Neural Control and Coordination

NCERT opens the chapter with a simple point. When you run, your legs work harder, so your muscles need more oxygen and glucose. To supply them the heart beats faster, the breathing rate rises, the blood vessels to the muscles widen and the liver releases glucose. Several organs are changing what they do at the same time, in the right direction, and they fall back to normal when you stop. This is what NCERT calls coordination: the process through which two or more organs interact and complement each other's functions. In humans two systems do this job together. The neural system provides an organised network of point-to-point connections, so its control is quick and precise. The endocrine system provides chemical integration through hormones, which is slower but longer-lasting. This chapter is about the first one; the next chapter is about the second.

The neural system of all animals is made of highly specialised cells called neurons, which can detect, receive and transmit different kinds of stimuli. How elaborate the system is depends on how complex the animal is. In Hydra the neural organisation is very simple: a diffuse network of neurons with no brain. In insects the system is better organised: there is a brain together with a number of ganglia and neural tissues. Vertebrates have a more developed neural system with a large, well-protected brain, a spinal cord and a full set of nerves. The examiner expects you to be able to quote these three examples in the same order as NCERT and to be able to define coordination in one sentence.

Keep the contrast between neural and hormonal control in mind because a short question often asks for it: neural control is fast, point-to-point, and its effect stops when the impulse stops; hormonal control is slower, reaches every cell that the blood reaches, and its effect lasts as long as the hormone stays in circulation. Both are needed, and the hypothalamus of the brain is the place where the two systems meet.

The human neural system is divided into two parts. The central neural system (CNS) includes the brain and the spinal cord and is the site of information processing and control. The peripheral neural system (PNS) comprises all the nerves of the body associated with the CNS. The nerve fibres of the PNS are of two types. Afferent fibres transmit impulses from tissues and organs to the CNS; efferent fibres transmit regulatory impulses from the CNS to the concerned peripheral tissues and organs. A simple memory aid: afferent arrives at the CNS, efferent exits it.

The PNS is further divided into two divisions on the basis of what it controls. The somatic neural system relays impulses from the CNS to skeletal muscles, so it handles voluntary actions. The autonomic neural system transmits impulses from the CNS to the involuntary organs and smooth muscles of the body, such as the heart, the gut and the glands. The autonomic system is itself divided into the sympathetic neural system and the parasympathetic neural system. NCERT does not list their individual actions in this chapter, but a school paper may expect the general idea: the two usually act in opposite directions on the same organ, the sympathetic division preparing the body for activity and the parasympathetic division restoring the resting state.

NCERT also names the visceral nervous system: it is the part of the peripheral neural system that comprises the whole complex of nerves, fibres, ganglia and plexuses by which impulses travel from the CNS to the viscera and from the viscera to the CNS. In practice this term overlaps with the autonomic system, and it is asked as a one-line definition.

Two numbers are not printed in the rationalised NCERT text but are so commonly asked in school papers that you should know them: humans have 12 pairs of cranial nerves, which arise from the brain, and 31 pairs of spinal nerves, which arise from the spinal cord. Cranial nerves may be sensory, motor or mixed; every spinal nerve is mixed, formed by the joining of a dorsal (sensory) root and a ventral (motor) root. Practise drawing the whole division as a flow chart: Neural system → CNS (brain, spinal cord) and PNS → afferent and efferent fibres → somatic and autonomic → sympathetic and parasympathetic. A 3-mark question often asks for exactly that chart.

A neuron is a microscopic structure composed of three major parts: the cell body, the dendrites and the axon. The cell body (cyton) contains cytoplasm with the typical cell organelles and, in addition, certain granular bodies called Nissl's granules. Short fibres that branch repeatedly and project out of the cell body are the dendrites; they also contain Nissl's granules and they transmit impulses towards the cell body. The axon is a long fibre whose distal end is branched; each branch terminates as a bulb-like structure called the synaptic knob, which possesses synaptic vesicles containing chemicals called neurotransmitters. Axons transmit nerve impulses away from the cell body to a synapse or to a neuro-muscular junction. In the NCERT figure the labels to learn are dendrite, cell body, nucleus, Nissl's granules, axon, Schwann cell, myelin sheath, node of Ranvier and axon terminal.

