Class 11 Biology Notes Chapter 21 (Chapter 21) – Examplar Problems (English) Book
Alright class, let's delve into Chapter 21: Neural Control and Coordination. This is a crucial chapter, not just for your Class 11 understanding, but also forms a significant part of syllabi for various government exams where Biology is a component. Pay close attention to the details.
Chapter 21: Neural Control and Coordination - Detailed Notes
1. Introduction
- Coordination: The process through which two or more organs interact and complement the functions of one another.
- In animals, coordination and integration of all organ activities are provided by the neural system and the endocrine system (hormones).
- The neural system provides rapid, point-to-point coordination.
2. Human Neural System
- Divided into two main parts:
- Central Neural System (CNS): Includes the brain and the spinal cord. It's the site of information processing and control.
- Peripheral Neural System (PNS): Consists of all the nerves associated with the CNS (arising from the brain and spinal cord).
- Nerve fibres of the PNS are of two types:
- Afferent fibres: Transmit impulses from tissues/organs to the CNS (Sensory).
- Efferent fibres: Transmit regulatory impulses from the CNS to the concerned peripheral tissues/organs (Motor).
- Nerve fibres of the PNS are of two types:
- PNS Divisions:
- Somatic Neural System: Relays impulses from the CNS to skeletal muscles (voluntary control).
- Autonomic Neural System (ANS): Transmits impulses from the CNS to the involuntary organs and smooth muscles of the body.
- Sympathetic Neural System: Generally involved in 'fight or flight' responses (increases heart rate, breathing rate, etc.).
- Parasympathetic Neural System: Generally involved in 'rest and digest' functions (decreases heart rate, promotes digestion).
3. Neuron as Structural and Functional Unit
- Structure:
- Cell Body (Soma/Cyton): Contains cytoplasm with typical cell organelles and certain granular bodies called Nissl's granules (contain RER, involved in protein synthesis).
- Dendrites: Short fibres branching repeatedly from the cell body. They transmit impulses towards the cell body.
- Axon: A long fibre, branched distally. Each branch terminates as a bulb-like structure called a synaptic knob, which possesses synaptic vesicles containing neurotransmitters. Axons transmit nerve impulses away from the cell body to a synapse or neuromuscular junction.
- Myelin Sheath: Insulating layer around the axon in many neurons, formed by Schwann cells (in PNS) or Oligodendrocytes (in CNS). Gaps in the myelin sheath are called Nodes of Ranvier. Myelinated fibres conduct impulses faster (Saltatory conduction). Non-myelinated fibres lack this sheath.
- Types of Neurons (based on number of axon and dendrites):
- Multipolar: One axon, two or more dendrites (e.g., cerebral cortex).
- Bipolar: One axon, one dendrite (e.g., retina of the eye).
- Unipolar: Cell body with one axon only (e.g., embryonic stage, dorsal root ganglia of spinal cord).
4. Generation and Conduction of Nerve Impulse
- Neurons are excitable cells because their membranes are in a polarised state.
- Resting Potential:
- The membrane potential of a neuron not conducting an impulse.
- Axonal membrane is more permeable to K+ ions and nearly impermeable to Na+ ions and negatively charged proteins in the axoplasm.
- Sodium-Potassium Pump (Na+/K+ ATPase): Actively transports 3 Na+ outwards for every 2 K+ inwards, maintaining the concentration gradients and the negative potential inside (-70mV approx). Outer surface is positively charged, inner surface is negatively charged.
- Action Potential (Nerve Impulse):
- When a stimulus is applied, the membrane at that site becomes freely permeable to Na+.
- Depolarisation: Rapid influx of Na+ leads to reversal of polarity (inside becomes positive, outside negative). This potential difference across the membrane is the action potential.
- Repolarisation: Permeability to Na+ decreases, while permeability to K+ increases. K+ diffuses out, restoring the resting potential (inside negative, outside positive).
- Hyperpolarisation: A brief period where the membrane potential becomes more negative than the resting potential due to slow closing of K+ channels.
- The Na+/K+ pump restores the initial ionic distribution.
- Conduction: The action potential generated at one site acts as a stimulus for the next site, causing sequential depolarisation along the axon. In myelinated axons, the impulse jumps from one Node of Ranvier to the next (Saltatory Conduction), which is much faster.
5. Synapse and Transmission of Impulse
- Synapse: A junction between two neurons (axon terminal of one and dendrite/cell body of the next) or between a neuron and an effector (e.g., neuromuscular junction).
- Types:
- Electrical Synapse: Pre- and post-synaptic membranes are in very close proximity. Impulse transmission is direct, similar to conduction along a single axon. Faster but rare in our system.
- Chemical Synapse: Pre- and post-synaptic membranes are separated by a fluid-filled space called the synaptic cleft. This is the most common type.
- Mechanism (Chemical Synapse):
- Action potential arrives at the axon terminal (synaptic knob).
