Class 11 Biology Notes Chapter 21 (Neural control and coordination) – Biology Book

Detailed Notes with MCQs of Chapter 21: Neural Control and Coordination. This is a critical chapter, not just for your Class 11 understanding, but also forms the basis for many questions in competitive government exams where Biology is a component. Pay close attention to the structures, their functions, and the mechanisms involved.

Neural Control and Coordination: Detailed Notes

1. Introduction to Coordination

• Coordination: The process through which two or more organs interact and complement the functions of one another to maintain homeostasis and respond to stimuli.
• In complex animals, two systems provide control and coordination:
  • Neural System: Provides rapid, point-to-point coordination via electrochemical signals (nerve impulses).
  • Endocrine System: Provides slower, widespread coordination via chemical messengers (hormones) transported through blood.
• This chapter focuses on the Neural System.

2. Human Neural System

• Highly specialized cells called neurons are the structural and functional units.
• The human neural system is divided into:
  • Central Neural System (CNS): Includes the brain and the spinal cord. It is the site of information processing and control.
  • Peripheral Neural System (PNS): Consists of all the nerves associated with the CNS (arising from the brain - cranial nerves, and spinal cord - spinal nerves). The PNS transmits sensory information to the CNS and motor commands from the CNS to the body parts.
    • Nerve Fibres of PNS:
      • Afferent Fibres: Transmit impulses from tissues/organs (receptors) to the CNS (Sensory).
      • Efferent Fibres: Transmit regulatory impulses from the CNS to the concerned peripheral tissues/organs (Effectors) (Motor).
    • Divisions of PNS:
      • 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 (involuntary control).
        • Sympathetic Nervous System: Generally prepares the body for 'fight-or-flight' responses (e.g., increases heart rate, dilates pupils, inhibits digestion).
        • Parasympathetic Nervous System: Generally promotes 'rest-and-digest' functions (e.g., slows heart rate, constricts pupils, stimulates digestion). These two systems often have antagonistic effects on the same organs.
• Visceral Nervous System: A part of the PNS comprising nerves, fibres, ganglia, and plexuses that innervate viscera (internal organs). Often considered part of the ANS.

3. Neuron: Structure and Function

• Structure:
  • Cell Body (Soma/Cyton): Contains the nucleus, cytoplasm with typical cell organelles, and characteristic granular bodies called Nissl's granules (involved in protein synthesis).
  • Dendrites: Short, branched fibres projecting from the cell body. They receive signals (nerve impulses) from other neurons or sensory receptors and transmit them towards the cell body.
  • Axon: A single, long fibre extending from the cell body. Its distal end is branched, terminating in bulb-like structures called synaptic knobs (containing neurotransmitters). The axon transmits nerve impulses away from the cell body to a synapse or a neuromuscular junction. The cytoplasm within the axon is called axoplasm, and its membrane is the axolemma. The point where the axon arises from the cyton is the axon hillock.
• Types of Neurons (based on number of axon and dendrites):
  • Multipolar: One axon and two or more dendrites (most common type, found in cerebral cortex).
  • Bipolar: One axon and one dendrite (found in the retina of the eye).
  • Unipolar: Cell body with only one axon (found usually in the embryonic stage; pseudo-unipolar neurons in dorsal root ganglia of spinal cord).
• Myelinated and Non-myelinated Axons:
  • Myelinated Axons: Enveloped with Schwann cells (in PNS) or Oligodendrocytes (in CNS) which form a myelin sheath around the axon. Myelin is a fatty insulating layer.
  • Nodes of Ranvier: Gaps between adjacent Schwann cells where the myelin sheath is absent.
  • Non-myelinated Axons: Enclosed by a Schwann cell but without the formation of a myelin sheath. Found in autonomic and somatic neural systems.
  • Myelinated fibres conduct impulses much faster due to saltatory conduction (impulse jumps from node to node). They appear white (forming the white matter of CNS). Non-myelinated fibres and cell bodies appear greyish (forming the grey matter).

