Part 4: Microanatomy, Graded Potentials, and Action Potentials (Textbook)

What are the two main types of cells in the nervous system?

Neurons and Glial cells.

How many neurons are in the human brain?

Approximately 80–90 billion.

What are neurons responsible for?

Receiving, integrating, and transmitting information throughout the nervous system.

What are glial cells?

Non-neuronal brain cells that provide structural, nutritional, and support functions to neurons.

What is the Neuron Doctrine?

The theory by Ramón y Cajal stating that neurons are individual, independent cells that communicate via synapses.

What are the two principles of the Neuron Doctrine?

1) Neurons are structurally, metabolically, and functionally independent. 2) Information is transmitted between neurons through tiny gaps called synapses.

About how many synapses exist in the human brain?

Approximately 10¹⁵ (10 quadrillion).

What is found in a neuron's cell body?

A nucleus containing DNA, ribosomes, mitochondria, and other organelles.

What occurs in the input zone of a neuron?

Dendrites receive information via synapses from other neurons.

What are dendritic spines?

Small protrusions on dendrites that increase surface area for receiving synaptic inputs.

What occurs in the integration zone of a neuron?

The cell body integrates incoming signals and decides whether to generate its own electrical signal.

What occurs in the conduction zone of a neuron?

The axon carries electrical impulses away from the cell body.

What are axon collaterals?

Branches of an axon that allow communication with multiple target neurons.

What occurs in the output zone of a neuron?

Axon terminals (synaptic boutons) transmit signals to other neurons’ dendrites.

What are motor neurons (motoneurons)?

Large neurons with long axons that connect to muscles and control contraction or relaxation.

What are sensory neurons?

Neurons that process sensory information, varying in shape depending on the stimulus type.

What are interneurons?

Neurons that connect and process information between other neurons; they make up the majority of neurons in the brain.

What are the three structural types of neurons?

Multipolar, Bipolar, and Unipolar.

What is a multipolar neuron?

A neuron with many dendrites and one axon; the most common type.

What is a bipolar neuron?

A neuron with one dendrite and one axon, commonly found in sensory systems like vision.

What is a unipolar neuron?

A neuron with a single extension that splits into two directions, carrying sensory information into the spinal cord.

What are the three principal components of a synapse?

Presynaptic membrane, synaptic cleft, and postsynaptic membrane.

What is the presynaptic membrane?

The specialized membrane of the axon terminal that releases neurotransmitters.

What is the synaptic cleft?

A 20–40 nm gap separating presynaptic and postsynaptic neurons.

What is the postsynaptic membrane?

The membrane on the dendrite or soma of the postsynaptic neuron that receives neurotransmitters.

What are synaptic vesicles?

Tiny sacs in the presynaptic terminal containing neurotransmitters.

What happens when synaptic vesicles fuse with the presynaptic membrane?

They release neurotransmitters into the synaptic cleft.

What are neurotransmitter receptors?

Proteins on the postsynaptic membrane that bind neurotransmitters and change the neuron’s excitation level.

What is neuroplasticity?

The ability of the nervous system to change and adapt its structure or connections in response to experience or environment.

What is the axon hillock?

The cone-shaped region where the axon originates from the cell body and where action potentials begin.

What is the function of the axon hillock?

It integrates synaptic inputs and initiates neural output signals.

What is axonal transport?

The movement of materials between the cell body and axon terminals.

What is anterograde transport?

The movement of newly synthesized materials from the cell body down the axon.

What is retrograde transport?

The movement of used materials from the axon terminal back to the cell body for recycling.

What are the four main types of glial cells?

Oligodendrocytes, Schwann cells, Astrocytes, and Microglial cells.

What do oligodendrocytes do?

Form myelin in the central nervous system (CNS).

What do Schwann cells do?

Form myelin in the peripheral nervous system (PNS).

What do astrocytes do?

Regulate blood flow, form the outer membrane of the brain, and secrete chemicals that affect synapses.

What do microglial cells do?

Clean up debris from injured or dead cells and act as immune defense cells in the brain.

What is myelin?

A fatty insulation around axons made by glial cells that speeds up signal conduction.

What is the myelin sheath?

The protective layer of myelin around axons that increases the speed of neural communication.

What are the Nodes of Ranvier?

Gaps between sections of myelin sheath where the axon membrane is exposed.

