The Nervous System – Structure and FunctionThe nervous system is built from a huge number of neurons. While details differ, they all have the same basic architecture:

There is an input end (frequently highly branched) called a dendrite.

There is a cell body, where the nucleus and much of the metabolic machinery is located.

There is an output end, called an axon. It is frequently branched at its tip. Each tip branch ends in a terminal (synaptic) knob.

In this example, the dendrites are short and the axon is long. These are the relative lengths seen in a typical motor neuron (one that innervates voluntary muscle, like the fingers I’m using to type). There is one ‘extra’ structure: the myelin sheath, each ‘sausage’ is a separate Schwann cell that insulates the axon and speeds conduction.

Other types of neurons have different lengths of axons and dendrites, but the same set of structures. A sensory neuron has a very long dendrite, reaching from the tip of your toe to the lower part of your spinal cord, but a short axon, extending only into the spinal cord.

The lengths of dendrites and axons are approximately equal in your brain, and both are branched.

The nervous system is divided into two major divisions:

The Central Nervous System (or CNS) made up of:

CerebrumCerebellumMedulla OblongataSpinal Cord

and the Peripheral Nervous System (or PNS) made up of:

Somatic (Voluntary) and theAutonomic (Involuntary)

Sympathetic and Parasympathetic

How do the PNS and CNS function together? The knee-jerk reflex…

In words first:

1.Your doctor hits your knee just below the kneecap. That stretches a stretch receptor (a sensory neuron)

in the patellar tendon.

2. The axon of that neuron carries the signal into the spinal cord through its dorsal root.

3. There is a synapse from the stretch receptor onto a spinal interneuron. Its axon has at least two synapses: one excites a motorneuron connecting to an extensor muscle and one inhibits the motorneuron for the opponent flexor muscle.

4. The signal is also transmitted through the spinal cord up to your brain. You feel that your knee has been struck, but you’ve already responded by…

5. Contraction of the extensor muscle and relaxation of the flexor muscle.

Diagrammatically:

This diagram has one error. It shows the motor neuron innervating the same muscle as the stretch receptor. Really, the stretch receptor is in a tendon, at one end of the muscle, and the reflex involves a completely different muscle contracting.

A little more now about Schwann cells and the myelin sheath…

Schwann cells are wrapped around the axon so that there are many layers of cell membrane insulating it. There are small gaps, called nodes of Ranvier, between individual Schwann cells. Conduction velocity goes from about 5m/s without the myelin sheath to ~150m/s with it.

The signal carried along the length of the dendrite and axon is a nerve impulse. It is a short (~10ms) electrical wave that passes down the dendrite and axon.

To understand the impulse, you first need to learn how neurons maintain a resting potential. The cell membrane of the neuron has proteins in it that act as ion-specific channels that are described as “gated” or voltage-dependent (K and Na), as well as a voltage independent K channel.

When the cell is at rest, the sodium channel is closed. The voltage-independent potassium channel permits ion movement, and the ATP-powered sodium-potassium pump pumps sodium out and potassium in, but it’s a coupled pump and moves more sodium out than potassium in.

Net result: organic ions (- charged) and more Na+ outside than K+ inside leaves the interior of the cell at a relativevoltage of ~-70mv.

Now something happens to reduce the potential on the membrane to a threshold.

The stimulus could be any of a number of things: the stretching of the stretch receptor membrane, a stimulus coming from another nerve cell by way of the synapse, …

The stimulus opens some of the Na channels. If enough sodium moves in to reduce the membrane potential to about -50mv, then the gated Na channels open and much more Na moves into the nerve cell. So much moves in that the interior becomes momentarily positive.

At peak the Na channels are closed. Now the voltage-dependent K channels open, and potassium rushes out of the cell.

The membrane potential once more becomes negative, even more negative (-75 to -80mv) than at rest. The K channels close.

However, now the K+ and Na+ ions are on the ‘wrong’ sides of the cell membrane. The coupled pump exchanges them, and restores the resting condition.

Propagation of the nerve impulse:

The nerve impulse is a local event, occurring at one point along an axon. To communicate, it must move down the axon to the point of communication with another cell.

