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)
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.
