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Neural Control and Coordination

NCERT Class 11 · Biology Based on NCERT Class 11 Biology textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explains how the neural system coordinates and integrates the functions of all organs to maintain homeostasis. It covers the structure and function of neurons, the generation and conduction of nerve impulses, and the transmission of impulses across synapses. The central and peripheral neural systems are described in detail, with a focus on the human brain's anatomy and functions. The one big idea is that the neural system, through electrical and chemical signaling, enables rapid, point-to-point coordination of body activities.
Key Concepts (47)
Neural System
The system composed of highly specialised cells called neurons which can detect, receive and transmit different kinds of stimuli.
e.g. When you touch a hot object, the neural system instantly sends a signal from your hand to your brain, and then back to your hand to pull away.
Central Neural System (CNS)
The part of the neural system that includes the brain and the spinal cord and is the site of information processing and control.
e.g. When you decide to write an answer, your brain (CNS) processes the thought and sends commands via the spinal cord.
Peripheral Neural System (PNS)
The part of the neural system comprising all the nerves of the body associated with the CNS (brain and spinal cord).
e.g. The nerves in your arm that carry signals from your spinal cord to your muscles are part of the PNS.
Afferent fibres
Nerve fibres that transmit impulses from tissues/organs to the CNS.
e.g. When you smell food, afferent fibres carry the signal from your nose to your brain.
Efferent fibres
Nerve fibres that transmit regulatory impulses from the CNS to the concerned peripheral tissues/organs.
e.g. When your brain decides to move your hand, efferent fibres carry the command from the CNS to the hand muscles.
Somatic neural system
The division of the PNS that relays impulses from the CNS to skeletal muscles.
e.g. When you voluntarily kick a football, the somatic neural system is at work.
Autonomic neural system
The division of the PNS that transmits impulses from the CNS to the involuntary organs and smooth muscles of the body.
e.g. The autonomic neural system controls your heartbeat and digestion without you thinking about it.
Sympathetic neural system
A division of the autonomic neural system that prepares the body for stress or emergency (fight or flight).
e.g. When you are scared, the sympathetic system increases your heart rate and dilates your pupils.
Parasympathetic neural system
A division of the autonomic neural system that calms the body and conserves energy (rest and digest).
e.g. After a meal, the parasympathetic system slows your heart rate and stimulates digestion.
Neuron
A microscopic structure composed of a cell body, dendrites, and axon; it is the structural and functional unit of the neural system.
e.g. A single neuron in your brain can receive signals from thousands of other neurons through its dendrites.
Nissl’s granules
Granular bodies present in the cytoplasm of the cell body and dendrites of a neuron.
e.g. Under a microscope, Nissl's granules appear as dark spots in the neuron's cell body.
Dendrites
Short fibres which branch repeatedly and project out of the cell body; they transmit impulses towards the cell body.
e.g. Dendrites of a neuron in your ear receive sound vibrations and carry the signal to the cell body.
Axon
A long fibre, the distal end of which is branched; it transmits nerve impulses away from the cell body to a synapse or neuro-muscular junction.
e.g. The axon of a motor neuron in your spinal cord can be over a metre long, reaching your foot muscles.
Synaptic knob
A bulb-like structure at the end of each branch of the axon, containing synaptic vesicles with neurotransmitters.
e.g. When a nerve impulse reaches the synaptic knob, it releases chemicals to signal the next neuron.
Neurotransmitters
Chemicals contained in synaptic vesicles that are involved in the transmission of impulses at chemical synapses.
e.g. Acetylcholine is a neurotransmitter that helps transmit signals from nerves to muscles.
Multipolar neuron
A neuron with one axon and two or more dendrites; found in the cerebral cortex.
e.g. Most neurons in your brain are multipolar, allowing them to integrate many signals.
Bipolar neuron
A neuron with one axon and one dendrite; found in the retina of the eye.
e.g. Bipolar cells in your retina help transmit visual signals from photoreceptors to ganglion cells.
