L6 Information transfer in the Nervous system Flashcards
Information Transfer in the nervous system
Complex pathway invovling generator potentials, action potentials and synaptic potentials (EPSP and IPSP)
Typical example:
- stimuli - receptive field
- primary afferent - to spine
- secondary afferent - to brain
- Complex processing in the brain - usually in thalamus
- upper motorneurone - to spine
- lower motorneurone - to muscle etc
Specialized nerve endings in the skin
N.B: whatever the stimulus, activation of the receptor causes a receptor graded potential to occur in the nerve terminal that may be either transient or sustained.
Fibres connected to A fibres - faster conduction:
- Merkel’s disc - light pressure, texture
- Meissner’s Corpuscle - light touch
- Pacinian Corpuscle - vibration
- Hair follicle receptor - movement
- Ruffini’s endings - skin stretch
Fibres connected to C fibres - slower conduction:
- Free nerve endings - warm, cold, itch, pain
INFO:
irrespective of modality, a nerve fibre conveys the A.P to the CNS
Frequency and pattern of the A.P’s that consitute the “signal” are produced by generator potentials in the sensory receptors
Frequency and pattern of the A.P in the nerve fibre encode information about the instensity (how big) and kinetics (how fast) of the sensory stimulus
sensory receptors can generate a Phasic and/or Tonic response
Phasic response
Adapting response that typically signals a change of state
Tonic response
Sustainted response
usually encodes information about the status quo - existing status of affairs
Weber’s law
used to quantify sensory responses
common feature of sensory systems that the stronger the stimulus the more difficult it becomes to discriminate.
E.g. harder to tell a 25kg weight from a 24kg weight than a 2kg weight from a 1kg weight even tho the diff is 1kg in each instance.
Generator Potential
Defintion
Produced by changes in the behaviour of ion channels
TRP ion channel family - good example
TRP= transient receptor potential
- local depolarization of the membrane potential at the end of a sensory neuron in graded response to the strength of a stimulus applied to the associated receptor organ, a pacinian corpuscle; if the generator potential becomes large enough (because the stimulus is at least of threshold strength), it causes excitation at the nearest node of Ranvier and a propagated action potential.
TRPV1
found in high levels in nociorecptors, may be involved in generatoring pain responses.
also opens in response to low (acidic) pH and some chemical stimuli (including capsaicin from chilli peppers
Often referred to as the Capsaicin receptor or VR1 (vanilloid receptor 1)
Generator Potentials
Job
Characteristic Features
N.B: ANALOGUE SIGNAL
Job:
• To initiate action potentials in the neuronal axon
CHARACTERISTIC FEATURES:
• Usually associated with non-specific cation channels (which can be found in non-neuronal cells too).
• modality specific: mechanoreceptors (high or low threshold), thermoreceptors (hot or cold), chemoreceptors, osmoreceptors, polymodal receptors (nociceptors), photoreceptors etc
• graded potentials
• transient (phasic, adapting) or sustained (non-adapting)
Generator Potentials:
Strengths & Limitations
STRENGTHS:
- localized: information on location of stimulus
- graded: information on intensity of stimulus
LIMITATIONS:
• Although generated by specific modalities, graded generator potentials do not contain modality specific information:
- modality-specific sensation requires labelled lines
Action Potentials
Job & Characteristic Features
N.B: Digital signal - all or nothing
JOB:
• Carry signal from point to point along axons
CHARACTERISTIC FEATURES:
- highly co-ordinated activity of voltage-gated Na + and K + ion channels
- all or nothing
- brief: usually ~2 - 5 ms
Action Potential
Strengths & limitations
STRENGTHS:
- signal size is maintained over distance and axonal branches
- Versatility of information coding: frequency encoding; pattern encoding
LIMITATIONS:
- membrane must be hyperpolarized to start
- system must be reprimed after use (refractory period)
EPSP
general info
EPSPs are not all or nothing they are graded.
Amplitude will depend on:
- Amount of neurotransmitter released
- Number of receptors
- State of receptors
Decay will usually depend on:
- Dissociation of ligand
- Diffusion and uptake (e.g. glutamate)
but may also depend on:
- Desensitization (AMPA-type glu receptors)
- Enzymatic destruction (ACh esterase)
Active process - not passive
EPSP: excitatory post-synaptic potential
Job & characteristic features
JOB:
• EPSPs are synaptic potentials that contribute to somatic depolarization leading to generation of an action potential at the axon hillock of the neuron.
CHARACTERISTIC FEATURES:
- EPSPs are graded - they are not all or none
- usually EPSPs are fast but slow EPSPs also occur:
- fast EPSPs ( ~ 10 - 100 ms) are usually due to activation of ligand-gated nons pecific (pass Na +, K + and sometimes Ca2+ p (p ) cation channels e. g. glu (AMPA, NMDA), ATP (P2X), ACh (nicotinic).
EPSP
Advantages & limitations
ADVANTAGES:
- versatility: different transmitters can act on the same postsynaptic cell using different receptors
- versatility: different receptors/ion channels can be regulated independently
- versatility: independent postsynaptic and presynaptic control of synaptic control of synaptic ‘strength’
LIMITATIONS:
- Metabolically expensive - v vulnerable to ischaemic attack
- Vulnerable to chemical attack (drugs and toxins)
IPSP: inhibitory post-synaptic potential
Job and characteristic features
JOB:
- IPSPs prevent somatic depolarization and generation of an action potential at somatic depolarization and generation of an action potential at the axon hillock..
CHARACTERISTIC FEATURES:
- EPSPs are graded - they are not all or none
- usually EPSPs are fast but slow EPSPs also occur:
- fast IPSPs ( ~ 10 - 500 ms) are usually due to activation of ligand-gated anion (Cl-) channels by GABA at GABAA receptors or glycine.
IPSP
advantages and limitations
same as EPSP
ADVANTAGES:
- versatility: different transmitters can act on the same postsynaptic cell using versatility: different transmitters can act on the same postsynaptic cell using different receptors (e.g. mixed epsp and ipsp)
- versatility: different receptors/ion channels can be regulated independently
- versatility: independent postsynaptic and versatility: independent postsynaptic and presynaptic presynaptic control of synaptic control of synaptic ‘strength’
LIMITATIONS:
- Metabolically expensive
- Vulnerable to chemical attack (drugs and toxins)
Convergence
Adding together of all signals +ve and -ve
Neuron makes a passive decision by adding together the criteria provided by the inputs.
Neuron will fire if the criteria are satisfied i.e. enough to cause an A.P etc
Synchronous activation of many inputs to a single neuron:
can give rise to a complex synaptic potential that involves the activity of many different types of ion channel.
Neuronal networks
Functional hierarchies where signals from multiple inputs can CONVERGE on neurons within a “nucleus” and be integrated.
Integrated output signals then DIVERGE to provide inputs to the next level of the hierachy
Divergence in networks is controlled by interneurons
Acuity/sharpness
- Acuity of sensation would be compromised due to divergence of the sensory signal at higher levels in the network
- Acuity can be restored due to ‘surround’ inhibition by negative inhibition by negative feedback
- forces signal down the central pathway
How can interneurons be configured to control activity in networks?
For example small inhibitory neurones can be introduced, in a a way a form of negative feedback etc.
Different config’s possible:
- Feed-forward inhibition
- feedback inhibition
- recurrent inhibition