Types of neurons. Based on the number of axons and dendrites, neurons are divided into three types. Multipolar neurons have one axon and two or more dendrites and are found in the cerebral cortex. Bipolar neurons have one axon and one dendrite and are found in the retina of the eye. Unipolar neurons have a cell body with one axon only and are usually found in the embryonic stage. The location is the part the examiner checks, so learn the three pairs together: multipolar–cerebral cortex, bipolar–retina, unipolar–embryo.

Myelinated and non-myelinated fibres. There are two types of axons. The myelinated nerve fibres are enveloped by Schwann cells, which form a myelin sheath around the axon; the gaps between two adjacent myelin sheaths are called nodes of Ranvier. Myelinated nerve fibres are found in spinal and cranial nerves. In non-myelinated (unmyelinated) fibres the axon is enclosed by a Schwann cell that does not form a myelin sheath around it, and such fibres are commonly found in the autonomous and the somatic neural systems. The functional difference matters for the next section: in a myelinated fibre the impulse jumps from node to node (saltatory conduction), so conduction is much faster than in a non-myelinated fibre where the whole membrane has to depolarise in turn.

A neuron is described as the structural and functional unit of the neural system because it is the cell that actually receives the stimulus (at the dendrites), conducts it (along the axon) and hands it on (at the synaptic knob). Every other structure in the chapter, from a nerve to the cerebral cortex, is built out of neurons and their supporting cells.

Neurons are excitable cells: their membranes are in a polarised state. To understand why, look at the axonal membrane. It carries different types of ion channels, and each channel is selectively permeable to a particular ion. When a neuron is not conducting any impulse, that is, when it is resting, the axonal membrane is comparatively more permeable to potassium ions (K⁺) and nearly impermeable to sodium ions (Na⁺). It is also impermeable to the negatively charged proteins present in the axoplasm. As a result the axoplasm inside the axon contains a high concentration of K⁺ and of negatively charged proteins and a low concentration of Na⁺, while the fluid outside the axon contains a low concentration of K⁺ and a high concentration of Na⁺. So there is a concentration gradient across the membrane for each ion.

These ionic gradients would run down by diffusion if nothing maintained them. They are maintained by the active transport of ions by the sodium–potassium pump, which transports 3 Na⁺ outwards for 2 K⁺ into the cell, using ATP. Because the pump pushes out three positive charges for every two it brings in, and because K⁺ keeps leaking out through the K⁺ channels while the negative proteins stay trapped inside, the outer surface of the membrane becomes positively charged and the inner surface negatively charged. The membrane is then said to be polarised, and the electrical potential difference across the resting plasma membrane is called the resting potential. NCERT does not give a value; the usual figure quoted is about −70 mV (inside negative relative to outside), and you may write it as an extra point but not in place of the explanation.

When the exercise asks you to explain the polarisation of the membrane of a nerve fibre, the marking scheme looks for four points in order: (1) unequal permeability, more to K⁺, nearly none to Na⁺, none to the negative proteins; (2) the resulting distribution, K⁺ and proteins high inside, Na⁺ high outside; (3) the Na⁺–K⁺ pump, 3 Na⁺ out for 2 K⁺ in, maintaining the gradient by active transport; (4) the conclusion, outer surface positive, inner surface negative, resting potential. Write them as four numbered lines and you will not miss a mark.

Now apply a stimulus to the axon at a site, call it site A, as in NCERT Fig. 18.2. The membrane at site A becomes freely permeable to Na⁺. This leads to a rapid influx of Na⁺, followed by a reversal of the polarity at that site: the outer surface of the membrane becomes negatively charged and the inner side becomes positively charged. The polarity of the membrane at site A is thus reversed and the membrane is said to be depolarised. The electrical potential difference across the plasma membrane at site A is now called the action potential, which is in fact the nerve impulse. So the marking points for 'depolarisation' are: stimulus → membrane freely permeable to Na⁺ → rapid Na⁺ influx → polarity reversed, inside positive and outside negative → action potential.

How does the impulse move? At the site immediately ahead, site B, the membrane is still at rest, so its outer surface is positive and its inner surface is negative. Because site A now has the opposite polarity, a current flows on the inner surface from site A to site B, and on the outer surface a current flows from site B to site A, completing the circuit of current flow. This local current reverses the polarity at site B: an action potential is generated at site B. The action potential, that is the impulse, generated at site A thus arrives at site B. The sequence is repeated along the length of the axon and in this way the impulse is conducted. Draw the two sites with their charges and the two arrows of current when you answer 'conduction of a nerve impulse along a nerve fibre'; the figure carries marks.

