Nerve tissue Flashcards
CNS vs PNS
CNS: brain and spinal cord
PNS: cranial, spinal and peripheral nerves + ganglia
cell components of nerve tissue + origin of tissue
Origin of both: neural crest of the ectoderm
- neurons (impulse generation at snyapses)
- ganglia (supporting cells that affect metabolism and activity of neurons)
!!! PLUS BLOOD BRAIN BARRIER –> rich vascular supply
Neuron functions
- receive signals
- integrate signals (determine whether or not they should be passed on)
- conduct signals to target cells (either neurons OR muscles/glands)
Properties of neurons
- excitability: ability to generate large responses from detection to small stimuli
- conductivity: ability to produce signals and trasmit them along nerve fibers
- functional polarity: asymmetry in distribution of cellular components which allows a unidireactional impulse travel
Somatic vs visceral neurones
SENSORY:
1. somatic afferent: convey sensory impulses (like pain) from body surface
2. visceral afferent: convey sensory stimuli from internal organs (glands/ blood vessels)
MOTOR:
1. somatic efferent: carry voluntary impulses to skeletal muscles
2. visceral efferent: carry involuntary impluses to smooth muscle, cardiac conducting cells (purkinje) and glands
SUMMARY: somatic sense the environment and visceral sense the interior milieu
grey vs white matter
GREY: cell bodies (unmyelinated)
WHITE: myelinated axons and axon terminals
Ways to classify neurones
- function (sensory, motor, inter)
- number of processes (uni,bi,pseudouni, multi polar)
- axon length (gogli type 1/2)
types of FUCNTIONAL neurons
- SENSORY: afferent, deliver info from PNS to CNS, located at dorsal root (posterior)
- MOTOR: efferent, deliver info from CNS to muscles/glands, located in ventral root (anterior)
- INTERNEURONS: connect motor and sensory and are fully located within the grey matter of the spinal cord
types of MORPHOLOGICAL neurons (number of processes)
- unipolar: one axon, no dendrites
- bipolar: one axon one dendrite
- peudounipolar: one process but one end extend to periphery and the other extends to CNS
- multipolar: one axon and 2+ dendrites
what types of neurons are associated with each classification of number of processes?
unipolar: sensory
bipolar: sensory of retina and olfactory nerves
multipolar: interneuron and motor
!! pyramidal: purkinje fibers of the cerebellum
types of AXON LENGTH neurons
GOLGI TYPE 1: projection neurons - long axons starting from CNS going to PNS
GOLGI TYPE 2: local circuit neurons: short axons residing in CNS and do not exit it (make connections with nearby cells)
Characteristics of the neuronal cell body
-large euchromatic nucleus with evident nucleolus
-Nissl bodies: dense basophilic perinuclear RER/free ribosomes
-abundant Golgi
-abundant mitochondria
-lysosomes and lipofuscin
-highly organised cytoskeleton
Cytoskeletal components and distribution within nerve cells
- microfilaments: peripheral, under plasma membrane of entire neuron
- neurofilaments: axon mainly and some in soma
- microtubules: perinuclear MTOC, axon and dendrites
Why do neurons need such high cytoskeletal organisation?
secretory proteins are made in the cell soma (due to presence of RER and Golgi) and then need transport through the axon to axon terminals for secretion (eg. neurotransmitters)
This occurs via motor proteins (kinesin/dynin)
Characteristics of dendrites
- receive impulses from other neurons
-similar cytoplasmic content to cell body but without Golgi
-larger diameter than axons
-UNMYELINATED
-variable branching at variable distances from soma
-SPINES: protrusions from plasma mebrane that are the sites of synapse connections with other terminal axons
Are spines stable or unstable?
