Neurons: Structure, Signaling, and Synaptic Transmission

Central nervous system (CNS) vs peripheral nervous system (PNS)

• CNS includes the brain and spinal cord.
• PNS includes all other neurons in the body, e.g., sensory neurons in eyes (sense light), ears (sense sound), skin (sense pressure, temperature, pain).
• The two divisions are connected and constantly communicate: peripheral input (e.g., touch) is sent to the brain via the spinal cord.
• Conceptual takeaway: CNS and PNS are separate divisions but operate as an integrated network.

General neuron layout and teaching approach

• A neuron typically has:
  • A cell body (soma) with a nucleus; basic cell functions are carried out here.
  • Dendrites: branching structures that receive input from other neurons; receiving end of the neuron.
  • Axon: a long projection that sends electrical signals to other neurons; the sending part.
  • Terminal buttons: end points where signals are transmitted to the next neuron.
  • Myelin sheath: fatty insulation around many axons that speeds signal transmission; not all neurons have myelin.
  • Nodes of Ranvier: gaps in the myelin sheath where the axon is exposed.
• Author’s teaching emphasis: focus on function (what parts do) rather than memorizing labels on a figure. For example, the amygdala’s function is what matters, not its exact location on a diagram.
• Overall goal: understand how neurons generate electrical signals and communicate across networks.

Neuron structure: function of key parts

• Cell body (soma): basic cellular functions; houses the nucleus.
• Dendrites: receive inputs from other neurons; multiple dendritic branches allow input from many neurons.
• Axon: single long projection that transmits electrical signals away from the cell body toward other neurons.
• Terminal buttons: end of the axon; release neurotransmitters into the synapse.
• Myelin sheath: fatty insulation around the axon to speed signal transmission (increases efficiency).
• Nodes of Ranvier: gaps in myelin where the axon is exposed, aiding rapid conduction
• Note: Myelin provides faster communication; breaks (nodes) are necessary for signal propagation efficiency.

Resting membrane potential and ion distribution

• Resting state (not sending a signal): neuron is at a negative charge relative to its surroundings.
• Inside vs outside fluid composition:
  • Outside: lots of Na^+ (sodium ions), positive charge.
  • Inside: lots of K^+ (potassium ions) and negatively charged proteins; negative overall inside.
• Resting potential: $V_{rest} \,\approx\, -70\text{ mV}$ (the exact value is not required to memorize, just that it is negative).
• Gate channels in the membrane:
  • At rest, Na^+ gates and K^+ gates are closed, preventing ion passage.
• Ionic asymmetry maintains the resting potential: higher Na^+ outside, higher K^+ inside, plus the presence of negatively charged proteins inside.
• Concept: resting potential is set by diffusion gradients and selective membrane permeability.

Generation of an electrical signal: the action potential (AP)

• Initiation: stimulus can come from another neuron or a sensory input; this starts the sequence of changes in the membrane.
• First event after stimulation: Na^+ gates open, Na^+ flows into the neuron due to the diffusion gradient (high outside, low inside).
  • Na^+ carries a positive charge, so inward Na^+ rises intracellular positivity.
  • If recorded, the membrane potential would rise from its resting negative value toward positive values (example trajectory: from -70 mV up toward +40 mV).
• Second event: K^+ gates open, K^+ flows out because of the gradient (high inside, low outside).
  • Loss of positively charged K^+ drives the membrane potential back toward negative values.
• Resulting sequence: the membrane potential moves from negative to positive and then back to negative. This brief, characteristic change is the action potential.
• Important: the sequence is strictly ordered—Na^+ influx (sodium in) occurs before K^+ efflux (potassium out); if they occurred simultaneously, there would be no net change in charge.
• Action potential characteristics:
  • Propagated response: once initiated, the AP travels the length of the axon to its terminals; it cannot be stopped mid-way.
  • All-or-none: a neuron fires fully or not at all; the size of the AP is constant (e.g., -70 mV to +40 mV to -70 mV trajectory is consistent).
  • Intensity encoding via rate of firing: stimulus intensity is coded by how many APs are generated per unit time, not by the amplitude of a single AP. For example, a weak stimulus might produce ~30 APs/sec, while a stronger stimulus could produce ~200 APs/sec.
• After an AP, the system must reset to its resting state to allow future signaling: the Na^+/K^+ pump actively exchanges Na^+ and K^+ to re-establish the original unequal concentrations (Na^+ outside, K^+ inside).
• Key takeaway of the AP process: sequence, propagation, and all-or-none nature underpin neural signaling and information transfer.

