Nervous

• Neural system

  The control system of the body
• Neurons

  • Highly specialised cells that make up the neural system
• Coreless, Cold, Destiny
• Coordination


  The process through which two or more organs of the body interact and complement the functions of each other
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• Endocrine system

  Interacts with the neural system as chemical messengers
• Central Nervous System (CNS)

  Includes the brain and the spinal cord
• Brain


  The site of information processing and control
• Afferent Fibres


  • Transmit impulses from tissues/organs to the CNS
• Efferent Fibres

  • Transmit regulatory impulses from the CNS to the concerned peripheral tissues/organs
• Cell Body (Cyton or Soma)


  Consists of cytoplasm, nucleus and cell membrane. Contains typical cell organelles like mitochondria, Golgi apparatus, rough endoplasmic reticulum, ribosomes, lysosomes, certain granular bodies and Nissl's granules
• Presence of Nissl's granules is the characteristic of all neurons
• Dendrites (Dendrons)


  Usually shorter, tapering and much branched processes that project out of the cell body. They also contain Nissl's granules and may be one to several in number. They conduct nerve impulses towards the neuron cell body and are called afferent processes (receiving processes).
• Axon


  • A single, usually very long process of uniform thickness. The part of cyton from where the axon arises is called axon hillock (most sensitive part of neuron).
• Synapse formation

  When terminal arborisations of the axon meet the dendrites of another neuron, each branch terminates as a bulb-like structure called synaptic knob which possesses mitochondria and secretory vesicles (containing chemicals called neurotransmitters). The axons transmit nerve impulses away from the cell body to a synapse or to a neuromuscular junction.
• Types of nerve fibres

  • Myelinated
  • Non-myelinated
• Myelinated nerve fibres

  • Schwann cells form myelin sheath around the axon. The gaps between two adjacent myelin sheaths are called nodes of Ranvier. Myelinated nerve fibres are found in cranial nerves, spinal nerves and white matter of brain.
• Non-myelinated nerve fibres

  • Schwann cells do not form myelin sheath around the axon and are without nodes of Ranvier. They are commonly found in autonomic and somatic neural systems.
• Types of neurons

  • Multipolar
  • Bipolar
  • Unipolar
• Multipolar neurons

  • Have several dendrites and an axon. They are found in cerebral cortex.
• Bipolar neurons

  • Have one dendrite and one axon. They are present in the retina of eye.
• Unipolar neurons


  • Have cell body with one axon only. These are usually found in the embryonic stage.
• Neural tissue

  • Excitability - Ability of nerve cells to generate an electrical impulse in response to a stimulus by altering the normal potential differences across their plasma membrane
  • Conductivity - Ability of nerve cells to rapidly transmit the electrical impulses as a wave from the site of its origin along with their length in a particular direction
• Nervous system


  • Controls and coordinates the working of all parts of the body, so that it functions as an integrated unit
  • Stores the impressions of previous stimuli and retrieves (recalls) these impressions in future
  • Helps in the maintenance of the body's internal environment (homeostasis)
• Nerve impulse conduction

  1. Permeability of axon membrane (axolemma)

  2. Osmotic equilibrium (electrical equivalence) between the axoplasm and Extracellular Fluid (ECF) present outside the axon
• Resting nerve fibre

  • Axoplasm (neuroplasm of axon) contains high concentration of K+ and negatively charged proteins and low concentration of Na+
  • Fluid outside axon contains low concentration of K+ and a high concentration of Na+ and thus forms a concentration gradient
• Nerve impulse


  A wave of bioelectric/electrochemical disturbance that passes along a neuron during conduction of an excitation
• Ionic gradients across the resting membrane

  Maintained by the active transport of ions by the sodium-potassium pump, which transports 3Na+ outwards and 2K+ inwards (into the cell)
• Resting membrane

  • Outer surface becomes positively charged
  • Inner surface becomes negatively charged
  • Polarised state
• Resting potential

  Electrical potential difference across the resting plasma membrane
• Depolarisation (Action Potential)

  1. Stimulus of adequate strength (threshold stimulus) applied to a polarised membrane

  2. Permeability of the membrane to Na+ ions greatly increased at the point of stimulation

  3. Rapid influx of Na+

  4. Reversal of polarity at that site

  5. Outer surface becomes negatively charged, inner side becomes positively charged

  6. Neuron at that point is depolarised
• Action potential

  Electrical potential difference across the plasma membrane at the site of depolarisation
• Repolarisation


  1. Rise in stimulus-induced permeability to Na+ is short lived

  2. Rise in membrane permeability to K+

  3. Na+ influx stops and K+ outflow begins

  4. Resting potential is restored at the site of excitation
• Refractory period


  Time taken for the neuron to restore its resting potential and become responsive to further stimulation
• Synapse


  • Junction formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron
  • Membranes separated by a fluid-filled space called synaptic cleft
• Central neural system

  The brain is the anteriormost part, located in the cranial cavity of the skull
• Coverings of the brain

  • Three membranes or meninges
  • Dura mater - fibrous membrane adhering close to the inner side of the skull
  • Arachnoid membrane - very thin layer
  • Pia mater - thin, very vascular, in contact with the brain tissue