UNSTABLE: can be synthesised and eliminated quickly (hence are linked with plasticity and memory)
characteristics of axons:
-single branch and uniform morphology
-have further branching at axon terminal
-originates at axon hillock (no nissl bodies)
-only organelle present is mitochondria
-MYELINATED
-high extent of cytoskeletal organisation
What is the axon hillock?
the origin of the axon from the nerve soma which lacks Nissl bodies (only mitohchondria are present)
2 types of axon transport and molecules used for
- FAST:
-contains both anterograde and retrograde (bidirectional)
-ANT used for vesicles, nutrients, membrane bound organelles, aa
-RET used for endocytosed toxins and viruses - SLOW:
-contains only anterograde (unidirectional)
-used for structural elements like neurofilaments and matrix proteins
What is the origin of the cytoskeletal elements in nerve cells
originate from MTOC and have their negative ends towards the nucleus and their positive ends towards the periphery
mechanism of axon transport
Anterograde: from soma to axon terminals
Retrograde: from axon terminals to soma
MOTOR PROTEINS: Kinesin for anterograde and dynin for retrograde
-proteins contain a head and tail component
-head binds to MTs and has ATPase activity
-tail binds to cargo
!! ATP hydrolysis allows change in conformation causing motor proteins to slide along MTs
Types of classifications for synapses (5)
- ACTION (chemical/ electrical)
- POST SYNAPTIC CELL
- SITE OF RELEASE (discrete/diffuse)
- EFFECT (excitatory/ inhibitory)
- SPEED (ionotropic/ metabotropic)
Electrical synapses mechanism
-present CNS where cells need to generate synchronous signals (eg. cardiomyocytes)
-contain gap junctions for ion flow
-flow of impulse can be bidirectional
chemical synapse mechanism
- AP reached preSN
- opening of Ca2+ channels (voltage gated)
- Ca2+ influx and fusion with vesicles
- Vesicles move to membrane
- Exocytosis of vesicles
- Neurostransmitter diffusion across cleft
- Binding to postSN receptors
- Signal initiated via influx of Na+ for depol
2 sources of synaptic vesicle release
- RESERVE POOL: located at greatest distance from active zone. While the RRP vesicles are exocytosed, the RP vesicles move along cytoskeleton to the active zone to replace the RRPs and be then secreted
- READILY RELEASABLE POOL: ready supply of active zone vesicles that are imediately released at the synapse
Process of vesicle release in synapse (6 STEPS)
- Dettachment of vesicle from cytoskeleton: Ca2+ activates Ca2+/calmodullin protein kinase that phosphorylates synapsins binding actin and vesicle and allows detachment
- mobilisation and direction towards active zone: vesicle membrane protein Rab3 interacts with active zone intrinsic protein Rim
- Interaction between V-snare (synaptobrevin) with T-snare on membrane (SNAP25 and sintaxin)
- Priming: interaction of V/T snares generates pulling force to cause vesicle contact with membrane
- Ca2+ binds to synaptotagmin, causes conformational shape allowing interaction with T-snares, binds to membrane phospholipids and causes FUSION PORE formation
- Endocytosus (either fast or slow) for recycling
how are neurotransmitters recycled (3)
- hydrolytic enzymes
- fast/slow endocytosis
- diffusion from cleft into blood and then removal via circulation
types of synapses classified depending on post synaptic cell
- axodendritic (dendritic spines)
- axosomatic (cell body
- axoaxonic (axon terminals)
- cells of the body that are not neurons (eg. effectors and glands)
Porocytosis def
the secretion of neurotransmitter without involving fusion of vesicle with the presynaptic membrane (transient fusion pore)
synapses classified by EFFECT
- EXCITATORY: opens cation channels causing Na+ influx and inducing depolarisation.
