Neural Control and Coordination | CBSE Class 11 Biology Notes

This note covers NCERT Class 11 Biology Chapter 18, Neural Control and Coordination: how the neural system is organised, the structure of a neuron, the resting potential, the generation and conduction of a nerve impulse, transmission across a synapse, and the parts of the human brain. It follows the NCERT chapter section by section.

What is coordination, and which systems bring it about?

Definition: Coordination is the process through which two or more organs interact and complement the functions of one another.

The functions of the organs and organ systems in our body must be coordinated to maintain homeostasis. NCERT's example is physical exercise.

1. When we exercise, the energy demand is increased for maintaining an increased muscular activity.
2. The supply of oxygen is also increased.
3. The increased supply of oxygen necessitates an increase in the rate of respiration, heart beat and blood flow via the blood vessels.
4. When the exercise is stopped, the activities of nerves, lungs, heart and kidney gradually return to their normal conditions.

The functions of muscles, lungs, heart, blood vessels, kidney and other organs are thus coordinated while performing physical exercise. In our body the neural system and the endocrine system jointly coordinate and integrate all the activities of the organs, so that they function in a synchronised fashion.

How is the neural system organised?

The neural system of all animals is composed of highly specialised cells called neurons, which can detect, receive and transmit different kinds of stimuli.

The human neural system

The human neural system is divided into two parts.

The nerve fibres of the PNS are of two types.

The PNS is divided into two divisions.

The visceral nervous system is the part of the peripheral nervous system that comprises the whole complex of nerves, fibres, ganglia and plexuses by which impulses travel from the central nervous system to the viscera, and from the viscera to the central nervous system.

Note: Afferent fibres carry impulses towards the CNS and efferent fibres carry them away from it. A simple way to keep them apart: afferent arrives at the CNS, efferent exits from it.

What is the structure of a neuron?

A neuron is a microscopic structure composed of three major parts: the cell body, the dendrites and the axon.

The cell body is at the top, with the nucleus at its centre and Nissl's granules in the cytoplasm. Short branched dendrites project from the cell body. A single long axon runs downwards from it. Along the axon are Schwann cells forming the myelin sheath, with gaps between them labelled node of Ranvier. At the lower end the axon branches into axon terminals, each ending in a synaptic knob. The labels are: dendrites, Nissl's granules, cell body, nucleus, Schwann cell, axon, myelin sheath, node of Ranvier, axon terminal and synaptic knob.

See Fig. 18.1 in your NCERT textbook

Types of neurons

Based on the number of axons and dendrites, neurons are divided into three types.

Types of axons

Note: Both kinds of fibre have Schwann cells. The difference is whether the Schwann cell forms a myelin sheath. Do not write that unmyelinated fibres lack Schwann cells.

Why is the membrane of a resting neuron polarised?

Neurons are excitable cells because their membranes are in a polarised state. Different types of ion channels are present on the neural membrane, and these ion channels are selectively permeable to different ions.

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⁺),
• nearly impermeable to sodium ions (Na⁺), and
• impermeable to the negatively charged proteins present in the axoplasm.

This difference forms a concentration gradient. The ionic gradients across the resting membrane are maintained by the active transport of ions by the sodium-potassium pump, which transports 3 Na⁺ outwards for 2 K⁺ into the cell.

As a result, the outer surface of the axonal membrane possesses a positive charge, while its inner surface becomes negatively charged. The membrane is therefore polarised.

Definition: The resting potential is the electrical potential difference across the resting plasma membrane.

Note: The pump moves three positive ions out for every two it brings in. Remember the numbers with the ions: 3 Na⁺ out, 2 K⁺ in. Questions are set on exactly which ion goes which way.

How is a nerve impulse generated and conducted along an axon?

NCERT explains the process with two neighbouring sites on the axon, A and B.

1. A stimulus is applied at site A on the polarised membrane. The membrane at site A becomes freely permeable to Na⁺.
2. This leads to a rapid influx of Na⁺, followed by the reversal of the polarity at that site: the outer surface of the membrane becomes negatively charged and the inner side becomes positively charged.
3. The polarity of the membrane at site A is thus reversed, and the membrane is depolarised. The electrical potential difference across the plasma membrane at site A is called the action potential, which is in fact termed a nerve impulse.
4. At the site immediately ahead, site B, the membrane still has a positive charge on the outer surface and a negative charge on its inner surface. As a result, a current flows on the inner surface from site A to site B, and on the outer surface from site B to site A, to complete the circuit of current flow.
5. Hence the polarity at site B is reversed, and an action potential is generated at site B. The impulse generated at site A has arrived at site B.
6. The sequence is repeated along the length of the axon, and consequently the impulse is conducted.

How is the resting potential restored?

