**Coordination** is the process through which two or more organs interact and complement each other's functions to maintain **homeostasis**. During physical exercise, for example, muscles demand increased energy, requiring greater oxygen supply, which necessitates increased respiration rate, heart beat, and blood flow. When exercise stops, all these systems gradually return to normal. This synchronised functioning of multiple organs is achieved through two major systems: the **neural system** (provides quick, point-to-point connections) and the **endocrine system** (provides chemical integration through hormones).
The neural system is composed of highly specialised cells called **neurons** that detect, receive, and transmit different kinds of stimuli.
**Evolution of neural organisation across animal groups:**
The human neural system is divided into two major parts:
**A. Central Neural System (CNS)**
**B. Peripheral Neural System (PNS)**
**Classification of PNS:**
**1. Somatic Neural System**
**2. Autonomic Neural System**
**Visceral Nervous System:** Part of PNS comprising nerves, fibres, ganglia, and plexuses by which impulses travel from CNS to viscera and vice versa.
A **neuron** is a microscopic cell specialised for transmission of nerve impulses, composed of three major parts:
**Structure of Neuron:**
**1. Cell Body (Soma)**
**2. Dendrites**
**3. Axon**
**Classification of Neurons Based on Number of Processes:**
**1. Multipolar Neurons**
**2. Bipolar Neurons**
**3. Unipolar Neurons**
**Classification of Axons Based on Myelination:**
**1. Myelinated Axons**
**2. Non-myelinated Axons**
**Definition:** The electrical potential difference across the resting plasma membrane of a neuron at rest is called **resting potential** (approximately **-70 mV**).
**Ionic Basis of Resting Potential:**
**Ion Distribution Across Membrane (at rest):**
**This creates concentration gradients maintained by:**
**Result of Ion Distribution:**
**Process:**
**Step 1: Stimulus Application**
**Step 2: Rapid Na⁺ Influx**
**Step 3: Polarity Reversal**
**Step 4: Action Potential**
**Conduction of Impulse Along Axon:**
Once impulse generated at site A:
**Local Current Flow:**
**Depolarisation of Adjacent Site:**
**Sequence of Conduction:**
**Repolarisation:**
**Synapse Definition:** A synapse is a junction formed by membranes of a pre-synaptic neuron and a post-synaptic neuron, separated or not separated by a gap.
**Types of Synapses:**
**Structure:**
**Mechanism:**
**Characteristics:**
**Structure:**
**Mechanism of Synaptic Transmission (Step-by-step):**
**Step 1: Arrival of Action Potential**
**Step 2: Depolarisation and Ca²⁺ Influx**
**Step 3: Vesicle Movement and Fusion**
**Step 4: Neurotransmitter Release**
**Step 5: Neurotransmitter Binding**
**Step 6: Ion Channel Opening**
**Step 7: Potential Generation in Post-synaptic Neuron**
**Step 8: Neurotransmitter Degradation and Reuptake**
**Characteristics of Chemical Synaptic Transmission:**
**Functions:**
**Protection of Brain:**
**Cranial Meninges** (three layers surrounding brain):
1. **Dura Mater:** Outer, tough, thick layer; protects brain from injury
2. **Arachnoid:** Very thin middle layer; web-like structure
3. **Pia Mater:** Inner layer in direct contact with brain tissue; highly vascularised
**Cerebrospinal Fluid (CSF):** Fills space between meninges; provides cushioning and nutrient supply
Brain divided into three major parts:
**Components:** Cerebrum, thalamus, and hypothalamus
**Structure:**
**Cerebral Cortex (Outer Layer):**
**White Matter (Inner Part):**
**Location:** Wraps around by cerebrum
**Functions:**
**Location:** Base of thalamus, forms floor of third ventricle
**Functions:**
**Components:** Inner parts of cerebral hemispheres plus associated deep structures including:
**Functions (with hypothalamus):**
**Location:** Between thalamus/hypothalamus of forebrain and pons of hindbrain
**Structures:**
**Functions:**
**Components:** Pons, cerebellum, and medulla oblongata
**Structure:** Bridge-like structure
**Composition:** Mainly fibre tracts (white matter)
**Functions:**
**Structure:**
**Internal Structure:**
**Functions:**
**Location:** Connected to spinal cord; forms base of brain stem
**Structure:** Continuation of spinal cord with enlargement
**Important Centres (Nuclei) in Medulla:**
**1. Respiratory Centre**
**2. Cardiovascular Reflexes Centre**
**3. Gastric Secretion Centre**
**Other Functions:**
**Definition:** Forms connections between brain and spinal cord
**Components:** Three parts:
1. **Midbrain**
2. **Pons**
3. **Medulla oblongata**
**Functions:**
| Brain Region | Primary Functions |
|---|---|
| **Cerebral Cortex** | Voluntary movement, sensation, memory, thought, language, emotion |
| **Thalamus** | Sensory and motor relay station; pain perception |
| **Hypothalamus** | Temperature, hunger, thirst, hormones, autonomic regulation |
| **Midbrain** | Visual and auditory reflexes; integrates sensory information |
| **Cerebellum** | Coordination, balance, muscle tone; refines movements |
| **Pons** | Interconnects brain regions; respiratory nuclei |
| **Medulla** | Vital reflexes (respiration, heart rate, blood pressure, digestion) |
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**Definitions to memorise:**
**Process to explain:**
**Comparisons frequently asked:**
**Diagrams essential for board exam:**
**Most asked questions:**
Q1. The Nissl's granules are found in which parts of a neuron?
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?
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:
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?
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?
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?
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?
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:
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:
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?
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.
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.
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.
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