neural control and coordination

• The human neural system is divided into two parts: CNS and PNS
• CNS includes the brain and the spinal cord
• PNS comprises all the nerves of the body associated with the CNS
• Nerve fibres of the PNS are of two types: afferent fibres and efferent fibres
• Afferent nerve fibres transmit impulses from organs to CNS
• Efferent fibres transmit regulatory impulses from the CNS to organs
• PNS is divided into two divisions: somatic neural system and autonomic neural system
• Somatic neural system relays impulses from the CNS to skeletal muscles
• Autonomic neural system transmits impulses from the CNS to the involuntary organs and smooth muscles of the body
• Autonomic neural system is further classified into sympathetic neural system and parasympathetic neural system
• A neuron is a microscopic structure composed of cell body, dendrites, and axon
• The cell body contains cytoplasm with typical cell organelles and granular bodies called Nissl’s granules
• Short fibres branching repeatedly and projecting out of the cell body are called dendrites
• Dendrites also contain Nissl’s granules
• Each dendrite branch terminates as a bulb-like structure called a synaptic knob
• Synaptic knobs possess synaptic vesicles containing neurotransmitters
• Myelinated nerve fibers are enveloped with Schwann cells
• Schwann cells form a myelin sheath around the axon
• Gaps between two adjacent myelin sheaths are called nodes of Ranvier
• When a neuron is not conducting any impulse:

  • The axonal membrane is more permeable to potassium ions
  • The axonal membrane is nearly impermeable to sodium ions
  • The membrane is impermeable to negatively charged proteins in the axoplasm
• Axoplasm inside the axon:

  • Contains a high concentration of K+ and negatively charged proteins
  • Contains a low concentration of Na+
• Fluid outside the axon:

  • Contains a low concentration of K+
  • Contains a high concentration of Na+
  • Forms a concentration gradient
• Ionic gradients across the resting membrane are maintained by the sodium-potassium pump:

  • It transports 3 Na+ outwards for 2 K+ into the cell
• Result of the sodium-potassium pump activity:

  • Outer surface of the axonal membrane possesses a positive charge
  • Inner surface of the axonal membrane becomes negatively charged
  • Axonal membrane is polarized
• The electrical potential difference across the axonal membrane is called the resting potential
• When a stimulus is applied at a site on the polarised membrane, the membrane at the site A becomes freely permeable to Na+
• This leads to a rapid influx of Na+ followed by the reversal of the polarity at that site:

  • The outer surface of the membrane becomes negatively charged
  • The inner side becomes positively charged
• The polarity of the membrane at the site A is reversed and hence depolarised
• The electrical potential difference across the plasma membrane at the site A is called the action potential, which is termed as a nerve impulse
• At sites immediately ahead, the axon membrane has a positive charge on the outer surface and a negative charge on its inner surface
• As a result, a current flows on the inner surface from site A to site B
• On the outer surface, current flows from site B to site A to complete the circuit of current flow
• A synapse is formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron
• The membranes may or may not be separated by a gap called synaptic cleft
• There are two types of synapses: electrical synapses and chemical synapses
• At electrical synapses, the membranes of pre- and post-synaptic neurons are in close proximity
• Electrical current can flow directly from one neuron into the other across electrical synapses
• Transmission of an impulse across electrical synapses is faster than across chemical synapses
• At chemical synapses, the membranes of pre- and post-synaptic neurons are separated by a fluid-filled space called synaptic cleft
• Neurotransmitters are involved in the transmission of impulses at chemical synapses