4. Generation and Conduction of Nerve Impulse

• Neurons are excitable cells because their membranes are in a polarized state.
• Resting Membrane Potential (RMP):
  • The potential difference across the resting neuronal membrane (axon). Typically around -70mV (inside negative relative to outside).
  • Maintained by:
    • Differential permeability: 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+ ions outwards for every 2 K+ ions moved inwards, consuming ATP. This maintains the concentration gradients.
• Action Potential (Nerve Impulse):
  • A rapid, transient change in membrane potential that occurs when a neuron is stimulated above a certain threshold level.
  • Depolarization: When a stimulus is applied, the membrane at that site becomes freely permeable to Na+. Rapid influx of Na+ leads to reversal of polarity (inside becomes positive, outside negative). This potential difference is the action potential.
  • Repolarization: Following depolarization, the permeability to Na+ decreases, and permeability to K+ increases. Rapid efflux of K+ restores the resting potential (inside becomes negative again).
  • Hyperpolarization: Briefly, the membrane potential may become slightly more negative than the RMP due to slow closure of K+ channels.
  • Conduction: The action potential generated at one site acts as a stimulus for the adjacent region of the membrane, causing depolarization there. This sequence repeats along the length of the axon, propagating the impulse.
  • Saltatory Conduction: In myelinated axons, the action potential 'jumps' from one Node of Ranvier to the next, significantly increasing the speed of conduction.

5. Transmission of Impulses at Synapses

• Synapse: A junction between two neurons (usually between the axon terminal of one neuron and the dendrite/cell body of the next) or between a neuron and an effector (like muscle or gland).
• Components:
  • Presynaptic Neuron: Neuron transmitting the impulse towards the synapse. Its terminal contains synaptic vesicles filled with neurotransmitters.
  • Synaptic Cleft: The fluid-filled space between the presynaptic and postsynaptic membranes.
  • Postsynaptic Neuron: Neuron receiving the signal. Its membrane has receptors specific for the neurotransmitter.
• Types of Synapses:
  • Electrical Synapse: Pre- and postsynaptic membranes are in very close proximity. Direct flow of ions through gap junctions. Transmission is very fast. Rare in humans compared to chemical synapses.
  • Chemical Synapse: Uses neurotransmitters. Slower than electrical synapses but allows for modulation.
• Mechanism of Transmission at Chemical Synapse:
  1. Action potential arrives at the axon terminal of the presynaptic neuron.
  2. Voltage-gated Calcium (Ca++) channels open, Ca++ enters the axon terminal.
  3. Increased Ca++ concentration triggers the fusion of synaptic vesicles with the presynaptic membrane.
  4. Neurotransmitters are released into the synaptic cleft by exocytosis.
  5. Neurotransmitters diffuse across the cleft and bind to specific receptors on the postsynaptic membrane.
  6. Binding opens ion channels on the postsynaptic membrane, causing a change in its potential (either excitatory - EPSP, causing depolarization, or inhibitory - IPSP, causing hyperpolarization).
  7. If the postsynaptic potential reaches threshold, an action potential is generated in the postsynaptic neuron.
  8. Neurotransmitter is quickly removed from the cleft (by enzymatic degradation or reuptake) to terminate the signal.
• Neurotransmitters: Examples include Acetylcholine (ACh), Dopamine, Serotonin, GABA (gamma-aminobutyric acid), Norepinephrine.

6. Central Neural System (CNS)

• Brain: The central information processing organ, acting as the 'command and control system'. Protected by the skull (cranium) and covered by three cranial meninges:

  • Dura mater: Outermost, tough layer.
  • Arachnoid mater: Middle, thin, web-like layer.
  • Pia mater: Innermost, delicate layer in contact with brain tissue.
  • Cerebrospinal Fluid (CSF): Fills the space between arachnoid and pia mater (subarachnoid space) and ventricles of the brain. Provides cushioning, nourishment, and waste removal.

