelinated fibers in grooves, where the Schwann cell's plasma membrane folds once and overlaps slightly.
    • The wrapping is the neurilemma.
  • Nerve fibers travel through individual channels, but small fibers are bundled, and a basal lamina surrounds the Schwann cell and fibers.
• Nerve signal conduction speed depends on fiber diameter and myelin presence.
  • Signal conduction occurs along the surface of a fiber, not in the axoplasm.
  • Large fibers have more surface area and conduct faster than small fibers.
    • Nerve signals travel at 0.5 to 2.0 m/s in small unmyelinated fibers and 3 to 15 m/s in myelinated fibers of the same size.
    • In large myelinated fibers, nerve signals travel at 120 m/s.
  • Slow unmyelinated fibers are sufficient for many processes, but fast myelinated fibers are vital for motor commands to skeletal muscles.
• PNS nerve fibers are vulnerable to trauma but can regenerate if the soma is intact and at least some neurilemma remains.
  • When a nerve fiber is cut, protein synthesis is impossible, the distal fiber and Schwann cells degenerate and macrophages clean up tissue debris.
  • The soma swells, its ER breaks up, and Nissl bodies disperse, the nucleus moves off-center, and some neurons die.
  • The axon stump sprouts growth processes while the distal end degenerates, and muscle fibers shrink (denervation atrophy).

Nerve Regeneration

• Near the injury site, Schwann cells, the basal lamina, and the neurilemma form a regeneration tube.
  • Schwann cells produce cell-adhesion molecules and nerve growth factors, enabling a neuron to regrow.
  • When a growth process finds its way into the tube, it grows rapidly (3-5 mm/day) and the others retract and return to original appearence.
  • The regeneration tube guides the sprout back to the original target cells, reestablishing synaptic contact, and the reinnervated muscle fibers regrow.
• Regeneration isn't perfect; nerve fibers may connect to the wrong muscle fibers or not find muscle fibers, and some motor neurons die, and nerve injury often results in functional deficit.
• Damaged nerve fibers in the CNS cannot regenerate but being enclosed in bone results in fewer traumas.

Electrophysiology of Neurons

• Santiago Ramon y Cajal showed that the nervous pathway is not a continuous "wire" or tube but a series of cells with gaps now called synapses
• Two key issues in neurophysiology: how a neuron generates an electrical signal and how it transmits a meaningful message to the next cell.
• Electrical potential is a difference in charged particle concentration, and electrical current is charged particle flow.
  • A battery with electrical potential is polarized.
• Living cells are polarized because a charge difference, the resting membrane potential (RMP), exists across the plasma membrane.
  • The RMP is -70 mV in a "resting" neuron, with more negative charges inside than outside.
• Electrical currents in the body are created by the flow of ions like Na+ and K+ through gated channels.
• The resting membrane potential exists is because electrolytes are unequally distributed between the extracellular fluid (ECF) and the intracellular fluid (ICF).
• Three factors determine the RMP: diffusion of ions down concentration gradients, selective membrane permeability, and electrical attraction of cations and anions.

Membrane Permeability

• Potassium ions (K+) have the greatest influence on RMP because the plasma is more permeable to K+ than any other ion.
  • Cytoplasmic anions cannot escape from the cell; K+ diffusion out of the cell leaves the ICF with a net negative charge.
  • The negative ICF attracts K+ back into the cell until equilibrium is reached between diffusion and electrical attraction (+40 times concentrated in the ICF).
    • At equilibrium, K+ is 40 times as concentrated in the ICF as in the ECF. If K+ were the only ion involved the RMP would be -90mV.
• Sodium ions (Na+) also influence the RMP.
  • There are 12 times more Na+ concentrated in the ECF than in the ICF.
  • Although the membrane is less permeable to Na+, it diffuses down its concentration gradient into the cell and is attracted by the ICF’s anions.
• The Na+-K+ pump compensates for Na+ and K+ leakage, consuming 1 ATP for every 3 Na+ pumped out and 2 K+ brought in.
  • The pump accounts for about 70% of the nervous system's ATP requirement.
• The net effect of K+ diffusion outward, Na+ diffusion inward, and the Na+-K+ pump's action is the -70 mV RMP.
• Membrane potential changes when a neuron is stimulated, starting at the dendrite, spreading through the soma, traveling down the axon, and ending at the synaptic knobs.
• A stimulus at the dendrite binds to receptors that open Na+ gates, allowing Na+ to flow into the cell and neutralize negative charge (depolarization).
  • Any change that shifts the membrane voltage to a less negative value is termed depolarization.