What happens back at site A? The rise in the stimulus-induced permeability to Na⁺ is extremely short-lived. It is quickly followed by a rise in the permeability to K⁺. Within a fraction of a second K⁺ diffuses outside the membrane and restores the resting potential of the membrane at the site of excitation, and the fibre becomes once more responsive to further stimulation. This restoration is called repolarisation, and the Na⁺–K⁺ pump then slowly restores the original ion distribution. The action potential is therefore a brief event: Na⁺ in (depolarisation, rising phase), K⁺ out (repolarisation, falling phase), pump resets. In a myelinated fibre the same events occur only at the nodes of Ranvier, so the impulse leaps from node to node and travels much faster; this is what the exercise wants when it asks you to distinguish conduction in myelinated and unmyelinated fibres.

Two consequences are worth stating in an answer. First, the action potential is all-or-none: a stimulus either reaches threshold and produces a full impulse or produces none. Second, the impulse travels in one direction along the axon under normal conditions because the region just behind is still recovering. Neither is stated in NCERT in those words, so use them as supporting sentences rather than as the core of the answer.

A nerve impulse is transmitted from one neuron to another through junctions called synapses. A synapse is formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron, which may or may not be separated by a gap called the synaptic cleft. That single sentence of NCERT is the definition to write when 'synapse' is asked as a short note, and it leads straight to the two kinds of synapse.

At an electrical synapse the membranes of the pre- and post-synaptic neurons are in very close proximity. Electrical current can flow directly from one neuron into the other across the synapse, so transmission is very similar to impulse conduction along a single axon. Impulse transmission across an electrical synapse is always faster than that across a chemical synapse, but electrical synapses are rare in our system. Three comparison points: no cleft (or almost none), no neurotransmitter, faster but rare.

At a chemical synapse the membranes of the pre- and post-synaptic neurons are separated by a fluid-filled space, the synaptic cleft. Chemicals called neurotransmitters are involved in the transmission of impulses at these synapses. The axon terminals contain vesicles filled with these neurotransmitters. NCERT gives the mechanism in a clear sequence, and the 3-mark or 5-mark question on 'mechanism of synaptic transmission' expects each step: (1) when an impulse (action potential) arrives at the axon terminal, it stimulates the movement of the synaptic vesicles towards the membrane; (2) the vesicles fuse with the plasma membrane and release their neurotransmitters into the synaptic cleft; (3) the released neurotransmitters bind to their specific receptors present on the post-synaptic membrane; (4) this binding opens ion channels, allowing the entry of ions which can generate a new potential in the post-synaptic neuron. The new potential developed may be either excitatory or inhibitory, depending on which ion channels are opened. The labelled figure has the axon terminal, synaptic vesicles, synaptic cleft, neurotransmitters, receptors and the post-synaptic membrane.

NCERT does not name individual neurotransmitters in this chapter, but acetylcholine is the one usually quoted in school papers as the transmitter at the neuro-muscular junction (it appears in the Locomotion chapter). Note also that the transmission at a chemical synapse is one-way: only the pre-synaptic knob has vesicles and only the post-synaptic membrane has the receptors. Because the transmitter must diffuse across the cleft, chemical transmission introduces a short synaptic delay, which is the reason an electrical synapse is faster.

The brain is the central information processing organ of our body and acts as the command and control system. NCERT gives a list of what it controls, and a 2-mark question can ask for any four of them: the voluntary movements, the balance of the body, the functioning of vital involuntary organs such as the lungs, heart and kidneys, thermoregulation, hunger and thirst, circadian (24-hour) rhythms of our body, the activities of several endocrine glands, and human behaviour. It is also the site for the processing of vision, hearing, speech, memory, intelligence, emotions and thoughts.

The human brain is well protected by the skull. Inside the skull the brain is covered by three cranial meninges. From outside inward they are the dura mater, the outer thick layer; the arachnoid, a very thin middle layer; and the pia mater, the inner layer which is in contact with the brain tissue. The order is asked as an MCQ almost every year, so learn it as D-A-P from outside to inside. The space between the arachnoid and the pia mater holds the cerebrospinal fluid, which cushions the brain; the same fluid fills the ventricles of the brain and the central canal of the spinal cord. That detail is from the exemplar rather than the main text, but it turns up in MCQs.

