Animal Form, Nutrition, and Control Systems

This is Unit Twelve of Biology, Part Three: the nervous system. Part Two ended with the cephalic phase of digestion, where the sight of food starts the process before any food arrives. This part is the network that did that. The through-line is speed and locality: signals here are electrical along a cell and chemical between cells, and the most important thing about the fastest responses is that they finish without the brain being consulted at all. By the end you will be able to name a neuron's three parts and say what glial cells do; say what makes an action potential all-or-nothing; tell a chemical synapse from an electrical one; say what long-term potentiation is and why craving outlasts withdrawal; and say why a reflex does not wait for the brain.

Three entries. The parts of a neuron, the four ways those parts are arranged, and the cells that are not neurons at all and make up a great deal of the brain.

Three parts.

Bio 2815 Neuron Structure: Dendrites, Soma, Axon

A neuron's soma houses its nucleus and organelles, dendrites receive signals from other neurons at synapses, and a single axon propagates the signal to the axon terminals. A human motor neuron's axon can run a metre, from spine to toes. Most neurons carry one or two axons, but some, like retinal amacrine cells, carry none.

Four arrangements.

Bio 2817 Four Neuron Types

Neurons divide structurally into unipolar, bipolar, multipolar and pseudounipolar types, distinguished by how many processes extend from the soma and how they branch. Most sensory neurons are pseudounipolar. Pseudounipolar is not the same as unipolar — its single process still splits into two functional branches.

Not a supporting cast.

Bio 2819 Glial Cell Types

Glial cells support neurons without transmitting signals: astrocytes regulate the chemical environment and form the blood-brain barrier, oligodendrocytes and Schwann cells produce myelin, and microglia clear debris. One oligodendrocyte myelinates several central axons; a Schwann cell myelinates only one peripheral axon. Glia make up a large share of brain tissue.

Three entries. What a neuron holds itself at when nothing is happening, what changes when something does, and the insulation that decides how fast the change travels.

Holding negative.

Bio 2821 Resting Membrane Potential

A resting neuron holds its interior near minus seventy millivolts relative to its exterior, maintained by more potassium leak channels than sodium and by the sodium-potassium pump. Potassium leaks out faster than sodium leaks in, leaving the interior negative. The exact resting value varies by neuron and species — the mechanism is stable, the number is not.

All, or nothing.

Bio 2823 Action Potential

An action potential is a brief, all-or-nothing reversal of membrane potential: once a stimulus depolarises the membrane to a threshold near minus fifty-five millivolts, sodium channels open and the neuron fully depolarises. After the peak, voltage-gated potassium channels open, repolarising and briefly overshooting into hyperpolarisation. A neuron reaching threshold always fires fully, never partially.

Jumping the gaps.

Bio 2825 Myelin and Saltatory Conduction

Myelin insulates an axon, forcing the action potential to jump between unmyelinated gaps called nodes of Ranvier — saltatory conduction, which speeds signal travel. Multiple sclerosis damages myelin, slowing conduction as current leaks from previously insulated stretches. The nodes are where the signal regenerates; without them channels would fire continuously along the whole axon.

Three entries. Two ways a signal gets from one neuron to the next, and how a receiving neuron turns everything arriving at once into a single decision about whether to fire.

Chemistry across a cleft.

Bio 2827 Chemical Synapse: Neurotransmitter Release

At a chemical synapse an arriving action potential opens calcium channels, triggering synaptic vesicles to release neurotransmitter that diffuses across the cleft and binds postsynaptic receptors. Acetylcholine at a neuromuscular junction opens sodium channels, depolarising the muscle cell. Chemical synapses carry a delay of roughly one millisecond, since the transmitter must diffuse.

Current, directly.

Bio 2829 Electrical Synapse: Gap Junctions

An electrical synapse passes current directly between neurons through gap junction channel proteins, transmitting virtually instantaneously and sometimes bidirectionally, unlike a chemical synapse. Electrical synapses in the thalamus help synchronise neurons involved in fast reflexes and sleep rhythms. They are fewer in number than chemical synapses despite their speed advantage.

Adding it up.

Bio 2831 EPSP, IPSP, and Summation

An excitatory postsynaptic potential depolarises toward threshold; an inhibitory one hyperpolarises away from it; summation at the axon hillock combines inputs into one firing decision. Multiple weak EPSPs arriving close together can summate past threshold even when no single input could. IPSPs can cancel EPSPs — the net summed voltage decides.