To understand how, all you need to recognize is that when the sodium rushes in, it spreads (in both directions) along the axon.

The sodium depolarizes a region further along the axon at least to the threshold level. Sodium then rushes in there, and all the rest of the stages of an impulse.

But that sodium also spreads out ….

Now we come to the point where information must be communicated from one neuron to another. This happens at synapses. In us virtually all synapses are chemical.

Inside the synaptic knob are large numbers of synaptic vesicles. Inside them are chemical transmitters. When an impulse arrives at the synaptic knob, it causes a number of these vesicles to fuse with the membrane, releasing their chemical contents into the synaptic cleft between the neurons.

The chemical diffuses to the post-synaptic membrane (that of the receiving cell), and opens ion channels, depolarizing or hyperpolarizing the cell.

The chemical is called a neurotransmitter. It is rapidly broken down on the post-synaptic membrane to limit how long it affects the receiving cell.

If the neurotransmitter depolarizes the cell, it is an excitatory transmitter; if it opens K channels, it causes a hyperpolarization and makes an impulse less likely – it is acting as an inhibitory transmitter.

Many different chemicals act as neurotransmitters. Among the most important are acetylcholine (ACH), serotonin, dopamine, and gammaaminobutyric acid (GABA). Most are exciteatory in some places and inhibitory in others.

This diagram (opening sodium channels) represents the way that ACH acts on a muscle cell membrane, exciting it.

Most of the drugs that police are interested in (morphine, heroin, cocaine, amphetamines, LSD, mescaline, …) have their effects by functioning as, blocking, or altering chemical synaptic activity.

Similarly, medicine uses drugs with known effects to treat various types of mental difficulty. Prozac increases serotonin presence at synapses. Valium and its relatives activate synaptic receptors for GABA.

Long-term use changes the synaptic chemistry. Both psychological and physical addiction can occur.

In the CNS (and many other places) there are many synapses connecting to a receiving cell. How it responds depends on how the potentials caused by synapses add together. The process is called summation.

Summation is clearly critical to CNS and brain function. We put together many sources of information to determine appropriate responses. The “putting together” is summation.

Some summation occurs in the spinal cord and associated dorsal root ganglia. Much more occurs in the cerebrum and cerebellum.

The brain and spinal cord function together.

However, the tracts in the spinal cord and regions of the brain involved in controlling the right side of your body travel up the left half of the spinal cord and are represented in the left half of the cerebrum.

The cerebrum has the cell bodies of its neurons in the ‘surface’ layer, called the cerebral cortex. It’s sometimes also called the gray matter. The axons and dendrites of these cells are myelinated, and are principally in a thick, deeper layer called the white matter (due to the whiteness of the myelin).

Extending outward from the brain itself are a set of 12 cranial nerves. There are many mnemonics to remember them (you don’t have to). The one I remember is:

On old Olympus’ towering tops a Finn and German viewed some hops.

The first letter of each of the 12 words is the first letter of the corresponding cranial nerve. For example, the first 3 Os stand for:

I. olfactoryII. opticIII.oculomotor

There are also spinal nerves coming to and leaving from each segment of the spinal cord.

Your sensory input and motor control are fully represented on the cortex of your cerebrum:

Touch sensation is arrayed on the somatosensory cortex.Control of voluntary muscle is similarly, but not identically arrayed on the motor cortex.

The details of how each is arrayed have been learned by stimulating the cortex of people undergoing brain surgery. The resulting ‘body images’ are referred to as a sensory and a motor homunculus:

motorcontrol

sensoryinput

Other parts of the brain are vital to how we function.

The reticular formation, an extended area in the brainstem, filters information and, as a result, is critical to wakefulness (alertness) and sleep.

The brain doesn’t stop when you’re asleep. Rather, it is partially cut off from sensory input. How alert you are to external information is quite evident in an EEG or electroencephalogram. External electrodes record a kind of summation of overall brain activity.

β waves

waves

δ waves

Emotions are associated with the amygdala and the hypothalamus.

An important aspect of laying down and bringing up memories seems associated with the hippocampus. However, tests on conscious surgery patients have found stimulating areas in the temporal lobe also bring up memories.