Unipolar neuron
A neuron with a cell body having one axon only; found usually in the embryonic stage.
e.g. Unipolar neurons are common in the developing nervous system of a foetus.
Myelinated nerve fibres
Nerve fibres enveloped with Schwann cells that form a myelin sheath around the axon; found in spinal and cranial nerves.
e.g. The nerves that carry signals from your spinal cord to your leg muscles are myelinated for fast conduction.
Nodes of Ranvier
The gaps between two adjacent myelin sheaths on a myelinated axon.
e.g. In a myelinated nerve, the impulse jumps from one node of Ranvier to the next, speeding up conduction.
Unmyelinated nerve fibre
A nerve fibre enclosed by a Schwann cell that does not form a myelin sheath around the axon; found in autonomic and somatic neural systems.
e.g. The nerves that control your digestive organs are often unmyelinated, conducting impulses more slowly.
Resting potential
The electrical potential difference across the resting plasma membrane of a neuron when it is not conducting any impulse.
e.g. A typical resting potential is about -70 millivolts, with the inside negative relative to the outside.
Action potential
The electrical potential difference across the plasma membrane at a site of excitation, also termed a nerve impulse.
e.g. When a neuron fires, the action potential rapidly reverses the membrane polarity from -70 mV to +40 mV.
Depolarisation
The reversal of polarity of the membrane at a stimulated site, where the outer surface becomes negatively charged and the inner side becomes positively charged.
e.g. Depolarisation occurs when sodium ions rush into the axon, making the inside positive.
Synapse
A junction 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 synaptic cleft.
e.g. The synapse between a motor neuron and a muscle fibre is called a neuromuscular junction.
Synaptic cleft
A fluid-filled space that separates the membranes of pre- and post-synaptic neurons at a chemical synapse.
e.g. Neurotransmitters diffuse across the synaptic cleft to bind to receptors on the post-synaptic membrane.
Electrical synapse
A synapse where the membranes of pre- and post-synaptic neurons are in very close proximity, allowing direct electrical current flow.
e.g. Electrical synapses are found in the heart, allowing rapid spread of excitation for coordinated contraction.
Chemical synapse
A synapse where the membranes of pre- and post-synaptic neurons are separated by a synaptic cleft, and transmission occurs via neurotransmitters.
e.g. Most synapses in the human brain are chemical synapses, allowing for modulation of signals.
Brain
The central information processing organ of the body, acting as the command and control system.
e.g. Your brain processes everything you see, hear, feel, and think, and controls your actions.
Cranial meninges
Three layers covering the brain: dura mater (outer), arachnoid (middle), and pia mater (inner).
e.g. Meningitis is an infection of the meninges, the protective layers around the brain.
Forebrain
The part of the brain consisting of the cerebrum, thalamus, and hypothalamus.
e.g. The forebrain is responsible for conscious thought, sensory processing, and voluntary movement.
Cerebrum
The major part of the human brain, divided longitudinally into two cerebral hemispheres connected by the corpus callosum.
e.g. The cerebrum's outer layer, the cerebral cortex, is responsible for language, memory, and reasoning.
Corpus callosum
A tract of nerve fibres that connects the left and right cerebral hemispheres.
e.g. The corpus callosum allows the two hemispheres of the brain to communicate with each other.
Cerebral cortex
The layer of cells covering the cerebral hemisphere, also called grey matter due to its greyish appearance.
e.g. The cerebral cortex is where most information processing occurs in the brain.
Grey matter
The region of the brain where neuron cell bodies are concentrated, giving a greyish appearance.
e.g. The cerebral cortex is grey matter, while the inner part of the cerebrum is white matter.
White matter
The inner part of the cerebral hemisphere, composed of myelinated nerve fibres, giving an opaque white appearance.
e.g. White matter consists of axons that connect different parts of the brain.
Thalamus
A structure in the forebrain that is a major coordinating centre for sensory and motor signaling.
e.g. All sensory information (except smell) passes through the thalamus before reaching the cerebral cortex.
Hypothalamus
A part of the forebrain at the base of the thalamus that controls body temperature, urge for eating and drinking, and secretes hypothalamic hormones.
