Neural Control and Coordination

All cells of a multicellular animal have to work as one body. Two systems do this coordination work. The endocrine system sends chemical messengers through the blood, which is slow but long lasting. The neural system sends electrical signals along cells that are wired to their targets, which is extremely fast but short lived. Neural control is what lets you pull your hand back from a hot plate before you have even thought about it.

Nervous systems are not equally complex in all animals. In lower invertebrates such as Hydra, the neural system is only a loose network of neurons spread through the body wall, with no brain and no clear direction of flow. Insects are a step ahead: they have a brain at the front end plus a chain of ganglia (clusters of nerve cell bodies) and associated neural tissue, so signals are sorted and processed at defined points. Vertebrates have the most developed arrangement, with a large brain, a spinal cord protected inside the vertebral column, and nerves reaching every organ.

The human neural system is described in three overlapping ways, and students often mix them up, so read the divisions slowly.

The central neural system (CNS) is the brain and the spinal cord. This is where information is received, compared with memory and past experience, and where commands are issued. It is the site of information processing and control.

The peripheral neural system (PNS) is made of all the nerves that lie outside the CNS and connect it with the rest of the body. Human beings have 12 pairs of cranial nerves arising from the brain and 31 pairs of spinal nerves arising from the spinal cord. The nerve fibres of the PNS are of two kinds. Afferent (sensory) fibres carry impulses from tissues and organs towards the CNS. Efferent (motor) fibres carry regulating impulses away from the CNS to the muscles and glands. A simple memory hook: afferent means arriving, efferent means exiting.

The PNS itself is divided by the kind of target it commands. The somatic neural system relays impulses from the CNS to skeletal muscles, so it drives voluntary movement. The autonomic neural system carries impulses from the CNS to the smooth muscles of the internal organs and to cardiac muscle and glands, so it works without your conscious control. The autonomic system is further split into the sympathetic division, which prepares the body for emergency and effort, and the parasympathetic division, which brings the body back to a calm, resting, energy-saving state.

The visceral nervous system is the part of the peripheral neural system that consists of the whole complex of nerves, fibres, ganglia and plexuses through which impulses travel from the CNS to the viscera and from the viscera back to the CNS. Do not treat visceral and autonomic as two totally separate systems in an answer; the visceral arrangement is the peripheral wiring that serves the internal organs, and its outgoing control is autonomic.

A neuron is a microscopic, excitable cell and it is the structural and functional unit of the neural system. Every neuron has three recognisable parts, and you can describe all three in words without any figure.

The cell body, also called the cyton or soma, is the widest part. It contains cytoplasm with all the usual cell organelles along with granular bodies called Nissl's granules, which are stacks of rough endoplasmic reticulum and are the protein factories of the neuron.

Short, branched, tapering processes come out of the cell body and are called dendrites. They also contain Nissl's granules. Dendrites receive signals and carry impulses towards the cell body.

One long process leaves the cell body and is called the axon. It is usually much longer than the dendrites and it carries the impulse away from the cell body. The axon ends in a branched set of fine endings, and each ending swells into a bulb-like structure called a synaptic knob. Synaptic knobs contain synaptic vesicles loaded with chemicals called neurotransmitters. An easy examination point: Nissl's granules are present in the cell body and dendrites but are absent from the axon, which is why the axon cannot make its own proteins and depends on the cell body.

Neurons are classified by how many processes leave the cell body. A unipolar neuron has only a single process (an axon) coming out of the cell body, and this type is usually seen in the embryonic stage. A bipolar neuron has one axon and one dendrite on opposite sides of the cell body, and this type is found in the retina of the eye. A multipolar neuron has one axon and two or more dendrites, and this is the type found in the cerebral cortex of the brain. Note the trap here: the count is about processes, not about function, so a multipolar neuron is not automatically a motor neuron.

Axons come in two kinds. In a myelinated nerve fibre, the axon is wrapped in Schwann cells which form a fatty myelin sheath around it. This sheath is not continuous; it is interrupted at regular intervals by unwrapped gaps called the nodes of Ranvier. Myelinated fibres are found in the spinal and cranial nerves. In a non-myelinated nerve fibre, the axon is still enclosed by a Schwann cell but no myelin sheath is formed around it, and there are no nodes of Ranvier. Non-myelinated fibres are commonly found in the autonomic neural system and in the somatic neural system.

