Describe the structure of neurons and nerves, explain action potential propagation and synaptic transmission, compare the central and peripheral nervous systems, and analyze reflex arcs

Learning goal: Describe the nervous system as neurons and supporting cells organized into nerves, the spinal cord and the brain, with the functions of sensation, integration and response; compare the structures and functions of the central and peripheral nervous systems; describe the neuron and neuroglia; explain action potential propagation using a graph of membrane potential against time, synaptic transmission by neurotransmitters and the reflex arc; identify the main structures of the brain, spinal cord and nerves, including the meninges and cerebrospinal fluid; and explain homeostatic imbalances such as the effects of drugs, spinal injuries, concussions, meningitis and multiple sclerosis.

Three jobs and two divisions. The nervous system has three functions that run in sequence. Sensation: receptors detect changes inside and outside the body and send signals inward. Integration: the brain and spinal cord process the signals, compare them with stored information and decide on a response. Response: signals go out to effectors, the muscles and glands, which act. The system is divided by location. The central nervous system (CNS) is composed of the brain and spinal cord, where integration occurs. The peripheral nervous system (PNS) includes the remaining nervous tissue: the cranial and spinal nerves and their branches, which carry signals to and from the CNS. Within the PNS, sensory (afferent) fibers carry signals toward the CNS and motor (efferent) fibers carry them away; motor output to skeletal muscle is the somatic division, under voluntary control, and output to smooth muscle, cardiac muscle and glands is the autonomic division, which runs involuntarily and has a sympathetic branch that prepares the body for action and a parasympathetic branch that favors rest and digestion.

Neurons and neuroglia. Nervous tissue has two cell classes. A neuron consists of dendrites, branching extensions that receive signals; a cell body containing the nucleus; and an axon, a single long extension that conducts impulses away from the cell body to its terminals, where it contacts the next cell at a synapse. Many axons are wrapped in myelin, a fatty insulating sheath laid down by neuroglial cells, with gaps called nodes between the wrappings. Neuroglial cells, roughly as numerous as neurons in the human brain, help to support neural function: in the CNS, astrocytes regulate the fluid around neurons and help form the barrier between blood and brain, oligodendrocytes make myelin, microglia remove debris and fight infection, and ependymal cells line the fluid-filled spaces; in the PNS, Schwann cells make myelin and satellite cells support cell bodies. A nerve is a bundle of many axons, wrapped in connective tissue and carrying its own blood vessels, that runs through the periphery; a tract is the equivalent bundle inside the CNS.

The resting membrane potential. A neuron at rest keeps its inside about 70 millivolts negative relative to the outside; this is the resting membrane potential, written as −70 mV. It exists because the membrane holds sodium ions mostly outside and potassium ions mostly inside, using a pump that spends ATP to move three sodium out for every two potassium in, and because the membrane at rest leaks potassium more easily than sodium, so positive charge drifts out and leaves the inside negative. The membrane also contains voltage-gated channels for sodium and potassium that are closed at rest and open when the potential changes.

The action potential, read from the graph. The diagram plots membrane potential against time for one point on an axon. At rest the trace sits at −70 mV. A stimulus, such as neurotransmitter arriving at the dendrites, lets some positive charge in and the potential rises. If it reaches the threshold, about −55 mV, voltage-gated sodium channels open, sodium rushes in and the potential shoots up to about +30 mV. Any change that makes the inside less negative is depolarization; in an action potential it overshoots zero, so the inside briefly becomes positive. At the peak the sodium channels close and inactivate while voltage-gated potassium channels open; potassium flows out and the potential falls back toward rest: repolarization. The potassium channels are slow to close, so the potential briefly overshoots to about −80 mV, more negative than rest: hyperpolarization, or the undershoot, after which the leak and the pump restore −70 mV. The whole event takes a few milliseconds. Two rules follow from the mechanism. It is all-or-none: a stimulus that reaches threshold produces a full-size action potential and one that does not produces none, so a stronger stimulus is signalled by more frequent action potentials, not bigger ones. And there is a refractory period: while the sodium channels are inactivated the membrane cannot fire again (the absolute refractory period; just afterwards, in the relative refractory period, only a stronger-than-usual stimulus can fire it), which is why the impulse travels in one direction and cannot reverse. Propagation means that the sodium entering at one point depolarizes the next patch of membrane to threshold, which fires in turn, and so on along the axon like a line of falling dominoes, each patch re-generating the full signal. In a myelinated axon the signal jumps from node to node, because current cannot leak through the sheath, and conduction is many times faster. Predicting from the mechanism: if a drug held the resting potential closer to threshold, the neuron would fire more easily; if the myelin were lost, as in multiple sclerosis, conduction would slow, become unreliable or fail.