e.g. When you are cold, the hypothalamus triggers shivering to generate heat.
Limbic system
A complex structure formed by the inner parts of cerebral hemispheres and associated deep structures like amygdala and hippocampus; involved in regulation of sexual behaviour, emotional reactions, and motivation.
e.g. The limbic system is responsible for feelings of pleasure, fear, and anger.
Midbrain
The part of the brain located between the thalamus/hypothalamus of the forebrain and pons of the hindbrain; contains the cerebral aqueduct and corpora quadrigemina.
e.g. The midbrain processes visual and auditory information and controls eye movement.
Cerebral aqueduct
A canal that passes through the midbrain.
e.g. Cerebrospinal fluid flows through the cerebral aqueduct from the third to the fourth ventricle.
Corpora quadrigemina
Four round swellings (lobes) on the dorsal portion of the midbrain.
e.g. The corpora quadrigemina are involved in visual and auditory reflexes.
Hindbrain
The part of the brain comprising the pons, cerebellum, and medulla (medulla oblongata).
e.g. The hindbrain controls vital functions like breathing, heart rate, and balance.
Pons
A part of the hindbrain consisting of fibre tracts that interconnect different regions of the brain.
e.g. The pons acts as a bridge between the cerebellum and the rest of the brain.
Cerebellum
A part of the hindbrain with a very convoluted surface, involved in coordinating voluntary movements and balance.
e.g. When you walk on a narrow beam, your cerebellum helps you maintain balance.
Medulla oblongata
The part of the hindbrain connected to the spinal cord; contains centres that control respiration, cardiovascular reflexes, and gastric secretions.
e.g. The medulla automatically adjusts your breathing rate when you exercise.
Brain stem
The region comprising the midbrain, pons, and medulla oblongata; forms the connections between the brain and spinal cord.
e.g. Damage to the brain stem can be fatal because it controls basic life functions.
Formulas & Equations (1)
3 Na+ outwards for 2 K+ into the cell (via sodium-potassium pump)
Reads as: Three sodium ions are pumped out for every two potassium ions pumped in
Variables: Na+ = sodium ions; K+ = potassium ions
Use when: Explaining how the resting potential is maintained in a neuron
⚠ Students often write the ratio as 2 Na+ out for 3 K+ in, which is the reverse of the correct ratio.
How It Works
Generation and Conduction of Nerve Impulse
  1. Step 1 — At rest, the axonal membrane is polarised: more permeable to K+ and nearly impermeable to Na+, with high K+ and low Na+ inside, and low K+ and high Na+ outside.
  2. Step 2 — The sodium-potassium pump actively transports 3 Na+ outwards for 2 K+ into the cell, maintaining the ionic gradient.
  3. Step 3 — A stimulus is applied at a site (e.g., point A) on the polarised membrane, making it freely permeable to Na+.
  4. Step 4 — Rapid influx of Na+ occurs, reversing the polarity at that site: the outer surface becomes negatively charged and the inner side becomes positively charged (depolarisation).
  5. Step 5 — The electrical potential difference at the excited site is called the action potential (nerve impulse).
  6. Step 6 — A current flows on the inner surface from site A to site B, and on the outer surface from site B to site A, causing depolarisation at site B.
  7. Step 7 — The sequence repeats along the length of the axon, conducting the impulse.
  8. Step 8 — The rise in Na+ permeability is short-lived, followed by a rise in K+ permeability; K+ diffuses out, restoring the resting potential (repolarisation).
💡 Students often forget that the sodium-potassium pump maintains the resting potential, not the action potential. Also, the ratio is 3 Na+ out for 2 K+ in.
Transmission of Impulse Across a Chemical Synapse
  1. Step 1 — An action potential arrives at the axon terminal (synaptic knob) of the pre-synaptic neuron.
  2. Step 2 — The arrival of the impulse stimulates the movement of synaptic vesicles towards the pre-synaptic membrane.
  3. Step 3 — The vesicles fuse with the plasma membrane and release their neurotransmitters into the synaptic cleft.
  4. Step 4 — The neurotransmitters diffuse across the synaptic cleft and bind to specific receptors on the post-synaptic membrane.
  5. Step 5 — Binding of neurotransmitters opens ion channels on the post-synaptic membrane, allowing entry of ions.
  6. Step 6 — This generates a new potential in the post-synaptic neuron, which may be excitatory or inhibitory.