Three entries. How a synapse strengthens or weakens with use — the cellular basis of learning — and then a widespread assumption about addiction, with the entry that answers it from the same mechanism.

Use it, or lose it.

Bio 2833 Long-Term Potentiation and Long-Term Depression

Long-term potentiation strengthens a repeatedly used synapse by inserting more receptors; long-term depression weakens an underused one by removing them — the cellular basis of learning and memory. NMDA receptor activation from rapid repeated stimulation drives AMPA receptor insertion. LTD is not synaptic failure: pruning weak connections sharpens the relative strength of potentiated ones.

The assumption.

Bio 2835 "Addiction Is a Failure of Willpower"

Picturing addiction as pure willpower failure overstates a real process. Some substances co-opt the same potentiation pathway that underlies learning and memory, so a drug that hijacks long-term potentiation strengthens drug-associated synapses the way ordinary repeated learning does. The neural mechanism this overlooks is the next entry.

The mechanism.

Bio 2837 Addiction as Hijacked Synaptic Plasticity

Addiction involves substances co-opting long-term potentiation, strengthening drug-associated synapses beyond what willpower governs, and synaptic strengthening from repeated exposure functions like an amplified version of ordinary reward learning. Those strengthened synapses persist long after the substance has cleared the body, which is why craving outlasts withdrawal.

Six entries. What protects the brain, the cortex and what each of its four lobes specialises in, the deep structures beneath, and the two regions that handle coordination and the functions a body cannot pause.

Three layers and a fluid.

Bio 2839 Meninges and Cerebrospinal Fluid

Three meningeal layers — dura, arachnoid and pia mater — cover the brain and spinal cord, cushioned by cerebrospinal fluid that also circulates nervous-system chemicals. Blocked drainage causes hydrocephalus, pressing the brain against the skull. The pia mater directly contacts brain tissue; the dura is the tough outermost layer.

Two halves, four lobes.

Bio 2841 Cerebral Cortex: Hemispheres and Lobes

The cerebral cortex folds into two hemispheres connected by the corpus callosum, each divided into frontal, parietal, temporal and occipital lobes. A split-brain patient whose corpus callosum is cut can pick up an object with one hand but not name it, depending on the visual field. The hemispheres' functions are largely redundant, despite some localisation.

What each lobe does.

Bio 2843 Frontal, Parietal, Occipital, Temporal Lobe Functions

The frontal lobe handles motor control, the parietal touch and body position, the occipital vision, and the temporal hearing and, through the hippocampus, memory. Recognising a familiar song relies on the temporal lobe; reading its title relies on the occipital. Any given behaviour draws on more than one lobe — these are specialisations, not boundaries.

Beneath the cortex.

Bio 2845 Basal Ganglia, Thalamus, and Hypothalamus

The basal ganglia regulate movement and motivation, the thalamus relays sensory and motor signals to the cortex, and the hypothalamus regulates hormone release and temperature homeostasis. Basal ganglia damage in Parkinson's disease produces a characteristic shuffling gait. The thalamus routes nearly all sensory information except olfaction — smell reaches the cortex separately.

Emotion and fear.

Bio 2847 Limbic System and the Amygdala

The limbic system, including the amygdala and parts of the thalamus and hypothalamus, regulates emotion, fear and motivation, and contributes to memory formation. The amygdala is central both to feeling fear and to recognising fearful expressions on other faces. The limbic system is a functional grouping of connected structures, not one anatomically bounded organ.

Coordination, and survival.

Bio 2849 Cerebellum and Brainstem

The cerebellum coordinates balance and motor learning; the brainstem connects brain to spinal cord and controls breathing, heart rate and alertness. Descending motor pathways cross within the brainstem, so the left hemisphere controls the body's right side. Brainstem damage threatens survival directly, since it regulates functions the body cannot pause.

Six entries. The spinal cord's own anatomy and the reflex that runs inside it, then the relay that reaches the internal organs and the three divisions built on it.

Grey inside, white around.

Bio 2851 Spinal Cord: Gray and White Matter

The spinal cord's cross section shows grey matter holding neuron and interneuron cell bodies, surrounded by white matter holding myelinated axons carrying signals to and from the brain. Dorsal regions carry mostly sensory information toward the brain, ventral regions mostly motor signals away. Spinal nerves do not regenerate on their own once severed.

Without asking the brain.