The rise in the stimulus-induced permeability to Na⁺ is extremely short-lived. It is quickly followed by a rise in 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. The fibre becomes once more responsive to further stimulation.

Two drawings of the same stretch of axon, shown as two parallel membranes. In the upper drawing Na is shown entering at point A, at the left end. There the charges are reversed, negative outside and positive inside, while the rest of the axon is still positive outside and negative inside. In the lower drawing the shaded region has spread to point B next to it, in a lighter shade, and small curved arrows between A and B show the local flow of current that carries the impulse forward.

See Fig. 18.2 in your NCERT textbook

The NCERT summary puts this in one line: the nerve impulse is conducted along the axon membrane in the form of a wave of depolarisation and repolarisation.

How is an impulse transmitted from one neuron to the next?

A nerve impulse is transmitted from one neuron to another through junctions called synapses.

Definition: 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.

Transmission of an impulse across an electrical synapse is very similar to impulse conduction along a single axon.

Transmission across a chemical synapse

1. The axon terminals contain vesicles filled with neurotransmitters.
2. When an impulse (action potential) arrives at the axon terminal, it stimulates the movement of the synaptic vesicles towards the membrane.
3. The vesicles fuse with the plasma membrane and release their neurotransmitters into the synaptic cleft.
4. The released neurotransmitters bind to their specific receptors, present on the post-synaptic membrane.
5. This binding opens ion channels, allowing the entry of ions, which can generate a new potential in the post-synaptic neuron.
6. The new potential developed may be either excitatory or inhibitory.

The swollen end of an axon, the axon terminal, sits against the surface of the next cell. Inside the terminal are round synaptic vesicles filled with neurotransmitters. Some vesicles have reached the pre-synaptic membrane and are releasing neurotransmitters into the narrow synaptic cleft. On the other side of the cleft, the post-synaptic membrane carries receptors. The labels are: axon, axon terminal, synaptic vesicles, pre-synaptic membrane, synaptic cleft, post-synaptic membrane, receptors and neurotransmitters; a bracket marks the pre-synaptic membrane, cleft and post-synaptic membrane together as the synapse.

See Fig. 18.3 in your NCERT textbook

What does the brain do, and how is it protected?

The brain is the central information processing organ of our body and acts as the "command and control system".

The human brain is well protected by the skull. Inside the skull, the brain is covered by the cranial meninges, which consist of three layers.

1. Dura mater: the outer layer.
2. Arachnoid: a very thin middle layer.
3. Pia mater: the inner layer, which is in contact with the brain tissue.

The brain can be divided into three major parts.

The brain is shown cut down the middle, from front to back. The large folded cerebrum (cerebral hemisphere) fills the upper part, with the corpus callosum as a curved band beneath it. Below the corpus callosum lie the thalamus and, under it, the hypothalamus. The cerebrum, thalamus and hypothalamus are bracketed together as the forebrain. Below them is the midbrain, with the cerebral aqueduct labelled. Below the midbrain the pons and the medulla run down into the spinal cord, and the cerebellum lies behind them; pons, cerebellum and medulla are bracketed as the hindbrain.

See Fig. 18.4 in your NCERT textbook

What are the parts of the forebrain?

Cerebrum

The cerebrum forms the major part of the human brain. A deep cleft divides it longitudinally into two halves, termed the left and right cerebral hemispheres. The hemispheres are connected by a tract of nerve fibres called the corpus callosum.

The cerebral cortex contains three kinds of regions.

• Motor areas.
• Sensory areas.
• Association areas: large regions that are neither clearly sensory nor motor in function. They are responsible for complex functions like intersensory associations, memory and communication.

Thalamus and hypothalamus

Limbic system

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. The NCERT summary adds that the limbic system is concerned with olfaction and autonomic responses as well.

What do the midbrain and the hindbrain do?

Midbrain

The midbrain 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.
• The dorsal portion of the midbrain consists mainly of four round swellings (lobes) called the corpora quadrigemina.
• The NCERT summary states that the midbrain receives and integrates visual, tactile and auditory inputs.

Hindbrain

The parts of the brain at a glance

Brain stem

Three major regions make up the brain stem: the midbrain, the pons and the medulla oblongata. The brain stem forms the connections between the brain and the spinal cord.

Note: The brain stem and the hindbrain are not the same. The hindbrain is pons, cerebellum and medulla. The brain stem is midbrain, pons and medulla. The cerebellum is in the hindbrain but not in the brain stem, and the midbrain is in the brain stem but not in the hindbrain.

How do you answer the comparison questions in the NCERT exercise?