💡 Students often think neurotransmitters directly open ion channels; actually, they bind to receptors which then open ion channels. Also, transmission is one-way at chemical synapses.
Transmission of Impulse Across an Electrical Synapse
  1. Step 1 — The membranes of pre- and post-synaptic neurons are in very close proximity.
  2. Step 2 — Electrical current flows directly from one neuron into the other across the synapse.
  3. Step 3 — Transmission is very similar to impulse conduction along a single axon.
  4. Step 4 — Impulse transmission is always faster than across a chemical synapse.
💡 Electrical synapses are rare in the human system. Students often forget to mention that they allow direct current flow and are faster than chemical synapses.
Exam Traps — Don't Make These Mistakes
⚠ Confusing afferent and efferent fibres
✗ Afferent fibres carry impulses from CNS to organs, and efferent fibres carry impulses from organs to CNS.
→ This is the exact opposite of the correct definitions. Afferent means 'carrying towards' the CNS, and efferent means 'carrying away from' the CNS.
✓ Afferent fibres transmit impulses from tissues/organs to the CNS. Efferent fibres transmit regulatory impulses from the CNS to the peripheral tissues/organs.
⚠ Mixing up depolarisation and repolarisation
✗ During depolarisation, K+ enters the axon and the inside becomes negative.
→ During depolarisation, Na+ enters the axon (not K+), making the inside positive. Repolarisation involves K+ leaving the axon.
✓ During depolarisation, the membrane becomes freely permeable to Na+, leading to rapid influx of Na+ and reversal of polarity (inside becomes positive). During repolarisation, K+ diffuses out, restoring the resting potential.
⚠ Forgetting the role of the sodium-potassium pump
✗ The resting potential is maintained by passive diffusion of ions.
→ Passive diffusion alone cannot maintain the concentration gradient; the sodium-potassium pump actively transports 3 Na+ out for 2 K+ in, using ATP.
✓ The resting potential is maintained by the active transport of ions by the sodium-potassium pump, which transports 3 Na+ outwards for 2 K+ into the cell, creating and maintaining the ionic gradient.
⚠ Confusing the functions of the thalamus and hypothalamus
✗ The thalamus controls body temperature and hunger, while the hypothalamus is a sensory relay centre.
→ These functions are reversed. The thalamus is a major coordinating centre for sensory and motor signaling, while the hypothalamus controls body temperature, hunger, and thirst.
✓ The thalamus is a major coordinating centre for sensory and motor signaling. The hypothalamus controls body temperature, urge for eating and drinking, and secretes hypothalamic hormones.
⚠ Misidentifying the parts of the brain stem
✗ The brain stem consists of the cerebrum, cerebellum, and medulla.
→ The brain stem does not include the cerebrum or cerebellum. It consists of the midbrain, pons, and medulla oblongata.
✓ The brain stem comprises three major regions: the midbrain, pons, and medulla oblongata. It forms the connections between the brain and spinal cord.
⚠ Thinking all synapses are chemical
✗ All synapses in the human body are chemical synapses.
→ There are two types of synapses: electrical and chemical. Electrical synapses are rare but do exist in the human system.
✓ There are two types of synapses: electrical synapses (where membranes are in close proximity and current flows directly) and chemical synapses (where a synaptic cleft is present and neurotransmitters are involved).
Compare & Contrast
Composition and function
Central Neural System (CNS)
Peripheral Neural System (PNS)
The CNS includes the brain and spinal cord and is the site of information processing and control; the PNS comprises all nerves of the body associated with the CNS and relays impulses to and from the CNS.
Presence of myelin sheath
Myelinated nerve fibres
Unmyelinated nerve fibres
Myelinated fibres are enveloped by Schwann cells forming a myelin sheath with nodes of Ranvier, found in spinal and cranial nerves; unmyelinated fibres are enclosed by a Schwann cell that does not form a myelin sheath, found in autonomic and somatic neural systems.
Membrane state and ion movement
Resting potential
Action potential
Resting potential is the polarised state of the membrane at rest (inside negative, outside positive) maintained by the sodium-potassium pump; action potential is the depolarised state during impulse conduction (inside positive, outside negative) caused by rapid Na+ influx.
Direction of impulse transmission
Dendrites
Axon