• Major Parts of the Brain:

  • (a) Forebrain (Prosencephalon):
    • Cerebrum: Largest part. Divided into two cerebral hemispheres (left and right) connected by a tract of nerve fibres called the corpus callosum. The outer layer is the cerebral cortex (grey matter) which is highly folded (gyri and sulci) to increase surface area. Inner part is cerebral medulla (white matter).
      • Cerebral Cortex: Responsible for higher functions like memory, communication, consciousness, sensory perception, voluntary muscle control. Contains motor areas, sensory areas, and large association areas (responsible for complex functions like intersensory associations, memory, communication).
      • Each hemisphere is divided into four lobes: Frontal, Parietal, Temporal, Occipital.
    • Thalamus: Located superior to the brainstem. Major coordinating centre (relay station) for sensory and motor signaling. Relays sensory information (except smell) to the cerebral cortex.
    • Hypothalamus: Lies at the base of the thalamus. Crucial role in:
      • Regulation of body temperature, urge for eating and drinking (hunger, thirst).
      • Control of pituitary gland function (neurosecretory cells).
      • Involvement in emotion, sleep-wake cycles, sexual behaviour.
  • (b) Midbrain (Mesencephalon): Located between the thalamus/hypothalamus and the pons. Contains cerebral aqueduct (canal passing through). Dorsal portion consists mainly of four round swellings called corpora quadrigemina (involved in visual and auditory reflexes).
  • (c) Hindbrain (Rhombencephalon):
    • Pons: Fibre tracts that interconnect different regions of the brain. Contains nuclei involved in regulating respiration.
    • Cerebellum: Located at the back, below the cerebrum. Has a highly convoluted surface. Coordinates voluntary movements, posture, balance, and equilibrium. Important for motor skill learning.
    • Medulla Oblongata: Connected to the spinal cord. Controls vital involuntary functions like respiration, cardiovascular reflexes (heart rate, blood pressure), gastric secretions, vomiting, coughing, sneezing.

• Brain Stem: Formed by the midbrain, pons, and medulla oblongata. Connects the forebrain to the spinal cord and controls many basic life functions.

• Spinal Cord:

  • Cylindrical structure extending from the medulla oblongata down through the vertebral canal. Protected by the vertebral column and meninges. CSF is present in the central canal.
  • Structure: Outer white matter (myelinated axons) and inner H-shaped grey matter (neuron cell bodies, dendrites, non-myelinated axons, interneurons).
  • Functions:
    • Conducts sensory impulses from the body to the brain.
    • Conducts motor impulses from the brain to the effectors.
    • Centre for spinal reflexes.

7. Reflex Action and Reflex Arc

• Reflex Action: An involuntary, rapid response to a stimulus, occurring without conscious thought. Mediated by the CNS (often spinal cord).
• Reflex Arc: The neural pathway involved in a reflex action. Components:
  1. Receptor: Detects the stimulus (e.g., pain receptors in skin).
  2. Afferent Neuron (Sensory Neuron): Transmits the impulse from the receptor to the CNS. Enters the spinal cord via the dorsal root.
  3. Integration Centre: One or more synapses within the CNS (grey matter of spinal cord or brainstem). May involve an interneuron.
  4. Efferent Neuron (Motor Neuron): Transmits the impulse from the CNS to the effector. Exits the spinal cord via the ventral root.
  5. Effector: The muscle or gland that responds to the motor command (e.g., muscle contracts).
• Example: Knee-jerk reflex (monosynaptic reflex - involves only one synapse between sensory and motor neuron). Withdrawal reflex (polysynaptic reflex - involves interneurons).

8. Sensory Reception and Processing

• Sensory organs detect changes in the environment (stimuli) and send information to the CNS.

