🧠 How It All Connects
Follow one biological story from the basic cell and neuron → the instructions that help build and regulate the system → the electrical signal inside the neuron → the chemical handoff to the next neuron.
🔁 The Core Loop — the piece that ties Weeks 1, 3, and 4 together
Week 1 gives you the parts and the direction. Dendrites receive → soma/cell body → axon sends → presynaptic terminals hand the message off.
The neuron receives graded input. Excitatory input moves it toward threshold; inhibitory input moves it away.
If the summed depolarization reaches threshold, the neuron triggers an action potential.
The action potential is the electrical event that propagates down the axon.
After firing, the membrane repolarizes, may briefly hyperpolarize, and goes through the refractory period as it returns toward readiness. This prevents “fire again” from meaning an instant no-reset repeat.
When the action potential reaches the presynaptic terminal, Ca²⁺ enters → vesicles release neurotransmitter → neurotransmitter crosses the synaptic cleft → binds receptors on the postsynaptic cell.
Those receptor effects create EPSPs or IPSPs in the postsynaptic neuron. They can add together and move that next neuron toward or away from threshold.
If the next neuron reaches threshold, that neuron generates its own action potential. Now the Week 3 electrical sequence happens inside the new neuron, followed by another Week 4 synaptic handoff.
Memory line: Receive → Sum → Threshold → Fire → Reset → Hand off → Next neuron receives.
The most important distinction: Week 3 is what happens inside one neuron as the electrical signal fires and travels. Week 4 is what happens between neurons when the message is handed across the synapse. Then the receiving neuron can begin its own Week 3 cycle if its inputs reach threshold.
Cells → neuron parts → sensory input → CNS processing → motor output
Genes/DNA → mRNA → proteins → biological structure and function; genes and environment interact
Resting potential → threshold → action potential → refractory/reset → propagation
Presynaptic terminal → Ca²⁺ → neurotransmitter → receptor → EPSP/IPSP → summation → possible threshold in the next neuron
Start with a cell
A neuron is a cell, so Week 1 starts with the basic parts that keep a cell working.
Memory: Membrane = border · Nucleus = chromosomes · Ribosomes = proteins · ER = transport · Mitochondria = energy.
Specialize that cell into a neuron
Now give the cell a shape designed for communication.
Dendrites receive → Soma home base → Axon sends → Terminals hand off.
Forward connection: Week 3 puts the action potential on the axon. Week 4 begins at the presynaptic terminal.
Move information through the nervous system
Week 1 also gives the direction of a basic sensory-to-motor pathway.
Afferent admits. Efferent exits.
Hot stove: heat → receptor → sensory/afferent input → CNS processing → motor/efferent output → muscle pulls the hand away.
Week 2: zoom underneath the structure to genes and proteins
Week 2 adds another layer: the nervous system is biological tissue built and regulated through genetic instructions interacting with environment.
DNA → mRNA → protein.
Connection to Brain City: Week 1 shows you the structures. Week 2 explains a course-level source of the biological instructions used to build and regulate those structures. It also emphasizes genes AND environment, not genes versus environment.
Week 3: the neuron sits ready at resting potential
Now zoom inside the neuron membrane. The resting neuron is electrically prepared to respond.
Resting does not mean inactive — it means ready.
Look back: this electrical state exists across the membrane of the neuron you learned in Week 1.
A stimulus pushes the membrane toward threshold
Inputs produce graded changes in membrane voltage. If depolarization reaches threshold, voltage-gated Na⁺ channels open.
Threshold is not the action potential. Threshold is what must be reached to trigger the action potential.
This is where your Week 3 graph starts turning from “ready” into “fire.”
The action potential fires, resets, and travels down the axon
Week 3 = electrical signal traveling inside the neuron.
Critical handoff: when that traveling action potential reaches the presynaptic terminal, Week 4 begins.
Week 4 starts: the action potential arrives at the presynaptic terminal
The presynaptic neuron is the sender. The postsynaptic neuron is the receiver. Between them is the synaptic cleft.
Pre sends. Post receives.
Week 3 → Week 4: electrical down the axon; chemical across the gap.
Calcium turns the electrical arrival into chemical release
AP arrives → Ca²⁺ enters → vesicles release → transmitter crosses → receptors bind.
The postsynaptic neuron becomes more or less likely to fire
The receiver does not automatically fire just because neurotransmitter arrived. The effect can move it toward or away from threshold.
Excitatory = toward threshold. Inhibitory = away from threshold.
Back to Week 3: threshold still matters. Week 4 explains how synaptic inputs can push the postsynaptic membrane toward or away from that Week 3 threshold.
Multiple inputs can add together — then the next neuron may start its own cycle
Postsynaptic graded potentials can combine.
If the summed depolarization reaches threshold, the postsynaptic neuron can generate its own action potential. That means Week 3 now happens inside this next neuron.
The repeating loop: receive/sum → threshold → action potential → refractory/reset → axon → presynaptic terminal → synapse → next neuron receives/sums.
🎯 The full story you should eventually be able to say
A neuron is a specialized cell with dendrites, a soma, an axon, myelin, and presynaptic terminals. Sensory information can enter through the PNS, travel toward the CNS, be processed, and leave through motor pathways to an effector. Genes and environment help shape the biological system, with DNA information flowing through mRNA to proteins. Inside a neuron, membrane gradients create a resting potential. Enough depolarization reaches threshold and triggers an action potential, which travels down the axon. The neuron also goes through repolarization, an undershoot/hyperpolarization, and a refractory/reset period as it returns toward readiness. When the action potential reaches the presynaptic terminal, calcium enters, vesicles release neurotransmitter, and the chemical crosses the synaptic cleft to receptors on the postsynaptic cell. That input can create an EPSP that moves the receiver toward threshold or an IPSP that moves it away. If summed excitation reaches threshold, the postsynaptic neuron generates its own action potential. In other words: Week 3 explains the electrical cycle inside a neuron, Week 4 explains the chemical handoff between neurons, and the receiving neuron can then begin its own Week 3 cycle.
This page is meant to grow. When a new week adds another piece to this biological story, we extend this same path rather than starting over.