Ch. 48 Neurons, Synapses, and Signaling

Molloy University Biology 1270

Specify Neurons

Are nerve cells that transfer information within the body

List the three stages that nervous systems process information

1. Sensory input - detects internal and external stimuli

2. Integration - processes and interprets information

3. Motor output - carries out responses (muscle contraction, gland secretion)

Define sensory neurons, interneurons and motor neurons

• Sensory neurons - transmit information about external stimuli such as light, touch, or smell

• Interneurons - integrate (analyze and interpret) the information

• Motor neurons - transmit signals to muscle cells, causing them to contract (response)

Define nerves

• Bundles of axons (from many neurons) wrapped in connective tissue

• Carry signals through the body

Specify the nervous system (Slide 4 of 4)

• Central Nervous System (CNS)

  • Brain or ganglia (simpler clusters)

  • Site of integration

• Peripheral Nervous System (PNS)

  • Carries info to and from CNS

• Glial cells (glia) (both from CNS and PNS)

  • Support, nourish, and protect neurons

Define voltage, membrane potential and resting potential

• Voltage – difference in electrical charge

• Membrane potential – voltage across the plasma membrane

• Resting potential – membrane potential when neuron is not signaling

• Action potential – change in membrane potential during signaling

Specify the formation of the resting potential (3 slides)

• K⁺ concentration is higher inside the cell

• Na⁺ concentration is higher outside the cell

Key components:

• Sodium-potassium pump (ATP required):

  • Pumps K⁺ into cell

  • Pumps Na⁺ out of cell

• Maintains concentration gradients

• Ion channels (pores):

  • Allow ions to diffuse across membrane

  • Selectively permeable

• At resting potential:

  • Many K⁺ channels open

  • Few Na⁺ channels open

  • K⁺ diffuses out → inside becomes more negative

• Ion gradients store potential energy used for signaling

Define equilibrium and Nernst equation

Equilibrium potential (Eion) = membrane voltage where ion is balanced

Eion​=62mV⋅log([ion]inside/ ​[ion]outside​​)

The equilibrium potential of K+ is ____, while the equilibrium potential of Na+ is ___.

• K⁺ (EK) → negative

• Na⁺ (ENa) → positive

What can be used to monitor the changes in membrane potential?

• Intracellular recording can be used to monitor the changes in membrane potential

Define voltage-gated ion channels

Open/close in response to changes in membrane voltage

Define hyperpolarization

• Increase in membrane potential magnitude

• Cell becomes more negative

Define depolarization

• Decrease in membrane potential magnitude

• Cell becomes less negative (more positive)

Define graded potentials

• variable changes in membrane potential

• Strength depends on stimulus intensity

Specify action potentials

• A rapid electrical signal in a neuron where the membrane potential quickly changes from negative to positive and back, allowing the nerve impulse to travel.

• Occurs when threshold is reached

Define threshold

• The minimum membrane potential (about −55 mV) that must be reached to trigger an action potential.

• Caused by opening of voltage-gated channels

Specify the five steps in generating the action potentials

• Resting state

  • Voltage-gated Na⁺ and K⁺ channels closed

• Depolarization

  • Na⁺ channels open → Na⁺ enters

• Rising phase

  • Threshold crossed → membrane approaches ENa

• Repolarization (falling phase)

  • Na⁺ channels inactivate

  • K⁺ channels open → K⁺ leaves

• Undershoot

  • Membrane becomes more negative than resting

  • K⁺ channels close → resting potential restored

Define refractory period

• Time after action potential when another cannot occur

• Caused by Na⁺ channel inactivation temporarily

What’s the relationship between the speed of an action potential and the axon’s diameter?

Larger diameter → faster signal conduction

Specify Myelin sheath

• Insulating layer around axons

• Increases speed of action potentials

Produced by:

• Oligodendrocytes (CNS)

• Schwann cells (PNS)

Define nodes of Ranvier and saltatory conduction

• Nodes of Ranvier – gaps in myelin

• Na⁺ channels concentrated here

• Saltatory conduction – signal “jumps” node to node → faster transmission

Define electrical and chemical synapses

• Electrical synapses

  • Direct current via gap junctions from one neuron to another

• Chemical synapses

  • Neurotransmitters carry signals between neurons

  • Most common type

How neurons communicate at synapses? (Slide 2 of 2)

1. Presynaptic neuron synthesized and packages the neurotransmitter in synaptic vesicles located in the synaptic terminal

2. Action potential triggers release of neurotransmitters

3. Neurotransmitter diffuses across synaptic cleft and is received by the postsynaptic cell

Specify the generation of postsynaptic potentials (2 slides)

• Ionotropic (ligand-gated) receptors:
  Neurotransmitters bind to these receptors and open ion channels, causing a postsynaptic potential.