• (A) Eye (Organ of Sight):

  • Location: Situated in sockets of the skull called orbits.
  • Structure: Roughly spherical. Wall composed of three layers:
    • Sclera: Outermost, tough, fibrous connective tissue (white of the eye). Anterior transparent part is the Cornea.
    • Choroid: Middle layer, vascular (supplies nutrients), pigmented (bluish), thin. Thickens anteriorly to form the ciliary body.
    • Retina: Innermost, light-sensitive layer. Contains photoreceptor cells.
  • Other Parts:
    • Iris: Pigmented, opaque structure, visible coloured portion. Continuation of the ciliary body. Controls pupil size.
    • Pupil: Aperture in the centre of the iris, regulated by iris muscles.
    • Lens: Transparent, biconvex crystalline structure held by ligaments attached to the ciliary body. Focuses light onto the retina.
    • Aqueous Humour: Watery fluid filling the space between cornea and lens (aqueous chamber).
    • Vitreous Humour: Transparent gel filling the space behind the lens (vitreous chamber). Helps maintain eye shape.
  • Retina Layers (Inside to Outside): Ganglion cells -> Bipolar cells -> Photoreceptor cells.
  • Photoreceptor Cells:
    • Rods: Contain the pigment rhodopsin (visual purple, derivative of Vitamin A). Responsible for twilight (scotopic) vision. Sensitive to dim light, do not detect colour. More numerous than cones.
    • Cones: Contain pigments (iodopsins) sensitive to red, green, or blue light. Responsible for daylight (photopic) vision and colour vision. Concentrated in the fovea.
  • Fovea: A thinned-out portion of the retina (in the macula lutea) where only cones are densely packed. Point of highest visual acuity (resolution).
  • Blind Spot: Point where the optic nerve leaves the eye and retinal blood vessels enter. No photoreceptor cells are present here.
  • Mechanism of Vision:
    1. Light rays focus on the retina (through cornea, lens).
    2. Light induces dissociation of retinal from opsin in photoreceptor pigments (rhodopsin/iodopsins).
    3. This causes changes in membrane permeability of photoreceptor cells.
    4. Potential differences are generated in photoreceptor cells.
    5. This generates signals that excite bipolar cells, which in turn excite ganglion cells.
    6. Action potentials are generated in the ganglion cells.
    7. Impulses are transmitted via the optic nerve to the visual cortex (occipital lobe) of the brain.
    8. Neural impulses are analysed, and the image formed on the retina is recognised.

• (B) Ear (Organ of Hearing and Balance):

  • Functions: Hearing (auditory function) and Maintenance of body balance (vestibular function).
  • Structure: Divided into three major sections:
    • Outer Ear:
      • Pinna: Collects sound waves.
      • External Auditory Meatus (Canal): Leads inwards, contains fine hairs and wax-secreting glands. Directs sound waves to the eardrum.
      • Tympanic Membrane (Eardrum): Thin membrane separating outer and middle ear. Vibrates in response to sound waves.
    • Middle Ear: Air-filled cavity.
      • Ear Ossicles: Three tiny bones - Malleus (hammer, attached to tympanic membrane), Incus (anvil), Stapes (stirrup, attached to the oval window of the cochlea). They amplify and transmit vibrations.
      • Eustachian Tube: Connects the middle ear cavity with the pharynx. Equalises pressure on both sides of the eardrum.
    • Inner Ear (Labyrinth): Fluid-filled cavity.
      • Bony Labyrinth: Series of channels inside temporal bone. Filled with perilymph.
      • Membranous Labyrinth: Lies within the bony labyrinth. Filled with endolymph.
      • Cochlea (Hearing): Coiled portion of the labyrinth. Contains three canals separated by membranes:
        • Scala Vestibuli (upper, ends at oval window, filled with perilymph)
        • Scala Media (middle, filled with endolymph)
        • Scala Tympani (lower, ends at round window, filled with perilymph)
        • Reissner's membrane: Separates scala vestibuli and scala media.
        • Basilar membrane: Separates scala media and scala tympani.
        • Organ of Corti: Located on the basilar membrane within scala media. Contains hair cells (auditory receptors) arranged in rows. The apical processes (stereocilia) of hair cells project into the overlying tectorial membrane.
      • Vestibular Apparatus (Balance): Located above the cochlea. Consists of:
        • Three Semicircular Canals: Oriented in different planes. Base of each canal is swollen (ampulla) containing a sensory ridge (crista ampullaris) with hair cells. Detect rotational/angular movements (dynamic equilibrium).
        • Otolith Organ (Utricle and Saccule): Located in the vestibule. Contain sensory patches (macula) with hair cells. Hair cells' cilia are embedded in a gelatinous layer containing calcium carbonate crystals (otoliths). Detect gravity and linear acceleration (static equilibrium).
  • Mechanism of Hearing:
    1. Pinna collects sound waves, directs them through the auditory canal to the tympanic membrane.
    2. Tympanic membrane vibrates.
    3. Vibrations are transmitted and amplified by the ear ossicles (malleus -> incus -> stapes).
    4. Stapes transmits vibrations to the perilymph of the scala vestibuli via the oval window.
    5. Pressure waves travel through perilymph (scala vestibuli -> scala tympani) and cause vibration of the basilar membrane.
    6. Movement of the basilar membrane causes the hair cells of the Organ of Corti to bend against the tectorial membrane.
    7. Bending of hair cells stimulates them, generating nerve impulses in the associated afferent neurons.
    8. Impulses are transmitted via the auditory nerve (cochlear part of vestibulocochlear nerve) to the auditory cortex (temporal lobe) of the brain.
    9. Impulses are interpreted as sound.
  • Mechanism of Balance: Movements of the head cause movement of endolymph within the semicircular canals (for rotation) or displacement of otoliths in the utricle and saccule (for linear acceleration/gravity). This bends the hair cells in the cristae or maculae, generating nerve impulses that travel via the vestibular part of the vestibulocochlear nerve to the cerebellum and brainstem for maintaining balance and posture.

