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Biology - The Nervous System (Brain, Spinal cord, Reflex action)

Grade 8ICSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The Nervous System is divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS consists of the Brain and the Spinal Cord.

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The basic unit of the nervous system is the Neuron (nerve cell), consisting of the Cyton (cell body), Dendrites, and a long Axon.

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Transmission of nerve impulses occurs through a junction called the Synapse, where chemical neurotransmitters like Acetylcholine facilitate the signal transfer.

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The Brain is protected by the cranium and three membranes called meninges, with Cerebrospinal Fluid (CSFCSF) acting as a shock absorber.

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The Cerebrum is the largest part of the brain, responsible for intelligence, memory, and voluntary actions.

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The Cerebellum (little brain) is located below the cerebrum and coordinates muscular activities and maintains body balance and posture.

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The Medulla Oblongata is the posterior-most part of the brain that controls involuntary actions like heartbeatheartbeat, breathingbreathing, and peristalsisperistalsis.

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The Spinal Cord is the extension of the medulla, protected by the vertebral column, and serves as the center for reflex actions.

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A Reflex Action is an automatic, sudden response to a stimulus without the conscious thought of the brain.

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The Reflex Arc is the shortest pathway taken by a nerve impulse to complete a reflex action: Stimulus→Receptor→Sensory Neuron→Spinal Cord→Motor Neuron→Effector→ResponseStimulus \rightarrow Receptor \rightarrow Sensory \ Neuron \rightarrow Spinal \ Cord \rightarrow Motor \ Neuron \rightarrow Effector \rightarrow Response.

📐Formulae

Reflex Arc:Stimulus→ReceptorSensory Neuron→CNSMotor Neuron→EffectorResponseReflex \ Arc: Stimulus \xrightarrow{Receptor} Sensory \ Neuron \xrightarrow{CNS} Motor \ Neuron \xrightarrow{Effector} Response

Nerve Impulse Velocity≈100 m/sNerve \ Impulse \ Velocity \approx 100 \ m/s

Synaptic Transmission:Electrical Signal→Chemical Signal (Neurotransmitter)→Electrical SignalSynaptic \ Transmission: Electrical \ Signal \rightarrow Chemical \ Signal \ (Neurotransmitter) \rightarrow Electrical \ Signal

💡Examples

Problem 1:

Identify the parts of the brain responsible for: (a) Remembering a friend's phone number, (b) Walking in a straight line, and (c) Salivation at the sight of food.

Solution:

(a) Cerebrum, (b) Cerebellum, (c) Medulla Oblongata.

Explanation:

The Cerebrum manages memory and intelligence; the Cerebellum manages balance and muscular coordination; the Medulla Oblongata manages involuntary reflexes like salivation.

Problem 2:

Trace the path of a nerve impulse when a person accidentally touches a hot plate.

Solution:

Heat (Stimulus)→Thermoreceptors (Skin)→Sensory Neuron→Spinal Cord→Motor Neuron→Hand Muscles (Effector)→Withdrawal (Response)Heat \ (Stimulus) \rightarrow Thermoreceptors \ (Skin) \rightarrow Sensory \ Neuron \rightarrow Spinal \ Cord \rightarrow Motor \ Neuron \rightarrow Hand \ Muscles \ (Effector) \rightarrow Withdrawal \ (Response)

Explanation:

This is a spinal reflex. The message travels to the spinal cord via sensory neurons, which immediately sends a command via motor neurons to the muscles to contract, bypassing the brain to save time and prevent injury.

Problem 3:

What is the role of the Myelin sheath in an axon?

Solution:

The Myelin sheath acts as an insulating layer, allowing the nerve impulse to travel faster by jumping between the gaps called Nodes of Ranvier.

Explanation:

Insulation prevents the loss of the electrical signal and ensures high-speed transmission, often reaching speeds up to 120 m/s120 \ m/s in myelinated fibers.