Lecture 2 Brain: A closer look at neurons and the cortex Flashcards
Neuronal signalling
Neurons receive, transform and transmit information
Role of a single neuron
Receive input information from other neurons (convergence)- Integrates/associated this input information to produce an output
- Sends this integrated output information to many other neurons (divergence)
Information = signal = variation in:
- The electrical membrane potential/voltage
- Post-synaptic potential
- Action potential
- The quantity of certain chemical molecules (neurotransmitters) released at the synapse
Electrical neuronal signalling within a neuron: 3 transmembrane properties
- Na+/K+ ion pump: always actively pumping ions in and out of the cell
- Na+ and K+ voltage-gated ion channels
- Let either Na+ or K+ ions through when open
- Closed at rest
- Open/close depending on the value of the electrical membrane potential
Electrical neuronal signalling within a neuron: Resting membrane potential
Na+/ K+ ion pump creates different concentrations of ions on each side of the neuron’s cell membrane
More positive ions in the extracellular space
Resting membrane potential is -70 mV
Voltage-gated ion channels are closed below -55 mV
Changes in the value of the membrane potential will carry information
When does the membrane potential change from rest ?
When a post-synaptic neuron receives input from other pre-synaptic neurons at dendrites/synapses
What happens if the pre-synaptic input makes the post-synaptic cellular membrane Less negative/more positive input
Depolarisation
Excitatory (PSP)
What happens if the pre-synaptic input makes the post-synaptic cellular membrane more negative/less positive than -70mV
hyperpolarisation
Inhibitory (PSP)
What happens if there is more excitatory than inhibitory input on the neuron’s membrane
Membrane depolarises overall
When will a neuron fire
If depolarisation reaches -55 mV threshold
Action potential definition
Fast depolarisation, repolarisation and hyperpolarisation of the neuron’s membrane
What happens in the cell membrane when a -55mV depolarisation threshold is reached
Voltage gated ion channels open
Na+ channels open faster
K+ channels open more slowly
Na+ ions first enter the neuron
Depolarization (membrane positivity increases up to +40 mV
K+ ions then exit the neuron
Repolarisation
From +40 mV to 70 mV back to resting state
Hyperpolarisation
below -70mV
Evolution of the membrane potential during an action potential
1.Na+ ion channels open (depolarisation above -55 mV)
2.K+ ion channels open (depolarisation slows down and peaks at 40 mV)
3.Na+ ion channels close (repolarisation, hyperpolarisation)
4.K+ ion channels close (back to resting membrane potential, thanks to ion pumps)
Time for full action potential process
About 2ms
AP propagation along the axon
Strong depolarisation at location A on the membrane spreads to next location B
-55 mV threshold is reached at location B, triggering AP
so on at the other locations along the axon
AP does not go back to location AP reaches axon terminal and becomes the input for the next neuron
Myelination
Myelin sheaths are protrusions from oligodendrocytes, a type of glial cell
Insulates axons and speeds up the propagation of APs
Aps can jump from one nod to the next- Speeds up AP propagation by 10 or 100-fold
-Only long axon are usually myelinated
-Myelination concentration is high in white matter
Key properties of APs
All or nothing phenomenon
Self-propagation
Unidirectionally
Does not dissipate
Chemical communication between neurons at the synapse: Action potential arriving at the pre-synaptic axon terminal riggers
Depolarisation of the axon terminal
Release of neurotransmitter molecules into the synaptic cleft
Depolarization or hyperpolarization of the membrane on the postsynaptic dendrite (excitatory or inhibitory PSP)
Post-synaptic potential (PSP)
Neurotransmitter receptors on post-synaptic neuronal membrane are linked to ion channels.