The fluid inside the axon is called axoplasm, and the membrane that encloses it is the axolemma or axonal membrane. Almost everything in the next section happens across this one membrane, so it is worth remembering that the axolemma is a selectively permeable membrane whose permeability to sodium and potassium is not fixed but changes from moment to moment.

A neuron that is not conducting any impulse is not doing nothing. It is holding a charge, like a small battery kept ready. This stored voltage across the axonal membrane is the resting potential, and in a typical neuron the inside of the axon is about -70 millivolts with respect to the outside. The minus sign simply means the inner surface of the membrane is negatively charged compared with the outer surface. A membrane in this charged, ready state is described as polarised.

Three facts, taken together, explain the resting potential completely, and you should be able to state all three in an examination answer without drawing anything.

First, the ions are unevenly distributed. The axoplasm inside the axon contains a high concentration of potassium ions (K⁺), a low concentration of sodium ions (Na⁺), and a large amount of negatively charged protein molecules. The fluid outside the axon is the reverse for the two cations: it has a high concentration of Na⁺ and a low concentration of K⁺.

Second, the resting axonal membrane is selectively permeable. It is comparatively more permeable to K⁺, only slightly permeable to Na⁺, and practically impermeable to the negatively charged proteins in the axoplasm. So potassium slowly leaks out down its concentration gradient, sodium hardly gets in, and the big negative proteins are stuck inside. The result of that mismatch is a net negative charge left behind on the inner side of the membrane.

Third, this arrangement would run down within seconds if nothing maintained it, because leakage steadily blurs the concentration difference. It is maintained by an active transport protein in the membrane called the sodium-potassium pump. The pump uses energy from ATP to move 3 Na⁺ out of the cell for every 2 K⁺ it moves into the cell. Both ions are pushed against their concentration gradients, which is why the transport is active and needs energy. Because more positive charge leaves than enters on each cycle, the pump also adds a small amount to the negativity of the inside, but its main job is to keep the ionic gradients ready for the next impulse.

Two cautions that catch students out. The resting potential is not caused by an absence of ions on one side; both sides are full of ions, and it is the difference in the kinds of ions and in the membrane's permeability that creates the voltage. And the sodium-potassium pump is not what produces the action potential; it is the housekeeper that recharges the battery, while the actual firing is done by ion channels opening.

Suppose a stimulus is applied at one point on a resting axon; call it site A. The membrane at site A suddenly becomes freely permeable to Na⁺. Sodium ions are far more concentrated outside and the inside is negative, so both the concentration gradient and the electrical attraction push them in. The result is a rapid influx of Na⁺, and so much positive charge rushes in that the polarity across the membrane is reversed. At site A the outer surface becomes negatively charged and the inner surface becomes positively charged, reaching about +30 mV. The membrane at site A is now depolarised and the potential difference measured there is the action potential. Because it is a signal that travels, it is called a nerve impulse.

Now look at what this does to the neighbouring stretch of membrane, say site B, which is still resting. At site A the outside is negative and the inside is positive; at site B the outside is positive and the inside is negative. This difference makes a small current flow between the two sites, and that local current is exactly the stimulus that opens the sodium channels at site B. So site B depolarises next, then the site after it, and the action potential is regenerated afresh at point after point along the axon. This is why a nerve impulse does not weaken as it travels: it is not a signal being passed along, it is a signal being rebuilt at every step.

Meanwhile site A does not stay depolarised. Almost immediately the membrane there becomes permeable to K⁺ and much less permeable to Na⁺, so K⁺ flows out of the axon. Losing positive charge from inside restores the original polarity, and site A returns to the resting state. This recovery is called repolarisation. For a very short period just after firing, that patch of membrane cannot be made to fire again however strong the stimulus; this is the refractory period, and it is the reason an impulse moves forward along the axon instead of running backwards into membrane it has just crossed.

Note carefully that the ionic movements during an action potential are tiny compared with the total store of ions. The gradients are not used up in one impulse. Over time, however, the sodium-potassium pump restores the exact ionic distribution so the axon stays ready.

A nerve impulse is also an all-or-none event. If the stimulus is below the threshold needed to open the sodium channels, nothing fires at all. Once the threshold is crossed, the action potential fires at its full size, and a stronger stimulus does not produce a bigger impulse. Intensity of a stimulus is signalled instead by how frequently impulses fire and by how many fibres are recruited.