Neurotransmitters = glutamine, Ach, serotonin - INHIBITORY: opens anion channels, causing influx of Cl- and inducing hyperpolarisation
Neurotransmitters = GABA, glycine
synapses classified by SITE of secretion
- DISCRETE: NTs only released at axon terminals (at active zone)
- DIFFUSE: NT secretion is not limited to active zone, typical of autonomic NS
-VARICOSITIES: repeated swellings containing NT vesicles
-EN PASSANT: singular bulge on axon with vesicles
Synapses classified by SPEED OF ACTION
- IONOTROPIC (fast): contain ligand gated ion channels as receptors to affect membrane permeability - rapid and short lasting effect
- METABOTROPIC (slow): receptors are GPCRs that trigger 2ndary messengers to activate channels
Types of glial cells and location
CNS: astrocytes, oligodendrocytes, microglial and ependymal cells
PNS: scwann and satellite cells
General functions of glial cells (6)
- physical support
- cell body insulation
- participation in BBB
- regulate CSF composition
- metabolic exchange
- neurotransmitter clearance
!! CAN PROLIFERATE, NON EXCITABLE
scwann cells: characteristics, origin and funciton
ORIGIN: neural crest, develop with expression of neuroregulin 1
CHARACTERISTICS:
-flattened cell and nuc
-low Golgi/mitochondria
FUNCTION: produce myelin sheath of peripheral neurones
Myelin sheath coat functions (3)
- electric insulation
- increases speed of impulses (saltatory propagation)
- provides scaffold for nerve regeneration
3 possible phenotypes of scwann cells and their functions
- myelinating - myelinate large diameter PNS axons
- Nonmyelinating - enclose small axons within grooves of membrane
- repair cells - provides scaffolding for nerve regeneration during injury
process of myelin sheath formation by scwann cells (X STEPS)
- SC surrounds axon and membrane becomes polarised – first few layers of sheath are called lamellae
- 2 ends of the SC prolongments meet and form the 3 domains of the early sheath (abaxomal/ mesaxon/ periaxonal)
- further wrapping forms the concentric (internal) layers of the sheath and the peripheral region where nucleus and cytoplasm are pushed (sheath of schwann)
- Final form of sheath containing: outer mesaxon, concentric myelin sheath and inner mesaxon
What are the 3 domains formed by the early myelin sheath?
- ABAXOMAL: PM of SC that is in contact with environement
- MESAXON: meeting place of the two prolongments
- PERIAXONAL: PM of SC directly in contact with axon
What are Ramek bundles?
unmyelinated groups of axons enclosed by non myelinating schwann cells.
axons distributed along periphery of the SC membrane
What are the components of a nerve fiber?
Axon + myelin sheath
What proteins are expressed within the myelin sheathof the PNS?
P0, MBP (myelin binding protein), PLP (Myelin Proteolipid Protein)
What are the visible areas of the myelin sheath under microscopy
- MDL (major dense lines): repeating adhesions of inner layer of PM. Dense bcos its rich in proteins
- IPL (intraperiod lines): spaces between MDLs due to fusion of outer layer. Faint bcos it is a phospholipi layer
Structure of a myelinated fiber across its length
- Internodal segments: myelinated portion of axon
- Nodes of Ranvier: space between internodal segments that only have a slight myelin cover due to interdigitation of SCs
- Schmidt-Lanterman clefts: small discontinuities of myelin lamellae –> small pockets of cytoplasm left behind during myelination which allow nutrient exhange between myelin-myelin or myelin-axon
How is resting potential maintained?
