Neural Control and Coordination
Touch a hot stove and your hand snaps back before you have even consciously registered pain — the whole exchange, sensor to muscle, is over in a fraction of a second. No hormone travelling in the blood could ever act that fast. That speed is the entire reason a neural system exists alongside the endocrine system: where hormones give the body slow, broadcast chemical integration, the neural system gives it an organised network of point-to-point wiring for instant coordination. When you exercise, both systems work together — muscles demand more oxygen, breathing quickens, the heart beats faster, blood vessels adjust, and the moment you stop, everything eases back to normal. That whole synchronised performance is coordination, and this chapter is about the fastest half of it. The story runs from a single cell to a whole organ system. It starts with the neuron itself — a cell so specialised for one job, carrying a signal, that its shape is almost pure wiring: a cell body, input fibres called dendrites, and one long output fibre called the axon. You will see exactly how a resting neuron holds a stored electrical charge across its membrane by pumping ions against their own concentration gradients, how a stimulus flips that charge in a travelling wave called the action potential, and how that wave leaps a genuine physical gap — the synapse — to reach the next cell, using chemical messengers rather than a direct electrical splice. Zoom out from the single cell and the same wiring logic scales up into the brain itself: a forebrain that thinks, feels and controls the body's internal thermostat, a midbrain that relays sensory traffic, and a hindbrain that keeps the heart beating and the lungs breathing without your ever having to ask them to. By the end, the trip from stimulus to reflex, and from neuron to brain, should feel like one continuous, well-engineered circuit.
- Coordination is how organs interact and complement each other's functions; the neural system provides fast, point-to-point wiring while the endocrine system provides slower chemical (hormonal) integration.
- The human neural system has two parts: the central neural system (CNS) — brain and spinal cord, for information processing and control — and the peripheral neural system (PNS) — all the nerves connecting the CNS to the body.
- PNS nerve fibres are afferent (carry impulses from organs to the CNS) or efferent (carry impulses from the CNS to organs); the PNS itself splits into the somatic neural system (to skeletal muscle) and the autonomic neural system (to involuntary organs and smooth muscle, further divided into sympathetic and parasympathetic).
- A neuron has three parts — cell body (with Nissl's granules), dendrites (carry impulses toward the cell body) and axon (carries impulses away, ending in synaptic knobs with neurotransmitter vesicles); neurons are classed as multipolar, bipolar or unipolar, and axons as myelinated (Schwann-cell sheath, nodes of Ranvier) or unmyelinated.
- A resting axon is polarised: the membrane is more permeable to than , and the sodium-potassium pump exports 3 for every 2 it imports, leaving the outside positive and the inside negative — the resting potential.
- A stimulus makes the membrane freely permeable to ; the resulting influx reverses the polarity at that site (depolarisation), generating the action potential (the nerve impulse), which self-propagates along the axon as each depolarised patch triggers the next, followed by efflux that restores the resting potential (repolarisation).
- A synapse links a pre-synaptic neuron to a post-synaptic neuron, sometimes across a fluid-filled synaptic cleft; electrical synapses pass current directly and fast, while at the far more common chemical synapses neurotransmitter release from synaptic vesicles crosses the cleft and binds post-synaptic receptors, opening ion channels that generate a new (excitatory or inhibitory) potential.
- The brain has three major divisions: forebrain (cerebrum, thalamus, hypothalamus, limbic system), midbrain (corpora quadrigemina around the cerebral aqueduct), and hindbrain (pons, cerebellum, medulla oblongata) — with the midbrain, pons and medulla together forming the brain stem, protected by three meninges: dura mater, arachnoid and pia mater.
- Key idea: neurons at rest hold a polarised resting potential maintained by the sodium-potassium pump, a stimulus reverses that polarity as a self-propagating action potential along the axon, chemical synapses relay this impulse between neurons via neurotransmitters, and the forebrain-midbrain-hindbrain organisation of the CNS processes and coordinates the resulting signals across the whole body.
A neuron's resting polarised membrane (maintained by the sodium-potassium pump favouring permeability) is reversed at a stimulated site into a self-propagating action potential, which crosses synapses chemically via neurotransmitters to reach the next neuron, and the CNS — organised into forebrain, midbrain and hindbrain — processes and integrates these impulses to coordinate the whole body faster than any hormone could.