Resting potential and action potential

Dendrites and axons

Cerebrum and cerebellum

Standard answers to three exercise questions

The exercise also asks about impulse conduction in myelinated and unmyelinated fibres, about cranial and spinal nerves, and about the most developed part of the human brain. The standard answers are:

• In a myelinated fibre the impulse jumps from one node of Ranvier to the next, so conduction is faster. In an unmyelinated fibre the impulse travels along the whole length of the membrane, so conduction is slower.
• Cranial nerves arise from the brain, and there are 12 pairs of them. Spinal nerves arise from the spinal cord, and there are 31 pairs of them.
• The most developed part of the human brain is the cerebrum, which forms the major part of the brain.

Glossary

• Coordination — The process through which two or more organs interact and complement the functions of one another.
• Neuron — A highly specialised cell of the neural system that can detect, receive and transmit different kinds of stimuli.
• Afferent fibres — Nerve fibres of the PNS that transmit impulses from tissues and organs to the CNS.
• Efferent fibres — Nerve fibres of the PNS that transmit regulatory impulses from the CNS to peripheral tissues and organs.
• Nissl's granules — Granular bodies found in the cytoplasm of the cell body and in the dendrites of a neuron.
• Synaptic knob — The bulb-like end of an axon branch, which possesses synaptic vesicles containing neurotransmitters.
• Nodes of Ranvier — The gaps between two adjacent myelin sheaths on a myelinated nerve fibre.
• Resting potential — The electrical potential difference across the resting plasma membrane of a neuron.
• Action potential — The electrical potential difference across the plasma membrane at a stimulated site; it is termed a nerve impulse.
• Synaptic cleft — The fluid-filled space that separates the membranes of the pre- and post-synaptic neurons at a chemical synapse.
• Neurotransmitters — Chemicals involved in the transmission of impulses at chemical synapses, stored in synaptic vesicles.
• Cranial meninges — The three coverings of the brain inside the skull: dura mater, arachnoid and pia mater.
• Corpus callosum — The tract of nerve fibres that connects the left and right cerebral hemispheres.
• Brain stem — The midbrain, pons and medulla oblongata together; it connects the brain and the spinal cord.

Common errors and misconceptions

• Misconception: The resting membrane is more permeable to Na⁺. Correct: At rest the membrane is comparatively more permeable to K⁺ and nearly impermeable to Na⁺.
• Misconception: The sodium-potassium pump moves 2 Na⁺ out and 3 K⁺ in. Correct: It transports 3 Na⁺ outwards for 2 K⁺ into the cell.
• Misconception: In a resting neuron the inside of the membrane is positive. Correct: At rest the outer surface is positive and the inner surface is negative. This is reversed only at a stimulated site.
• Misconception: Depolarisation is caused by K⁺ leaving the axon. Correct: Depolarisation is caused by a rapid influx of Na⁺. The outward diffusion of K⁺ restores the resting potential.
• Misconception: Chemical synapses are faster than electrical synapses. Correct: Impulse transmission across an electrical synapse is always faster, but electrical synapses are rare in our system.
• Misconception: Dendrites carry impulses away from the cell body. Correct: Dendrites transmit impulses towards the cell body. The axon transmits them away from it.
• Misconception: The grey matter is grey because of the myelin sheath. Correct: The grey matter is grey because neuron cell bodies are concentrated there. Myelin gives the white matter its white appearance.
• Misconception: The cerebellum is part of the brain stem. Correct: The brain stem is made up of the midbrain, pons and medulla oblongata. The cerebellum is part of the hindbrain.

Exam-style questions with model answers

Q1. Name the three layers of the cranial meninges, from the outside inwards. [1 mark]

1. The cranial meninges are the dura mater (outer layer), the arachnoid (a very thin middle layer) and the pia mater (inner layer, in contact with the brain tissue).

Q2. Distinguish between afferent and efferent nerve fibres. [2 marks]

1. Afferent nerve fibres transmit impulses from tissues and organs to the central neural system.
2. Efferent nerve fibres transmit regulatory impulses from the central neural system to the concerned peripheral tissues and organs.

Q3. What is the role of the sodium-potassium pump in a resting neuron? [2 marks]

1. The sodium-potassium pump maintains the ionic gradients across the resting membrane by active transport, carrying 3 Na⁺ outwards for 2 K⁺ into the cell.
2. As a result the outer surface of the axonal membrane is positively charged and the inner surface negatively charged, so the membrane stays polarised.

Q4. Differentiate between myelinated and unmyelinated nerve fibres. [3 marks]

1. Myelinated nerve fibres are enveloped with Schwann cells that form a myelin sheath around the axon. In unmyelinated fibres the Schwann cell encloses the fibre but does not form a myelin sheath.
2. Myelinated fibres have nodes of Ranvier, the gaps between two adjacent myelin sheaths. Unmyelinated fibres do not have them.
3. Myelinated fibres are found in spinal and cranial nerves. Unmyelinated fibres are commonly found in the autonomous and the somatic neural systems.