The synapse: communication between neurons

• Synapse definition: the junction where one neuron (presynaptic) communicates with another neuron (postsynaptic).
• Presynaptic vs postsynaptic:
  • Presynaptic neuron: the neuron sending the signal.
  • Postsynaptic neuron: the neuron receiving the signal.
• Structure at the synapse:
  • Electrical signal travels down the presynaptic axon to the terminal buttons.
  • Terminal buttons release a chemical neurotransmitter into the synaptic gap (synapse).
  • Neurotransmitter diffuses across the gap and binds to receptor sites on the postsynaptic dendrite.
  • Binding can alter the postsynaptic neuron’s electrical state.
• Important distinction: neurotransmitter release is chemical, bridging the gap between neurons; electrical signal itself does not jump the gap directly.
• Two possible postsynaptic outcomes:
  • Excitatory response: postsynaptic neuron becomes more likely to fire (action potential may be triggered).
  • Mechanism example: neurotransmitter binding may open Na^+ gates in the postsynaptic membrane.
  • Inhibitory response: postsynaptic neuron becomes less likely to fire (net suppression of activity).
  • Mechanism example: the neurotransmitter may stabilize the membrane to resist depolarization, making it harder to trigger an AP.
• The classroom depiction often emphasizes that a neuron can either push the next neuron toward firing (excitatory) or suppress firing (inhibitory).
• Visual note: a typical synapse illustration shows the presynaptic terminal ending near the postsynaptic dendrite, with neurotransmitter-containing vesicles ready to release into the gap and receptors on the dendritic membrane.

Neurotransmitters: a focus on serotonin

• Types: there are many neurotransmitters (e.g., serotonin among others); the course emphasizes one key example.
• Serotonin:
  • A neurotransmitter involved in regulating mood and emotion.
  • Dysregulation is associated with mood-related disorders such as depression and anxiety.
• SSRIs (selective serotonin reuptake inhibitors):
  • Medications that elevate serotonin levels in the brain by affecting its reuptake.
  • They are commonly used to treat depression and, in some cases, anxiety.
  • The name SSRIs highlights serotonin as the target (S for serotonin).
• Real-world relevance: understanding serotonin helps explain mood regulation and the pharmacology of antidepressants.

Connections to broader concepts and practical implications

• The nervous system is a vast, interconnected network: sensory input, CNS processing, and motor output rely on rapid signaling across neurons.
• The alternation between resting potential, action potential, and neurotransmitter-mediated synaptic transmission is the fundamental mechanism by which information is processed in the brain.
• Signal strength is encoded by rate (frequency) of action potentials, not by the size of a single AP, which is always consistent.
• Myelination increases conduction speed, enhancing overall nervous system efficiency; nodes of Ranvier are essential for maintaining rapid signaling along myelinated axons.
• The balance of excitation and inhibition across synapses shapes neural circuit activity and behavior.
• Ethical, philosophical, and practical implications: understanding neural signaling informs debates about consciousness, mental health treatment, and the biological basis of emotions.

Quick conceptual checks (based on lecture emphasis)

• When the neuron is at rest, where are Na^+ and K^+ ions concentrated relative to the inside of the cell?
  • Outside: high Na^+; Inside: high K^+ and negative charged proteins.
• What is the resting membrane potential commonly around?
  • Approximately $V_{rest} \,\approx\, -70\text{ mV}$ (negative inside).
• What sequence occurs during the action potential?
  • First: Na^+ gates open and Na^+ flows into the neuron (inside becomes more positive).
  • Second: K^+ gates open and K^+ flows out of the neuron (inside returns toward negative).
• What is the role of the sodium–potassium pump?
  • Actively restores the resting ion distributions (Na^+ outside, K^+ inside) after an action potential, enabling repeated signaling.
• How does the brain encode stimulus intensity if action potentials are all-or-none?
  • Through the rate/frequency of action potentials, not the size of each AP.
• What happens at the synapse when an action potential arrives at the presynaptic terminal?
  • Neurotransmitters are released into the synaptic gap, bind to receptors on the postsynaptic neuron, and can produce an excitatory or inhibitory response.
• How can a neuron influence another neuron to fire or to stop firing?
  • Excitatory neurotransmitters can increase the chance of postsynaptic firing; inhibitory neurotransmitters can decrease it, effectively modulating neural activity.
• Why is serotonin important in this context?
  • It regulates mood and emotion; abnormalities are linked to depression/anxiety, and SSRIs raise serotonin levels to alleviate some symptoms.
• What does the term presynaptic vs postsynaptic refer to?
  • Presynaptic: the neuron sending the signal; Postsynaptic: the neuron receiving the signal.

Note on figures and exam expectations (teaching point)

• Do not worry about labeling parts on a figure for exams.
• The emphasis is on understanding the function of parts and processes (e.g., what the amygdala does, how signaling propagates, and how neurotransmitters influence the postsynaptic neuron).