Local and Action Potentials

• Incoming Na+ ions diffuse and produce a current (local potential) that travels toward the trigger zone.
• Four characteristics distinguish local potentials from action potentials:
  • Local potentials are graded, varying in magnitude depending on the stimulus strength.
  • Local potentials are decremental, weakening as they spread from the point of stimulation.
  • Local potentials are reversible if stimulation ceases.
  • Local potentials can be excitatory or inhibitory, resulting in hyperpolarization (increase in negative potential).
• An action potential is a rapid voltage shift produced by voltage-gated ion channels in the plasma membrane.
  • Occur only with a high density of voltage-gated channels.
    • The trigger zone contains 350 to 500 voltage-gated channels per µm², compared to 50 to 75 gates per µm² in the soma.
  • A strong local potential can open channels and generate an action potential.
  • The current arrives at the axon hillock, depolarizing the membrane.
• The local potential must rise to the threshold (-55 mV) to open the voltage-gated channels for the neuron to produce an action potential where the voltage-gated Na+ channels open quickly, while K+ gates open more slowly, depolarizing the membrane in a positive feedback cycle.
  • As the rising potential passes 0 mV, Na+ channels are inactivated and close, peaking at +35 mV.
  • The slow K+ gates fully open, and K+ exits to repolarize the membrane.
  • The K+ channels stay open longer; the membrane voltage drops more negative than the RMP (hyperpolarization or afterpotential).
  • Na+ diffusion into the cell and astrocyte removal of extracellular K+ restore the RMP.
    • Only one in a million ions crosses the membrane to produce an action potential, which affects distribution near the membrane.
• Action potentials differ from local potentials:
  • Follow an all-or-none law and are not graded; if threshold is not reached, no action potential is generated.
  • Are nondecremental; they do not weaken.
  • Are irreversible; they go to completion if initiated and cannot be stopped.
• During and after an action potential, the neuron cannot be stimulated to fire again (refractory period).

Refractory Periods

• The refractory period has absolute and relative phases.
  • During the absolute refractory period, no stimulus triggers a new action potential (Nat channels are open).
  • During the relative refractory period, a strong stimulus will trigger a new action potential (K+ channels are still open).
• The refractory period is only on a membrane patch, not the entire neuron.
• Signal conduction is different in unmyelinated and myelinated fibers.
• An unmyelinated fiber has voltage-gated channels along its length.
  • Na+ enters and depolarizes, exciting distal channels.
  • This chain reaction occurs to the axon end.
    • An action potential stimulates a new action potential just ahead of it, making it different from a nerve signal which is a traveling excitation wave.
    • Nerve signals are chain reactions of action potentials.
  • Action potentials don't travel backward because the membrane is in its refractory period.
    • a nerve signal is not like a current in a wire; it moves much more slowly, and the last action potential has the same voltage as the first one.
• Voltage-gated ion channels are scarce in myelinated fibers (internodes), fewer than 25/µm² compared with 2,000 to 12,000/µm² in the nodes of Ranvier.
  • A nerve signal travels as Na+ diffuses down the fiber under the axolemma in the internode.
  • A node of Ranvier occurs every millimeter or less with voltage-gated channels.
  • The diffusing ions reaching a node open channels, and a new action potential is generated; the nerve signal jumps from node to node (saltatory conduction).
    • Saltatory conduction is why myelinated fibers transmit signals faster (120 m/s) than unmyelinated fibers (2 m/s).

