Week 5 - Lecture 1 - Alterations in neuronal impulse conduction Flashcards
What are the two principle type of cells of nervous tissue
neuroglia and neurones
What are neuroglia cells
small cells that surround and wrap delicate neurons
in the central nervous system there’s astrocytes, microglial, ependymal and oliodendrocytes
in the PNS there is satellite and Schwann cells
what are neurons
excitable cells that transmit electrical signals
5 neuron characteristics
- extreme longevity
- 100 years or more - amitotic ( do not divide)
- high metabolic rate
- require continuous supply of oxygen and glucose - large, highly specialised cells - conduct impulses
- All have cell body and processes
- processes are dendrites and an axon
Functional classification of neurones
grouped by direction in which nerve impulse travels relative to CNS
- sensory - afferent
- motor - efferent
- interneurones (association)
- lie between motor and sensory neurones
- 99% of body’s neurones
dendrites
dendrite are the receptive (input) region of a neurone, they receive incoming signals toward cell body as graded potentials (changes in membrane potential)
Axons
Axons are the conducting region of a neurone
What is the myelin sheath
The myelin sheath is a whitish, protein lipoid substance
- it is segmented sheath around most long, or large diameter axons
myelinated fibres
What are the functions of myelin
protects and electrically insulates axon
increase speed of nerve impulse transmission
nonmyelinated fibres conduct impulses slowly
Plasma membranes of myelinating cells have less protein, why
because there are no channels or carriers, good electrical insulators and interlocking proteins bind adjacent myelin membranes
what are and why are there myelin sheath gaps
gaps between adjacent Schwann cells
called nodes of ranvier
regions rich in sodium channels to promote nerve impulse over long distances
Membrane potentials
neurons communicate with other neurons in the body through action potential (nerve impulse/electrical signal)
- nerve impulse is always in the form of an action potential regardless of stimulus (eg. pain, light, heat)
- Action potential is a result off charged ions
What are the two main types of ion channels
leakage (non gated) and gated
Leakage non gated channel
always open
Gated channels
part of protein changes shape to open/close channel
3 channels
- chemically gated (ligand - gated) channel
- voltage gated channel
- mechanically gated channel
chemically gated ( ligand - gated) channel
open with binding of a specific neurotransmitter
voltage gated channels
open and close in response to changes in membrane potential
mechanically gated channels
open and close in response to physical deformation of receptors, as in sensory receptors
The resting membrane potential
the bulk solution inside and outside of the cell are electrically neutral
potential difference exists in a very defined area only across the membrane of resting cells
- approx - 70mV In neurons
(cytoplasmic side of membrane negatively changed relative to outside)
membrane termed polarised
This is generated by the differences in ionic makeup of ICF and ECF
differential permeability of the plasma membrane
- ECF has higher concentration of sodium NA+ than ICF, balanced chiefly by chloride ions (Cl-)
-ICF has higher concentration of K+ than ECF, balanced by negatively charged proteins
K+ plays most important role in membrane potential
Differences in plasma membrane permeability
slightly permeable to Na+ (through leakage channels)
- sodiums diffuses into cell down concentration gradient
25 times more permeable to K+ than sodium (more leakage channels)
- potassium diffuses out of cell down concentration gradient
more potassium diffuses out of the cell than sodium diffuses into the cell
- cell more negative inside
- establishes resting membrane potential
sodium potassium pump stabilises resting membrane potential
How do membrane potentials change
membrane potential changes when
- concentrations of ions across membrane change
- membrane permeability to ions changes
Changes in membrane potential
terms describing membrane potential relative to resting membrane potential
Depolarisation
- decreased in membrane potential (towards zero and above)
- inside of membrane becomes less negative than resting membrane potential
- increase probability of producing a nerve impulse
Hyperpolarisation
- an increase in membrane potential (away from zero)
- inside of cell more negative than resting membrane potential)
- reduces probability of producing a nerve impulse
threshold
not all depolarisation events produce an action potential
for an axon to ‘fire’ depolarisation must reach threshold
- that voltage at which the action potential is triggered
at threshold
- membrane has been depolarised by 15 to 20 mV
- Na+ permeability increases
- Na+ influx exceeds K+ efflux
an action potential either happens completely, or it does not happen at all
Propagation of an action potential
propagation allow an action potential to serve as a signalling device
Na+ influx causes local currents
- local currents depolarisation of adjacent membrane areas in direction away from action potential origin (toward axon’s terminal)
- local currents trigger an action potential there
- this causes the action potential to propagate away from the origin
since Na+ channels closer to an action potential origin are inactivated no new action potential is generated there
once initiated an AP is self propagating
The synapse
nervous system works because information flows from neurone to neurone
neurons functionally connected by synapses
- junctions that mediate information transfer
1. from one neurone to another neurone
