Neural Control and Coordination: NEET notes

1. Coordination

NCERT §18 (opening)

• Organs must be coordinated to keep homeostasis. Coordination is the process by which two or more organs interact and complement one another's functions.
• During physical exercise, working muscles need more energy and more oxygen. To supply it, the rate of breathing, the heart beat and blood flow through the vessels all rise.
• When exercise stops, the activities of nerves, lungs, heart and kidneys gradually return to normal. Muscles, lungs, heart, blood vessels and kidneys are thus coordinated during exercise.
• The neural system and the endocrine system together coordinate and integrate the activities of all organs so that they work in a synchronised way.
• The neural system gives an organised network of point-to-point connections for quick coordination; the endocrine system gives chemical integration through hormones.

2. Neural system in animals

NCERT §18.1

• In all animals the neural system is made of neurons, highly specialised cells that can detect, receive and transmit different kinds of stimuli.
• Lower invertebrates have a very simple arrangement: in Hydra the neural system is a network of neurons.
• Insects have a better organised system, with a brain, a number of ganglia and neural tissues.
• Vertebrates have a still more developed neural system.

3. Human neural system

NCERT §18.2

• The human neural system has two parts: the central neural system (CNS) and the peripheral neural system (PNS).
• The CNS is the brain and the spinal cord; it is where information is processed and control is exerted.
• The PNS is all the nerves of the body that are connected to the CNS. Its nerve fibres are of two kinds: afferent and efferent.
• Afferent fibres carry impulses from tissues and organs to the CNS; efferent fibres carry regulating impulses from the CNS out to the tissues and organs concerned.
• The PNS has two divisions. The somatic neural system carries impulses from the CNS to skeletal muscles; the autonomic neural system carries impulses from the CNS to involuntary organs and smooth muscles.
• The autonomic neural system is divided further into the sympathetic and the parasympathetic neural systems.
• The visceral nervous system is the part of the PNS made of all the nerves, fibres, ganglia and plexuses that carry impulses from the CNS to the viscera and from the viscera back to the CNS.

4. Structure of a neuron

NCERT §18.3

• A neuron is microscopic and has three main parts: the cell body, the dendrites and the axon.
• The cell body holds cytoplasm with the usual organelles and granular bodies called Nissl's granules.
• Dendrites are short, repeatedly branched fibres that project from the cell body; they also contain Nissl's granules and carry impulses towards the cell body.
• The axon is a long fibre whose far end branches. Each branch ends in a bulb-like synaptic knob holding synaptic vesicles filled with neurotransmitters.
• Axons carry impulses away from the cell body, either to another neuron at a synapse or to a muscle at a neuromuscular junction.
• By the number of axons and dendrites: multipolar neurons (one axon, two or more dendrites) are in the cerebral cortex; bipolar neurons (one axon, one dendrite) are in the retina; unipolar neurons (cell body with one axon only) are usually found in the embryonic stage.
• Axons are myelinated or non-myelinated. In myelinated fibres, Schwann cells form a myelin sheath round the axon; the gaps between neighbouring sheaths are the nodes of Ranvier. Myelinated fibres are found in spinal and cranial nerves.
• In a non-myelinated fibre, a Schwann cell encloses the axon without wrapping a myelin sheath round it; such fibres are common in the autonomous and somatic neural systems.

5. Resting potential

NCERT §18.3.1

• Neurons are excitable because their membranes are polarised. The neural membrane carries different ion channels, each selectively permeable to particular ions.
• At rest, the axonal membrane is fairly permeable to K⁺, nearly impermeable to Na⁺, and impermeable to the negatively charged proteins of the axoplasm.
• So the axoplasm has a high concentration of K⁺ and negative proteins and a low concentration of Na⁺, while the fluid outside has low K⁺ and high Na⁺: a concentration gradient.
• The sodium-potassium pump maintains these gradients by active transport, moving 3 Na⁺ out of the cell for every 2 K⁺ it brings in.
• The result is a positive charge on the outside of the axonal membrane and a negative charge on its inner face: the membrane is polarised.
• This potential difference across the membrane of a resting neuron is called the resting potential.

6. Nerve impulse

NCERT §18.3.1

• When a stimulus hits a site on the polarised membrane (site A), the membrane there becomes freely permeable to Na⁺.
• Na⁺ rushes in, and the polarity at that site reverses: the outer surface turns negative and the inner surface positive. The site is depolarised.
• The potential difference across the membrane at site A is the action potential; this is the nerve impulse.
• Just ahead, at site B, the membrane is still positive outside and negative inside. So current flows along the inner surface from A to B, and along the outer surface from B to A, completing the circuit.
• This reverses the polarity at B and an action potential forms there: the impulse has moved from A to B. Repeated along the axon, the impulse is conducted.
• The rise in Na⁺ permeability lasts only a very short time. It is quickly followed by a rise in K⁺ permeability; within a fraction of a second K⁺ diffuses out, restoring the resting potential at the site, which can then respond to a new stimulus.
• The impulse is thus a wave of depolarisation followed by repolarisation travelling along the axon membrane.