• Are permeable to both K+ and Na+

Postsynaptic potentials:

• EPSPs (excitatory): Depolarize the membrane, bringing it closer to threshold.

• IPSPs (inhibitory): Hyperpolarize the membrane, moving it farther from threshold.

Define summation and temporal

Summation: Multiple postsynaptic potentials (EPSPs/IPSPs) combine to change membrane potential; helps determine if threshold is reached.

Temporal summation: EPSPs from the same synapse arrive rapidly (before reset) and add together, making a stronger depolarization.

Define spatial summation

• EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron add together

• The combination of E P S P s through spatial and temporal summation can trigger an action potential

The combination of EPSPs _____ can tigger an action potential

• Through summation

  • an IPSP can counter the effect of an EPSP

The summed effect of EPSPs and IPSPs determines

• whether an axon hillock will reach threshold and generate an action potential

Specify the termination of neurotransmitter signaling

• Chemical Synapse Reset (After Signal)

  • After a response, the synapse returns to its resting state

  • Neurotransmitters are removed from the synaptic cleft

  How Neurotransmitters Are Cleared

  • Enzymatic hydrolysis: enzymes break them down

  • Reuptake: taken back into the presynaptic neuron

  Why This Matters

  • Clearing neurotransmitters is essential to stop the signal and allow new signals

  • If not cleared → signals keep firing uncontrollably

  Real-World Example

  • Sarin gas blocks the enzyme that breaks down neurotransmitters (like acetylcholine)

  • This causes continuous muscle stimulation → paralysis and death

Specify neurotransmitters

• One neurotransmitter can bind to multiple receptors

• Could excite/inhibit postsynaptic cells expressing one receptor/ expressing a different receptor

Specify acetylcholine (2 slides)

• A very common neurotransmitter found in both vertebrates and invertebrates

• Plays key roles in muscle contraction, memory, and learning

• Works through two types of receptors:

  • Ligand-gated (ion channels) → fast responses

  • Metabotropic (G-protein coupled) → slower, longer-lasting effects

Toxins affecting acetylcholine:

• Certain substances disrupt ACh signaling, including:

  • Nicotine

  • Sarin (nerve gas) → causes paralysis and can be fatal

  • Botulinum toxin (from bacteria) → blocks muscle contraction

Other neurotransmitters:

• ACh is just one of 100+ neurotransmitters

List the 5 classes of neurotransmitters

• Acetylcholine

• Amino acids

• Biogenic Amines

• Neuropeptides

• Gases

List the names and functions of three amino acid neurotransmitters

Amino Acid Neurotransmitters:

• Glutamate → a major neurotransmitter in both vertebrates and invertebrates; typically excitatory

• Glycine → functions at inhibitory synapses in parts of the central nervous system (mainly outside the brain, like the spinal cord)

• GABA (gamma-aminobutyric acid) → the main inhibitory neurotransmitter in the brain

Quick way to remember:

• Glutamate = Excites (turns signals ON)

• Glycine & GABA = Inhibit (turn signals OFF)

List four biogenic amines of neurotransmitters

Biogenic Amines:

• Include norepinephrine, epinephrine, dopamine, and serotonin

• Norepinephrine is synthesized from the amino acid tyrosine

• These neurotransmitters play important roles in mood, movement, and overall brain function

Clinical connection:

• Imbalances in biogenic amines are linked to nervous system disorders

• Parkinson’s disease is associated with a lack of dopamine in the brain, leading to movement problems like tremors and stiffness

Quick memory tip:

• Biogenic amines = brain chemistry regulators (mood + movement)

Which class of neurotransmitters do substance P and endorphins belong?

Neuropeptides

• These are short chains of amino acids that act as neurotransmitters

• Examples: substance P (pain signaling) and endorphins (pain relief)

Why opiates can be used as painkillers?

• Opiates bind to the same receptors as endorphins

• This mimics the body’s natural pain-relief system and reduces pain perception

Specify NO and CO as gas neurotransmitters

• Nitric oxide (NO) and carbon monoxide (CO) act as gaseous neurotransmitters in the PNS

• NO:

  • Not stored in vesicles; made on demand

  • Acts locally and breaks down quickly (within seconds)

• CO:

  • Produced in small amounts in the body (vertebrates)

  • Can function as a neurotransmitter despite being toxic in large amounts