S3: Modulatory and Arousal Systems Flashcards
Describe the specific pathway and given an exampple
Specific pathways are associated with sensory, motor and cognitive processing.
- They carry information that is highly specific and event related (i.e. the pattern of neuronal firing encodes a specific meaning and is based on the event that occurred).
- These pathways are information rich and primarily have inotropic receptors (they still have some metabotropic receptors though). This allows fast and time dependent synaptic transmission, avoid delay and allows important information to be shared rapidly.
- The anatomy of specific pathways is that they are precisely localised and they project distinctly to their targets.
- E.g. primary visual pathway
Describe a modulatory pathway and give examples
The modulatory pathways control the state of specific systems and they are not involved in information processing and so the pattern of firing in their axons isn’t encoding information.
- These system increase/decrease their AP firing in order to change the state of the sleep-wake cycle, behavioural state etc. For example, by causing a specific neurone to open its K+ channels quicker so that it can fire again quickly.
- The primary receptor type are metabotropic hence there is slow synaptic transmission because they work via intracellular cascades. These cascades can even promote permanent changes in the post synaptic cell for example, by up/down regulating expression of particular gene.
- The anatomy of this pathway is diffuse connections so they also have a diffuse wide spread of information.
- E.g. occurs in the brainstem and basal forebrain.
What is an electroencephalogram (EEG)?
Electrodes are applied on surface of scalp and they can measure activity in the cerebral cortex.
- There is correlation between the pattern of EEG and brain state.
- The EEG represents the summed activity of millions of nerve cells.
Describe pattern of EEG when awake or asleep
- Awake: High frequency of activity with low amplitude. This is because an individual will be alert and attentive so the population of cells in the cortex will be processing information. Each individual cell will be firing its unique pattern reflecting the information it is carrying, Because each cortex cell is firing in its own unique manner, the population will be firing asynchronously, this means their summed activity will largely cancel out and this is what we see in the EEG (low amplitude).
- Slow wave sleep (deepest): Large amplitude of signals and lower frequency of oscillations. The cortex is receiving waves of synchronous activity from the thalamus so the cortical neurones will also fire synchronously at regular intervals. Each neurones AP will summate at the same time and stop at the same time and so the EEG shows large peaks and troughs.
Describe the awake pathway in our brain
When awake, the thalamus is acting as a transferor if information.
- When awake the retinal cell will respond to sensory (visual) input. The retinal cell will be excited and send impulses down its afferent to the thalamic cell. The thalamic cells will then pass this information quite accurately to the cortex via projecting axons.
- For the thalamic cell to do this, it needs to be supported by modulatory inputs which are inhibitory interneurones acting on them.
Describe the asleep pathway in our brain
When asleep, these modulatory inputs (inhibitory interneurone) stop so the thalamic cells start to fire spontaneously (depolarise), sending waves of impulses when cell reaches threshold. These are at regular intervals.
- It is now ignoring sensory input form the retina, as there will be many of these cells in the thalamus that would usually be collecting info from the visual system they will all start firing these waves synchronously.
- Hence cortex is no longer receiving information from the outside world and instead it is receiving waves of meaningless impulses carrying no information which explains our EEG while we are asleep.
List stages of the sleep-wake cycle with EEG
- Awake: EEG low amplitude high frequency (20-40 Hz activity).
- Starts to fall asleep: Amplitude goes up and frequency goes down.
- Drops down from sleep 1 to 4: 4 is the deepest stage of sleep due to synchronisation of firing in the thalamus and cortex. There us about 4Hz activity (4 cycles a second or less).
- The individual will remain in deep sleep for some time and then sleep will begin to lighten and pass back up to stage 3 and 2 sleep.
- Instead of entering stage 1 sleep, the person will enter a period of REM sleep.
- This is one cycle and lasts about 90 minutes. Over the course of the night, we go through multiple cycles. As the night goes on, the period of REM sleep gets longer and period of deep sleep shorter.
Describe REM sleep
REM is rapid eye movement sleep.
- The EEG of a person in REM sleep looks like that of a person who is awake, especially in the frontal lobe.
- The cortex is active and processing information (no longer synchronous activity in the thalamus and cortex from stage 4 sleep).
- The eyes dart around below the lids and the muscles in the ear are active.
- Heart and respiratory rate will increase and if you wake a person in this stage, it is likely they will report that they were dreaming.
- During REM sleep the majority of skeletal muscle in the body is paralysed to prevent the sleeper acting out their dreams.
- If we wake up naturally it is usually in the period of REM sleep.
Can EEG change when person is awake?
Yes. Faster EEG rhythm are associated with attention and concentration. The modulatory systems are asjusting the activity of the cortex.
List the main ‘modulatory’ pathways
- Acetylcholine (Pontomesencephalic tegmentum and Basal forebrain).
- Noradrenaline (Locus coeruleus).
- Dopamine (Substantia Nigra) and Ventral tegmental area).
- Histamine (Hypothalamus).
- Orexin/hypocretin (Hypothalamus).
- Serotonin (raphe nuclei).
What are the two origins for cholinergic modulatory input in the brain?
This is where the cell bodies are and where the axons will project from.
- Pontomesencephalic tegmentum.
- Basal forebrain.
Describe the pontomesencephalic tegmentum as a cholinergic modulatory input
Pontomesencephalic tegmentum is close to the pontine region of the brainstem. The cells in this region primarily project to the thalamus.
- The Ach modulatory pathways in the thalamus primarily act to desynchronise thalamic cells.
- It also increases thalamic responsiveness to external stimuli coming in when a person is awake and thus help restore connection with the outside world when a person is waking up.
- However this pathway doesn’t do this on its own, it is working as part of what wakes us up.
Describe the basal forebrain as a cholinergic modulatory input
The axons from the basal forebrain project throughout the neocortex and into the hippocampal complex. The axons mainly start from the basal nucleus of meynert or the medial septal nucleus.
- It has widespread connections.
- The modulatory projections from the basal forebrain to the cortex will modulate cortical cells by increasing their reponse strength and selectivity, they will also promote plasticity.
- So increased activity in these areas are associated with being awake, attentive, increased cognitive activity and increased activity for learning and memory.
Describe the big pathology associated with the ACh (cholinergic) modulatory system
It is Alzheimer’s. Alzheimer’s damages the frontal cortex and so damages the circuitry in the regions we use to think with and also damages the pathways that enhance the effectiveness of that circuitry.
- This is a double whammy impact that causes cognitive decline and memory issues.
- The only effective treatment is to give acetylcholinesterase inhibitors which will prevent metabolism of Ach in the synaptic cleft and thus its removal. Hence it enhances the effectiveness of any remaining cholinergic input to the cortex.
Describe the noradrenaline modulatory system
The origin of the NA modulatory axons is the Locus coruleus. This isn’t a single location rather it spreads out as a long thin line of cells running down the brainstem.
- The locus coruleus projects everywhere including down the spinal cord (involved in producing analgesia.
- Stimulation of the cortical cells by these modulatory noradrenergic fibres increases the cortical neurones response amplitude and selectivity and increases plasticity.
- Hence this modulatory system is associated with shifting from sleep to wakefulness, in being awake and vigilant especially being alert to novel stimuli (e.g. when foraging and need to be alert to danger). It also is involved in learning and memory through supporting LTP at synapses.