Dendrites transmit impulses towards the cell body; the axon transmits nerve impulses away from the cell body to a synapse or neuro-muscular junction.
Function
Thalamus
Hypothalamus
The thalamus is a major coordinating centre for sensory and motor signaling; the hypothalamus controls body temperature, urge for eating and drinking, and secretes hypothalamic hormones.
Location and function
Cerebrum
Cerebellum
The cerebrum forms the major part of the forebrain and is responsible for voluntary movements, sensory processing, memory, and intelligence; the cerebellum is part of the hindbrain and coordinates voluntary movements and balance.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 SA (2-3 marks), 1 LA (5 marks) — typically a diagram-based question on neuron or brain, or a detailed explanation of impulse conduction.
Most asked: Structure of a neuron and types of neurons · Generation and conduction of nerve impulse (resting potential, action potential, depolarisation, repolarisation) · Transmission of impulses across synapses (chemical and electrical) · Divisions of the human neural system (CNS, PNS, somatic, autonomic, sympathetic, parasympathetic) · Parts of the brain (forebrain, midbrain, hindbrain) and their functions
Don't skip: Draw a labelled diagram of a neuron and explain the generation and conduction of a nerve impulse.; Describe the structure and functions of the human brain with a labelled diagram.; Differentiate between chemical and electrical synapses, and explain the mechanism of synaptic transmission.; Explain the role of Na+ and K+ in the generation of action potential.
One-Line Revision (read aloud before the exam)
  1. The neural system coordinates and integrates functions of all organs to maintain homeostasis.
  2. The human neural system is divided into the central neural system (CNS) and the peripheral neural system (PNS).
  3. The CNS includes the brain and spinal cord; the PNS comprises all nerves of the body associated with the CNS.
  4. Afferent fibres carry impulses from tissues/organs to the CNS; efferent fibres carry impulses from the CNS to peripheral tissues/organs.
  5. The PNS is divided into the somatic neural system (to skeletal muscles) and the autonomic neural system (to involuntary organs).
  6. The autonomic neural system is further divided into the sympathetic and parasympathetic neural systems.
  7. A neuron has three parts: cell body, dendrites (transmit impulses towards cell body), and axon (transmits impulses away from cell body).
  8. Nissl's granules are granular bodies found in the cell body and dendrites of a neuron.
  9. Myelinated nerve fibres have a myelin sheath formed by Schwann cells and have nodes of Ranvier; unmyelinated fibres lack this sheath.
  10. The resting potential is the electrical potential difference across the resting membrane, maintained by the sodium-potassium pump (3 Na+ out for 2 K+ in).
  11. An action potential is generated when a stimulus causes rapid Na+ influx, reversing membrane polarity (depolarisation).
  12. Impulse conduction along an axon involves a wave of depolarisation followed by repolarisation due to K+ efflux.
  13. A synapse is a junction between a pre-synaptic and post-synaptic neuron, separated by a synaptic cleft in chemical synapses.
  14. At chemical synapses, neurotransmitters are released from synaptic vesicles and bind to receptors on the post-synaptic membrane.
  15. Electrical synapses allow direct current flow and are faster than chemical synapses but are rare in humans.
  16. The brain is protected by the skull and three layers of cranial meninges: dura mater, arachnoid, and pia mater.
  17. The forebrain consists of the cerebrum, thalamus, and hypothalamus.
  18. The cerebrum is divided into two hemispheres connected by the corpus callosum; its outer layer is the cerebral cortex (grey matter).
  19. The thalamus is a major coordinating centre for sensory and motor signaling.
  20. The hypothalamus controls body temperature, hunger, thirst, and secretes hypothalamic hormones.
  21. The limbic system (including amygdala and hippocampus) regulates sexual behaviour, emotional reactions, and motivation.
  22. The midbrain contains the cerebral aqueduct and corpora quadrigemina; it processes visual and auditory inputs.
  23. The hindbrain comprises the pons, cerebellum, and medulla oblongata.
  24. The cerebellum coordinates voluntary movements and balance.
  25. The medulla oblongata controls respiration, cardiovascular reflexes, and gastric secretions.
  26. The brain stem consists of the midbrain, pons, and medulla oblongata, connecting the brain to the spinal cord.