What this chapter covers
The chapter builds the neural system from the smallest working part outward. We start with the big picture of organisation — how the human neural system splits into the central neural system (brain and spinal cord) and the peripheral neural system (afferent and efferent fibres, divided into somatic and autonomic, sympathetic and parasympathetic). We then zoom into the neuron itself — its three-part structure, its classification by polarity and myelination — before tackling the two mechanisms that make it a signalling cell: the ionic basis of the resting and action potential, and the conduction of the impulse along the axon. From one neuron we move to two: the synapse, where an impulse crosses from a pre-synaptic to a post-synaptic neuron, electrically in rare cases but almost always chemically. The chapter closes by scaling up to the brain — forebrain, midbrain, hindbrain — and the specific job each division does. Three worked derivations trace the ionic mechanism of the resting and action potential, the propagation of the impulse along an axon, and the step-by-step events at a chemical synapse.
The core ideas
Organisation of the human neural system
The neural system of animals is built from specialised, excitable cells called neurons, which detect, receive and transmit stimuli — but its organisation scales enormously across the animal kingdom. In Hydra, a lower invertebrate, it is nothing more than a diffuse network of neurons. Insects show a better-organised system, with a brain plus a number of ganglia and neural tissues. Vertebrates, including humans, have the most developed neural system of all. The human neural system is divided into two parts. The central neural system (CNS) consists of the brain and the spinal cord and is the site of information processing and control — the body's command centre. The peripheral neural system (PNS) comprises all the nerves of the body that connect the CNS to every organ and tissue. The nerve fibres of the PNS are of two functional types: afferent fibres, which transmit impulses from tissues and organs to the CNS (the sensory, incoming traffic), and efferent fibres, which transmit regulatory impulses from the CNS to the peripheral tissues and organs (the motor, outgoing traffic). The PNS itself is further divided into two divisions. The somatic neural system relays impulses from the CNS to skeletal muscles — this is the system you consciously control when you move a limb. The autonomic neural system transmits impulses from the CNS to involuntary organs and the smooth muscle of the body — the system running your gut, heart rate and glands without conscious input. The autonomic system is classified further into the sympathetic neural system and the parasympathetic neural system, which typically act as opposing accelerators and brakes on the same involuntary organs. A related term, the visceral nervous system, refers to the part of the peripheral nervous system comprising the whole complex of nerves, fibres, ganglia and plexuses that carries impulses between the CNS and the viscera (the internal organs) in both directions.
The neuron: structure of the functional unit
A neuron is the structural and functional unit of the neural system — a microscopic cell built almost entirely for one purpose, moving a signal. It has three major parts. The cell body (soma) contains cytoplasm with the typical cell organelles, plus distinctive granular bodies called Nissl's granules. Short fibres that branch repeatedly and project out of the cell body — also containing Nissl's granules — are the dendrites; their job is to transmit impulses towards the cell body. The axon is a single, usually long fibre whose distal end branches; each branch ends in a bulb-like synaptic knob, which houses synaptic vesicles filled with chemical neurotransmitters. The axon's job is to transmit impulses away from the cell body, toward a synapse or a neuro-muscular junction. Based on the number of axons and dendrites, neurons fall into three structural classes. Multipolar neurons have one axon and two or more dendrites, and are found in the cerebral cortex. Bipolar neurons have one axon and one dendrite, and are found in the retina of the eye. Unipolar neurons have a cell body with only one axon (no separate dendrites), and are usually found at the embryonic stage. Axons are also of two types by their sheathing. Myelinated nerve fibres are enveloped by Schwann cells, which wrap around the axon to form an insulating myelin sheath; the gaps between successive myelin sheaths, where the axon membrane is exposed, are called nodes of Ranvier. Myelinated fibres are found in spinal and cranial nerves. Unmyelinated nerve fibres are also enclosed by a Schwann cell, but that cell does not wrap around to form a myelin sheath; these are commonly found in the autonomic and somatic neural systems.