7. Transmission of impulses

NCERT §18.3.2

• Impulses pass from neuron to neuron at junctions called synapses. Each synapse is built from two membranes, one of the pre-synaptic neuron and one of the post-synaptic neuron; a gap, the synaptic cleft, may or may not lie between them.
• There are two types of synapse: electrical and chemical.
• At an electrical synapse the two membranes lie very close together and current flows directly from one neuron into the next. Transmission is much like conduction along a single axon, and is always faster than at a chemical synapse. Our body has few electrical synapses.
• At a chemical synapse the membranes are separated by a fluid-filled synaptic cleft, and chemicals called neurotransmitters carry the signal across. The axon terminals contain vesicles filled with neurotransmitter.
• When an impulse arrives at the axon terminal, it makes the synaptic vesicles move to the membrane, fuse with it and release neurotransmitter into the cleft.
• The neurotransmitter binds specific receptors on the post-synaptic membrane. This opens ion channels; the ions that enter can set up a fresh potential in the next neuron.
• The new potential may be excitatory or inhibitory.

8. The brain

NCERT §18.4

• The brain is the body's central information-processing organ, its 'command and control system'.
• It controls voluntary movements, balance, the working of vital involuntary organs such as the lungs, heart and kidneys, thermoregulation, hunger and thirst, circadian (24-hour) rhythms, the activity of several endocrine glands, and behaviour.
• It is also where vision, hearing, speech, memory, intelligence, emotions and thoughts are processed.
• The skull protects the brain. Inside the skull, the cranial meninges cover it in three layers: the outer dura mater, a very thin middle arachnoid, and the inner pia mater, which touches the brain tissue.
• The brain has three major parts: the forebrain, the midbrain and the hindbrain.

9. Forebrain

NCERT §18.4.1

• The forebrain consists of the cerebrum, the thalamus and the hypothalamus. The cerebrum forms the major part of the human brain.
• A deep cleft divides the cerebrum lengthwise into the left and right cerebral hemispheres, which are joined by a tract of nerve fibres, the corpus callosum.
• The cell layer covering each hemisphere is the cerebral cortex. It is thrown into prominent folds and is called grey matter because neuron cell bodies are concentrated there.
• The cortex has motor areas, sensory areas, and large association areas that are neither clearly sensory nor motor; these handle complex functions such as intersensory associations, memory and communication.
• Myelin-covered tract fibres form the inner part of each hemisphere; their opaque white look gives this layer the name white matter.
• The cerebrum wraps round the thalamus, a major coordinating centre for sensory and motor signalling.
• The hypothalamus lies at the base of the thalamus. Its centres control body temperature and the urge to eat and drink, and its neurosecretory cells secrete hypothalamic hormones.
• The inner parts of the hemispheres with deep structures such as the amygdala and hippocampus form the limbic lobe or limbic system. With the hypothalamus it regulates sexual behaviour, the expression of emotions (excitement, pleasure, rage, fear) and motivation. The chapter summary also links the limbic system with smell (olfaction) and autonomic responses.

10. Midbrain and hindbrain

NCERT §18.4.2–18.4.3

• The midbrain lies between the thalamus and hypothalamus of the forebrain and the pons of the hindbrain. A canal, the cerebral aqueduct, runs through it.
• The dorsal part of the midbrain is mainly four round swellings (lobes), the corpora quadrigemina. The chapter summary adds that the midbrain takes in and integrates what we see, hear and touch.
• The hindbrain comprises the pons, the cerebellum and the medulla (medulla oblongata).
• The pons is made of fibre tracts that link different regions of the brain.
• The cerebellum has a very convoluted surface, which gives room for many more neurons. The summary adds that it integrates signals arriving from the ear's semicircular canals and from the auditory system.
• The medulla is connected to the spinal cord and contains centres that control respiration, cardiovascular reflexes and gastric secretions.
• The midbrain, pons and medulla oblongata together make up the brain stem, which connects the brain with the spinal cord.

