Neuron

Neuron:
• human body has at least 100 billion neurons
• smallest functional unit of nervous system; a neuron is a nerve cell and its processes
• morphologically and biochemically designed to receive stimuli and transmit impulses (it is excitable); plasma membrane has a electrical voltage gradient between the inside and outside the cell
• large complex cells with a cell body and one or more slender cytoplasmic processes that project (called axons and dendrites – often called nerve fibres)
• characteristically have a long life (last over the lifetime of the person) ; amitotic (unable to divide); have a high metabolic rate
• neuron has 3 functional regions – receptor (input), conducting (transmission), secretory (output)
• neurons are very ‘irritable’ (excitable); when stimulated  electrical impulse is conducted along its length (they are polarised cells)
• cytoplasm (the axoplasm) flows around the neuron in a system of microtubules, microfilaments and neurofilaments (these structural proteins help maintain shape as wells as assist with the intracellular transport)
• neurofilaments are unique to neurons (microtubules and microfilaments occur in other cells); made up of fibrous monomers that are twisted and packed together; modified neurofilaments are a pathological feature of Alzheimer’s disease
• spectrin and ankyrin group of proteins link this cytoskeleton to the plasma membrane
• anterograde transport is the transport of materials to the axon terminals (proteins and transmitter substances up to 400mm/day; axoplasm transported at rate of up to 3mm/day); retrograde is the transport away from axon terminals (less than anterograde; returns used vesicles and other substances to cell body)

Cell body (soma/perikaryon):
large spherical centrally located nucleus surrounded by granular cytoplasm
ranges in size from 5 to 140 μm
‘biosynthetic’ centre of neuron
most of the cell bodies are located in the CNS (protected by bones of cranium and vertebral column)
clusters of cell bodies in CNS – called nuclei; in PNS, called ganglia
nucleus – usually located centrally in cell body; chromosomes are in an uncoiled state (as do not need to duplicate self); usually only a single prominent nucleolus

Neuron processes (neurites):
cytoplasmic extensions from cell body of all neurons
bundles of neuron processes in CNS called tracts; in PNS, called nerves
two types – dendrites and axons:

Dendrites:
(need to state what they are)
in motor neurons they are short, tapering extensions of cell body
receptive or input region by providing large surface area for reception from other neurons
they conduct electrical signals towards cell body
have ‘thorny’ appendages – dendritic spines – for close contact (synapses) with other neurons
rarely myelinated

Axon:
conducting component of neuron  generate and transmit nerve impulses away from cell body
each neuron has one axon arising from cone shaped area of cell body (called axon hillock)  then narrows to form a slender process
axon hillock is the part of the cell body that has no Nissel substance – it is where the axon originates
the ‘initial segment’ is the first part of the axon where the action potentials are initiated
varies in length. Long axons are called nerve fibres.
occasionally axons give off branches – axon collaterals
can be myelinated or unmyelinated
branches at end – usually 10 000+ branches – called telodendria
end of telodendria is bulbous – called ‘axon terminals’, ‘synaptic knobs’, ‘boutons’  the secretary component of the neuron – neurotransmitters are stared in vesicles and released into extracellular space in response to nerve impulse

Synapse:
• site of communication between neurons; occurs between axon of one neuron and the dendrite of another neuron; communication is only in one direction (axon  dendrite)
• most neurons connect to 1000+ other neurons
• two types – chemical (neurons communicate by neurotransmitters) and electrical (neurons connected by gap junctions that ions diffuse across)
• 20-40nm space between neurons

Myelin Sheath:
proteolipid membrane that covers many nerve fibres
protects and electrically insulates fibres from one another to increase speed of transmission of impulse
unmyelinated fibre transmit impulses slower
formed by Schwann cells in PNS and oligodendrocyte in CNS – wraps around neuron several times
part of Schwann cell that surround the myelin sheath  called the neurilemma (or sheath of Schwann)
Schwann cells along a neuron do not touch each other – the gaps are called nodes of Ranvier or neurofibril nodes; there is a greater current density in these regions; occur every 0.5-2mm along an axon
demyelinating diseases (eg Multiple sclerosis) lead to a slowing of propagation of action potential
regions of CNS that has dense collections of myelinated fibres  white matter; Grey matter is mostly cell bodies and unmyelinated fibres.

Types of neurons:
Structural types:
multipolar (many dendrites and one long axon extend from cell body; most common type)
bipolar (one dendrite and one axon extend from cell body; receptor neurons in special senses, such as retinal bipolar cells)
unipolar (single process extends short distance from cell body  divides to form two processes; common in PNS – posterior root ganglia of spinal cord; one process goes to a peripheral structure and other process enters CNS)
Functional types:
• sensory/afferent (transmit impulses from sensory receptors toward the CNS)
• motor/efferent (transmit impulses from CNS to an effector)
• association/interneuron (transmit impulses within the CNS; make up over 99% of neurons in body)
Types based on size:
• Golgi type 1 neurons (long axon, up to 100cm; form long fibres in tracts of brain and spinal cord and fibres of peripheral nerves)
• Golgi type 2 neurons (short axon; in cerebral and cerebellar cortex)

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