Multiple Choice Questions (MCQs)

1. 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

2. The myelin sheath around axons in the peripheral nervous system (PNS) is formed by:
   a) Oligodendrocytes
   b) Astrocytes
   c) Schwann cells
   d) Microglia

3. During the transmission of a nerve impulse, the depolarization of the axonal membrane is primarily due to:
   a) Influx of K+ ions
   b) Efflux of K+ ions
   c) Influx of Na+ ions
   d) Efflux of Na+ ions

4. The junction between two neurons is called a:
   a) Synapse
   b) Node of Ranvier
   c) Neuromuscular junction
   d) Receptor site

5. Which of the following structures is part of the hindbrain?
   a) Thalamus
   b) Cerebrum
   c) Pons
   d) Corpus callosum

6. In the human eye, the photoreceptor cells responsible for colour vision and daylight vision are:
   a) Rods
   b) Cones
   c) Bipolar cells
   d) Ganglion cells

7. The sequence of ear ossicles starting from the tympanic membrane is:
   a) Incus, Malleus, Stapes
   b) Malleus, Stapes, Incus
   c) Stapes, Incus, Malleus
   d) Malleus, Incus, Stapes

8. A reflex arc typically includes the following components in order:
   a) Effector, Efferent Neuron, CNS, Afferent Neuron, Receptor
   b) Receptor, Efferent Neuron, CNS, Afferent Neuron, Effector
   c) Receptor, Afferent Neuron, CNS, Efferent Neuron, Effector
   d) Effector, Afferent Neuron, CNS, Efferent Neuron, Receptor

9. The part of the inner ear responsible for static equilibrium (sensing gravity and linear acceleration) is the:
   a) Cochlea
   b) Semicircular canals
   c) Organ of Corti
   d) Utricle and Saccule (Otolith organ)

10. The sympathetic and parasympathetic nervous systems are subdivisions of the:
    a) Central Nervous System (CNS)
    b) Somatic Neural System
    c) Autonomic Neural System (ANS)
    d) Visceral Nervous System

Answers to MCQs:

1. c) Hypothalamus
2. c) Schwann cells
3. c) Influx of Na+ ions
4. a) Synapse
5. c) Pons
6. b) Cones
7. d) Malleus, Incus, Stapes
8. c) Receptor, Afferent Neuron, CNS, Efferent Neuron, Effector
9. d) Utricle and Saccule (Otolith organ)
10. c) Autonomic Neural System (ANS)

Revise these notes thoroughly. Understanding the flow of information, the specific roles of different parts, and the mechanisms of impulse generation and transmission is key for your exams. Good luck!