2. or from one neurone to an effector cell
synapses are electrical or chemical in nature
axodendritic
between axon terminals of one neurone and dendrites of others
axiomatic
between axon terminals of one neurone and soma of others
Electrical synapses
less common than chemical synapses
neurones electrically coupled (joined by gap junctions that connect cytoplasm of adjacent neurones and can allow the passage of charged ions)
- communication very rapid
- may be unidirectional or bidirectional
More abundant in embryonic nervous tissue
chemical synapses
specialised for release and reception of chemical neurotransmitters
typically composed of two parts
1. axon terminal of presynaptic neurone
- contains synaptic vesicles filled
two parts separated by synaptic cleft : fluid filled space
electrical impulse changed to chemical across synapse, then back into electrical
transmission across synaptic cleft
- chemical event (slower than an electrical one)
- depends on
- release of neurotransmitter
- diffusion of neurotransmitter
- receptor binding of neurotransmitter
- ensure unidirectional communication between neurones
information transfer across chemical synapses 9 stages
- action potential arrives at axon terminal of presynaptic neurone
- causes voltage-gated Ca2+ channels to open, Ca2+ floods into the cell
- intracellular proteins binds Ca2+ and promote fusion of synaptic vesicles with axon membrane
- exocytosis of neurotransmitter into synaptic cleft occurs
- neurotransmitter diffuses across synapse
- binds to receptors on post synaptic neurone
- ion channels are opened
- causes an excitatory or inhibitory event
- neurotransmitter effects terminated
termination of neurotransmitter effects
within a few milliseconds neurotransmitter effect terminated in of three ways
- reuptake
- by astrocytes or axon terminals - degradation
- by enzymes - diffusion
- away from synaptic cleft
post-synaptic potentials
neurotransmitter receptors cause changes in membrane potential that vary in strength with
- amount of neurotransmitter released and
- time neurotransmitter stays in the area
types of post synaptic potentials
- EPSP - excitatory postsynaptic potentials
- IPSP - inhibitory postsynaptic potentials
Neurotransmitters
50 or more neurotransmitters have been identified
most neurones make two or more neurotransmitters
- neurons can evert several influences
classified by chemical structure and by function
Acetylcholine
first identified : best understood
released at
- neuromuscular junctions
- some ANS neurons
- some CNS neurones
- synthesised from acetic acid and choline by enzyme choline acetyltransferase
- Degraded by enzyme acetylcholinesterase (AChE)
Biogenic amines
- catecholines
- dopamine, norepinephrine (NE) and epinephrine
- excitatory/inhibitory - indolamines
- serotonin (inhibitory) and histamine (excitatory/inhibitory)
Broadly distributed in brain
- play role in emotional behaviour and biological clock
- imbalanes associated with mental illness
Amino acids
glutamine : excitatory
- stroke neurotransmitter
- excess release : cell stimulated to death
aspartate : excitatory
glycine : inhibitory
- spinal cord main inhibitory
GABA - gamma - aminobutyric acid : inhibitory
= main inhibitory neurotransmitter of the brain
classification of neurotransmitters : function
great diversity of functions
can classify by
effects: excitatory versus inhibitory
actions: direct versus indirect
function of the neurotransmitter is determined by the receptor to which it bunds
effects - excitatory versus inhibitory
- neurotransmitter effects can be excitatory (depolarising) and or inhibitory (hyper polarising)
- GABA and glycine usually inhibitory
- glutamine usually excitatory
- acetylcholine and NE bind to at least two receptor types with opposite effects
ACh excitatory at neuromuscular junction in skeletal muscles
ACh inhibitory in cardiac muscles
Direct vs indirect actions
direct action
- neurotransmitter bind to and opens ion channels
- promotes rapid responses by altering membrane potential
- examples : Each and amino acids
indirect action
- neurotransmitter acts through intracellular second messengers, usually G protein pathways
- broader, longer lasting effects similar to hormones
- example : biogenic amines, neuropeptides and dissolved gases
neurotransmitter receptors
- channel linked receptors : mediate fast synaptic transmission
- ligand gated ion channels
- action is immediate and brief - G protein linked receptor : mediate slow synaptic responses
- responses are indirect, complex, slow, and often prolonged
neurotransmitters vs neuromodulators
nitric oxide, adenosine often referred to as neuromodulators
neuromodulators
chemical messengers released by the neurone
does not can EPSP or IPSP
affects the strength of synaptic transmission (may inhibit, potentiate, or prolong the effects of neurotransmitters)
Processes of neuronal injury
mature neurones do not divide
- new neurones do not replace damaged neurone that lost function
loss of neurones leads to impaired neurologic transmission
impaired neurologic transmission leads to neurologic disorders
- cell damage
- ageing
manifestations reflect
- site of injury
- functions that are controlled by the dysfunctional neuronal signals
processes of neuronal injury
result of injury at the cellular level
chromatolysis
- for of apoptosis
- swelling of a neurone and dissolution of chromophil substance in neurones
- in certain pathological conditions or following an injury to the cell or axon
atrophy
- decrease in cell size
neuronophagia
- phagocytosis and inflammatory responses causes by a dead neurone damaging neighbouring cells
intraneuronal inclusions
- distinctive structures formed in the nucleus or cytoplasm