The brain can be divided into three major parts: the forebrain, the midbrain and the hindbrain. The forebrain consists of the cerebrum, thalamus and hypothalamus. The midbrain lies between the forebrain and the hindbrain. The hindbrain consists of the pons, cerebellum and medulla oblongata. The midbrain and hindbrain together form the brain stem, which continues below as the spinal cord. When the examiner asks you to 'briefly describe the structure of the brain', open with the meninges, then the three parts in this order, then the parts of each, and finish with the brain stem. The next three sections give you the content for each part.

The spinal cord is the other component of the CNS. It runs down the vertebral canal, is covered by the same three meninges and has grey matter in the centre (in the shape of a butterfly or an H) surrounded by white matter, the reverse of the arrangement in the cerebrum. It carries impulses to and from the brain and is the centre for spinal reflexes, which is where it reappears in section 18.5.

The cerebrum forms the major part of the human brain. A deep cleft, the longitudinal fissure, divides the cerebrum longitudinally into two halves, the left and right cerebral hemispheres. The hemispheres are connected by a tract of nerve fibres called the corpus callosum. The layer of cells which covers each cerebral hemisphere is called the cerebral cortex, and it is thrown into prominent folds, which increase its surface area. The cerebral cortex is referred to as the grey matter because of its greyish appearance, which is due to the concentration of neuron cell bodies there. The cortex contains motor areas, sensory areas and large regions that are neither clearly sensory nor motor in function. These regions are called association areas and are responsible for complex functions like intersensory associations, memory and communication. Beneath the cortex, the fibres of the tracts are covered with the myelin sheath; they constitute the inner part of the cerebral hemisphere and give it an opaque white appearance, hence white matter. So in the cerebrum grey matter is outside and white matter inside, while in the spinal cord it is the reverse.

The cerebrum wraps around a structure called the thalamus, which is a major coordinating centre for sensory and motor signalling. Almost every sensory input on its way to the cortex is relayed through the thalamus. Lying at the base of the thalamus is the hypothalamus. It contains a number of centres which control the urge for eating and drinking and the body temperature. It also contains several groups of neurosecretory cells, which secrete hormones; this is how the neural and endocrine systems are linked, and it is why the hypothalamus is called the master clock of the body (it sets the circadian rhythm) and the controller of the pituitary in the next chapter. When the exercise asks which part of the central neural system acts as a master clock, the answer is the hypothalamus.

The inner parts of the cerebral hemispheres and a group of associated deep structures like the amygdala and the hippocampus form a complex structure called the limbic lobe or limbic system. Along with the hypothalamus it is involved in the regulation of sexual behaviour, the expression of emotional reactions (for example excitement, pleasure, rage and fear) and motivation. Which part of the human brain is the most developed? The cerebrum, specifically its cerebral cortex, which is the seat of intelligence, memory, reasoning and speech.

Two comparison questions come from this section. Thalamus versus hypothalamus: the thalamus is a relay and coordinating centre for sensory and motor signals; the hypothalamus lies below it, houses the centres for hunger, thirst and temperature and produces hormones through neurosecretory cells. Cerebrum versus cerebellum: the cerebrum is the largest part, in the forebrain, with grey matter outside and association areas for memory and intelligence; the cerebellum is in the hindbrain, controls balance and posture and the coordination of voluntary movements, and has a highly convoluted surface.

The midbrain is the smallest of the three divisions and NCERT gives it three sentences, but each one is an examination fact. It is located between the thalamus and hypothalamus of the forebrain and the pons of the hindbrain. A canal called the cerebral aqueduct passes through the midbrain; it connects the third ventricle of the forebrain with the fourth ventricle of the hindbrain and carries cerebrospinal fluid. The dorsal portion of the midbrain consists mainly of four round swellings, called lobes, known as the corpora quadrigemina. The word means 'four bodies': the upper pair (superior colliculi) is concerned with visual reflexes and the lower pair (inferior colliculi) with auditory reflexes, though NCERT only asks you to know the name and position. The ventral part of the midbrain contains the crura cerebri (cerebral peduncles), the bundles of fibres connecting the forebrain to the hindbrain; this label appears in many school diagrams although it is not in the rationalised text.