Saltatory conduction explains why myelinated fibres are so much faster. The myelin sheath is a fatty insulator, so ions cannot cross the axonal membrane where the sheath covers it. Ion exchange, and hence depolarisation, can only happen at the bare gaps, that is, at the nodes of Ranvier. The action potential therefore appears at one node, then at the next node, then the next, effectively jumping over the insulated stretches instead of creeping along every micrometre of membrane. The word saltatory comes from the Latin for leaping. Since fewer patches of membrane have to be depolarised, less time and less ionic movement are needed, so conduction along a myelinated fibre is far faster and more energy-efficient than the continuous, point-by-point conduction seen in a non-myelinated fibre.

A nerve impulse travels beautifully along one axon, but the axon has to end somewhere. The junction where an impulse is handed from one neuron to the next neuron, or to an effector such as a muscle, is called a synapse. A synapse is formed by the membranes of a pre-synaptic neuron (the one delivering the impulse) and a post-synaptic neuron (the one receiving it). These two membranes may be separated by a fluid-filled gap or they may lie tightly together, and that difference gives us the two types of synapse.

In an electrical synapse, the membranes of the pre-synaptic and post-synaptic neurons are in very close proximity, so the electrical current can flow directly from one neuron into the next. Transmission across an electrical synapse is very similar to impulse conduction along a single axon and is faster than transmission across a chemical synapse. However, electrical synapses are rare in our system.

In a chemical synapse, the two membranes are separated by a fluid-filled space called the synaptic cleft. Electricity cannot jump this gap, so the message is converted into a chemical form. Chemicals called neurotransmitters do this job. Acetylcholine is a well-known example.

Here is the sequence at a chemical synapse, step by step. The nerve impulse arriving down the axon reaches the synaptic knob at the axon terminal. Its arrival causes the knob's membrane to open channels for calcium ions (Ca²⁺), and Ca²⁺ enters the knob from outside. The calcium ions make the synaptic vesicles move towards the pre-synaptic membrane and fuse with it. Fusion opens the vesicles to the outside, and the neurotransmitter stored inside is released into the synaptic cleft. The transmitter diffuses across the narrow gap and binds to specific receptors present on the post-synaptic membrane. Binding opens ion channels in the post-synaptic membrane, ions move through them, and a new potential is generated in the post-synaptic neuron.

The important point is what kind of potential is generated. The new potential developed may be excitatory, in which case it pushes the post-synaptic membrane towards firing its own action potential, or it may be inhibitory, in which case it makes the post-synaptic neuron less likely to fire. So a synapse is not a simple wire joint; it is a decision point. A post-synaptic neuron typically receives many excitatory and inhibitory inputs at once and fires only if the excitatory influence wins. This is where a large part of the nervous system's actual computing happens.

Two more consequences of chemical transmission are worth noting. Transmission across a chemical synapse takes a small but real amount of time, called synaptic delay, because diffusion is slower than current flow. And because the vesicles are only on the pre-synaptic side and the receptors are only on the post-synaptic side, a chemical synapse allows transmission in one direction only, from pre-synaptic to post-synaptic neuron. This one-way traffic is what keeps signals in the nervous system organised and prevents messages from sloshing back and forth.

The brain is the command and coordination centre of the body. It sits inside the cranium for protection and is wrapped in three connective tissue membranes called the meninges. From outside inwards these are the dura mater, the arachnoid and the pia mater. The brain is divided into three major regions: forebrain, midbrain and hindbrain.

The forebrain consists of the cerebrum, thalamus and hypothalamus. The cerebrum forms the major part of the human brain. A deep longitudinal cleft divides it into two halves, the left and right cerebral hemispheres, and these two halves are joined by a thick band of nerve fibres called the corpus callosum, which lets them exchange information. The layer of cells covering the cerebral hemisphere is the cerebral cortex. It is thrown into folds and ridges, which pack a large surface area into a small skull, and it looks grey to the naked eye because it is packed with nerve cell bodies, which is why it is called grey matter. The cortex contains motor areas that issue commands to muscles, sensory areas that receive incoming information, and large association areas that are responsible for complex functions such as memory, communication and reasoning. Beneath the cortex, the inner part of the cerebrum consists of tracts of myelinated axons, and the whitish colour of myelin gives this region the name white matter.

