Neurons and Glia Flashcards
How many cells are in the human brain
- rational number - 100 billion neurons, 10X more glial cells
Cells in the human brain by number
- Neurons 10%
- Gila 90%
Cells in the human brain by volume
- Neurons 50%
- Astrocytes 15%
- Oligodendroglia 15%
- Microglia 7%
- Extracellular Fluid 10%
- Blood 3%
Estimates for number of neurons in. the human brain
- 21-26 billion neurons in cerebral cortex, 101 billion in cerebellum
- most recent numbers - 85 billion neurons, amount the same number of glial cells
Number of cells in human brain
- regarding 10 X more glial cells, it is true only in subcortical areas like thalamus
- In cerebral cortex - glial cells outnumber neurons by <2 times
- In cerebellum - neurons outnumber glial cells by 25:1
What are the 2 advantages of the human brain
- densely packed neurons, space-saving arrangement
- largest brain in the space-saving arrangement of primates
- but most characteristics of human brain are as expected of primates
Describe a neuron
- functional units of the nervous system
- responsible for information processing & communication
- highly specialized excitable cells, use electrical impulses & chemicals to communicate
- have specialized process called dendrites & axons
Who discovered the different shapes & sizes of neurons
- Santiago Ramon Y Cajal - Father of modern neuroscience
Classification of neurons by morphology
- Golgi Type I: long axon, projection (project their info to distant far away places), Pyramidal cells, Purkinje cells
- Golgi Type II: short axon, remains local inside the CNS, smaller than Golgi Type I, they influence info processing in their neighboring areas
Who made the classification of neurons by morphology
- Camillo Golgi
How can we classify neurons functionally
- Functionally classified as secretory (neurosecretory) cells
- Neurons are heterogeneously shaped & highly compartmentalized
Define polarity
- Heterogenous distribution of cellular structures & functions among distinct compartments of cells
Where is polarity evident
- Epithelial cells (neurons are highly polarized specialized epithelial cells - neuroepithelium)
- Apical and basolateral domains
What is contained in the basolateral domain and the apical domain
- Basolateral: cell body and dendrites
- Apical: axon, axon terminals, and synapses
Describe the structural and functional polarity of neurons
- Cell body & Dendrites: input function (receptive component)
- Axon Hillock (initial segment): trigger (summing or integration)
- Axon: conduction
- Axon Terminals & Synapse: secretion of neurotransmitter
Classification based on polarity of neurons
- Pseudounipolar: exclusively sensory (afferent) & almost exclusively in the PNS and DRGs (dorsal root ganglions)
- Bipolar: exclusively sensory (afferent), bring in visual information from the retina and smell from the olfactory epithelium
- Multipolar: most CNS neurons are multipolar, motor (efferent) neurons, interneurons, & autonomic ganglia, and are responsible for executing higher order functions like cognition, learning, & memory
Difference between afferent and efferent neurons
- Afferent neurons: transmit information from the periphery to the CNS & are responsible for bringing in sensations of touch, temperature, pressure, etc. towards the CNS
- Efferent neurons: send signals towards the peripheral organs to execute a function, the cells that can tell a muscle to contract, the heart, to beat, or a gland to secrete
What percentage of neurons are pseudo unipolar or bipolar
- 1-2% of cells
Classification of neurons based on structure/histogically
- Morphology: multipolar, bipolar, pseudo polar
- Shape: mitral, basket, granule, stellate, pyramidal
- Location: ganglia, nucleus, cortex
- Neurotransmitter: cholinergic, glutamatergic, adrenergic, GABAergic
- Eponym: Purkinji, Renshaw, Cajal, Golgi, Betz, Waldeyer
- Axon length: Golgi I (long), Golgi II (short)
- Embryonic origin: neural crest, alar lamina, basal lamina
Classification of neurons based on function
- Direction of information conduction: afferent, efferent, association (interneurons)
- Effect: excitatory (let information through them), inhibitory (stops information)
Neurons in adult brain do not divide except for in what 2 regions
- Olfactory bulb (granule cell layer)
- Hippocampus (dentate gyrus)
Where do the cells in the olfactory bulb and hippocampus come from
- they come from the sub-ventricular zone (SVZ), which retains some neural stem cells into adulthood
What cells regenerate
- local
- interneurons
What cells are not known to divide
- long projection neurons
Describe Nissl substance
- Named after Franz Nissl
- RER (rough endoplasmic reticulum) & ribosomes
- Protein synthesis, transported to dendrites/axons
Define dendritic spines
- sites of synapses with other neurons
What are axons responsible for
- responsible for sending information out
What are dendrites responsible for
- responsible for bringing information in
What is the axolemma
- plasma membrane of axon
What is the axoplasm
- cytoplasm of axon
- devoid of any Nissl granules or Golgi complex
- dependent on transport of substances for survival
What is the axon hillock
- conical elevation form where axon arises (starting point of the axon)
What is the initial segment
- first 50-100 microns of the axon as it leaves axon hillock
- most excitable part of axon
- contains densely packed voltage-gated ion channels
- the site at which action potential originates
Describe myelin
- lipid rich sheath that provides electrical insulation to axons
- allows for fast (saltatory) conduction of electrical impulses
What cells wrap the axon in a myelin sheath in the PNS and CNS
- PNS: Schwann cells
- CNS: Oligodendrocytes
Define saltatory conduction
- when current jumps from one node to the next one
What is the function of neurofilaments and microtubules
- provide structure and help in intracellular transport
What does the cytoskeletal transport transport
- Organelles
- Secretory material
- Vesicles
- Membrane precursors
- Mitochondria
- Etc.
