neurones, electrical communication Flashcards
neuronal structure and functions:
- Cell body: Biosynthesis; signal integration
- Dendrites receives incoming signals;
Ligand-gated and/or GPCR gated
ion channels - Axon Impulse conduction
(action potentials);
Voltage-gated Na and K channels - Nerve termini Secretion of neurotransmitter; Voltage-gated
Na, K and Ca channels
types of neurones based on the number of poles:
slide 14
types of neurones based on functions (afferent VS efferent):
- Motor or efferent neurons:
- Carry impulses from CNS to
peripheral effector organs e.g.,
muscles/glands/blood vessels - Generally each motor neurons
has long axon and short
dendrites - Sensory or afferent neurons:
- Carry impulses from periphery to
CNS - Generally each neuron has short
axon and long dendri
types of neurones based on the length of axon:
- Golgi Type I neurons:
- Have long axons
- Cell body situated in CNS and their axon reaches remote
peripheral organs - Golgi type II neurons:
- Have short axons
- Present in cerebral cortex and spinal cord
soma are present in:
- Grey matter of CNS
- Nuclei of brain e.g., cranial N. Nuclei/Basal
ganglia/Ganglia of CNS - All neurons contain soma
- All processes do not survive without soma
!!* Each neuron has centrally placed one nucleus in soma - Prominent nucleoli which contains ribose nucleic acid
- No centrosome – loss power of division!!
what is soma:
also known as the cell body or perikaryon, is the central part of a neuron that contains the nucleus
(contains genetic material)
what are nissl granules/bodies:
- Small basophilic granules or membrane bound
cavities found in clusters or clumps in soma - Present in cell body and dendrites but absent in axon
and axon hillock
!! They are made up of rough endoplasmic reticulum (RER) and ribosomes. The primary role of Nissl granules is related to protein synthesis!!
some components of neurones:
- Neurofibrillae (Microtubules &
microfilaments): - Thread like structure present all over cell
- Consists of microtubules and
microfilament - Mitochondria:
- Present in soma and axon
- Form the power house of the nerve cell
where ATP is produced - Golgi Apparatus
- Same of Golgi Apparatus of other cells
- Concerned with processing and packing of
proteins into granules
dendrites composition + function
- Tapering and branching extension of soma
- Dendrites of cerebral cortex and cerebellar cortex show
knobby projections called dendritic spine - May be absent. If present there is one or more
- Conduct impulses towards the cell body
- Generate local potential not action potential as well as
integrate activity - Has Nissl granules and neurofibrils
- Dendrites and soma constitute input zone
axons composition + function:
- Each neuron has only one axon
- Arises from axon hillock of soma
- Carry impulses away from cell body
- Cannot synthesize own protein depends upon soma
- Branched only at its terminal end called synaptic knob,
terminal button, axon telodendria - Axon may be medullary or non medullary
- Contains granules or vesicles which contain synaptic
transmitters - Specialized to convert electrical signal (AP) to chemical signal
Axis cylinder composition + function:
- Has long central core of cytoplasm- axoplasm
- Axoplasm covered by membrane – axolemma continuation of
cell membrane of soma - Axoplasm along with axolemma- axis cylinder
- Contains mitochondria, neurofibrils and axoplasm, vesicles
- Axis cylinder covered by neurilemma in non myelinated
nerve fiber - Nerve fiber insulated by myelin sheath – myelinated nerve
fiber
!! carries electrical signals from the cell body to the synaptic terminals, where the signals are transmitted to other neurons or target tissues !!
Synapses:
neurones = connected to other cells/ neurones by synapses
(neuron-neuron, neuron-skeletal muscle, neuron-gland synapse)
Neurones communication:
- electrical communication: mvmts of ions in and out of cell membrane
- chemical communication: release of neurotransmitters to nearby transmitter receptors at synapses
Stimuli, activator of neurons:
- neurons = excitable, activated by stimuli to send signals to other cells
what is stimulus?
