Neurones

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Learning objectives

  • Understand the functions and structure of the CNS
  • Role of Neurons

Cellular building blocks of the CNS

The brain contains 1011 neurons and approximately 10-50 times as many glial cells. About 40% of the human genome is involved in some part with its development and structure. The excitability of neurons is key to how the brain works. The brain requires an uninterrupted supply of ATP derived from oxidative phosphorylation to preserve this excitability. The Brain consists of two main types of cells - neurons and glial cells.

Neurons are the main "brain cells" involved in information processing, information storage, sensing input and generating output. Their cell bodies which and when many are packed together are known as grey matter and can be seen on the cortical surface and subcortical nuclei and brainstem nuclei and cerebellum and are peripherally called ganglia. The structure of individual neurons also varies depending on the individual role they play. Neurons are packed within the central nervous system at about 80,000 per mm3 of cortex and this is uniform across most of the cortex. The exception is the high density of 200,000 per mm3 in the primary visual cortex.

  • Neurons are highly variable in their appearance and the function they perform, from Purkinje cells of the cerebellum to those that lie in the retina (the eye is an outgrowth of the brain) and to hair cells in the auditory system. Neurons in their simplest form consist of one end with a cell body which receives input from the dendrites. It is the inputs here which if occurring together or closely related in time can cause the neuron to fire off an action potential. This arises on the distal side of the cell body at an area called the axon hillock. There is localised depolarisation and this, in turn, passes along the axon to the axon terminal with what are called terminal boutons at the end which releases neurotransmitters into a synaptic cleft and can cause depolarisation of the next neurone through its dendrites. Collections of cell bodies are the grey matter and the often myelinated "wire like" axons are the white matter.
  • Direction of information flow : input → dendrite neuron 1 → cell body → axon → terminal bouton → synapse → dendrite of neuron 2
  • Neurons have a cell nucleus surrounded by a soma or cell body rich in mitochondria, Golgi complexes, ribosomes, and smooth endoplasmic reticulum. Huge amounts of protein synthesis happen within the cell body and the dendrites but not in the axons. A sign of the protein production is the presence of Nissl substance which represents ribosomes. The axons transport substances including neurotransmitters, mitochondria, enzymes to make neurotransmitters from the cell body to the axon terminal. This is called anterograde flow. This is done actively using microtubules and molecules developed for the purpose. There are two kinds of anterograde transport - fast (30 cm/day) and slow (1 cm/day). Fast or rapid transplant carries proteins, vesicles, mitochondria. slow transport contains more structural elements. The soma also sends brain-derived neurotrophic factor which maintains axonal and synaptic health. Old organelles can be transported retrogradely at 15 cm/day. Anterograde transport uses tubulin and ATPases. Retrograde uses dynein ATPase. Retrograde transport is important in the pathogenesis of some diseases such as tetanus and rabies.