Synapses

• A synapse exists at the end of an axon, where the first neuron (presynaptic) releases neurotransmitter, and the second (postsynaptic) responds.
• A presynaptic neuron may form an axodentritic, axosomatic, or axoaxonic synapse.
• A single neuron may have synapses, such as a spinal motor neuron (10,000 synaptic knobs) and a cerebellum neuron (100,000).
• Otto Loewi discovered acetylcholine in 1921.

Types of Synapses

• Electrical communication between cells was impossible due to synaptic clefts, but excitatory cells, like cardiac muscle, have electrical synapses through gap junctions.
• Chemical synapses rely on neurotransmitters.
• A chemical synapse includes the synaptic knob, containing synaptic vesicles.
  • Many vesicles are "docked" at the membrane, while others form a reserve pool.
  • The postsynaptic neuron lacks vesicles but contains receptor proteins and ligand-gated ion channels.
• More than 100 confirmed or suspected neurotransmitters fall into four major categories based on composition.

Neurotransmitter Categories

• Neurotransmitters divided into four categories include:
  • Acetylcholine, a class by itself, is formed from acetic acid and choline.
  • Amino acid neurotransmitters include glycine, glutamate, aspartate, and y-aminobutyric acid (GABA).
  • Monoamines (biogenic amines) are synthesized from amino acids by removing the -COOH group.
    • Epinephrine, norepinephrine, dopamine, histamine, and serotonin form a subclass called catecholamines.
  • Neuropeptides are chains of 2 to 40 amino acids, such as cholecystokinin (CCK) and substance P.
    • They are stored in secretory granules, function as hormones or neuromodulators, and are produced by neurons and the digestive tract (gut-brain peptides).
• A given neurotransmitter may have different effects in the body, and the receptor determines a neurotransmitter’s effect on a cell.
• Three neurotransmitter action modes include excitatory cholinergic synapses, inhibitory GABA-ergic synapses, and excitatory adrenergic synapses.

Synapse Types

• An excitatory cholinergic synapse employs acetylcholine (ACh).
  • A nerve signal opens voltage-gated calcium channels, entering the knob and triggering exocytosis of synaptic vesicles to release ACh.
  • Empty vesicles drop back into the cytoplasm to be refilled, and synaptic vesicles in the reserve pool move to active sites to release ACh.
  • ACh diffuses across the cleft, binds to ligand-gated channels, opens them, and allows Na+ to enter and K+ to leave.
  • Na+ spreads, producing a local voltage shift called the postsynaptic potential that, if strong and persistent, triggers an action potential.
• An inhibitory GABA-ergic synapse employs y-aminobutyric acid, where GABA is released and binds to ion channels.
  • The GABA receptor is a chloride channel; when it opens, Cl- enters the cell, making the resting membrane potential more negative and preventing neuron firing.
• An excitatory adrenergic synapse employs norepinephrine (NE).
  • The receptor is a transmembrane protein associated with a G protein in a second-messenger system.
    • NE binding causes the G protein to dissociate and bind to adenylate cyclase, stimulating it to convert ATP to cyclic AMP (cAMP).
  • Cyclic AMP can induce several alternative effects inside the cell, produce a chemical that binds to a ligand-gated ion channel from inside the membrane, activate cytoplasmic enzymes, and induce genetic transcription.
  • Although slower than cholinergic and GABA-ergic synapses, adrenergic synapses have the advantage of enzyme amplification where a single NE molecule can form many cAMPs.
• Synaptic events require 0.5 ms from the signal's arrival at the axon terminal to the beginning of an action potential in the postsynaptic cell (synaptic delay).
• A signal stops when neurotransmitter release stops and neurotransmitters are removed from the synaptic cleft.
  • The presynaptic nerve fiber stops action potentials.
  • Neurotransmitter is removed from the synaptic cleft in three ways: diffusion, reuptake, and degradation.
    • Diffusion: Neurotransmitter leaves the synapse and enters the ECF, where astrocytes absorb and return to neurons.
    • Reuptake: The synaptic knob reabsorbs amino acids and monoamines by endocytosis and breaks them down with monoamine oxidase (MAO).
    • Degradation: Acetylcholinesterase (AChE) breaks down ACh into acetate and choline, reabsorbed by the synaptic knob.
• Neuromodulators are hormones, neuropeptides, and messengers.
  • Nitric oxide (NO) diffuses into a postsynaptic cell and activates second-messenger pathways.
  • Neuropeptides include enkephalins and endorphins, which inhibit spinal neurons from transmitting pain signals to the brain.

