Neural Control and Coordination

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XI, Chapter 18.

How can your hand move before you feel the pain?

Touch a hot pan and your hand is already moving back when the burn registers. Signals race along nerve cells at well over 100 metres per second in the fastest fibres. No chemical messenger released into the blood could be that quick. This chapter is about the wiring that makes such speed possible, the electrical trick each wire uses to carry a message, the gaps where one wire hands over to the next, and the command centre, the brain, that sits at the top.

Two systems, one job

Coordination is the process by which two or more organs interact and complement each other's functions. Your body runs two coordination systems side by side. The neural system works like a telephone network: fast, point to point and short lived. The endocrine system works like a radio broadcast: slower, spread through the blood and long lasting. You will meet the second one in the next chapter, Chemical Coordination and Integration.

The complexity of the neural system grows as you go up the animal kingdom.

Organisation of the human neural system

The neural system has two parts. The central neural system (CNS) is the processing and control site and consists only of the brain and spinal cord. The peripheral neural system (PNS) is made of all the nerves connected to the CNS, so every spinal and cranial nerve belongs here, never to the CNS.

PNS nerve fibres are of two kinds, defined by direction of travel. Afferent fibres carry impulses from tissues and organs to the CNS. Efferent fibres carry regulatory impulses from the CNS to peripheral tissues and organs. A simple way to hold this: afferent = arriving at the CNS, efferent = exiting the CNS.

The visceral nervous system is the part of the PNS made of the nerves, fibres, ganglia and plexuses that carry impulses between the CNS and the viscera (internal organs).

Why it matters. The two autonomic divisions usually work as opposites. The sympathetic division speeds up the body for an emergency, and the parasympathetic division restores calm, resting activity. That is why a fright makes your heart race and the settling afterwards slows it down again.

Pause and predict: a nerve carries a touch signal from your fingertip towards the spinal cord. Is it afferent or efferent, and which part of the neural system does it belong to?

Check your answer: afferent, because it travels toward the CNS, and part of the PNS, because it lies outside the brain and spinal cord.

The neuron: the unit of the neural system

A neuron is a highly specialised, excitable cell that detects, receives and transmits stimuli. It is the structural and functional unit of the neural system. Every neuron has three parts.

Three types of neurons are named by the number of processes on the cell body.

Two types of axons are named by their covering.

Beyond the textbook. For the exam: myelinated fibres have nodes of Ranvier and are found in spinal and cranial nerves. In reality: myelin is fatty insulation, and it makes the impulse faster. In a myelinated fibre the impulse jumps from one node to the next instead of crawling along the whole membrane, which is called saltatory conduction, and it also saves energy. Schwann cells make myelin in peripheral nerves, while in the brain and spinal cord other cells called oligodendrocytes do the same job.

The nerve impulse: a wave of changing polarity

A nerve impulse is not a current flowing through a wire like electricity in copper. It is a wave of change in membrane polarity that is renewed at every point along the axon. To follow it you need only three ideas: who is inside, who is outside and which ion can cross.

The resting state. In a neuron at rest, the axonal membrane is more permeable to K⁺, nearly impermeable to Na⁺ and impermeable to the negatively charged proteins inside the axoplasm. So the axoplasm holds a lot of K⁺ and protein anions and little Na⁺, while the fluid outside holds a lot of Na⁺ and little K⁺. The sodium potassium pump keeps this uneven distribution going by pushing 3 Na⁺ out for every 2 K⁺ in, using ATP. The net result is that the outer face of the membrane is positive and the inner face is negative. The membrane is polarised, and the potential difference across it is the resting potential, about −70 mV inside.

The impulse. When a stimulus acts at one spot, the membrane there suddenly becomes freely permeable to Na⁺. Na⁺ rushes in, and the polarity reverses: the outside becomes negative and the inside positive. This is depolarisation, and the potential difference produced across the membrane is the action potential, which is the nerve impulse itself. The reversed spot sets up a local current that depolarises the next spot ahead, which does the same to the next, so the impulse travels down the axon.

The reset. The rise in Na⁺ permeability lasts only a moment. It is followed by a rise in K⁺ permeability, so K⁺ diffuses out, the outside becomes positive again and the inside negative again. This is repolarisation, and now the fibre is ready for the next stimulus.

The cycle in one line: resting potential → stimulus → Na⁺ influx (depolarisation, action potential) → current depolarises next site → K⁺ efflux (repolarisation) → resting potential again.

Pause and predict: a drug blocks the Na⁺ entry into an axon. When you stimulate it, what happens to the impulse?

Check your answer: there is no depolarisation, so no action potential and no impulse. Local anaesthetics numb a patch of tissue in exactly this way, by stopping sodium from entering the sensory axons.

Beyond the textbook. For the exam: resting potential is about −70 mV, and the action potential reverses the polarity. In reality: the peak reaches roughly +30 to +40 mV, and the action potential is all or none. A stimulus too weak to reach the threshold produces nothing, and one that reaches it produces the full impulse. The patch of axon that has just fired also needs a short rest, a refractory period, before it can fire again, and that is why the impulse cannot go backwards.