The midbrain and the hindbrain together form the brain stem. Remember that the brain stem is the part that is continuous with the spinal cord and that all the fibre tracts between the cerebrum and the body pass through it, so an injury there is far more dangerous than an injury to the cortex of a similar size.

For the short note on the midbrain, which is exercise 5(c), the marking points are: position (between forebrain and pons), the cerebral aqueduct, the corpora quadrigemina on the dorsal side, and its membership of the brain stem. Four lines, four marks' worth of content, and a small labelled sketch of the sagittal section with the midbrain shaded will make the answer complete.

The hindbrain comprises the pons, the cerebellum and the medulla (also called the medulla oblongata). The pons consists of fibre tracts that interconnect different regions of the brain; it lies in front of the cerebellum and above the medulla and acts as a bridge (that is what 'pons' means) between the cerebellum, the cerebrum and the medulla. It also contains centres that help the medulla regulate breathing, which is how many school papers phrase its function.

The cerebellum has a very convoluted surface in order to provide additional space for many more neurons. Functionally, the cerebellum coordinates voluntary movements and maintains the posture and balance (equilibrium) of the body. This is the answer to the popular question 'which part of the nervous system is responsible for maintaining balance and coordination of body movements?' A person with cerebellar damage can still move but moves clumsily and cannot stand steadily.

The medulla of the brain is connected to the spinal cord. It contains centres which control respiration, cardiovascular reflexes and gastric secretions. Because the centres for breathing and for heart rate sit here, the medulla is the most vital part of the brain: injury to it is fatal even when the rest of the brain is intact. Together with the midbrain, the pons and the medulla form the brain stem.

Exercise 5(d) asks for a short note on the hindbrain and exercise 8(d) asks for cerebrum versus cerebellum. For the short note write three lines, one for each part with its function. For the difference, place the two side by side: cerebrum (forebrain, largest part, grey matter outside, seat of intelligence, memory, voluntary control) against cerebellum (hindbrain, second largest part, convoluted surface, balance, posture and coordination of movement). A helpful way to keep the hindbrain functions straight is to ask what stops working when the part is damaged: cerebellum, you fall over; medulla, you stop breathing; pons, the traffic between the parts of the brain is cut.

You must have experienced a sudden withdrawal of a body part which comes in contact with something that is extremely hot, cold, pointed or an animal that is scary. The entire process of response to a peripheral nervous stimulation that occurs involuntarily, that is without conscious effort or thought, and requires the involvement of a part of the central neural system, is called a reflex action. That is the NCERT definition and it has three parts the examiner looks for: a response to a peripheral stimulus, involuntary, and involving a part of the CNS (usually the spinal cord). The last part is what students forget; a reflex is involuntary but it is not independent of the CNS.

The reflex pathway comprises at least one afferent neuron (receptor) and one efferent neuron (effector or excitor) appropriately arranged in a series. The afferent neuron receives the signal from a sensory organ and transmits the impulse via the dorsal nerve root into the CNS, at the level of the spinal cord. The efferent neuron then carries the signal from the CNS to the effector, leaving through the ventral nerve root. The stimulus and the response thus form a reflex arc. In many reflexes an interneuron (association neuron) inside the grey matter of the spinal cord connects the afferent and efferent neurons; in the simplest reflexes, such as the knee jerk, the afferent neuron synapses directly with the efferent neuron.

NCERT Fig. 18.5 shows the knee-jerk reflex, and this is the diagram to practise. A tap below the kneecap stretches the thigh muscle; the receptor (muscle spindle) in the muscle generates impulses; the sensory (afferent) neuron, whose cell body lies in the dorsal root ganglion, carries them through the dorsal root into the spinal cord; there the signal passes (directly or through an interneuron) to the motor (efferent) neuron whose cell body lies in the grey matter; the motor neuron's axon leaves through the ventral root and reaches the effector, the thigh muscle, which contracts and the leg kicks. Write the sequence receptor → afferent neuron → dorsal root → spinal cord (integrating centre) → efferent neuron → ventral root → effector.

Why do reflexes exist? Because the pathway is short and does not wait for the brain, the response is faster than a conscious action and protects the body from damage. The brain is informed afterwards, which is why you feel the pain of the hot pan a moment after your hand has already moved. Distinguish a reflex from a voluntary action in a 2-mark answer: reflex, involuntary, spinal cord as the centre, very fast, stereotyped; voluntary, conscious, cerebrum involved, slower, variable.