Q5. Explain the role of Na⁺ in the generation of an action potential. [3 marks]

1. At rest the axonal membrane is nearly impermeable to Na⁺, and the fluid outside the axon has a high concentration of Na⁺. The outer surface is positive and the inner surface negative.
2. When a stimulus is applied, the membrane at that site becomes freely permeable to Na⁺, and there is a rapid influx of Na⁺.
3. This reverses the polarity at the site: the outer surface becomes negative and the inner side positive. The membrane is depolarised, and the potential difference across it is the action potential, or nerve impulse.

Q6. Describe the three types of neurons based on the number of axons and dendrites. [3 marks]

1. Multipolar neurons have one axon and two or more dendrites, and are found in the cerebral cortex.
2. Bipolar neurons have one axon and one dendrite, and are found in the retina of the eye.
3. Unipolar neurons have a cell body with one axon only, and are found usually in the embryonic stage.

Q7. Explain the transmission of a nerve impulse across a chemical synapse. [5 marks]

1. At a chemical synapse the membranes of the pre- and post-synaptic neurons are separated by a fluid-filled space called the synaptic cleft. The axon terminals contain vesicles filled with neurotransmitters.
2. When an impulse (action potential) arrives at the axon terminal, it stimulates the movement of the synaptic vesicles towards the membrane.
3. The vesicles fuse with the plasma membrane and release their neurotransmitters into the synaptic cleft.
4. The neurotransmitters bind to their specific receptors on the post-synaptic membrane. This binding opens ion channels, allowing the entry of ions.
5. The entry of ions can generate a new potential in the post-synaptic neuron, which may be either excitatory or inhibitory.

Q8. Describe the structure of the human forebrain. [5 marks]

1. The forebrain consists of the cerebrum, thalamus and hypothalamus. The cerebrum forms the major part of the brain and is divided by a deep cleft into left and right cerebral hemispheres, connected by the corpus callosum.
2. The cerebral cortex covers the hemispheres and is thrown into folds. It is the grey matter, because neuron cell bodies are concentrated there. It contains motor areas, sensory areas and association areas.
3. The inner part of the hemisphere is the white matter, made of tracts of fibres covered with myelin sheath.
4. The cerebrum wraps around the thalamus, a major coordinating centre for sensory and motor signalling. The hypothalamus, at the base of the thalamus, controls body temperature and the urge for eating and drinking, and secretes hypothalamic hormones.
5. The inner parts of the hemispheres, with deep structures like the amygdala and hippocampus, form the limbic system, which with the hypothalamus regulates sexual behaviour, emotional reactions and motivation.

Key takeaways

• The neural system gives quick, point-to-point coordination, and the endocrine system gives chemical integration through hormones; together they maintain homeostasis.
• The human neural system has a CNS of brain and spinal cord and a PNS of nerves, with afferent and efferent fibres.
• A neuron has a cell body, dendrites that carry impulses towards it and an axon that carries impulses away from it.
• At rest the membrane is more permeable to K⁺ than to Na⁺, and the sodium-potassium pump moves 3 Na⁺ out for 2 K⁺ in.
• A stimulus makes the membrane freely permeable to Na⁺; the influx of Na⁺ reverses the polarity and produces the action potential.
• At a chemical synapse, neurotransmitters released from synaptic vesicles cross the synaptic cleft and bind to receptors on the post-synaptic membrane.
• The forebrain has the cerebrum, thalamus and hypothalamus; the hindbrain has the pons, cerebellum and medulla.
• The medulla controls respiration, cardiovascular reflexes and gastric secretions; the midbrain, pons and medulla form the brain stem.

Test yourself

Which two systems jointly coordinate the activities of the organs?

The neural system and the endocrine system jointly coordinate and integrate all the activities of the organs.

What does the central neural system include?

The central neural system includes the brain and the spinal cord, and is the site of information processing and control.

Where are bipolar neurons found?

Bipolar neurons, which have one axon and one dendrite, are found in the retina of the eye.

How many ions does the sodium-potassium pump move in each direction?

The sodium-potassium pump transports 3 Na⁺ outwards for 2 K⁺ into the cell.

Which ion restores the resting potential after an impulse has passed?

K⁺ diffuses outside the membrane within a fraction of a second and restores the resting potential at the site of excitation.

Which type of synapse transmits impulses faster?

Impulse transmission across an electrical synapse is always faster than across a chemical synapse.

What connects the two cerebral hemispheres?

The two cerebral hemispheres are connected by a tract of nerve fibres called the corpus callosum.

Which part of the brain controls respiration and cardiovascular reflexes?

The medulla contains centres which control respiration, cardiovascular reflexes and gastric secretions.