Nervous system

• The nervous system


  Detecting and reacting to what is happening in and outside the body through the sensory input, integration and motor output

  
• Sensory input


  Information gathered through sensory receptors to monitor changes occurring both inside and outside the body
• Integration


  The sensory input is processed and interpreted to decide what should be done at each moment
• Motor output


  The response of the nervous system through activation of effector organs (muscles and glands)
• Central nervous system (CNS)


  Integrating sensory information (from peripheral system) and dictate motor output coordinating both conscious and unconscious activities

  
• Peripheral nervous system (PNS)


  Conducting two way signal between brain and body, governing basic muscle reflexes without input from the brain

  
• Astrocytes

  • Help regulate blood flow in the brain
  • maintain the composition of the fluid that surrounds neurons
  • regulate communication between neurons at the synapse
  • contribute to the formation of the blood-brain barrier

  
• Microglia


  • Related to the macrophages of the immune system and act as scavengers to remove dead cells and other debris
• Ependymal cells


  • Line the ventricles of the brain and the central canal of the spinal cord, have hair-like cilia that beat to promote circulation of the cerebrospinal fluid found inside the ventricles and spinal canal

  
• Oligodendrocytes


  • Produce myelin, the insulating substance that forms a sheath around the axons of many neurons
• Satellite glial cells


  • Cover the cell bodies of neurons in PNS ganglia, thought to support the function of the neurons and might act as a protective barrier
• Schwann cells


  • Produce myelin, the insulating substance that forms a sheath around the axons of many neurons

  
• Neuron


  Consists of cell body, dendrites, and axon

  
• Gray matter


  Contains neuron cell bodies and dendrites, and microglial cells
• White matter


  Contains neuron myelinated axons, oligodendrocytes/Schwann cells, and astrocytes
• Brain cortex


  • Frontal lobe: motor cortex, Temporal lobe: auditory cortex, Parietal lobe: somatosensory cortex, Occipital lobe: visual cortex, Cerebellum: tone, posture, balance, Brainstem: respiratory and cardiac function

  
• Afferent neurons


  Carry information to the CNS
• Efferent neurons


  Carry information away from the CNS
• Spinal nerves


  31 pairs, carry impulses to and from the spinal cord
• Cranial nerves


  12 pairs, carry impulses to and from the brainstem
• Meninges


  Dura mater, arachnoid, pia mater
• Cerebrospinal fluid


  100-160 mL, limpid, ~15 mmHg, contains proteins, glucose, electrolytes, produced in brain ventricles, kept in motion by cilia of ependymal cells
• Blood-brain barrier


  Selective barrier separating blood and interstitial liquid in CNS, formed by endothelial cells, basal membrane, astrocytes, pericytes
• Resting membrane potential


  • 65mV, due to ion channels and Na/K pump
• Gated ion channels


  Ligand-gated, mechano-gated, voltage-gated
• Excitatory and inhibitory post synaptic potentials

  Net influx of positive charge → depolarization → excitatory, Net influx of negative charge → hyperpolarization → inhibitory
• Action potential


  All-or-nothing, caused by permeability changes in the plasma membrane, Na+ channels open causing depolarization, K+ channels open later causing repolarization and hyperpolarization
• Net influx of positive charge


  Depolarization → excitatory post synaptic potential (EPSP) (it makes it more likely for an action potential to occur)
• Net influx of negative charge (Cl-)


  Hyperpolarization → inhibitory post synaptic potential (IPSP) (it makes it less likely for an action potential to occur)
• Action potential: all-or-nothing
• Action potential


  1. Na+ channels open, causing depolarization

  2. K+ channels open later, over a longer time interval, causing repolarization and hyperpolarization

  3. The action potential is caused by permeability changes in the plasma membrane
• Saltatory conduction in myelinated axons


  Voltage-gated channels are located only in the axon. The myelin sheat does not have voltage-gated channels. Nodes of Ranvier
• Synapse


  Junction that mediates information transfer from one neuron to the next or from a neuron to an effector cell (muscle cell or gland cell). The neuron conducting impulses toward the synapse is the presynaptic neuron, and the neuron transmitting the electrical signal away from the synapse is the postsynaptic neuron.
• Electrical synapse


  Transmission of the electrical excitation between two cells via ion current. The transmission is without loss of time and can be bidirectional. For example in retina.
• Electrical synapse


  1. Depolarisation of the presynaptic cell through an action potential

  2. Development of a potential gradient

  3. Ion current through the gap junctions

  4. Depolarisation of the postsynaptic cell (= electric coupling)

  5. Release of an action potential at the postsynaptic cell
• Chemical synapse

  Most common type. Transmission of information via messenger substance (=neurotransmitter). Transmission only unidirectional.
• Chemical synapse

  1. Presynapse: conversion of electrical signal (action potential) into chemical signal (release of transmitter)

  2. Synaptic cleft: release of transmitter of the presynapse

  3. Postsynapse: the transmitter binds to receptors whereby the chemical signal is transformed into an electrical signal (postsynaptic potential)

  4. Termination of the signal transmission via degradation or reuptake
• Neurotransmitters

  • Acetylcholine
  • Glutamate
  • GABA
  • Glycine
  • Dopamine
  • Serotonin
• Neurotransmitter reuptake


  A neurotransmitter is bound to a receptor for a very short time (ms). The synaptic cleft is kept clean by removing the neurotransmitter in different ways: Reuptake by the presynaptic terminal or astrocytes, Degradation by enzymes, Diffusion away from the synapse
• Substances that change the levels of neurotransmitters

  • Stimulate the release of a neurotransmitter
  • Inhibit the reuptake of a neurotransmitter
  • Be an agonist: produce the same effect (bind to same receptor and give same signal)
  • Be an antagonist: produce the opposite effect (bind to same receptor but prevent signal)