Nervous Control: A First Look

In 1850, in a frog’s leg, Hermann von Helmholtz timed a nerve signal and found it ran at a few tens of metres per second: about the speed of a car on a motorway.

Helmholtz showed that the nerve signal has a speed you can calculate from a ruler and a clock. From that result, this lesson meets the cell that carries the signal, compares how four animal groups arrange those cells, traces the simplest circuit from a sense organ to a muscle, and looks at the division that runs your heart and gut without asking you.

One neuron: in at the dendrites, out along the axon

A neuron is a nerve cell: a cell specialized to carry electrical signals and pass them to other cells. A typical vertebrate motor neuron has four parts. The dendrites are short branches that receive signals from other cells. The cell body (soma) holds the nucleus and gathers that input. The axon is a single long extension that carries the signal away from the cell body, sometimes for a metre or more in a large mammal. The axon terminals at its far end pass the signal on.

The signal travelling along the axon is an action potential, or nerve impulse: a brief, self-renewing electrical change across the cell membrane. It is all-or-none. Once the stimulus passes a threshold, the impulse fires at full size; below threshold, nothing travels. So a stronger stimulus does not make an impulse taller. It makes the neuron fire more often, and it can bring more neurons into action. How the impulse is made, by ions moving through channels in the membrane, is the subject of the action potential and the resting membrane in Neuroscience. Figures often show a resting voltage of -70 mV; that is an illustration, and real values vary between cells.

Where an axon meets the next cell is a synapse. At most synapses the cells do not touch: the arriving impulse makes the terminal release a chemical, a neurotransmitter, which crosses a narrow gap, the synaptic cleft, and acts on the next cell. Some synapses are electrical instead, with ions passing straight from cell to cell. The steps of release are in synaptic transmission.

Many vertebrate axons are wrapped in myelin, layers of membrane from glial cells (Schwann cells outside the brain and spinal cord, oligodendrocytes inside). The impulse is renewed only at gaps between wrappings (myelination; for neurons as separate cells, the neuron doctrine).

How fast? Helmholtz times a frog nerve

Helmholtz used an isolated frog gastrocnemius (calf) muscle still attached to its sciatic nerve. He stimulated the nerve with a brief electric shock near the muscle and timed the delay before the muscle began to twitch. Then he stimulated it farther up and timed it again. He timed these tiny intervals with a galvanometer method and with a recording myograph that wrote the twitch on a moving surface.

His teacher Johannes Müller had doubted that the speed could be measured at all. Helmholtz saw that the farther stimulus point always added a small, repeatable delay. The experiment works by subtraction. The time the junction and the muscle take to respond is the same in both trials, so the extra delay belongs only to the extra length of nerve. His estimate came out at a few tens of metres per second.

That rules in a physical process of finite speed in this preparation. It does not show what the signal is, and the nerve of an ectothermic frog, an animal whose body temperature follows its surroundings, at room temperature is not a human nerve at 37 degrees C.

conduction velocity v = difference in distance / difference in time

Worked problem. These numbers are illustrative, not Helmholtz’s.

1. Velocity from two points. Two stimulus points on a frog nerve are 30 mm apart. Contraction starts 1.2 ms later from the far point than from the near one. v = 0.030 m / 0.0012 s = 25 m per s.
2. A model reflex. Suppose a reflex path in a tall adult human has a 1.0 m sensory axon and a 1.0 m motor axon, both conducting at 50 m per s, joined by one synapse with a 1.0 ms delay. Ignore the time the sense organ, the nerve-muscle junction and the muscle take. Each axon takes 1.0 m / 50 m per s = 0.020 s. Total = 0.020 + 0.020 + 0.001 = 0.041 s, or 41 ms.
3. The same path without speed. Make both axons conduct at 1 m per s. Each now takes 1.0 s, and the total is 1.0 + 1.0 + 0.001 = 2.001 s, about 49 times longer (2.001 / 0.041 = 48.8).

• sensory axon time
• synaptic delay
• motor axon time

Conduction velocity, both axons 50 m per s Axon length, each 1.0 m (1.0 m is the lesson’s tall-adult path; shorter values are illustrative) Inset: distance between stimulus points 30 mm

Each axon: time = length / velocity

Total latency = sensory + synapse + motor

Compared with the same path at 50 m per s

Share of the total taken by the synapse

Inset: a two-point velocity measurement (illustrative nerve at the velocity setting above, not Helmholtz’s value)

Extra delay = distance / velocity

Velocity from the subtraction: difference in distance / difference in time

In this sketch: one tall adult human path, the same velocity in both axons, one synapse of 1.0 ms, illustrative numbers. This is a model sum, not a measured knee-jerk latency; it ignores the time the sense organ, the nerve-muscle junction and the muscle take. Only the direct excitatory limb of the knee jerk is monosynaptic. The inset’s numbers are illustrative, not Helmholtz’s: his frog nerve at room temperature is not a human nerve at 37 degrees C.

Rule to keep: time = distance / velocity for each axon, and the total is the sum of the axon times and the synaptic delay; a two-point measurement removes the delays common to both trials by subtraction.

This is a model sum, not a measured knee-jerk latency. Animals have reached fast conduction in two ways. The giant axon of the squid Loligo has no myelin and is fast because it is very wide, typically about half a millimetre across and sometimes wider. Vertebrate motor axons are far narrower, a few to about 20 micrometres, yet with myelin they conduct as fast or faster. Hodgkin and Huxley’s 1952 analysis of the impulse used that squid axon (action potential).