The resting membrane: why a neuron is polarised
Neurons are excitable cells precisely because their membranes sit in a polarised state. This polarisation comes from selectively permeable ion channels on the neural membrane. When a neuron is at rest — not conducting an impulse — the axonal membrane is comparatively more permeable to potassium ions () and nearly impermeable to sodium ions (); it is also impermeable to the negatively charged proteins present in the axoplasm. As a result, the axoplasm inside the axon holds a high concentration of and negatively charged proteins but a low concentration of , while the fluid outside the axon holds a low concentration of and a high concentration of — a standing concentration gradient across the membrane. This gradient is not a passive accident; it is actively maintained by the sodium-potassium pump, which transports 3 ions outward for every 2 ions it brings in. Because more positive charge leaves than enters, the outer surface of the axonal membrane ends up positively charged while the inner surface becomes negatively charged — the membrane is polarised. The electrical potential difference across this resting plasma membrane is the resting potential (typically around , inside negative relative to outside, in a real neuron — the exact figure is standard physiology built on the qualitative NCERT description).
Generation and conduction of the nerve impulse
When a stimulus is applied at a site on the polarised membrane, that site suddenly becomes freely permeable to . The resulting rapid influx of reverses the polarity right there: the outer surface becomes negatively charged and the inner surface becomes positively charged. This local reversal is depolarisation, and the electrical potential difference across the membrane at that site — the nerve impulse itself — is called the action potential. Immediately ahead of the depolarised site, the axon membrane still carries its resting charge (positive outside, negative inside). This charge difference between the depolarised site and the still-resting site next to it drives a local current: on the inner surface, current flows from the depolarised site toward the resting site ahead; on the outer surface, current flows the opposite way, completing the circuit. This local current is enough to reverse the polarity at the next site too, generating a fresh action potential there — and the whole sequence repeats, patch by patch, along the length of the axon. That is how the impulse is conducted, not by any single charge travelling the axon's length, but by each depolarised patch triggering depolarisation in the patch immediately ahead of it. The stimulus-induced rise in permeability is extremely short-lived. It is quickly followed by a rise in permeability: within a fraction of a second diffuses back out of the membrane, restoring the resting potential at the site of excitation — this is repolarisation — and the fibre becomes responsive to further stimulation once again.
The synapse: transmission of impulses between neurons
A nerve impulse passes from one neuron to the next through a junction called a synapse, formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron, which may or may not be separated by a fluid-filled gap called the synaptic cleft. There are two kinds of synapses. At an electrical synapse, the pre- and post-synaptic membranes lie in very close proximity, and electrical current flows directly from one neuron into the other — transmission here is very similar to impulse conduction along a single axon, and it is always faster than transmission across a chemical synapse. Electrical synapses are rare in the human system. At a chemical synapse — by far the more common type in humans — the pre- and post-synaptic membranes are separated by the synaptic cleft. The axon terminal holds vesicles filled with neurotransmitters. When an action potential arrives at the axon terminal, it triggers the synaptic vesicles to move to the membrane, fuse with it, and release their neurotransmitters into the synaptic cleft. These neurotransmitters diffuse across and bind specific receptors on the post-synaptic membrane, opening ion channels there. The resulting ion entry generates a new potential in the post-synaptic neuron, which may be either excitatory or inhibitory, depending on the neurotransmitter and receptor involved.