The thalamus is wrapped by the cerebrum and is the major coordinating centre for sensory and motor signalling; almost all sensory information is relayed through it on its way to the cortex. The hypothalamus lies at the base of the thalamus and is small but powerful. It contains centres that control body temperature and the urge for eating and drinking, and it also contains several groups of neurosecretory cells which secrete hypothalamic hormones. This is where the neural and endocrine systems meet.

The inner parts of the cerebral hemispheres, together with a group of associated deep structures such as the amygdala and the hippocampus, form a ring-like arrangement called the limbic system or limbic lobe. Along with the hypothalamus, the limbic system is involved in the regulation of sexual behaviour, in the expression of emotional reactions such as excitement, pleasure, rage and fear, and in motivation.

The midbrain lies between the thalamus and hypothalamus of the forebrain in front and the pons of the hindbrain behind. A narrow canal called the cerebral aqueduct passes through it. The dorsal portion of the midbrain mainly consists of four rounded swellings or lobes called the corpora quadrigemina. Midbrain centres handle reflex responses to sight and sound, such as turning the head towards a sudden noise.

The hindbrain is made of the pons, cerebellum and medulla oblongata. The pons consists of fibre tracts that interconnect different regions of the brain. The cerebellum has a very convoluted surface, which provides additional space for many more neurons than a smooth surface would allow; it looks after balance, posture and the smooth coordination of movement. The medulla oblongata is the lowest part of the brain and is connected to the spinal cord. The medulla contains centres that control respiration, cardiovascular reflexes and gastric secretions, which is why damage here is so serious.

The brain stem is the stalk that connects the brain with the spinal cord, and it is made of the midbrain, the pons and the medulla oblongata. Take care with this one: the cerebellum lies behind the brain stem and belongs to the hindbrain, but it is not counted as a part of the brain stem itself.

Touch something scalding hot and your hand is already away from it before you consciously register the heat. That involuntary, extremely fast, automatic response to a peripheral nervous stimulus is a reflex action. The value of a reflex is speed. Sending the message all the way up to the cerebrum, waiting for a decision and sending it back down would cost time that the tissue does not have.

The pathway a reflex signal actually takes is called the reflex arc, and it has five links. First comes the receptor, the sense organ or sensory ending that detects the stimulus, such as pain receptors in the skin of the finger. Second is the afferent (sensory) neuron, which receives the signal from the sensory organ and transmits the impulse into the central neural system. In the case of a spinal reflex, this fibre enters the spinal cord through the dorsal root of the spinal nerve. Third is the central neural system itself, in most reflexes the spinal cord, where the incoming signal is passed on either straight to a motor neuron or through one or more connecting interneurons. Fourth is the efferent (motor) neuron, which carries the command out of the spinal cord through the ventral root of the spinal nerve. Fifth is the effector, the muscle or gland that actually performs the response, for example the biceps that flexes the arm and pulls the hand away.

Remember the root rule as a pair: dorsal root carries sensory fibres in, ventral root carries motor fibres out. Swapping these two is an easy slip, so fix the pair in memory: dorsal in, ventral out.

The classic example is the knee-jerk reflex. A tap just below the kneecap stretches the tendon of the thigh muscle. Stretch receptors in the muscle fire, the sensory neuron carries the impulse into the spinal cord, it passes directly to a motor neuron supplying the same thigh muscle, and the muscle contracts, so the lower leg kicks forward. In this particular reflex the sensory neuron synapses directly on the motor neuron with no interneuron in between, which is why it is described as a monosynaptic reflex and why it is so quick. A withdrawal reflex, such as pulling back from a hot object, uses one or more interneurons inside the spinal cord and is therefore polysynaptic.

Notice what the brain does and does not do here. The reflex response itself is completed by the spinal cord, so it happens whether or not the brain is paying attention. At the same time, the sensory impulse also travels upward along the spinal cord to the brain, which is why you feel the pain a moment after your hand has already moved. That mismatch in timing is direct everyday evidence that the reflex did not wait for the brain's decision.

One last distinction to keep clear. A reflex is not a voluntary action. A voluntary movement is planned in the motor areas of the cerebral cortex and can be started, altered or stopped at will, which makes it flexible but slower. A reflex runs along a fixed circuit, gives the same response to the same stimulus each time, and is completed at the spinal level, which is exactly why it is so fast. Not every reflex is a spinal one either: blinking when an object rushes towards the eye, and the narrowing of the pupil in bright light, are controlled from the brain stem rather than the spinal cord. The five links of the arc, however, are the same in every case, and naming them in order is the safest way to answer any question on this topic.