Nerve nets, cords and brains in four groups

Animals with neurons all carry signals from where they are sensed to where they are acted on, but they arrange the neurons differently (the groups are in animal body plans).

• Hydra, a freshwater cnidarian, has a nerve net: neurons spread through the body wall and connected in a mesh, with no brain.
• A planarian, a free-living flatworm, has two nerve cords running along its underside, joined by cross-connections like the rungs of a ladder, and a cluster of neurons, a ganglion, in the head.
• The earthworm Lumbricus, an annelid, has a single ventral nerve cord with a ganglion in each body segment, linked to a pair of ganglia above the pharynx near the front.
• Vertebrates, such as a frog or a human, have a dorsal hollow nerve cord, the spinal cord, running along the back, with a brain at its front end.

Flatworm and earthworm cords are ventral and solid; the vertebrate cord is dorsal and hollow. None is a step toward another: Hydra’s net is a working nervous system in a living lineage, not an unfinished brain.

In vertebrates the central nervous system (CNS) is the brain and spinal cord. The peripheral nervous system (PNS) is everything outside them: the nerves and ganglia. Its motor output has two parts. The somatic division drives skeletal muscle. The autonomic division drives cardiac muscle, smooth muscle and glands. Somatic does not mean voluntary, as the next section shows. The full map: nervous system map.

The reflex arc

A reflex is a rapid, automatic response to a stimulus. Its pathway, the reflex arc, has five parts: a receptor that detects the stimulus, a sensory neuron that carries it into the CNS, an integrating centre in the CNS, a motor neuron that carries the output, and an effector, the muscle or gland that responds.

In the knee-jerk (stretch) reflex of humans, a tap on the tendon below the kneecap stretches the quadriceps muscle at the front of the thigh. Stretch receptors in the muscle, the muscle spindles, fire. Their sensory fibres (called Ia fibres) enter the spinal cord and synapse directly on the motor neurons of the same quadriceps; the muscle contracts and the leg kicks. That limb has one synapse in the cord: it is monosynaptic. At the same time, branches of the Ia fibres excite small interneurons, neurons that lie wholly within the CNS, which inhibit the motor neurons of the opposing hamstring muscles. Only the direct excitatory limb is monosynaptic. The knee jerk is a somatic reflex: its effector is skeletal muscle, and no decision is involved.

The withdrawal reflex, pulling your foot off a tack, works through interneurons from the start. Pain receptors excite sensory neurons, which excite interneurons in the cord. These activate motor neurons of flexor muscles that lift the limb, while other interneurons inhibit the opposing extensors. Many reflexes use interneurons, though the direct limb of the knee jerk does not. Charles Sherrington, working with mammals, made such reflexes the units of coordination; more is in motor units and reflexes.

Two autonomic divisions and their exceptions

The autonomic route to an organ usually has two neurons in series: a preganglionic neuron in the CNS, and a postganglionic neuron whose cell body sits in a peripheral ganglion and whose axon reaches the organ. John Newport Langley, who named the autonomic nervous system, divided its outflow into two, and set the gut’s own enteric network apart. In humans, the sympathetic division leaves from the thoracic and upper lumbar spinal cord (thoracolumbar). The parasympathetic division leaves from the brainstem, in cranial nerves such as the vagus, and from the sacral cord (craniosacral). A 2016 mouse study proposed calling the sacral part sympathetic; that is disputed, and this course keeps the standard grouping.

Human target	Sympathetic	Parasympathetic
Heart pacemaker	Raises heart rate (norepinephrine)	Lowers heart rate (acetylcholine, vagus)
Pupil	Dilates (radial iris muscle)	Constricts (circular iris muscle)
Gut	Inhibits peristalsis	Promotes peristalsis and digestion
Most blood vessels	Main neural control of tone	Little or no direct supply
Sweat glands	Supplied, and the fibres release acetylcholine	No supply

The divisions oppose each other at the heart, the iris and the gut. They do not oppose everywhere. Most vessels and the sweat glands have only a sympathetic supply, so their control is turned up and down by one division. In the rabbit parotid salivary gland, sympathetic stimulation increased flow on top of a parasympathetic background secretion, so there the two worked in the same direction. For hypothalamic control see autonomic control and the hypothalamus; for transmitters and drugs, transmitter systems. Hormones, the second kind of control, are in the endocrine system, and the pacemaker in circulation and the heart.

Design problem

The numbers below are illustrative, in the pattern of human drug-blockade studies such as Jose and Collison’s (1970). Healthy young adults at rest have a heart rate of 70 beats per minute. After atropine, which blocks the parasympathetic effect on the heart, it is 110. After propranolol, which blocks the sympathetic effect through beta receptors, it is 62. After both drugs, it is 100. Which division dominates the resting heart rate, and how strong is each?

One way to do it

With both divisions blocked, the pacemaker runs at its intrinsic rate, about 100 beats per minute here. The resting rate, 70, is 30 below that, so the net autonomic effect at rest is braking.

With only parasympathetic input left (propranolol), the rate is 62: vagal braking takes it 38 below intrinsic. With only sympathetic input left (atropine), it is 110: sympathetic drive adds 10. So at rest the parasympathetic division dominates, and sympathetic drive adds a little. The two effects, -38 and +10, sum to -28, close to the observed -30 but not exactly, so they do not combine as simple addition.

Two checks. The conclusion holds only if each drug fully blocks its own division, and nothing else, at the dose used. And the divisions oppose at this pacemaker, which does not carry over to sweat glands or most vessels, with one supply, or to the rabbit parotid, where they add.