- Voltage-gated Ca2+ channels open, Ca2+ enters the axon terminal.
- Ca2+ influx causes synaptic vesicles to fuse with the pre-synaptic membrane.
- Neurotransmitters (e.g., Acetylcholine, Dopamine, Serotonin, GABA) are released into the synaptic cleft via exocytosis.
- Neurotransmitters bind to specific receptors on the post-synaptic membrane.
- This binding opens ion channels, generating a new potential (Excitatory Postsynaptic Potential - EPSP, or Inhibitory Postsynaptic Potential - IPSP) in the post-synaptic neuron.
- Neurotransmitter is quickly removed from the cleft (by enzymatic degradation or reuptake) to allow for repolarisation and new signals.
6. Central Neural System (CNS)
- Brain: Central information processing organ, acts as the 'command and control system'. Protected by the skull and cranial meninges (outer Dura mater, middle Arachnoid mater, inner Pia mater). Space between meninges contains Cerebrospinal Fluid (CSF) which provides cushioning.
- Forebrain (Prosencephalon):
- Cerebrum: Largest part. Divided into two cerebral hemispheres connected by the Corpus Callosum (tract of nerve fibres). The outer layer is the Cerebral Cortex (grey matter due to neuron cell bodies), highly folded (gyri and sulci) to increase surface area. Inner part is white matter (myelinated axons). Cortex contains motor areas, sensory areas, and large association areas (responsible for complex functions like intersensory associations, memory, communication). Lobes: Frontal, Parietal, Temporal, Occipital.
- Thalamus: Major coordinating centre for sensory and motor signalling. Relays sensory information (except smell) to the cerebrum.
- Hypothalamus: Lies at the base of the thalamus. Controls body temperature, urge for eating and drinking (homeostasis). Contains centres for regulating sexual behaviour, emotional reactions (limbic system component). Secretes hypothalamic hormones controlling the pituitary gland.
- Midbrain (Mesencephalon): Located between thalamus/hypothalamus and pons. Contains centres for reflex movements of the head, neck, and trunk in response to visual and auditory stimuli (Corpora quadrigemina). Cerebral aqueduct passes through it.
- Hindbrain (Rhombencephalon):
- Pons: Fibre tracts interconnecting different regions of the brain. Contains a respiratory rhythm centre.
- Cerebellum: Highly convoluted surface. Coordinates voluntary muscle movements, posture, and balance (equilibrium).
- Medulla Oblongata: Connects to the spinal cord. Controls respiration, cardiovascular reflexes (heart rate, blood pressure), gastric secretions, vomiting, coughing, sneezing.
- Brain Stem: Includes Midbrain, Pons, and Medulla Oblongata. Connects the forebrain to the spinal cord and controls many basic life functions.
- Forebrain (Prosencephalon):
- Spinal Cord: Cylindrical structure extending from the medulla oblongata down the vertebral column. Protected by vertebrae and meninges. Central canal contains CSF. Outer white matter, inner H-shaped grey matter. Conducts impulses to and from the brain. Centre for spinal reflexes.
7. Reflex Action and Reflex Arc
- Reflex Action: Rapid, involuntary, unconscious response to a stimulus.
- Reflex Arc: The pathway followed by the nerve impulses during a reflex action. Components:
- Receptor: Detects the stimulus.
- Afferent Neuron (Sensory): Transmits impulse from receptor to CNS.
- Integration Centre: One or more synapses within the CNS (spinal cord or brainstem). May involve an interneuron.
- Efferent Neuron (Motor): Transmits impulse from CNS to effector.
- Effector: Muscle or gland that responds.
- Example: Knee-jerk reflex (monosynaptic), Withdrawal reflex (polysynaptic).
8. Sensory Reception and Processing
- Sensory organs detect changes in the environment and send signals to the CNS.
- Eye (Organ of Sight):
- Layers of Eyeball: Outer Sclera (tough, fibrous, anterior part is transparent Cornea), Middle Choroid (vascular, thin, forms Ciliary body and Iris anteriorly), Inner Retina.
- Iris: Pigmented, opaque structure, visible coloured portion. Controls pupil diameter.
- Pupil: Aperture surrounded by the iris. Regulated by iris muscles.
- Lens: Transparent, crystalline, biconvex structure held by ligaments attached to the ciliary body. Focuses light on the retina. Accommodation (adjusting focal length) is achieved by ciliary muscles.
- Retina: Contains photoreceptor cells (Rods - twilight/scotopic vision, contain rhodopsin pigment; Cones - daylight/photopic vision and colour vision, contain different photopsins). Also contains bipolar cells and ganglion cells.
- Optic Nerve: Axons of ganglion cells bundle together to form the optic nerve, leaving the eye at the Blind Spot (no photoreceptors).
- Fovea (in Macula Lutea): Point of greatest visual acuity (resolution), densely packed with cones only.
- Aqueous Humor: Watery fluid in the space between cornea and lens (aqueous chamber).