MCQs — 10 Questions with Answers

Q1. The Nissl's granules are found in which parts of a neuron?

  • A. Cell body and dendrites only ✓
  • B. Axon and synaptic knob only
  • C. Cell body, dendrites, and axon terminal
  • D. Synaptic vesicles only

Answer: A — Nissl's granules are granular bodies present in the cell body and also in the short branched dendrites, but not in the axon.

Q2. Which statement about resting potential is CORRECT?

  • A. The outer surface of the axonal membrane is negatively charged at rest
  • B. The inner surface of the axonal membrane is positively charged at rest ✓
  • C. The resting potential is maintained by passive diffusion of ions only
  • D. The typical value of resting potential is approximately +70 mV

Answer: B — At rest, the inner (intracellular) surface is negatively charged and the outer (extracellular) surface is positively charged, creating a potential difference of about -70 mV.

Q3. The sodium-potassium pump (Na+-K+ ATPase) transports ions in the ratio of:

  • A. 2 Na+ out for 1 K+ in
  • B. 3 Na+ out for 2 K+ in ✓
  • C. 1 Na+ out for 1 K+ in
  • D. 3 K+ out for 2 Na+ in

Answer: B — The Na+-K+ pump actively transports 3 sodium ions outward and 2 potassium ions inward using ATP energy.

Q4. When a stimulus is applied at point A on an axonal membrane, what is the FIRST event that occurs?

  • A. The membrane becomes freely permeable to K+ ions
  • B. The membrane becomes freely permeable to Na+ ions ✓
  • C. The inner surface becomes positively charged
  • D. Repolarisation begins immediately

Answer: B — Upon stimulation, sodium channels open and the membrane becomes freely permeable to Na+ ions, allowing rapid Na+ influx and depolarisation.

Q5. Which of the following is NOT a function of the peripheral nervous system?

  • A. Transmitting impulses from tissues to the CNS via afferent fibres
  • B. Processing information and making decisions about responses ✓
  • C. Transmitting regulatory impulses from CNS to organs via efferent fibres
  • D. Relaying impulses from CNS to skeletal muscles through the somatic system

Answer: B — Information processing and decision-making are functions of the CNS (brain and spinal cord), not the PNS.

Q6. The nodes of Ranvier are gaps found in which type of nerve fibre?

  • A. Unmyelinated nerve fibres
  • B. Myelinated nerve fibres in the autonomic nervous system
  • C. Myelinated nerve fibres in spinal and cranial nerves ✓
  • D. All nerve fibres regardless of myelination status

Answer: C — Nodes of Ranvier are gaps between successive myelin sheaths on myelinated nerve fibres found in spinal and cranial nerves, facilitating saltatory conduction.

Q7. During repolarisation of an axonal membrane, which ion movement RESTORES the resting potential?

  • A. Na+ influx increases the negativity inside
  • B. K+ efflux removes positive charges from inside ✓
  • C. Cl- ions move into the axon
  • D. Protein ions are pumped out actively

Answer: B — During repolarisation, K+ channels open and K+ ions diffuse out of the axon, removing positive charges from inside and restoring the negative inner surface.