Must-know facts

1. Coordination: organs interacting and complementing each other; the neural system is quick and point-to-point, the endocrine system works through hormones.
2. Exercise raises breathing rate, heart beat and blood flow to meet the extra oxygen demand.
3. Hydra: network of neurons. Insects: brain plus ganglia. Vertebrates: most developed.
4. CNS = brain + spinal cord; PNS = all nerves joined to the CNS.
5. Afferent fibres carry impulses to the CNS; efferent fibres carry them from the CNS.
6. Somatic system: CNS to skeletal muscles. Autonomic system: CNS to involuntary organs and smooth muscles; split into sympathetic and parasympathetic.
7. Neuron = cell body (with Nissl's granules) + dendrites (towards the cell body) + axon (away from it), ending in synaptic knobs with vesicles.
8. Multipolar: cerebral cortex. Bipolar: retina. Unipolar: usually embryonic stage.
9. Myelin sheath is formed by Schwann cells; the gaps are nodes of Ranvier. Myelinated fibres in spinal and cranial nerves.
10. Resting axon: permeable to K⁺, nearly impermeable to Na⁺; inside high K⁺ and negative proteins, outside high Na⁺.
11. Sodium-potassium pump: 3 Na⁺ out for 2 K⁺ in. Outside positive, inside negative = polarised.
12. Stimulus: membrane freely permeable to Na⁺, Na⁺ rushes in, polarity reverses (depolarised) = action potential = nerve impulse.
13. Current flows inside from A to B and outside from B to A, so the impulse moves along.
14. Then K⁺ permeability rises, K⁺ diffuses out and the resting potential is restored.
15. Synapse: pre-synaptic + post-synaptic membranes, with or without a synaptic cleft.
16. Electrical synapses: membranes very close, current passes directly, always faster, rare in humans.
17. Chemical synapse: impulse → vesicles fuse → neurotransmitter into cleft → binds receptors → ion channels open → new potential, excitatory or inhibitory.
18. Meninges from outside in: dura mater, arachnoid, pia mater.
19. Forebrain: cerebrum, thalamus, hypothalamus. Hemispheres joined by corpus callosum.
20. Cortex = grey matter (cell bodies); inner myelinated tracts = white matter.
21. Hypothalamus: body temperature, eating, drinking; hypothalamic hormones.
22. Limbic system + hypothalamus: sexual behaviour, emotions, motivation.
23. Midbrain: cerebral aqueduct, corpora quadrigemina. Hindbrain: pons, cerebellum, medulla.
24. Medulla: respiration, cardiovascular reflexes, gastric secretions. Brain stem = midbrain + pons + medulla.

Common traps

Key terms

Coordination

The process by which two or more organs interact and complement each other's functions.

CNS

Central neural system: the brain and spinal cord, where information is processed and control is exerted.

PNS

Peripheral neural system: all the nerves of the body connected to the CNS.

Afferent fibres

Nerve fibres that carry impulses from tissues and organs to the CNS.

Efferent fibres

Nerve fibres that carry regulating impulses from the CNS to tissues and organs.

Somatic neural system

The PNS division relaying impulses from the CNS to skeletal muscles.

Autonomic neural system

The PNS division relaying impulses from the CNS to involuntary organs and smooth muscles; sympathetic and parasympathetic.

Visceral nervous system

The nerves, fibres, ganglia and plexuses carrying impulses between the CNS and the viscera in both directions.

Nissl's granules

Granular bodies in the cell body and dendrites of a neuron.

Dendrites

Short branched fibres that carry impulses towards the cell body.

Axon

The long fibre that carries impulses away from the cell body.

Synaptic knob

Bulb-like ending of an axon branch holding vesicles of neurotransmitter.

Myelin sheath

Covering formed round an axon by Schwann cells.

Nodes of Ranvier

Gaps between neighbouring myelin sheaths on an axon.

Resting potential

The electrical potential difference across the resting neural membrane.

Sodium-potassium pump

Active transport that moves 3 Na⁺ out for 2 K⁺ in, keeping the ion gradients.

Action potential

The potential difference across a depolarised site of the membrane; the nerve impulse.

Depolarisation

Reversal of membrane polarity (inside positive, outside negative) as Na⁺ enters.

Synapse

Junction formed by the membranes of a pre-synaptic and a post-synaptic neuron.

Synaptic cleft

The fluid-filled gap between the two membranes at a chemical synapse.

Neurotransmitter

Chemical released into the synaptic cleft that carries the impulse to the next neuron.

Meninges

The three coverings of the brain: dura mater, arachnoid and pia mater.

Corpus callosum

Tract of nerve fibres joining the left and right cerebral hemispheres.

Association areas

Cortex regions neither clearly sensory nor motor, handling memory, communication and intersensory links.

Hypothalamus

Forebrain part at the base of the thalamus controlling temperature, eating and drinking, and secreting hypothalamic hormones.

Limbic system

Inner hemisphere parts with amygdala and hippocampus; with the hypothalamus, governs emotions, sexual behaviour and motivation.

Corpora quadrigemina

Four round swellings on the dorsal side of the midbrain.

Brain stem

Midbrain, pons and medulla oblongata; links the brain with the spinal cord.