Post-synaptic potential (PSP) Neurotransmitter binding on the receptor opens the ion channel, which either:
Hyperpolarizes the membrane potential - inhibitory post synaptic potential
Depolarises the membrane potential - excitatory post-synaptic potential
Properties of PSPs
They are near the post-synaptic dendrite
They dissipate
They are smaller in amplitude than APs
Many EPSPs must combine for the resting membrane potential to reach the AP threshold
From PSPs to action potentials
Summation of PSPs occur at the neuron’s hillock
If enough EPSPs sum together to reach threshold, an AP is generated
If too many inhibitory PSPs sum together, prevent AP in post-synaptic neuron
Neuronal computation
If the post synaptic neuron ‘integrates’ the input information from many pre-synaptic neurons to produce an AP
Neuronal coding
Information is coded differently at different stages of neuronal signalling
Neurotransmitter releases in the synapse: a stronger stimulus means more neurotransmitter
EPSP and IPSP: strong stimulus leads to stronger EPSP or IPSP
Neuronal coding: APs
Amplitude does not change
Stronger stimulus = more AP firing = increased firing rate
Neurotransmitters: types of neurotransmitter molecules
- Amino acids mostly in CNS
- Glutamate: mainly excitatory, 20% CNS synapses
- Gamma-aminobutyric acid mainly inhibitory 40% of synapses
System neurotransmitters mostly in specific brain circuits: Acetylcholine
cholinergic system
System neurotransmitters mostly in specific brain circuits: Serotonin
serotoninergic system
System neurotransmitters: mostly in specific brain circuits: Noradrenaline
noradrenergic system
System neurotransmitters: mostly in specific brain circuits: dopamine
dopaminergic system
Cerebral cortex: Functional subdivisions based on
- Neurological ’neuropsychological lesion studies
- Neuroimaging or neurostimulation studies in human participants
- Neurophysiological in animal models
Types of functional cortical regions
Sensory cortex
Motor cortex
Association cortex
Primary sensory cortex
Where most of the sensory information arrives from peripheral sensory organs
Secondary sensory cortex
Less input from peripheral sensory organs, more input from the rest of the cortex
High level cognitive processing
Primary motor cortex
Sends information to muscles
Secondary motor cortex
Planning of motor actions
Brodmann area BA17
Primary visual cortex
Brodmann area BA22
primary auditory cortex
Brodmann area BA1, 2, 3
primary somatosensory cortex
Brodmann area BA4
Primary motor cortex
Somatosensory and motor cortex maps
of the body
Visual cortex maps
of the visual field
More receptor density
larger cortical areas
Auditory cortex maps
maps of sound frequencies
olfactory and gustatory cortex maps
unclear
Motor and sensory maps
- Different parts of the somatosensory cortex receive tactile information from different parts of the skin (pain, vibration, temperature
- More dexterous or sensitive parts of the body are magnified compared to others
- Maps are contralateral
Visual cortical maps
Visual hemifields are represented contralaterally
Top visual field is represented ventrally, bottom dorsally
Centre for the visual field is magnified compared to the periphery
Secondary visual cortex contains many more maps
Auditory cortical maps
Cochlea in the inner ear breaks down sounds into their component frequencies
Primary auditory cortex contains map of frequencies
Both primary and secondary auditory cortex probably contain several maps
Each ear sends information bilaterally to the auditory cortex
Association cortex
- Everything that is not exclusively sensory or exclusively motor
- Integrates information from multiple sensory modalities, or performs computations that are not tied to a specific sensory modality
- Complex cognitive abilities (object recognition, memory, attention, action planning, …)
- Frontal, parietal and temporal lobes
Prefrontal cortex subdivides into:
Lateral, orbital, and medial prefrontal regions
What is the prefrontal cortex involved in
- Executive functions: planning, guidance and evaluation of behaviour e.g.,
- organise coherent sequential behaviours
- Predict consequence of our actions
- Moral reasoning
- Speech and auditory processing
- Uniquely human abilities
Frontal lobe: Lateral prefrontal cortex involved in:
- Executive functions
- Working memory
- Language
Frontal lobe: Orbital prefrontal cortex involved in
Emotional processing
Frontal lobe: Medial prefrontal cortex involved in
Decision making
error monitoring
Parietal lobe involved in:
- Spatial processing
- Attention
- Integration/association of information: across different sensory modalities, between sensory information, internal memory representations and actions
Temporal lobe: medial side
- Memory (hippocampus and surrounding cortex)
- Emotional processing (amygdala)
Temporal lobe: lateral side
- Visual object recognition (ventral temporal lobe)
- Auditory object recognition (secondary auditory cortex in superior temporal cortex)
White matter tracts
- White matter connects different parts of the cerebral cortex
- White matter tracts are the highways of white matter: large bundles of white matter connecting the same two cortical regions
Superior longitudinal fasciculus:
- connects frontal and parietal/occipital lobes
- Important for attentional and executive control