Q8. Based on the number of axons and dendrites, a multipolar neuron is correctly described as having:

  • A. One axon and one dendrite
  • B. One axon and two or more dendrites ✓
  • C. Multiple axons and multiple dendrites
  • D. One axon only with no dendrites

Answer: B — Multipolar neurons possess one axon and two or more dendrites and are found in structures like the cerebral cortex.

Q9. Assertion (A): Myelinated nerve fibres conduct impulses faster than unmyelinated fibres. Reason (R): Myelinated fibres have myelin sheath gaps called nodes of Ranvier that allow saltatory conduction. Choose the correct option:

  • A. Both A and R are correct; R is the correct explanation of A ✓
  • B. Both A and R are correct; R is not the correct explanation of A
  • C. A is correct but R is incorrect
  • D. Both A and R are incorrect

Answer: A — Myelinated fibres conduct faster due to saltatory conduction jumping between nodes of Ranvier, so both assertion and reason are correct and logically connected.

Q10. If the concentration of K+ inside the axon decreases due to experimental manipulation, what immediate effect would occur on the resting potential?

  • A. The resting potential would become more negative (hyperpolarised)
  • B. The resting potential would become less negative (depolarised) ✓
  • C. The resting potential would remain unchanged as Na+ would compensate
  • D. The resting potential would reverse to positive

Answer: B — Decreased intracellular K+ reduces the ionic gradient, making the membrane less negative inside relative to the normal -70 mV, thus depolarising the membrane.

Flashcards

What is the function of dendrites in a neuron?

Dendrites are short branched fibres that receive impulses and transmit them towards the cell body.

Define resting potential and state its typical value.

Resting potential is the electrical potential difference across the resting plasma membrane, with a value of approximately -70 mV (inner surface negative relative to outer surface).

How does the sodium-potassium pump maintain the resting potential?

The Na+-K+ pump actively transports 3 Na+ ions out and 2 K+ ions into the cell, creating and maintaining ionic concentration gradients across the axonal membrane.

What happens to the axonal membrane permeability when a stimulus is applied?

The membrane becomes freely permeable to Na+ ions, causing rapid influx of Na+ and depolarisation of the membrane at that site.

Define action potential and explain its relationship to nerve impulse.

Action potential is the electrical potential difference across the plasma membrane when it is depolarised; it is the actual nerve impulse that travels along the axon.

Distinguish between myelinated and unmyelinated nerve fibres.

Myelinated fibres are enveloped by Schwann cells forming a myelin sheath with nodes of Ranvier gaps, while unmyelinated fibres are enclosed by Schwann cells without myelin sheath formation.

What is the role of synaptic vesicles in a neuron?

Synaptic vesicles located in the synaptic knob contain neurotransmitter chemicals that are released to transmit impulses across a synapse.

Explain how impulse conduction occurs from site A to site B along an axon.

Depolarisation at site A creates current flow on the inner surface from A to B and outer surface from B to A, reversing polarity at B and generating action potential there.

What is the function of K+ efflux after Na+ influx during impulse conduction?

K+ efflux restores the resting potential by moving out of the axon, bringing the membrane back to its polarised state through repolarisation.

Compare the role of the somatic and autonomic neural systems in the PNS.

The somatic neural system relays impulses to skeletal muscles while the autonomic neural system transmits impulses to involuntary organs and smooth muscles.

Important Board Questions

Define resting potential. Why is the axonal membrane polarised in the resting state? [2 marks]

State that resting potential ≈ -70 mV is the potential difference across resting membrane. Explain Na+-K+ pump maintains ionic gradient: high K+ inside (negative proteins), high Na+ outside, creating outer positive and inner negative charge.

Explain the mechanism of impulse conduction from point A to point B along an axon during nerve signal transmission. What role do Na+ and K+ ions play in this process? [5 marks]

Step 1: Stimulus at A → Na+ channels open → Na+ influx → Depolarisation (polarity reversal at A). Step 2: Action potential at A creates current flow (inner A→B, outer B→A) → Point B depolarises. Step 3: K+ channels open → K+ efflux → Repolarisation restores resting potential. Include that this sequence repeats along the axon length.