The central neural system: forebrain
The brain is the body's central information processing organ, the "command and control system." It controls voluntary movements, body balance, the functioning of vital involuntary organs (lungs, heart, kidneys and others), thermoregulation, hunger and thirst, circadian (24-hour) rhythms, the activity of several endocrine glands, and human behaviour generally. It is also the site of vision, hearing, speech, memory, intelligence, emotion and thought processing. Protected by the skull, the brain is further covered by three cranial meninges: an outer dura mater, a very thin middle arachnoid, and an inner pia mater that is in direct contact with the brain tissue. The brain divides into three major parts: the forebrain, the midbrain, and the hindbrain. The forebrain consists of the cerebrum, the thalamus and the hypothalamus. The cerebrum forms the major part of the human brain; a deep longitudinal cleft divides it into left and right cerebral hemispheres, connected by a thick tract of nerve fibres called the corpus callosum. The layer of cells covering each cerebral hemisphere is the cerebral cortex, thrown into prominent folds, and known as grey matter because the concentrated neuron cell bodies there give it a greyish appearance. The cortex contains motor areas, sensory areas, and large association areas — neither clearly sensory nor motor — responsible for complex functions such as intersensory associations, memory and communication. Beneath the cortex, myelin-sheathed fibre tracts form the inner white matter, so called for its opaque white appearance. The cerebrum wraps around the thalamus, a major coordinating centre for sensory and motor signalling. At the base of the thalamus lies the hypothalamus, which contains centres controlling body temperature and the urge for eating and drinking, and also contains groups of neurosecretory cells that secrete hypothalamic hormones. The inner parts of the cerebral hemispheres, together with a group of deep associated structures such as the amygdala and hippocampus, form the limbic lobe or limbic system, which, along with the hypothalamus, regulates sexual behaviour, the expression of emotional reactions (excitement, pleasure, rage, fear) and motivation.
The midbrain and hindbrain: brain stem and cerebellum
The midbrain sits between the thalamus/hypothalamus of the forebrain and the pons of the hindbrain. A canal called the cerebral aqueduct runs through it, and its dorsal portion consists mainly of four round swellings called the corpora quadrigemina. The hindbrain comprises the pons, the cerebellum, and the medulla (medulla oblongata). The pons consists of fibre tracts that interconnect different regions of the brain. The cerebellum has a very convoluted surface, which provides extra room to pack in far more neurons. The medulla is directly connected to the spinal cord and contains centres that control respiration, cardiovascular reflexes, and gastric secretions. Together, the midbrain, pons and medulla oblongata make up the brain stem, which forms the connection between the brain and the spinal cord.
Worked derivations
The goal is to trace, ion by ion, exactly why a resting axon carries a stored charge across its membrane and exactly what happens to that charge the instant a stimulus arrives — the mechanism examiners most often ask students to "explain" rather than merely name, since both the resting potential and the action potential rest on the same handful of facts about selective membrane permeability and the sodium-potassium pump.
Step 1. Start with the resting, unstimulated axon. Its membrane is comparatively more permeable to than to , and essentially impermeable to the negatively charged proteins inside the axoplasm. Left alone, tends to leak out along its concentration gradient more easily than can leak in.
Step 2. The sodium-potassium pump actively corrects and maintains this imbalance, using cellular energy to transport 3 ions out of the cell for every 2 ions it brings in. Because three positive charges leave for every two that enter, there is a net export of positive charge with every pump cycle.
Step 3. The cumulative effect is a standing charge separation: the outer surface of the axonal membrane becomes positively charged, the inner surface negatively charged (helped further by the trapped negative proteins inside). This charge difference across the resting membrane is the resting potential — the membrane is said to be polarised.
Step 4. Now apply a stimulus at one site on the membrane. That site suddenly becomes freely permeable to , and rushes in along its steep concentration gradient (it was kept high outside, low inside, precisely by Steps 1 to 3). This rapid influx of positive charge flips the local charge distribution: outside now negative, inside now positive.
Step 5. This local flip is depolarisation, and the potential difference it produces is the action potential — the nerve impulse. The rise in permeability is brief; it is quickly followed by a rise in permeability, so diffuses back out within a fraction of a second, restoring the original resting charge distribution at that site. This restoration is repolarisation, and once it is complete the site is ready to respond to a fresh stimulus. Check: the whole cycle — polarised at rest, depolarised on stimulation, repolarised immediately after — depends on exactly two permeability switches ( in, then out) riding on a concentration gradient that the sodium-potassium pump built and keeps rebuilding; remove the pump and the gradient (and therefore every subsequent step) collapses.
Go deeper · Derivation B · 2 minConduction of the nerve impulse along the axon
The goal is to show mechanically how a nerve impulse generated at one point on an axon ends up, unaided, at the far end of that axon — without imagining that a single charge or ion physically travels the whole distance, which is the single most common misreading of this topic.
Step 1. Take two neighbouring sites on the axon membrane, call them site A and site B, with B lying just ahead of A in the direction the impulse will travel. Before any stimulus, both A and B sit at the resting potential: positive outside, negative inside.
