Neurons

Who first proposed that neurons are discrete, individual units rather than a continuous network?

Santiago Ramón y Cajal, based on his study of Golgi-stained tissue.

What did Camillo Golgi discover, and what did he believe about the nervous system?

He discovered the Golgi stain, but believed (like Joseph von Gerlach) that the nervous system was a continuous network

What is a Nissl stain vs. a Golgi stain?

Nissl stain labels only the cell body; Golgi stain fills the entire cell (body and all processes) but only stains about 1–2% of neurons, letting individual cells be distinguished.

Approximately how many neurons are in the human brain, per this module?

About 100 billion (note: a different module in this course cites ~86 billion

What are the three main components of a neuron?

Dendrites, soma (cell body), and axon.

What do dendrites do?

Extend from the soma, branch repeatedly, and receive input from thousands of other neurons

What does the nucleus do?

Located in the soma; contains genetic information, directs protein synthesis, and supplies energy/resources the neuron needs.

What does the axon do?

Extends away from the soma and carries the action potential to other neurons

What is a terminal button?

The structure at the end of the axon that forms synapses with dendritic spines (or the soma/axon) of another neuron.

What are spines?

Protrusions on a dendrite that form synapses with the terminal buttons of a presynaptic axon.

What is a synapse?

The junction between the presynaptic terminal button of one neuron and the dendrite, axon, or soma of another (postsynaptic) neuron.

What is the synaptic gap (cleft), and how big is it?

The tiny space (~5 nm) between the presynaptic terminal button and the postsynaptic membrane.

What are synaptic vesicles?

Packets within the terminal button that hold groups of neurotransmitters.

Name the three functional types of neurons.

Sensory neurons (receive info about the world), motor neurons (initiate movement/behavior), and interneurons (process sensory input, plan responses, connect to motor neurons).

What are the three structural types of neurons, and their functions?

Unipolar (one axon, no dendrites

What is a pyramidal neuron?

A common type of multipolar neuron named for its triangular/pyramid-shaped soma.

What are glia cells and what do they do?

Support cells in the brain (not involved in signal communication like neurons); include oligodendroglia (form myelin sheaths), and microglia/astrocytes (digest dead-neuron debris, deliver nutrients, regulate extracellular fluid).

What is resting membrane potential, and what is its value?

The baseline electrical charge difference inside vs. outside a neuron at rest; approximately -70 mV.

What two forces determine ion movement at rest?

Diffusion (movement from high to low concentration) and electrostatic pressure (like charges repel, opposite charges attract).

What is the equilibrium potential?

The membrane potential at which the force of diffusion exactly balances electrostatic pressure for a given ion, so no net ion flow occurs.

How are Na+, K+, Cl-, and A- distributed and controlled at rest?

A- (anions): high inside, impermeable to the membrane. K+: high inside, permeable, diffusion pushes it out while electrostatic pressure pulls it in (forces oppose). Cl-: high outside, permeable, diffusion pushes it in while electrostatic pressure pushes it out (forces oppose). Na+: high outside, membrane not very permeable; both diffusion and electrostatic pressure push it in, but it mostly stays out until channels open.

What does the sodium-potassium pump do?

Uses ATP to pump 3 Na+ ions out of the cell in exchange for bringing 2 K+ ions in.

What is an action potential?

An all-or-nothing electrical current that travels down the axon once the membrane reaches the threshold of excitation; a depolarization from -70 mV toward positive values (peaking around +40 mV).

What is the threshold of excitation?

The specific membrane potential (about -50 mV) that must be reached to trigger an action potential.

What are EPSPs and IPSPs?

EPSPs (excitatory postsynaptic potentials) are depolarizing, graded currents that push the membrane potential toward the threshold. IPSPs (inhibitory postsynaptic potentials) are hyperpolarizing, graded currents that push the membrane potential further from threshold. Unlike action potentials, both are graded (variable in strength), not all-or-nothing.

What does it mean for EPSPs and IPSPs to "summate"?

They add together in time and space

What happens during the depolarizing (rising) phase of an action potential?

Voltage-dependent Na+ channels open, Na+ rushes into the cell, making it very positively charged (up to ~+40 mV).

What happens during the hyperpolarizing (falling) phase?

Voltage-dependent K+ channels open; K+ flows out of the cell (driven by both diffusion and electrostatic pressure), returning the cell toward resting potential.

What is the refractory period?

The period right after an action potential when Na+ channels are closed and cannot reopen until the cell returns to resting membrane potential

What are nodes of Ranvier, and what is saltatory conduction?

Nodes of Ranvier are gaps in the myelin sheath; saltatory conduction is the process by which the action potential "jumps" from node to node, speeding up transmission and saving energy.

What are ionotropic receptors?

Receptors on ion channels that open in a lock-and-key fashion when a specific neurotransmitter binds, allowing ions to flow in or out.

What neurotransmitter is the brain's main excitatory transmitter, and what receptor does it activate?

Glutamate, which activates NMDA receptors (opens Na+/Ca+ channels, causing an EPSP).

What neurotransmitter is the brain's main inhibitory transmitter, and what receptor does it activate?

GABA (gamma-aminobutyric acid), which activates GABA receptors (opens Cl-/K+ channels, causing an IPSP).

What is reuptake?

The process where a neurotransmitter that doesn't bind to a receptor is taken back into the presynaptic terminal button (rather than being broken down by enzymes or glial cells).

Who won the Nobel Prize for describing the ionic basis of the action potential, and using what animal?

Hodgkin and Huxley (1963 Nobel Prize), using the giant axon of the squid.