Elaborate on how the human neural system coordinates organ functions during physical exercise. Describe the roles of the CNS, PNS divisions, and the ionic mechanisms involved in nerve impulse conduction that enable rapid coordination. [6 marks]

Part A: Explain CNS processes information, somatic PNS activates skeletal muscles, autonomic PNS increases heart rate and respiration. Part B: Detail how afferent fibres carry sensory input to CNS, efferent fibres carry commands to organs. Part C: Describe polarisation maintained by Na+-K+ pump, depolarisation via Na+ influx creating action potential, repolarisation via K+ efflux—this rapid electrical mechanism enables point-to-point quick coordination necessary during exercise.

Assertion-Reason Questions for Neural Control and Coordination

Q1. Assertion (A):

The axonal membrane of a resting neuron is more permeable to K+ ions than to Na+ ions.

Reason (R):

The selective permeability of the membrane is maintained by specific ion channels that control the movement of different ions across the membrane.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true, and the reason correctly explains why the assertion is true. The selective permeability of ion channels creates the differential permeability to K+ and Na+ at rest.

Q2. Assertion (A):

Synaptic knobs are located at the distal end of axon branches and contain neurotransmitters.

Reason (R):

Neurotransmitters in synaptic knobs are released into the synaptic cleft during repolarisation of the axon.

Answer: C — A is true but R is false — The assertion is true—synaptic knobs do contain neurotransmitters. However, the reason is false because neurotransmitters are released during depolarisation and the arrival of an action potential, not during repolarisation.

Q3. Assertion (A):

Myelinated nerve fibres conduct action potentials faster than unmyelinated nerve fibres.

Reason (R):

The myelin sheath insulates the axon and allows saltatory conduction, where depolarisation jumps between nodes of Ranvier.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true. The myelin sheath insulation enables saltatory conduction, which significantly increases the speed of impulse conduction compared to continuous conduction in unmyelinated fibres.

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Case-Based Questions (CBQ) for Neural Control and Coordination

Case Study:

A medical student observed a patient experiencing sudden paralysis in the lower limbs after a spinal cord injury. During examination, the doctor applied electrical stimulation to the skin below the injury site, and surprisingly, the muscles did not respond. However, when the doctor stimulated the muscles directly with an electrical current, the muscles contracted normally. This indicated that the sensory and motor pathways through the spinal cord were disrupted, but the muscles and motor nerves remained functionally intact.

Q1. Which part of the peripheral nervous system was most likely damaged in this patient?

  • A. The afferent nerves carrying sensory information
  • B. The central neural system in the spinal cord ✓
  • C. The synaptic knobs of motor neurons
  • D. The neuromuscular junctions below the injury site

Answer: B

Q2. Why did the muscles respond when stimulated directly but not when skin stimulation was applied below the injury?

  • A. The myelin sheath was damaged, preventing conduction
  • B. The spinal cord damage disrupted both afferent and efferent pathways, but peripheral nerves and muscles remained intact ✓
  • C. The sodium-potassium pumps were non-functional in the spinal cord
  • D. The synaptic vesicles in the spinal cord were depleted of neurotransmitters

Answer: B

Q3. This case demonstrates that the coordination of body activities depends on:

  • A. The integrity of only the peripheral nerves
  • B. Normal resting potential in the sensory receptors
  • C. The presence of myelin sheath on all axons
  • D. Proper functioning of the central neural system and intact communication pathways ✓

Answer: D

Q4. If this patient received treatment that restored conduction in the spinal cord, which neural pathway would need to be restored first for the patient to perceive sensation?

  • A. The efferent pathways from muscles to spinal cord
  • B. The pathways within the cerebral cortex
  • C. The afferent pathways from sensory receptors to the spinal cord ✓
  • D. The pathways in the autonomic nervous system

Answer: C

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