Step 2. A stimulus depolarises site A: rushes in there, and A's polarity flips to negative outside, positive inside, generating an action potential at A. Site B, immediately ahead, is still fully at rest.
Step 3. A now has the opposite charge distribution to B. This charge difference between adjacent, oppositely charged patches of membrane drives a local electric current: on the inner (axoplasm) side, current flows from the positively charged region at A toward the still-negative interior at B; on the outer side, current flows the opposite way, from B back to A, closing the circuit.
Step 4. This local current is itself the "stimulus" that depolarises site B — it is strong enough to flip B's permeability and reverse its polarity too, generating a fresh action potential exactly at B, while site A, its job done, immediately begins repolarising via efflux and returns to rest.
Step 5. The sequence in Steps 2 to 4 now repeats with B as the new "site A" and the next patch of membrane as the new "site B," and so on down the entire length of the axon. The impulse is therefore conducted as a self-regenerating wave of depolarisation followed by repolarisation, each patch triggering the next — never as one ion or charge migrating the full length. In myelinated fibres this local-current mechanism can only occur at the exposed nodes of Ranvier (myelin insulates the membrane in between), so the impulse effectively jumps node to node — a much faster form of conduction than the patch-by-patch spread along an unmyelinated fibre. Check: at every step the current triggering the next site's depolarisation comes from the potential difference between an already-depolarised patch and its still-resting neighbour — remove that adjacent resting patch (as myelin does, except at the nodes) and ordinary local-current spread cannot occur there, which is exactly why myelinated conduction must skip from node to node.
Go deeper · Derivation C · 2 minSynaptic transmission at a chemical synapse
The goal is to walk through, in strict order, how an electrical signal (the action potential) inside one neuron becomes a chemical signal crossing the synaptic cleft and then a fresh electrical signal inside the next neuron — the conversion at the heart of every chemical synapse, and a favourite short-answer target because each step has a distinct, nameable event.
Step 1. An action potential, generated and conducted as in the earlier derivations, arrives at the axon terminal (synaptic knob) of the pre-synaptic neuron. The synaptic knob already contains synaptic vesicles loaded with a specific neurotransmitter.
Step 2. The arriving action potential stimulates these synaptic vesicles to move toward the pre-synaptic membrane. On reaching it, the vesicles fuse with the membrane, a process that opens them to the outside and releases their neurotransmitter content into the synaptic cleft — the fluid-filled gap separating the pre- and post-synaptic membranes.
Step 3. The released neurotransmitter molecules diffuse across the narrow synaptic cleft and bind to specific receptor proteins embedded in the post-synaptic membrane. This binding is highly specific — a given receptor recognises only its matching neurotransmitter.
Step 4. Neurotransmitter binding opens ion channels on the post-synaptic membrane, allowing particular ions to enter the post-synaptic neuron. This ion entry generates a brand-new electrical potential in the post-synaptic cell — an electrical signal has now been regenerated, one neuron downstream of where it started.
Step 5. Whether this new potential is excitatory (pushing the post-synaptic neuron closer to firing its own action potential) or inhibitory (pushing it further away) depends on which neurotransmitter was released and which receptor and ion channel it opened. A single post-synaptic neuron typically receives many synapses at once, and its eventual response reflects the combined balance of excitatory and inhibitory inputs it is receiving. Check: the chain — action potential arrives, vesicles fuse, neurotransmitter released, receptor binds, ion channel opens, new potential generated — has exactly one chemical step (neurotransmitter diffusion across the cleft) sandwiched between two electrical ones, which is precisely why chemical synaptic transmission is slower than electrical synaptic transmission, where current passes directly with no such chemical intermediate.
Worked examples
Board level - Explain why the resting axonal membrane is said to be polarised
Step 1. Identify the ionic backdrop: at rest, the axonal membrane is comparatively more permeable to and nearly impermeable to and to the negatively charged proteins in the axoplasm, giving high /protein but low inside, and the reverse outside.
Step 2. Bring in the sodium-potassium pump, which actively transports 3 ions out for every 2 ions in, so more positive charge leaves the cell than enters with each pump cycle.