active transport pump and natural increased permeability of axon membrane to K+ (leaky channels):
2K+ in and 3Na+ out –> leads to smaller conc of positive charge inside than outside –> -70mV
Relationship between speed of impulse, axon diameter and myelin thickness
all in direct proportion
Response of nerve fiber to injury
-nucleus pused to periphery
-stop in protein synthesis
-terminal axon atrophies
-fragments are removed by microglial cells (acting as macrophages)
-neural bodies synthesise components needed to replace organelles and PM
-SC differentation into repair schwann cells
-proliferation of SC to form channels though which axon is generated
!! unmyelinated axons have less efficient repair due to decreased layers of SC
Satelite cells characteristics and functions
-cover neuron soma at ganglion
-cuboidal cells
-VERY EVIDENT NUCLEI
-support neurons and provide electrical insulation
Astroglial cells funcion
-moves metabolites
-contributes to tight junctions in BBB
-covers portions of CNS axons with less/no myelin (nodes of ranvier and synapses)
-control of K+ concentration (K+ buffering)
2 type of astroglial cells
- protoplasmic: numerous and short prolongments, present in CNS grey matter, large soma
- fibrous: fewer prolongments, smaller, found in CNS white matter
Characteristics of astrocytes
-large and star shaped
-branched prolongments
-expression of GFAP that makes up intermediate filaments
-vimentin and nestin proteins expressed during development
-glycogen cytoplasmic granules
-gap junctions in terminal portions form the astrocytic syncytium
3 forms of microglial cells
- amoeboid: early form
- ramified: resting form
- reactive: active form
function of microglial cells
-act as specialised macrophages
-clean CNS via phagocytosis
-release metabolic factors that help regulate homeostasis
Components of the blood brain barrier (4) and its function
4 components make up 1 NEUROVASCULAR UNIT:
- blood vessel endothelium (not fenestrated, has tight junctions)
- astrocytes
- basement membrane
- pericytes
FUNCTIONS:
-only allow substances with weight < 500Da to pass freely (hence provides controlled tranport)
-mantains electrolyte balance
-prevents entrance of toxins
-controls intake of nutrients
ependymal cells characteristics
-columnar cells
-line ventricles and central spinal cord canal
-cilliated + with microvilli (for CSF movement)
-contain apical junctions (tight and communicating)
functions of ependymal cells
-secrete and monitor CSF
-AT CHOROID PLEXUS: they produce the CSF
tanycytes roles and characteristics
-specialised ependymal cells that are only found in 3rd ventricle
-long basal processes surrounding blood vessels
-secrete/absorb substances needed for regulating CSF
-contain tight junctions to regulate transport
oligodendrocytes, 2 types and function
-control the myelin shealth of axons in the CNS
- PERINEURAL: grey matter
- INTERFASCICULAR: white matter
oligodendrocytes characteristics
-cytoplasm rich in RER/MTs/Golgi
-1 oligodendrocyte can myelinate multiple axons (different to SCs)
-express different myelin specific proteins: MAG/MOG (+MBP/PLP)
Pathology associated with oligodendrocytes/myelin
MULTIPLE SCLEROSIS:
destruction of myelination of CNS axons due to targeting of myelin specific proteins by the immune system (causing degeneration)
structure of a peripheral nerve
- epineurium: surrounds entire peripheral nerve , dense irregular CT (contain fibroblasts, macrophages, mast cells)
HIGH VASCULARISATION - perineurium: surrounds fascicles, forms blood nerve barrier, contains perineural cells
- endoneureum: surrounds individual nerve fibers, dense irregular CT, associated with adipose
composition and location of the blood nerve barrier
at the level of Perineurium
-specialised CT containing perineural cells
-2-4 layers of squamous cells with basal lamina on BOTH SIDES of the layer
-tight junctions which express ZO1 protein
-cells are contractile due to actin in cytoplasm
-collagen 3 fibers between layers
FUNCTION: maints ionic milieu so that there are no drastic conc changes that affect andoneurium
peripheral nerve definition
a bundle of nerves that are held together by CT
ganglion definition
a bundle of nerve bodies in the PNS
differences between the myelin sheath of the CNS and the PNS (6)
- P made by SC, C of oligodendrocytes
- SC only myelinate 1 axon whereas ODC can myelinate multiple
- different myelin specific proteins
- Saltatory conduction more efficient in CNS bcos nodes of ranvier are larger
- SC have an external lamina, ODC dont
- CNS myelin has less SL clefts bcos metabolism is mainly regulated by astrocytes
3 types of sensory receptors
- exteroreceptors (external stimuli like temp/smell/touch)
- enteroreceptors (stimuli from within body like stretching of organs)
- proprioreceptors (sense of body positioing)
why are nerve cells wavy in appearance
to allow them to stretch with the movement of the tissue
function, location and structure of pacinian corpuscles
-located all around the body (often in hypodermis)
-sensory endings acting as proprioreceptors –> distinguishes between diff levels of pressure exerted
-made of lammellae layers of CT separated by a gel