Bio 2853 The Spinal Reflex Arc

A spinal reflex arc produces a fast, unconscious motor response through as few as one synapse between a sensory and a motor neuron, with no involvement from the brain. The knee-jerk reflex a doctor tests runs through a single spinal synapse. A reflex can still send information to the brain afterward, through interneurons, but the response does not wait for it.

Two synapses.

Bio 2855 Autonomic Nervous System: Two-Synapse Relay

The autonomic nervous system controls internal organs without conscious control, relaying through two synapses: a preganglionic neuron synapsing onto a postganglionic neuron that reaches the target organ. One preganglionic neuron often synapses onto many postganglionic neurons, amplifying one signal across many cells. The sensory-somatic system uses only one synapse.

Fight or flight.

Bio 2857 Sympathetic Nervous System

The sympathetic nervous system drives the fight-or-flight response, accelerating heart rate and inhibiting digestion to prepare the body for exertion under threat. Postganglionic sympathetic neurons release norepinephrine on target organs while the adrenal medulla adds more into circulation. Activation is pervasive by design — a whole-body rush, not a local effect.

Rest and digest.

Bio 2859 Parasympathetic Nervous System

The parasympathetic nervous system produces rest-and-digest responses, slowing heart rate and stimulating digestion, resetting organ function after sympathetic activation subsides. Acetylcholine released by parasympathetic postganglionic neurons slows the heart and stimulates digestive activity. The two systems are typically antagonistic on the same organs, not independent controls.

In and out, consciously.

Bio 2861 Sensory-Somatic Nervous System

The sensory-somatic nervous system carries sensory information from skin and muscle inward and motor commands outward, through twelve cranial and thirty-one spinal nerve pairs. The olfactory nerve carries only sensory information, while the oculomotor nerve carries mostly motor commands to eye muscles. Unlike the autonomic relay, these use only one synapse.

Three entries. Two classes of disorder named at the level of mechanism and nothing further, then the chapter's own walkthrough of a signal from a fingertip to a muscle.

Neurons lost.

Bio 2863 Neurodegenerative Disorders as Neuron Loss

A neurodegenerative disorder progressively kills neurons, with symptoms tracing to where in the nervous system that loss occurs. Parkinson's disease specifically loses dopamine neurons in the substantia nigra, a midbrain structure regulating movement. This entry names a mechanism only and offers no guidance about diagnosis or treatment.

Wiring, differently.

Bio 2865 Neurodevelopmental Disorders as Atypical Wiring

A neurodevelopmental disorder disturbs nervous-system development itself, producing a spectrum of effects on communication, learning, attention or motor function. Reduced synaptic pruning during development is one pattern researchers associate with some of these conditions. This entry names a mechanism only and offers no guidance about diagnosis or treatment.

Hand to muscle.

Bio 2867 Tracing a Reflex Arc Signal Path, Worked

A hand touches something sharp. A sensory neuron's dendrite detects it and depolarises past threshold, generating an action potential that propagates to the spinal cord and enters the dorsal grey matter, forming a chemical synapse onto a motor neuron directly, or through one interneuron. Calcium influx triggers transmitter release, depolarising the motor neuron past its own threshold; its axon exits the ventral root and releases acetylcholine at the neuromuscular junction, contracting the muscle. Assuming the brain initiates the withdrawal is the error — the arc completes locally, and the brain learns afterward.

Five things to carry out of this part. A neuron has a soma holding the nucleus, dendrites receiving signals, and one axon carrying the signal out — while glial cells, which transmit nothing, regulate the chemical environment, build the blood-brain barrier, make myelin and clear debris, and compose a large share of brain tissue. An action potential is all-or-nothing: cross threshold near minus fifty-five millivolts and the neuron depolarises fully, never partially, and myelin makes that signal jump between nodes of Ranvier rather than crawl. A chemical synapse releases neurotransmitter across a cleft and costs about a millisecond; an electrical synapse passes current directly through gap junctions almost instantly, and is much rarer despite being faster. Long-term potentiation strengthens a repeatedly used synapse by adding receptors, which is the cellular basis of learning — and some substances co-opt that same pathway, strengthening drug-associated synapses that persist after the substance has cleared, which is why craving outlasts withdrawal. And a reflex does not wait for the brain: the arc completes inside the spinal cord through as few as one synapse, pulling the hand away before the sensation has finished reaching awareness, with the brain informed afterward by a separate pathway. Part Four takes up the senses — where all this signalling starts.