What is anterograde (Kinesin)
- Forward transport
- Rapid transport: 100-400mm/day & transports membrane and transmitter proteins
- Slow transport: 0.1-3mm/day & transports cytoskeletal proteins
What is retrograde (Dynein)
- Intermediate transport: 150-200mm/day & transports growth factor and neurotransmitters for recycling
What happens to the cytoskeletal transport if Tau protein malfunctions
- it will cause the transport system to breakdown
- becomes sticky and can lead to clogging
What is the relationship between Tau protein and Alzheimer’s dementia
- abnormal Tau proteins tend to ‘stick’ to each other which leads to Neurofibrillary Tangles (NFTs) causing disintegration of the microtubule transport system
- Neurofibrillary Tangles (NFTs) are hallmark of Alzheimer’s
What does herpes simplex, herpes zoster, poliomyelitis, & rabies all have in common
- they all use axonal transport to spread virus to different parts of the body
Relationship between speed of electrical impulse conduction and the thickness/diameter of the axon
- speed of the electrical impulse conduction increases with an increase in axon diameter
How does the cell body react to injury
- damage to the cell body or axons distally causes Chromatolysis of the cell body
- cell body swells, Nissl granules move peripherally, & nucleus displaced to periphery
How does the axon react to injury
- if injury is just to the myelin & the oligodendrocytes/Schwann cells are still alive then the axon can be remyelinated
- reaction to injury depends on severity & site of injury
What is the most abundant cell in the CNS
- astrocytes
- make up 20-40% of all glial cells
- function with intricate processes but do not function as axons or dendrites
Describe the types of astrocytes
- Fibrous astrocyte: mostly in white matter, more spread out or oval like shaped
- Protoplasmic astrocyte: mostly in gray matter, more circularly centralized shaped
What are the functions of astrocytes
- Provide structural framework for the nervous system
- Provide nutrition to neurons: store glycogen in cytoplasm, release glucose & lactate to surrounding neurons, & by using perivascular endfeet
- Wrap around synapses: insulate synaptic information, release their own transmitters ‘gliotransmitters’, & modulate synaptic transmission
- Maintain homeostasis: reuptake of excess neurotransmitters & ions
- they have their own network, form tight junctions (called gap junctions), communicate with each other over distance using calcium waves
- scaffolding for migrating immature neurons
- phagocytosis cells: glial scars after injury
Relationship between astrocytes and chronic pain
- astrocytic hypertrophy around synapses of pain pathways potentiate & prolong pain sensation
What are oligodendrocytes
- responsible for formation of myelin sheath of nerve fibers in CNS
- fewer processes
- found in rows along myelinated nerve fibers
- Myelin sheath: 80% lipid and 20% protein
- insulate axons, can wrap around same or different axons, can form as many as 60 internodal segments
Damage to the myelin sheath and oligodendrocytes
- when the myelin sheath is damaged it leads to nerve conduction problems in the myelinated axons as seen in multiple sclerosis due to autoimmune reasons and the oligodendrocytes try to repair the damaged myelin sheath every time they are damaged but every subsequent repair work is of poorer quality leading to progression of the symptoms & the disease process
Why should we be careful when applying heat modalities to multiple sclerosis (MS)
- heat disrupts the conduction process in axons and leads to worsening of the symptoms
Functions of oligodendrocytes
- myelination increases speed of nerve conduction: Saltatory propagation of action potentials
- conduction speed of myelinated axons increases linearly with the axon diameter, whereas speed of unmyelinated axon increases only with the square root of the diameter: efficient increase in speed
What happens when the myelin sheath is damaged in a patient with multiple sclerosis
- symptoms get worse with increasing temperature & improve with cooling
- effect on duration of action potential, avoid use of heat therapy
- MS only effects upper motor neurons
What are microglia
- smallest sized cells
- developmentally unrelated to neurons or other glial cells
- resident macrophages or immune cells of the CNS
- distributed in largely non-overlapping regions throughout CNS
- efficient process for scanning
- scavenging & phagocytosis of cell debris, pathogens & even unnecessary synapses: synaptic pruning
- immediate first line of defense against infectious agents in the absence of antibodies from the rest of the body
How do microglia react to a local brain injury
- minimize injury process by sequestering injured areas in the nervous tissue
- upon local brain injury, microglial processes rapidly & autonomously converge on the site of injury without cell body movement, establishing a potential barrier between the healthy and injured tissue
What are ependymal cells
- they line cavities of the brain & central canal of the spinal cord
- they are involved in production, secretion, and circulation of CSF (cerebral spinal fluid)
Describe ventricles
- cavities in the brain
- primary job is to produce the CSF (cerebral spinal fluid)
