Neurological pathways and brain areas Flashcards
What is the role and structure of the mesolimbic pathway?
Known as the ‘reward pathway’
The cell body is found in the ventral tegmental area (VTA) and the synapses in the nucleus accumbens
What neurotransmitter is involved in the mesolimbic pathway?
Dopamine acts on D1/D2 receptors in the pathway
Where can dopamenergic neurones be found?
In the mesolimbic pathway (reward), the frontal cortex (cognition) and the nigrostriatal pathway (movement)
Also in the tuberoinfundibular pathway and the mesocorticolimbic system (affection and emotion) as well as in the brain stem (vomiting)
How does cocaine affect dopamenergic neurones?
Inhibits reuptake of DA by blockade of DAT on presynaptic terminal
Therefore DA concentration in the synaptic cleft increases
How does amphetamine affect dopamenergic neurones?
Inhibits reuptake of DA by blockade of DAT on presynaptic terminal
Also increases the release of DA
Therefore DA concentration in the synaptic cleft increases
How do opiates affect mesolimbic pathway?
They disinhibit the mesolimbic pathway
Occurs by acting on mu opioid receptors (Gi coupled) on GABAergic neurones causing inhibition of that neuron
There is therefore less inhibition of DA release in the mesolimbic pathway
How does Ethanol affect the mesolimbic pathway?
Best theory is that it blocks K+ channels which reduces the afterpolarisation in the mesolimbic pathway
Therefore there is increased firing of the nerves in the mesolimbic pathway
How does nicotine affect mesolimbic pathway?
Nicotine acts on nicotinic receptors (ionotropic) on DA neurons increasing Na+ influx
Therefore making action potnetials in DA neurones easier to form
How does cannabis affect mesolimbic pathway?
Acts on CB1 receptors on GABA neurones inhibiting them acting on DA neurones in mesolimbic pathway
Therefore overall effect is disinhibition of mesolimbic pathway
Where are NA nerves found?
Originates in locus coerulus and is also found in descending pain pathway
Diffuse innervation of cereberal cortex and hippocampus
What is the tuberoinfundibular (TI) system?
A nerve path connecting from the hypothalamus to the pituitary
DA antagonists can act on this pathway and cause increased pro-lactin output which can cause galactorrhoea
Involved in pituitary hormone output
5HT pathways in the CNS
Arises from raphe nucleus
Dorsal/median rahe nuclei connects to forebrain and cerebellum
Caudal raphe nuclei connects spinal cord and cerebellum
ACh pathways in the CNS
Produced by neurones with cell bodies in the brain stem that project out into cortex, hippocampus and thalamus
Also has local interneurones connecting to basal ganglia
What is the septo-hippocampal pathway?
Involved in learning and memory
Degenerates in Alzheimer’s
What are the functions of the basal ganglia?
Motor control (Degeneration here with motor disorders)
>co-ordinates the will to move, direction to move and puts all the information together
>filters out unwanted motor activity
A part of the mid-brain that acts as a bridge between many areas of the brain
What does the choroid plexus do?
Produces cerebrospinal fluid (Nacl/glucose solution)
CSF used to provides buoyancy and cushioning for brain and also serves to compensate for changes in brain volume
CSF can be used for diagnostic purposes via lumbar puncture
Histamine pathways in the brain
Synthesised and localised in tuberomammilary nucleus in the hypothalamus
Describe the structure of the ascending pain pathway
Free nerve ending of C-fibres pick up nocicepetive signals in the periphery and transmits AP’s through the dorasl root to the dorsal cord of the spine where it meets the postsynaptic projection neuron ( A Delta)
What neurotransmitters are used in the ascending pain pathway?
H+, ATP, K+ all activate C-fibres
The c-fibres will release Substance P which acts on NK-1 receptors (Gq coupled) which opens Na+/Ca2+ channels as well as activating PIP3 pathway
C-fibres also release glutamate which activates AMPA (very fast) and NMDA (fast)
Describe Gate control of nociceptive transmission
A alpha/A beta mechanoreceptors (touch receptors) runs along the ascending pathway for pain.
Stimulation of this pathway will increase activity of GABAergic neurones connected to A delta pain nerves
Therefore there is increased inhibition of post synaptic pain signals
Describe supraspinal control of pain (descending pain pathway)
Periaquaductal grey recieves input from hypothalamus (stress) and amygdala (fear/stress) which increases activity of nucleus raphe
Nucleus raphe releases 5HT which inhibits the A delta neuron therefore less pain signals sent upto to the brain
Locus coerulus also releases NA which has the same effect
What is ‘cortical spreading depression’ (CSD)?
