The Nervous system I

• A nervous system is a structure that enables us to react/respond to a situation/stimulus
• Logical order of a nervous system

  • Stimulus
  • Response of the organism
  • Assessment of the stimulus
  • Evaluation of the impact of the response
  • Communication to responding structures
  • Preparation of the response
• In principle, that logical structure is also followed by single-celled organisms like amoeba or individual mammalian cells
• Single-celled organisms such as protists and bacteria can detect changes in their environment and respond to stimuli using receptor proteins embedded in their cell membranes
• A nervous system is not that different. Individual cells respond via receptors and signaling cascades but the responses of individual cells may be accumulated
• The Hippocampal Trisynaptic Circuit, involving the hippocampus and cortex, plays a role in learning, forming memories, and retrieving them
• The Hippocampal Trisynaptic Circuit comprises regions with neurons in layers and constitutes a circular loop receiving information from layer 2 of the Entorhinal Cortex and returning it to the EC in layer 5
• The formation of memories is an example of neural plasticity – the ability of the brain or neuron to adjust in response to a new situation or environment. Changes can strengthen or weaken signaling at a synapse
• Short-term memory is accessed via the hippocampus. The hippocampus also plays a role in forming long-term memory, which is stored in the cerebral cortex. Some consolidation of memory is thought to occur during sleep
• Nervous system structures in different organisms

  • Hydra (cnidarian)
  • Sea star (echinoderm)
  • Planarian (flatworm)
  • Leech (annelid)
  • Salamander (vertebrate)
  • Insect (arthropod)
  • Chiton (mollusc)
  • Squid (moll)
• Types of nervous systems

  • The simplest animals with nervous systems, the cnidarians, have neurons arranged in nerve nets; no ganglia and no centralization
  • More complex animals have nerves, which are bundles that consist of the axons of multiple nerve cells
  • Bilaterally symmetrical animals exhibit cephalization, the clustering of sensory organs at the front end of the body
  • Annelids and arthropods have segmentally arranged clusters of neurons called ganglia
  • In vertebrates, the CNS is composed of the brain and spinal cord
  • The peripheral nervous system (PNS) is composed of nerves and ganglia
• Nerve net

  A series of interconnected nerve cells
• Some consolidation of memory is thought to occur during sleep
• Cephalization


  The clustering of sensory organs at the front end of the body
• Central nervous system (CNS)


  Consists of the brain and spinal cord
• Peripheral nervous system (PNS)

  Composed of nerves and ganglia
• Efferent neurons


  Neurons that carry signals away from the central nervous system
• Afferent neurons


  Neurons that carry signals towards the central nervous system
• The Peripheral Nervous System transmits information to and from the CNS and regulates movement and the internal environment
• The autonomic nervous system regulates smooth and cardiac muscles and is generally involuntary
• Divisions of the autonomic nervous system

  • Sympathetic division
  • Parasympathetic division
  • Enteric division
• The motor system carries signals to skeletal muscles and is voluntary
• Components of the PNS

  • Motor system
  • Autonomic nervous system
• NS are found in

  • Brain
  • Spinal cord (dorsal nerve cord)
  • Central nervous system (CNS)
  • Peripheral nervous system (PNS)
  • Cranial nerves
  • Ganglia outside CNS
  • Spinal nerves
  • Sensory ganglia
• Organization of the Vertebrate Nervous System

  1. Efferent neurons

  2. Afferent neurons

  3. Central Nervous System (information processing)

  4. Peripheral Nervous System

  5. Sensory receptors

  6. Internal and external stimuli

  7. Autonomic nervous system

  8. Motor system

  9. Control of skeletal muscle

  10. Sympathetic division

  11. Parasympathetic division

  12. Enteric division

  13. Control of smooth muscles, cardiac muscles, glands
• The sympathetic division regulates arousal and energy generation (“fight-or-flight” response)
• The parasympathetic division has antagonistic effects on target organs and promotes calming and a return to “rest and digest” functions
• In the parasympathetic system, the cell body of the preganglionic neurons is located within the CNS. They extend a long axon to the postganglionic neuron located close to the target organ. Acetylcholine is the neurotransmitter released by the postganglionic neuron
• The enteric division controls activity of the digestive tract, pancreas, and gallbladder
• Gray matter consists of neuron cell bodies, dendrites, and unmyelinated axons
• In the sympathetic system, the preganglionic neuron is shorter originating within the spinal cord and synapse with the postganglionic neuron that extends a long axon to the target organ. The catecholamines (epinephrine and norepinephrine) are released on the target organs
• Cerebrospinal Fluid is produced by the secretory epithelium of the choroid plexus at a rate of ∼0.4 ml·min−1·g tissue−1; 500-600ml/day
• Cerebrospinal Fluid is a clear, colourless liquid composed mostly of water
• Cerebrospinal Fluid in the CNS amounts to 150ml volume and is replaced 3-4 x daily
• White matter consists of bundles of myelinated axons
• Cerebrospinal Fluid is formed in the ventricles and then circulates in the subarachnoid space then absorbed into venous circulation
• Cerebrospinal Fluid supplies water, amino acids, ions and removes metabolic wastes
• Any condition that causes CSF accumulation – overproduction or obstruction causes serious neurological deficits (e.g., hydrocephalus as seen in a fetus or newborn)
• Choroid plexus is produced at a rate of ∼0.4 ml·min−1·g tissue−1; 500-600ml/day in CNS amounts to 150ml volume and is replaced 3-4 x daily
• Formation of Choroid plexus and CSF circulation

  1. Formed in the ventricles, circulates in the subarachnoid space, then absorbed into venous circulation

  2. Supplies water, amino acids, ions

  3. Removes metabolites