- Vitreous Humor: Transparent gel in the space behind the lens (vitreous chamber).
- Mechanism of Vision: Light rays -> Cornea -> Aqueous Humor -> Pupil -> Lens -> Vitreous Humor -> Retina. Light induces dissociation of retinal from opsin in photoreceptors -> changes membrane potential -> generates action potentials in ganglion cells -> transmitted via optic nerve to visual cortex (occipital lobe) for processing.
- Ear (Organ of Hearing and Balance):
- Outer Ear: Pinna (collects sound waves) and External Auditory Meatus (canal) leading to the Tympanic Membrane (eardrum).
- Middle Ear: Air-filled cavity containing three ossicles - Malleus, Incus, Stapes (amplify sound vibrations). Connected to the pharynx by the Eustachian tube (equalises pressure). Stapes is attached to the Oval Window of the cochlea.
- Inner Ear (Labyrinth): Fluid-filled.
- Bony Labyrinth: Series of channels filled with perilymph.
- Membranous Labyrinth: Floats within the bony labyrinth, filled with endolymph. Consists of:
- Cochlea: Coiled portion responsible for hearing. Contains three canals: Scala vestibuli (ends at oval window), Scala media (contains the Organ of Corti), Scala tympani (ends at round window). Organ of Corti, located on the basilar membrane, contains hair cells (auditory receptors).
- Vestibular Apparatus: Responsible for balance/equilibrium. Composed of Semicircular Canals (three, detect rotational/angular acceleration) and Otolith Organ (Utricle and Saccule, detect linear acceleration and gravity/head position). Both have hair cells stimulated by movement of endolymph or otoliths.
- Mechanism of Hearing: Sound waves -> Pinna -> External Auditory Meatus -> Tympanic Membrane (vibrates) -> Malleus, Incus, Stapes (amplify vibrations) -> Oval Window -> Perilymph of Scala Vestibuli -> Endolymph of Scala Media -> Basilar membrane vibrates -> Hair cells of Organ of Corti bend against Tectorial membrane -> Nerve impulse generated -> Transmitted via Auditory Nerve to Auditory Cortex (temporal lobe).
- Mechanism of Balance: Movements of the head cause movement of endolymph (semicircular canals) or otoliths (utricle, saccule), stimulating hair cells -> Nerve impulse via Vestibular Nerve to cerebellum and brainstem.
Multiple Choice Questions (MCQs)
-
Which part of the human brain is primarily responsible for regulating body temperature, hunger, and thirst?
a) Cerebellum
b) Thalamus
c) Hypothalamus
d) Medulla Oblongata -
During the transmission of a nerve impulse across a chemical synapse, which ion influx triggers the release of neurotransmitters from the pre-synaptic terminal?
a) Na+
b) K+
c) Cl-
d) Ca2+ -
The myelin sheath around axons in the peripheral nervous system (PNS) is formed by:
a) Oligodendrocytes
b) Astrocytes
c) Schwann cells
d) Microglia -
Which of the following structures is part of the inner ear and primarily responsible for dynamic equilibrium (detecting rotational movements)?
a) Cochlea
b) Utricle
c) Saccule
d) Semicircular canals -
Rods and cones are the photoreceptor cells located in which layer of the human eye?
a) Sclera
b) Choroid
c) Retina
d) Cornea -
The reflex arc components include Receptor, Afferent neuron, Integration centre, Efferent neuron, and Effector. Which component is typically located entirely within the CNS?
a) Receptor
b) Afferent neuron
c) Integration centre (often involving interneurons)
d) Effector -
The parasympathetic nervous system is characterized by which of the following actions?
a) Increased heart rate
b) Dilation of pupils
c) Stimulation of digestion
d) Increased breathing rate -
Nissl's granules, found in the cyton and dendrites of neurons, are aggregations of:
a) Mitochondria and Golgi apparatus
b) Ribosomes and Rough Endoplasmic Reticulum
c) Lysosomes and Peroxisomes
d) Microtubules and Neurofilaments -
Saltatory conduction of nerve impulses occurs in:
a) Non-myelinated axons
b) Myelinated axons
c) Dendrites only
d) Synaptic clefts -
The corpus callosum connects the:
a) Cerebrum and Cerebellum
b) Two cerebral hemispheres
c) Thalamus and Hypothalamus
d) Pons and Medulla Oblongata
Answer Key for MCQs:
- c) Hypothalamus
- d) Ca2+
- c) Schwann cells
- d) Semicircular canals
- c) Retina
- c) Integration centre (often involving interneurons)
- c) Stimulation of digestion
- b) Ribosomes and Rough Endoplasmic Reticulum
- b) Myelinated axons
- b) Two cerebral hemispheres
Make sure you revise these notes thoroughly. Understanding the structure and function of each part of the neural system, along with the mechanisms of impulse transmission and sensory perception, is vital. Good luck with your preparation!