Synapses and neurotransmitters. At the axon terminal the electrical signal becomes a chemical one. The arriving action potential opens calcium channels; calcium entry makes vesicles fuse with the membrane and release neurotransmitter molecules into the narrow synaptic cleft. They diffuse across and bind receptors on the next cell, opening channels that either depolarize it toward threshold (an excitatory effect, as with glutamate or acetylcholine at a muscle) or hyperpolarize it away from threshold (an inhibitory effect, as with GABA). The neurotransmitter is then removed, by an enzyme, as acetylcholinesterase breaks down acetylcholine, or by reuptake into the terminal, as with serotonin and dopamine, so that the signal ends. A single neuron sums thousands of excitatory and inhibitory inputs and fires only if the total crosses threshold; this summing is the cellular basis of integration. Drugs act here. Stimulants such as caffeine or amphetamines increase excitatory transmission; depressants such as alcohol enhance inhibitory transmission and slow the brain. Opioids bind their own receptors on neurons in the spinal cord and brain, reducing the transmission of pain signals, and they also increase dopamine release in the brain's reward pathway, which is why they produce craving and dependence; in overdose they suppress the brainstem centers that drive breathing. Environmental neurotoxins such as lead and mercury interfere with neurotransmitter release and with development of the young brain, and claims about them should be judged on peer-reviewed evidence.

The brain. The brain consists of three major parts. The cerebrum, the largest, is divided into left and right hemispheres joined by a thick band of axons, the corpus callosum, and its folded surface, the cortex, is divided into lobes: the frontal lobe (voluntary muscle control through the motor cortex, planning, judgment, speech production), the parietal lobe (touch and body position), the temporal lobe (hearing, understanding language, memory), and the occipital lobe (vision). Emotions and memory formation involve structures deep within the hemispheres. Each hemisphere controls the opposite side of the body, and some functions are lateralized, language usually on the left; but the popular idea that people are "left-brained" or "right-brained" in personality is not supported by brain imaging, which shows both hemispheres active in nearly every task. Beneath the cerebrum, the thalamus relays sensory signals to the cortex and the hypothalamus runs homeostasis and the endocrine system. The cerebellum, behind and below the cerebrum, is primarily responsible for balance and coordination, comparing intended movements with actual ones and smoothing them. The brainstem, made of the midbrain, pons and medulla oblongata, connects the brain to the spinal cord and contains the centers that regulate the basic life functions run by the autonomic nervous system, such as breathing, heart rate and blood pressure; damage here is immediately life-threatening.

Protection: meninges and cerebrospinal fluid. The CNS is wrapped in three membranes, the meninges: the tough outer dura mater, the web-like arachnoid mater and the delicate pia mater on the surface of the brain and cord. Cerebrospinal fluid (CSF), made in fluid-filled chambers inside the brain called ventricles, fills the space beneath the arachnoid and the ventricles, so the brain floats and is cushioned against knocks and its chemical environment is kept steady. Meningitis is an infection and inflammation of the meninges, dangerous because the swelling presses on the brain and because CSF can carry the infection throughout the CNS.

The spinal cord and nerves. The spinal cord is a continuation of the brainstem that runs down the vertebral canal. In cross-section it shows a butterfly of gray matter, containing cell bodies and synapses, surrounded by white matter, the bundles of myelinated tracts that transmit signals between the brain and the body. Thirty-one pairs of spinal nerves leave the cord through gaps between vertebrae, each formed by a dorsal root carrying sensory axons in and a ventral root carrying motor axons out. Twelve pairs of cranial nerves leave the brain directly and serve mainly the head, including the optic nerve for vision and the vagus nerve to the heart and gut. Spinal nerves branch and merge to form the nerves of the limbs; the sciatic nerve, the largest in the body, runs from the lower spinal nerves down the back of the thigh to the leg and foot. A spinal cord injury (by cutting, crushing or bruising) damages the tracts, so sensation and voluntary movement are reduced or lost below the level of the damage, completely or partly depending on how much of the cord is affected, and because CNS axons regenerate poorly the loss is often permanent; a concussion is a brain injury from an impact that shakes the brain within the skull, disturbing function without necessarily any visible damage, and repeated concussions increase the risk of lasting harm.

Reflex arcs. A reflex is a rapid, automatic response that runs through the spinal cord or brainstem without waiting for the brain to decide. The simplest nerve pathway is the reflex arc: a receptor detects the stimulus; a sensory neuron carries the signal into the cord; an integration center, which is either a direct synapse onto the motor neuron or one or more interneurons in between; a motor neuron carries the command out; an effector responds. When a tendon below the kneecap is tapped, the thigh muscle is stretched, its stretch receptors fire, the sensory neuron synapses directly on the motor neuron in the cord and the muscle contracts, kicking the leg; this knee-jerk reflex has one synapse and takes a few tens of milliseconds. Pulling a hand from a hot pan is a withdrawal reflex with an interneuron, still complete before the pain is consciously felt; the brain receives a copy of the signal and learns of the event afterward. Reflex time is therefore far shorter than reaction time, which is the time for a signal to reach the brain, be recognized, and produce a voluntary command.

