Neural Control and Coordination

Coordination and Homeostasis

  • Functions of organs/organ systems must be coordinated to maintain homeostasis.
  • Coordination involves interaction of two or more organs to complement each other’s function.
    • Example: During physical exercise, oxygen demand increases, necessitating:
    • Increased respiration rate
    • Higher heart rate
    • Increased blood flow through vessels
  • After exercise, the activities of nerves, lungs, heart, and kidneys gradually return to normal.

Neural and Endocrine Coordination

  • The neural system and endocrine system jointly coordinate and integrate activities of organs for synchronized functioning:
    • Neural system: Provides quick coordination via point-to-point connections.
    • Endocrine system: Facilitates chemical integration through hormones.

Chapter Overview

  • Neural System is composed of specialized cells called neurons.
    • Sections to be covered include:
    • 18.1: Neural System
    • 18.2: Human Neural System
    • 18.3: Neuron as Structural and Functional Unit
    • 18.4: Central Neural System

18.1 Neural System

  • Composed of highly specialized cells called neurons that detect, receive, and transmit stimuli.
  • Simple neural organization in lower invertebrates (e.g., Hydra)
  • More complex systems in insects and vertebrates.

18.2 Human Neural System

  • Divided into two main parts:
    1. Central Neural System (CNS):
    • Comprises the brain and spinal cord; responsible for processing and control.
    1. Peripheral Neural System (PNS):
    • Includes all nerves associated with the CNS.
    • Divided into:
      • Afferent fibres: Carry impulses from organs to CNS.
      • Efferent fibres: Transmit regulatory impulses from CNS to organs.
    • Further divided into:
      • Somatic Neural System: Relays impulses to skeletal muscles.
      • Autonomic Neural System: Relays impulses to involuntary organs and smooth muscles; consists of:
        • Sympathetic system.
        • Parasympathetic system.

18.3 Neuron as Structural and Functional Unit of Neural System

  • Neurons have three main parts:
    • Cell body: Contains cytoplasm and organelles (e.g., Nissl’s granules).
    • Dendrites: Short fibers conducting impulses towards the cell body.
    • Axon: Long fiber that transmits impulses away from the cell body.
    • Ends in synaptic knobs containing neurotransmitters.
  • Classification of neurons based on structure:
    1. Multipolar: One axon, two or more dendrites (e.g., in cerebral cortex).
    2. Bipolar: One axon, one dendrite (e.g., in retina).
    3. Unipolar: One axon only, usually found in embryonic stages.
  • Axon types:
    • Myelinated: Surrounded by myelin sheath (Schwann cells); has nodes of Ranvier.
    • Unmyelinated: No sheath; common in autonomous and somatic systems.

18.3.1 Generation and Conduction of Nerve Impulse

  • Neurons are excitable cells; their membranes are polarized:
    • Higher permeability to K^+ ions and low permeability to Na^+ ions during resting state.
    • Results in a resting potential difference (negative inside, positive outside).
  • When stimulated:
    • Depolarization: Influx of Na^+ leads to reversal of membrane potential.
    • Action potential: Defined as the nerve impulse.
    • Propagation of impulse involves a sequence of depolarization along the axon.
    • Return to resting potential is facilitated by the efflux of K^+ ions.

18.3.2 Transmission of Impulses

  • Nerve impulses are transmitted across synapses (junctions between neurons).
    • Two types of synapses:
    1. Electrical Synapse: Direct electrical current flow between neurons.
    2. Chemical Synapse: Separated by the synaptic cleft; neurotransmitters facilitate transmission.
  • Mechanism of chemical synapse:
    • Arrival of action potential causes release of neurotransmitters into the synaptic cleft.
    • Binding to receptors on post-synaptic membrane opens ion channels, generating a new potential (excitatory or inhibitory).

18.4 Central Neural System

  • The brain is the central processing organ, controlling:
    • Voluntary movements, vital involuntary organs, thermoregulation, hunger/thirst, circadian rhythms, endocrine activities, and behavior.
  • Protected by skull and covered with meninges:
    • Dura mater (outer), Arachnoid (middle), and Pia mater (inner).
  • Major divisions:
    1. Forebrain: Cerebrum, thalamus, hypothalamus.
    2. Midbrain: Between forebrain and hindbrain.
    3. Hindbrain: Pons, cerebellum, medulla oblongata.

18.4.1 Forebrain

  • Comprises:
    • Cerebrum: Major part divided into left and right hemispheres, connected by corpus callosum.
    • Outer layer (cerebral cortex) = grey matter (neuron cell bodies).
    • Inner layer = white matter (myelinated fibres).
    • Thalamus: Sensory and motor signalling coordination.
    • Hypothalamus: Controls body temperature, hunger, thirst, and endocrine hormone secretion.
  • Limbic system (inner structures) involved in emotion regulation and motivation.

18.4.2 Midbrain

  • Located between forebrain and hindbrain, includes:
    • Corpora quadrigemina: Four round swellings for sensory processing.

18.4.3 Hindbrain

  • Comprised of:
    • Pons: Connects different brain regions.
    • Cerebellum: Coordinates voluntary movements, balance.
    • Medulla: Controls respiration, cardiovascular reflexes, and gastric secretions.
    • Brain Stem: Composed of midbrain, pons, and medulla; connects brain to spinal cord.

Summary

  • The neural system integrates metabolic and homeostatic activities of organs.
  • Neurons as excitable cells exhibit polarized resting potentials.
  • Nerve impulses travel as waves of depolarization and repolarization along axons.
  • Synapses (chemical/electrical) crucial for impulse transmission.
  • The CNS (brain + spinal cord) crucial for processing and control, divided into forebrain, midbrain, and hindbrain, each with specific functions.

Exercises

  1. Describe the structure of the brain.
  2. Compare CNS vs PNS and resting potential vs action potential.
  3. Explain:
    • (a) Membrane polarization.
    • (b) Membrane depolarization.
    • (c) Neurotransmitter roles at chemical synapses.
  4. Draw neurons and brain diagrams.
  5. Write short notes on neural coordination and components of the CNS.
  6. Describe synaptic transmission mechanisms.
  7. Explain role of Na^+ in action potential generation.
  8. Differentiate various neuronal structures and systems in the CNS.