Neural Integration

• Neural integration is is the ability of neurons to process information, store, recall, and make decisions.
• Neural integration is based on postsynaptic potentials from neurotransmitters.
• A voltage change raising membrane potential closer to the threshold (from -70 mV to -55 mV) is an excitatory postsynaptic potential (EPSP).
• A voltage change hyperpolarizing the membrane and making it more negative than the RMP is an inhibitory postsynaptic potential (IPSP).
• All neurons fire at a background rate due to ion leakage, and EPSPs/IPSPs change the firing rate.
  • Glutamate and aspartate produce EPSPs; glycine and GABA produce IPSPs; acetylcholine and norepinephrine are excitatory/inhibitory for cells.
• Summation, facilitation, and inhibition influence a neuron's integration of inputs.
  • Summation is adding up ESPSs and ISPSs and responding to their net effect and enables the nervous system to make decisions.
    • A single action potential doesn't produce enough activity to fire; at least 30 EPSPs are needed.
  • ESPSs can be added up temporally or spatially.
    • Temporal summation: a single synapse produces EPSPs very quickly.
    • Spatial summation: EPSPs from different synapses add up to threshold at the axon hillock.
• Neurons work in groups to modify actions: facilitation or presynaptic inhibition.
  • Facilitation enhances the effect of another neuron.
    • A neuron firing alone may be unable to induce firing but the firing combined of two neurons will.
  • Presynaptic inhibition suppresses another's action (often blocking transmission with an inhibitory neurotransmitter).
• Neural coding converts information to meaningful action potential patterns.

Importance of Neural Pools

• Neurons function in ensembles called neural pools, composed of interneurons where the functions are determined by their neural circuit.
• Information arrives at a neural pool through input neurons, where that neuron alone can make postsynaptic cells fire in a discharge zone.
• In a facilitated zone, the neuron synapses with still other neurons and can stimulation them with the help of other input neurons.
• A neural pool's neural circuit consists of pathways among neurons, resulting in neural functions: diverging, converging, reverberating, and parallel after-discharge circuits.
  • In a diverging circuit, one nerve fiber branches and synapses with postsynaptic cells to produce output through neurons.
  • In a converging circuit, input from never fibers is funneled to a neuron or neural pool, allowing different sensory systems to be evaluated, like in balance.
  • In a reverberating circuit, neurons are stimulated in a linear fashion, and some send axon collaterals leading back to initial neurons and restimulate them.
  • In a parallel after-discharge circuit, an input neuron diverges to stimulate neuron chains, which reconverge on an output neuron.

Types of Memory

• The physical basis of memory is a pathway through the brain called a memory trace which adds, takes away, or modifies synapses (synaptic plasticity).
  • Synapses in a certain pathway become modified so signals travel more easily across them (synaptic potentiation).
• Immediate memory is the ability to hold something in mind for just a few seconds, is necessary for continuity/ important in reading, and might be based on reverberating circuits.
• Short-term memory (STM) lasts from seconds to hours, where working memory is STM for performing an action where facilitated synapses store.
  • Tetanic stimulation, the arrival of repetitive signals at a synapse, can induce facilitation, so Ca²+ accumulates in the synaptic knob, causing EPSPs to become stronger.
• Memories lasting for a few hours may involve posttetanic potentiation when level in the knob stays elevated, releasing bursts of neurotransmitter.
• Long-term memory (LTM) lasts up to a lifetime and is less limited than STM where declarative memory (facts) and procedural memory (motor skills) form.
  • Some LTM involves synapse remodeling or new synapse formation through axon terminal/dendrite growth and branching.
• LTM can also involve long-term potentiation when receptors bind glutamate and are subjected to tetanic stimulation.
  • Ca2+ entries have effects, the neuron produces more receptors, synthesizes remodeling proteins, and sends signals back to make release of neurotransmitter.