The synapse: where one neuron hands over to the next

A synapse is the junction between the membranes of a pre-synaptic neuron and a post-synaptic neuron, which may or may not be separated by a synaptic cleft, the fluid filled gap. Impulses cross synapses to move from one neuron to the next.

How a chemical synapse works. The same steps happen every time.

The sequence in one line: action potential reaches the axon terminal → synaptic vesicles fuse with the pre-synaptic membrane → neurotransmitter is released into the synaptic cleft → it binds specific receptors on the post-synaptic membrane → ion channels open → a new potential is generated, which may be excitatory or inhibitory.

Why it works this way. Vesicles are only on the pre-synaptic side and receptors only on the post-synaptic side. So the signal can cross a chemical synapse in one direction only, which keeps traffic in the neural system flowing the right way round.

Pause and predict: a student says, "chemical synapses are faster than electrical ones because they are the more common type." What is wrong with this?

Check your answer: common does not mean fast. Electrical synapses are faster since current passes straight across, but they are rare in humans. Chemical synapses are slower because the neurotransmitter has to be released, diffuse across the cleft and bind its receptor. They are favoured because they allow both excitation and inhibition.

Beyond the textbook. For the exam: the neurotransmitter is released, binds the receptor and a new potential is generated. In reality: the neurotransmitter must also be switched off, either broken down by an enzyme in the cleft or taken back up by the terminal. If it lingered, the post-synaptic neuron would keep firing. Acetylcholine, broken down by the enzyme acetylcholinesterase, is the standard example.

The human brain

The brain is the central information processing organ, the command and control system of the body. It controls voluntary movement, balance, the working of vital involuntary organs, body temperature, hunger and thirst, the 24 hour rhythm, and the activity of many endocrine glands. It also handles vision, hearing, speech, memory, intelligence, emotion and thought.

Protection. The skull encloses the brain, and inside it the cranial meninges form three coverings. Read from outside to inside they are dura mater (thick, outer), arachnoid (thin, middle) and pia mater (innermost, in contact with brain tissue). A handy order is D, A, P: Dura, Arachnoid, Pia.

The brain has three major divisions: forebrain, midbrain and hindbrain.

The cerebrum. A deep cleft divides the cerebrum into the left and right cerebral hemispheres, joined by a thick tract of nerve fibres, the corpus callosum. Each hemisphere has four lobes: frontal, parietal, temporal and occipital. The surface layer is the cerebral cortex. Because it is made of neuron cell bodies it appears grey, and so it is called the grey matter. It is folded, which fits more neurons into the same space. The cortex has motor areas, sensory areas and association areas, which are neither clearly sensory nor motor and handle intersensory association, memory and communication. Beneath the cortex lies the white matter, made of myelinated nerve fibres, which gives the white colour.

The rest of the forebrain. The thalamus is a major coordinating centre for sensory and motor signalling. The hypothalamus, at the base of the thalamus, holds centres for body temperature, hunger and thirst. Its neurosecretory cells secrete hormones, so it is the point where the neural and endocrine systems meet. The inner parts of the cerebral hemispheres together with deep structures such as the amygdala and hippocampus form the limbic system. With the hypothalamus it regulates sexual behaviour, emotional reactions such as excitement, pleasure, rage and fear, and motivation.

The midbrain. It lies between the forebrain (thalamus and hypothalamus) and the hindbrain (pons). A canal, the cerebral aqueduct, passes through it and connects the third and fourth ventricles, the fluid filled chambers of the brain. Its dorsal side carries four round swellings, the corpora quadrigemina.

The hindbrain. The pons is made of fibre tracts that connect different regions of the brain. The cerebellum has a highly convoluted surface which gives extra room for neurons, and it integrates information from the semicircular canals of the ear and the auditory system to control balance and smooth movement. The medulla oblongata holds the centres that control respiration, cardiovascular reflexes and gastric secretions, and it joins the spinal cord. The midbrain, pons and medulla together form the brainstem, the link between the brain and the spinal cord.

Pause and predict: after an accident a person can think and speak clearly but walks with an unsteady, jerky gait. Which part of the brain is most likely affected?

Check your answer: the cerebellum. The cerebrum handles thought and speech, so it is working. The cerebellum coordinates balance and movement, so the person staggers.

Beyond the textbook. For the exam: medulla = respiration, cardiovascular reflexes and gastric secretions. In reality: because the brainstem holds these life sustaining centres, damage to it is far more dangerous than damage to most of the cerebral cortex, and in many countries the legal definition of death is built on the loss of all brainstem function.

Exam traps

60-second revision

Where this chapter connects

• Previous: Locomotion and Movement shows what the neural system commands: a motor neuron triggers every skeletal muscle contraction.
• Next: Chemical Coordination and Integration picks up at the hypothalamus, whose neurosecretory cells control the pituitary and its hormones.
• Also related: Breathing and Exchange of Gases explains how the respiratory centres of the brain set the rhythm of breathing, and Excretory Products and Their Elimination shows the hypothalamus acting on the kidney.