Investigation: reaction time. A partner holds a ruler vertically with the zero mark level with your open thumb and finger, then drops it without warning; you catch it and read the distance it fell. The ruler falls freely, so distance and time are related by distance = ½ × 9.8 × time², which gives time = square root of (2 × distance ÷ 9.8) with distance in meters. A 20 centimeter drop means √(0.40 ÷ 9.8) = √0.0408 ≈ 0.20 seconds. Take ten trials, discard the first two as practice, and average. Then compare with a reflex: a gentle tap below the kneecap with the side of the hand while the leg hangs relaxed gives a kick that begins almost immediately. Record drop distances, calculated times and observations, and compare with a partner and with a distraction such as counting backwards. A reaction time of roughly 0.15 to 0.25 seconds against a reflex that is faster than you can time by hand supports the claim that the reflex arc bypasses the brain's decision. The test measures the whole pathway from eye to hand; it cannot separate how long is spent in sensing, deciding and moving, and the ruler method has a resolution of about a centimeter, which limits precision to about 0.01 seconds.

Misconceptions. "Nerves are single cells" confuses a nerve with an axon; a nerve is a cable of thousands. "A stronger stimulus makes a bigger action potential" contradicts the all-or-none rule. "Reflexes are controlled by the brain" is wrong for spinal reflexes, whose response is produced in the spinal cord without waiting for any decision by the brain. "Sensation adapts because the nerve is tired" misreads adaptation: many receptors are built to stop signaling a constant stimulus, which is why you stop feeling a watch strap, so that changes stand out.

Worked reading of the membrane potential graph: From the diagram: at time 0 to 1 ms the trace is flat at −70 mV, the resting potential. At about 1.5 ms it reaches −55 mV, threshold, and rises steeply to +30 mV at 2 ms: depolarization, caused by voltage-gated sodium channels opening and sodium entering. From 2 to 3 ms it falls to −70 mV: repolarization, caused by sodium channels inactivating and potassium channels opening so potassium leaves. At 3.5 ms it dips to −80 mV: hyperpolarization, because the potassium channels close slowly. By 5 ms it is back at −70 mV. Total excursion from rest to peak: 100 mV. While the sodium channels are inactivated (the absolute refractory period, covering the spike and most of the fall) no second impulse can start there, so the signal cannot run backward; late in repolarization and during the undershoot (the relative refractory period) a stronger-than-usual stimulus can trigger one.

Worked prediction from a changed membrane potential: Suppose a toxin holds the resting potential at −60 mV instead of −70. The gap to threshold shrinks from 15 mV to 5 mV, so much smaller stimuli would fire the neuron; it would be hyperexcitable, and muscles might twitch or cramp. (Condition: if the membrane were held depolarized for a long time, many sodium channels would stay inactivated, and the neuron could instead become harder to fire; the simple gap-to-threshold reasoning applies to a modest, short-lived shift.) If instead the resting potential were held at −85 mV, the gap would be 30 mV and the neuron would be hard to fire, so responses would be sluggish or absent. If the myelin sheath were destroyed, the peak would be unchanged but propagation would be slow and might fail between damaged patches, which is the mechanism of the symptoms of multiple sclerosis.

Worked reflex arc, knee-jerk: Stimulus: a tap on the tendon below the kneecap stretches the quadriceps. Receptor: stretch receptors inside the muscle. Sensory neuron: carries the signal through the dorsal root into the spinal cord. Integration: the sensory neuron synapses directly on the motor neuron (one synapse, no interneuron); a branch also excites an interneuron that inhibits the hamstrings so they do not fight the kick. Motor neuron: carries the command out through the ventral root. Effector: the quadriceps contracts and the leg kicks. The brain receives a copy of the sensory signal; no conscious decision or circuit through the brain is needed to produce the kick, although signals descending from the brain can turn the reflex up or down.

Worked reaction time calculation: Three catches at 12, 20 and 30 centimeters. Convert to meters: 0.12, 0.20, 0.30. Time = √(2 × d ÷ 9.8): √(0.0245) = 0.157 s; √(0.0408) = 0.202 s; √(0.0612) = 0.247 s. Average ≈ 0.20 seconds. The knee-jerk reflex, by contrast, begins within a few hundredths of a second of the tap, which cannot be timed with a ruler, so the comparison is qualitative: reaction time is several times longer than reflex time because it includes conscious recognition and decision in the brain.

Representation task: Construct a labeled model of a neuron (dendrites, cell body, axon, myelin sheath, nodes, axon terminals, synapse) and beside it draw the membrane potential graph with rest, threshold, depolarization, repolarization and hyperpolarization labeled with their millivolt values, then draw a reflex arc with its five components. On an outline of the brain, label the cerebrum with its four lobes, the cerebellum and the brainstem, and write one function beside each.

Independent performance: Carry out the ruler-drop reaction time investigation and the knee-jerk observation, and write a report that also compares the central and peripheral nervous systems. Success criteria: (a) at least eight drop distances per person are recorded for two people, converted to times with the formula shown, averaged and compared under one changed condition; (b) the report explains why reflex time is shorter than reaction time by tracing both pathways and naming where integration occurs in each; (c) a table compares the CNS and PNS by structures, functions and protection (meninges, CSF, bone); (d) one imbalance from drugs, spinal injury, concussion, meningitis and multiple sclerosis is explained by the structure or process it disrupts, and one prediction is made about how a change in membrane potential would affect action potential propagation; (e) two limits of the ruler method are stated.