A slowly propagating wave of depolarisation perhaps triggered by K+ disturbance
There is also a regional decrease in cerebral blood flow
How do we feel migraine pain?
CNS does not have any of it’s own pain receptors, the pain sensation comes from:
> intracranial blood vessels in the dura mater
arteries in the circle of willis
The nerves that innervate these blood vessels have cell bodies in the sympathetic/parasympathetic/sensory nervous system
What is the trigeminovascular pathway?
It is central to the neurovascular theory of how migraines occur.
Involves:
>intracranial arteries
>trigeminal ganglion (TG)
>trigeminal nucleus caudalis (TNC)
How does the trigeminovascular work?
Initiated by CSD and/or brainstem nuclei dysfunction
Neurones from TG release CGRP
This causes transient vasodilation in the cortex, pia and dura blood vessels leading to local neuorgenic inflammation
Activates TG/TNC to activate rostral areas involved in pain
This causes central sensitization to pain
What is wernicke’s area?
An area of the cortex involved with understanding and reading
What Broca’s area?
An area of the cortex involved with writing and speaking.
Neurological dysfunctions in MDD/Bipolar disorder
Disruptiion of hypothalamic-pituitary-adrenal axis
-Disruption in DA/NA/5HT levels
Decreased rate of neurogenesis in the hippocampus
Decreased activity in the cortex (reduction in cognition/attention)
Inccreased activity in the amygdala (increased emotionality)
What is generalised whole cortex bilateral seizures and its sub types?
Generalised is where the whole brain is synchronised
>Tonic-clonic (grand-mal) last for 5-10 seconds
>Absence (petit-mal) lasts for minutes
> Tonic- Seizure causes rigid muscles
Atonic- Flacid muscles
Myoclonic- brief periods of contraction
All involve a loss of consciousness
What are partial seizures and its sub-types?
Also known as focal or localised, they don’t involve the whole cortex
> Simple- localised seizure, does not spread across brain, no loss of consciousness
Complex- initiates in one area but can spread, some loss of consciousness possible
Secondary generalised is where it starts of as localised but spreads to involve whole cortex
Patient may experience auras associated with the brain area the focal point is in
Where do seizures occur most often?
30-40% of seizures occur in the temporal cortex
> also the area where patients are likely to be drug refactory (drugs ineffective) hence targetted for surgical resection
What is the cortico-spinal tract?
Allows for voluntary control of movement
Cell body is located in the motor cortrex and axon extends down into cortico-spinal tract
The path crosses over- commands for the right side of the body start in the left hemisphere of the brain
What is the rubro-spinal tract?
Very little use in adults but is found to control movement in babies and other primates
What are the ventro-medial pathways?
3 of them:
Vestibulospinal tract from vestibular nucleus that innervates both sides of the spine.
Tectospinal tract from superior coliculus and crosses over in the spine
Reticulo spinal tract- two nerves coming from pontine/mocullary and don’t cross over in the spine
These pathways are involved in balance, posture, reflex movements and fine motor control
Areas of the basal ganglia: Striatum
Made up of the caudate and putamen
>Degeneration here causes huntington’s disease resulting in impaired striatal-nigra and striato-transmission
Receives input from cortex and SNc
Inhibits GPi/SNr in direct motor pathway and inhibits GPe in indirect motor pathway
Areas of the basal ganglia: External/internal globus pallidus (GPe/GPi)
Involved in filtering out unwanted motor signals
Areas of the basal ganglia: Substantia nigra (SN)
2 parts: Zona compacta (SNc) which is involved in regulating caudate/putamen via D1/D2 receptors. Known as the nigro-striatal pathway and has significant degeneration in Parkinson's
Zona reticulata (SNr) which receives inhibitory input from caudate/putamen and inhibits the thalamus
How is the thalamus involved in movement?
Not part of the basal ganglia but receives significant input from it
Sends excitatory signals to the cortex and is inhibited by the GPi/SNr
Function of cerebellum
Involved in fine motor control, gait and co-ordination
Has input from many areas of the brain
Degeneration here can cause cerebral ataxia
Nucleus accumbens
In the ventral striatum is involved in reward and aversion
Hippocampus
Involved in cognitive function and memory
