Neuroscience Lecture 5 2024.pdf

Neuroscience Lecture 5 Sleep and wakefulness Reward  Sleep All animals sleep. (Even worms and jellyfish!) But amount varies enormously (eg, giraffes and elephants sleep 4 h and bats up to 19 h /day) Amount of sleep related...

Neuroscience Lecture 5 Sleep and wakefulness Reward  Sleep All animals sleep. (Even worms and jellyfish!) But amount varies enormously (eg, giraffes and elephants sleep 4 h and bats up to 19 h /day) Amount of sleep related to many factors: diet, habitat, prey/predator status, social network, metabolic rate, brain complexity…. Sleeping = opportunity cost / vulnerable (hence most animals sleep at night) Birds and aquatic mammals (eg dolphins) can sleep with half their brain at a time. Image by NOAA - NOAA, CC BY-SA 3.0  Sleep and Health Sleep is important for many brain functions. Any cognitive function (attention, speed, mood, memory …) Eating and obesity (and diabetes). Effects on ghrelin and leptin and responsiveness to insulin. Activity in sympathetic nervous system: impacts on the body: eg, Heart health, 24% increase in heart attacks on day after daylight savings starts.  What keeps us awake? Noradrenaline neurons in Locus Coeruleus, Serotonin neurons in Raphe nuclei and Acetylcholine neurons in Pons, are responsible for arousal (all in the brainstem) Disconnecting the forebrain from brainstem (“decerebration”) causes profound and almost continuous sleep. Neurons active while awake, and quiet during sleep. Electrical stimulation of these neurons Pons can wake a sleeping animal. Locus Raphe Effects of stimulant drugs Coeruleus Nuclei (amphetamines, ecstasy).  What keeps us awake? Noradrenaline neurons in Locus Coeruleus, Serotonin neurons in Raphe nuclei and Acetylcholine neurons in Pons, are responsible for arousal (all in the brainstem) Disconnecting the forebrain from brainstem (“decerebration”) causes profound and almost continuous sleep. Neurons active while awake, and quiet during sleep. Electrical stimulation of these neurons Pons can wake a sleeping animal. Locus Raphe Effects of stimulant drugs Coeruleus Nuclei (amphetamines, ecstasy). What sends us to sleep? Pre-optic area (anterior hypothalamus) Destruction of pre-optic area can cause insomnia; Pre-optic area Neurons in Pre-optic area contain GABA; these neurons inhibit ACh, 5HT, and NA arousal systems in brainstem stimulation can induce sleep. What sends us to sleep? Adenosine builds up in brain while we are awake. Being awake for longer increases amount of adenosine Adenosine increases sleepiness (“sleep Pre-optic area pressure”) by inhibiting alertness centres (esp ACh in pons) and stimulating sleep Pons centres (pre-optic area). Adenosine levels Caffeine (coffee, tea) blocks adenosine receptors. 12am 12am 12am 12pm 3 6 9 3 6 9 12pm 3 6 9 3 6 9 12pm 3 6 9 3 6 9 What sends us to sleep? Suprachiasmatic nucleus controls release of hormone melatonin from pineal gland. Melatonin signals night-time to whole body. Suprachiasmatic nucleus Circadian changes in melatonin  Sleep Cycles When we’re awake, the electrical activity in our brains is high frequency and “noisy” Sleep characterised by slow rhythmic patterns of electrical activity in the brain (“slow-wave sleep”). Slow-Wave Sleep Role of reciprocal connections between thalamus and cortex in orchestrating synchronised neuronal activity. thalamus  REM sleep At multiple times through the night, the brain waves become desynchronised – Just like when we’re awake. During these periods, the eyes dart back and forth (“rapid eye movement sleep”, or REM sleep) And if woken during REM, we typically report we were dreaming. Amount of REM increases during the night, but decreases across life span. (The foetus has a lot of REM! 50% for new born, decreasing to 20% in adults.) Alcohol reduces REM sleep (including in utero).  Brain regions REM sleep caused by neurons in Pons that contain acetylcholine and stimulate neurons in the thalamus, which project to the visual cortex. ACh neurons Using fMRI… in pons During REM, various areas become very active: visual regions at back of brain, motor regions, and limbic system (esp amygdala and cingulate, even more active than when we’re awake). Suppression of particular regions of frontal lobes (lateral prefrontal cortex; important for logic reasoning and self-control).  During REM sleep, we are paralysed. ACh neurons in pons stimulate neurons in medulla, which ultimately inhibit motor neurons in the spinal cord  paralysis. Destruction of these neurons in medulla causes animal to become very active during REM sleep (no longer paralysed) Said to behave as if “acting out its dreams”. Neural Mechanism of Reward The concept of reward Motivation to engage in a particular behaviour Animals (including humans) will act in order to receive some reward. How do rewards work?  Neural Mechanism of Reward Electrical self-stimulation of the brain: Olds & Milner accidentally implanted an electrode into the medial forebrain bundle (mfb), (a bundle of noradrenaline & dopamine fibres travelling from brainstem to forebrain) Their rat kept returning to place where it had received electrical brain stimulation. O&M went on to show that rats with electrode in mfb would readily learn to perform many acts in order to receive electrical stimulation If allowed, rats would do nothing else but press bar; would prefer to starve or even tolerate shocks through floor in order to continue self-stimulating. YouTube: [7min] Brain mechanisms of pleasure and addiction  Neural Mechanism of Reward Dopamine axons in mfb make greatest contribution to rewarding effect of electrical stimulation.  Effects greatly reduced by destruction of dopamine fibres. Many addictive drugs Accumbens (eg, amphetamines, cocaine) m.f.b. VTA directly boost release of dopamine in mfb Other drugs (eg, heroin, nicotine) indirectly increase dopamine release. + rats will work (eg bar press) in order to receive injections of amphetamines or opiates into nucleus accumbens.