What causes the action potential (AP) to occur?

A sudden influx of Na+ ions into the axon (Hodgkin & Katz, 1949).

What happens during the resting phase?

K+ flows out and Na+ flows in slowly; membrane potential ≈ –65 mV.

What happens during depolarization?

Voltage-gated Na+ channels open, Na+ rushes in, and membrane potential rises to about +40 mV.

What happens at the peak or overshoot of an AP?

Na+ channels close and K+ channels reopen slowly.

What occurs during repolarization?

K+ exits the neuron rapidly, restoring the membrane potential toward –65 mV.

What causes hyperpolarization?

Some K+ channels stay open too long, leading to excess K+ leaving and a more negative potential.

What restores ionic balance after an action potential?

The Na+/K+ pump (3 Na+ out, 2 K+ in).

What are refractory periods?

Intervals during which a neuron cannot or is less likely to fire another action potential.

What is the absolute refractory period?

A brief period of total insensitivity to new stimuli; Na+ channels are inactivated.

What is the relative refractory period?

A period of reduced sensitivity when only strong stimuli can trigger a new AP; Na+ channels closed, K+ still open.

What determines a neuron’s maximum firing rate?

The overall duration of its refractory periods.

How does an action potential propagate along the axon?

Each AP depolarizes the next segment of the axon, regenerating identical APs down the axon.

Why do APs travel in only one direction?

Because the area behind the AP is in its refractory phase and lacks active Na+ channels.

What is conduction velocity?

The speed at which an AP travels along the axon (~150 m/s in large mammalian fibers).

What factors increase conduction velocity?

Larger axon diameter and myelination.

What is myelin?

A fatty insulation formed by glial cells that increases the speed of electrical conduction.

What are Nodes of Ranvier?

Gaps between myelinated segments where the axon membrane is exposed.

What is saltatory conduction?

The process by which APs “jump” from one Node of Ranvier to the next, greatly speeding signal transmission.

What evolutionary advantage does saltatory conduction provide?

It allows vertebrates faster, more efficient neural communication than invertebrates.

What is multiple sclerosis (MS)?

An autoimmune disorder where antibodies attack myelin, disrupting saltatory conduction.

What does “multiple sclerosis” mean in Latin?

“Many scars.”

What are the effects of MS on neurons?

Demyelination slows or blocks neural communication.

What are common symptoms of MS?

Blurry vision, numbness, loss of coordination, stiffness, walking difficulty, fatigue.

Is there a cure for MS?

No, but symptoms can be managed.

What therapies help manage MS?

Physical therapy and hormone therapy (e.g., estrogen during pregnancy may reduce symptoms).

What lifestyle factor worsens MS?

Smoking increases risk and accelerates disease progression.

What is a neurotransmitter (NT)?

A chemical released from the presynaptic terminal that communicates across a synapse.

What does “presynaptic” mean?

Located on the transmitting side of a synapse.

What does “postsynaptic” mean?

Located on the receiving side of a synapse.

What is a postsynaptic potential (PSP)?

A local change in membrane potential at a synapse, which can be excitatory or inhibitory.

How do PSPs spread?

Passively across the dendrites and soma, decreasing in strength with time and distance.

What is an excitatory postsynaptic potential (EPSP)?

A depolarizing potential that increases the likelihood of the postsynaptic neuron firing an AP.

What is an inhibitory postsynaptic potential (IPSP)?

A hyperpolarizing potential that decreases the likelihood of the postsynaptic neuron firing.

Which ion causes IPSPs when it enters the cell?

Cl– ions entering make the inside more negative.

What happens when an excitatory presynaptic neuron fires?

Na+ channels open in the postsynaptic cell, causing depolarization (EPSP).

What happens when an inhibitory presynaptic neuron fires?

Cl– enters or K+ exits, causing hyperpolarization (IPSP).

Are EPSPs and IPSPs graded or all-or-none?

They are graded potentials.

What is spatial summation?

The combined effect of PSPs from different synapses arriving at different dendritic locations simultaneously.

What is temporal summation?

The combined effect of multiple PSPs arriving in quick succession at the same synapse.

What happens if EPSPs and IPSPs occur together?

They can cancel each other out, affecting whether the neuron reaches threshold.

What determines whether a neuron fires an AP?

The total sum of excitatory and inhibitory inputs integrated over space and time.