any change in the environment that triggers a change in the neuron (mechanical, electrical, chemical)
- activation = through activation of channels on dendrites (stimulus = changes shape of channel = transmits via electrical or chemical changes inside the cell
types of ion channels found in neurones:
- mechanically gated channels: in response to physical forces
- voltage-gated channels: in response to electrical forces
- ligand-gated channels: when chemicals or neurotransmitters bind
- leaky channels: always open
(Ionic flux through a channel is always passive), when gate opens= conformational change that lets ionic flux across the membrane
channels = selective for specific ions:
Channels are Selective for Specific Ions
Ion channel pores have selectivity filters
* Charge selectivity
- The aqueous pore of a channel may contain multiple amino acid residues with the opposite charge of the ion it’s selective for
- An intrapore positive charge would repel cations, thus making the channel selective for anions and vice versa
* Size selectivity
- Allows selectivity within a charge class (i.e., monovalent cation channels selective for Na+ vs K+ vs Li+)
(An anion selectivity filter
contains positive charge)
ion channel gates criteria to opens
- Different ion channel proteins are classified by different
gating mechanisms:
1. Ligand-gated: a signal molecule binds to a receptor domain
on the channel protein, leading to the conformational change,
opening the channel
2. Phosphorylation-gated: the ion channel is phosphorylated
and remains open as long as the PO4- group is attached
3. Voltage-gated: a change in membrane potential typically
opens the channel
4. Mechanically-gated: stretch or pressure opens the channel
5. Non-gated: constitutively open
ions channel and when the flow can change:
*Ions tend to flow from an area of high concentration to an area of low concentration.
*In the presence of a voltage gradient, there may be no flow of ions despite unequal
concentrations.
*Ion channels can be open or closed.
*Opening is brought about by changing the voltage across the membrane, or binding a
chemical substance to a receptor.
*Most important role is that they provide the neuron with electrical excitability.
*Found in all parts of the neuron and to a lesser extent in the neuroglial cells
fundamental properties of an ion channel:
1.It is made up of a number of protein sub units, sitting
across the membrane, allowing ions to cross from one
side to the other. (Transmembrane pore)
2.The channel must be able to move from open to
closed state, and back.
3.Must be able to open in response to the appropriate
stimuli.
Some channels respond to chemical stimulus
(particularly at the synapse). These channels have
specific receptors for that chemical, that leads to
channel opening
electrical communication=
occurs by movement of ions in and out of the membrane.
electrical gradients are measured as membrane potential
membrane potential:
= amount of electrical charge present on the inside of the cell (ICF) compared to outside of cell (ECF)
- positive membrane potential = positive more inside than outisde (+ de K+ inside ou more Cl- outside)
- negative membrane potential:
(opposites ac Na+ et tralala)
slide 41 !!
Ion accumulation membrane:
- normalement entre in and out of membrane electrical charge = 0 mais when there’s a difference = electrical gradient
- the ions will accumulate close to the membrane on either sides due to the attraction of negative charge to positive charge mais they can’t pass sans channel pcq hydrophobic (ions). donc they wait for signal et en attendant they’re stored very locally at the cell.
Magnitude of membrane potential:
increasing number of ions that accumulate at the membrane = increases magnitude
- normalement magnitude at rest = negative pcq: proteins A- inside + K+ inside + Na+ outside
membrane potential at rest = -70mV
NOTHING MOVES WITHOUT CHANNELS
depolarisation VS hyperpolarisation:
depo = positive
hyper = negative
case of leaky K+
when at rest, passive, k+ channels let them leak to near to equilibrium
k+ PERMEABILITY = high at rest but not complete et le contraire pr Na+
Electrical signaling types:
- Graded potential: ( = in response to stimulus, a graded potential is variable in strength and can either depolarize or hyperpolarize the membrane), small local change, can vary in size, does not travel far, can build-up to become a larger trigger)
- The amplitude of the voltage deflection is variable and dependent upon the
stimulus intensity = graded potential
excitatory = positive ions move IN, inhibitory = positive ions move OUT/ negative = move IN
(* Graded potentials can occur in any part of the membrane in any cell - Graded potentials also can result from the process of summation, where two or more stimuli affect the membrane potential over time or space.)
- Action potential: large, fast, long distance, same magnitude every time it occurs
SLIDES 59- 60- 61
graded potential then action potential:
- In membrane domains that contain voltage-gated channels, if the
magnitude of the graded potential reaches threshold, an action potential is developed. - This is a maximum, all-or-none voltage deflection (i.e., it’s not graded)
action potential pattern:
1) membrane rest
2) treshold
3) rising phase: depolarisation
4) falling phase: repolarisation
5) DIP: hyperpolarisation
6) return to rest
Voltage gated channels:
- Voltage gated Na+ channels: closed at -70mV, opens rapidly at -50/-55, closes by separate inactivation gate
- // // K+: closed at -70, opens slowly (same mV), closes slowly
speed of impulse propagation:
slide 67
Factors influencing Action potential:
thanks to conduction velocity: 2 factors that influence it:
- myelination: increased myelination = increased speed
- axon diameter: large diameter axons = fast conduction (large diameter = low resistance + fast conduction, small diameter = high resistance + slow conduction)
- space = limited, if axon needs fast transmission = will be both large and myelinated
nervous system tissue:
the fatter the fibre, the faster it flies
continuous vs saltatory conduction:
slide 73
nerve transmission:
slide 76
graded vs action potential
slide 77