Step 3. State the consequence: the outer surface of the membrane ends up positively charged while the inner surface is negatively charged — a stored charge difference across the membrane, which is what "polarised" means, and this potential difference is called the resting potential.
Answer: The resting axonal membrane is polarised because it is selectively permeable to over , and because the sodium-potassium pump exports 3 for every 2 it imports, leaving the outer surface positively charged and the inner surface negatively charged — the resting potential.
Board level - Distinguish between resting potential and action potential
Step 1. Define both in one line each: the resting potential is the electrical potential difference across the membrane of an unstimulated neuron (outside positive, inside negative); the action potential is the potential difference at a site that has just been stimulated (outside negative, inside positive).
Step 2. Contrast the mechanism: the resting potential is maintained by the sodium-potassium pump's steady, active 3-out/2-in exchange against the concentration gradient; the action potential is produced by a sudden, passive rush of down its concentration gradient once the membrane becomes freely permeable to it.
Step 3. Contrast duration and role: the resting potential is a stable baseline the neuron returns to between impulses; the action potential is a brief, transient reversal that is, in fact, the nerve impulse itself.
Answer: Resting potential is the stable, pump-maintained polarised state (outside positive, inside negative) of an unstimulated neuron; action potential is the brief, stimulus-triggered reversal of that polarity (outside negative, inside positive) caused by a rapid Na+ influx, and it is this transient reversal that constitutes the nerve impulse.
Competitive level (NEET) - Identify the incorrect statement about the neuron and the nerve impulse
A student is given four statements and asked to identify the one that is incorrect: (A) Dendrites carry impulses toward the cell body, while the axon carries impulses away from it. (B) Myelinated nerve fibres conduct impulses faster than unmyelinated fibres of similar diameter. (C) During depolarisation, the outer surface of the axonal membrane becomes positively charged and the inner surface negatively charged. (D) The sodium-potassium pump transports 3 sodium ions out of the cell for every 2 potassium ions it brings in.
Step 1. Check A and D against the chapter's exact wording: dendrites carry impulses toward the cell body and axons carry them away (correct); the pump transports 3 outward for every 2 inward (correct, matches exactly).
Step 2. Check B: myelin restricts local-current generation to the exposed nodes of Ranvier, so the impulse jumps node to node — faster than continuous patch-by-patch spread — so B is correct too.
Step 3. Check C against the mechanism: depolarisation is the reversal of the resting charge distribution. At rest, outside is positive and inside is negative; on depolarisation, influx makes the outer surface negative and the inner surface positive — the exact opposite of what C claims. Statement C has described the resting state, not depolarisation, so it is the incorrect one.
Answer: Statement C is incorrect. During depolarisation the outer surface of the membrane becomes negatively charged and the inner surface becomes positively charged — the reverse of the resting condition, not a continuation of it. Timing note: allow about 60 to 75 seconds for this style of question. Recall which state (resting or depolarised) is "outside positive" versus "outside negative" as a fixed pair, then scan the four statements for any that silently swaps this pairing — that swap is almost always where the trap is hidden.
Common traps
Students frequently picture the action potential as a single charged particle physically travelling the length of the axon like a bead sliding along a wire — it is not. What actually happens is a self-regenerating wave: each depolarised patch of membrane generates a local current that depolarises the next patch, with the ions themselves moving only a very short distance across the membrane at each site, not along its length. A second frequent confusion is between depolarisation and the resting state — students sometimes state the charge pattern backwards, calling depolarisation "outside positive, inside negative" (that is the resting pattern) instead of the correct reversed pattern. A third trap is treating "myelinated" as "insulated everywhere" — myelin insulates the internode segments precisely so that local current, and therefore new depolarisation, can only happen at the nodes of Ranvier, which is what makes conduction faster, not slower. Students also confuse the thalamus (a relay and coordinating centre for sensory and motor signals) with the hypothalamus (temperature, hunger, thirst, hypothalamic hormones) simply because the names sound alike and both sit in the forebrain. Finally, many assume all synapses work like the familiar chemical synapse; electrical synapses exist too, transmit current directly with no neurotransmitter step, and are faster — they are simply rare in the human system.
You should be able to…
- Describe the two-way organisation of the human neural system: CNS versus PNS, afferent versus efferent fibres, and somatic versus autonomic (sympathetic and parasympathetic) divisions of the PNS.
- Label the three parts of a neuron (cell body, dendrites, axon) and identify Nissl's granules, synaptic knobs and synaptic vesicles on a diagram.
- Classify a neuron as multipolar, bipolar, or unipolar, and an axon as myelinated or unmyelinated, and name where each type is typically found.
- Explain, in ionic terms, why a resting axonal membrane is polarised, naming the role of selective permeability and the sodium-potassium pump.
- Trace the sequence of events from a stimulus to a generated action potential, and explain depolarisation and repolarisation in terms of Na+ and K+ movement.
- Explain how a nerve impulse is conducted along an axon as a self-propagating local-current wave, and why myelinated fibres conduct faster.
- Distinguish electrical synapses from chemical synapses, and describe the step-by-step events of transmission at a chemical synapse.
- Name the three major divisions of the brain (forebrain, midbrain, hindbrain) and locate the cerebrum, thalamus, hypothalamus, limbic system, corpora quadrigemina, pons, cerebellum and medulla within them.
- State which three structures together form the brain stem and name the three cranial meninges in order from outside to inside.
Ask the tutor: Want to walk through how a simple reflex arc links a stimulus at your skin all the way to muscle contraction, or see how the hypothalamus and limbic system hand off control to the endocrine system in the next chapter?
Why does the sodium-potassium pump move 3 sodium ions out for every 2 potassium ions in — does that not make the cell run out of charge balance?
The pump's job is not to balance the total number of ions moved but to actively maintain a specific concentration gradient the neuron can later use to generate an action potential. By exporting more positive charge (3 Na+) than it imports (2 K+) each cycle, the pump keeps the inside of the resting membrane net negative relative to the outside — the resting potential. This "imbalance" is exactly the stored charge difference a stimulus later taps into: when a site becomes permeable to Na+, that steep, pump-maintained gradient drives the rapid Na+ influx that produces the action potential. Far from being a problem, the 3-out/2-in imbalance is the whole point.
What is actually different between an electrical synapse and a chemical synapse, apart from speed?
At an electrical synapse the pre- and post-synaptic membranes sit extremely close together and current passes directly from one neuron into the next, much like current spreading along a single axon — there is no neurotransmitter and essentially no delay. At a chemical synapse the two membranes are separated by a fluid-filled synaptic cleft, and the signal must convert from electrical (action potential) to chemical (neurotransmitter diffusing across the cleft) and back to electrical (a new potential in the post-synaptic cell). That extra chemical step is why chemical synapses are slower, but it also lets them be tuned: because the outcome depends on which neurotransmitter binds which receptor, a chemical synapse can be either excitatory or inhibitory, letting the neural system combine many inputs on one post-synaptic neuron — a flexibility electrical synapses cannot offer, which is why they remain rare in the human system despite being faster.
What is the real difference between grey matter and white matter in the cerebrum?
Grey matter is the outer cerebral cortex, and it looks greyish because it is packed with neuron cell bodies (along with dendrites and unmyelinated fibres) — this is where sensory, motor and association processing actually happens. White matter lies beneath the cortex and consists of axon tracts wrapped in myelin sheaths; the fatty myelin gives this layer its opaque white appearance. Grey matter is where signals are generated and processed; white matter is the fast, myelinated "wiring" that carries those already-processed signals between regions of the cerebrum and to and from other parts of the brain.
How is a chemical synapse different from a neuromuscular junction?
They share the same underlying principle — a pre-synaptic terminal releasing neurotransmitter across a small gap to trigger a response — but the "next cell" differs. At a chemical synapse the post-synaptic partner is another neuron, and the new potential generated may be excitatory or inhibitory depending on the neurotransmitter and receptor. At a neuromuscular junction, the axon terminal instead releases neurotransmitter onto a muscle fibre, and the resulting potential triggers muscle contraction rather than passing the impulse to another neuron. The chapter notes that axons transmit impulses to a synapse or to a neuro-muscular junction, treating both as destinations for the same kind of axon-terminal signalling.