Limbic System Flashcards
What is the limbic system?
Group of brain structures found deep beneath the cerebral cortex - just above the brainstem
-Main function = regulating emotions - particularly those related to survival e.g., fear, anger, hunger, sexual beh & higher mental functions - e.g., learning & memory
-Emotions, survival instincts, memory formation
What structures make up the limbic system?
-Amygdala = deep & medial in temporal lobes –> emotional centre - fear conditioning (associative process we learn fear) = anger, violence, fear, anxiety
-Hippocampus = in temporal lobe - role in forming & storing new memories - particularly consolidating short term memories into long term memories
–> sends C-shaped axonal projections through forebrain, around diencephalon into hypothalamus via the fornix
–> so fornix connects hippocampus & hypothalamus
-Hypothalamus = group of nuclei - many functions e.g., regulates endocrine system (such as pit. gland), ANS (symp & para), sleep wake cycle in response to light
-Thalamus = large mass of grey matter cells that make up the main relay centre in brain –> where sensory & motor signals are sent to & from spinal cord & cerebrum - here influences emotions
-Cingulate gyrus = med. aspect of brain - sup to corpus callosum - regulates pain & emotion
What did Paul Broca think about the limbic system -> emotions?
That the limbic system was mainly olfactory - sense of smell
Role of olfactory areas in the limbic system?
The olfactory cortex = links to limbic system
–> smell recognition can evoke emotions
-Connections made between the smell & what you’re feeling/experiencing
-So memory & smell = link!
-So odours/smells can trigger +ve & -ve emotions - as these smells may draw upon specific memories
-Olfaction evokes emotion (+ve or -ve) -> can influence decision making
Give the names of areas found in the olfactory area.
-Olfactory tubercle
-Optic chiasm = where optic nerves cross over (CNII)
-Olfactory tract
-Olfactory bulb
Label this image of the olfactory area of the brain.
What is the cerebral cortex histologically divided into?
-Paleocortex/piriform cortex = olfactory in nature -> more ancient organisation (3 layers)
-Neocortex - motor cortex
-Neocortex - visual cortex
-Archicortex/hippocampus
What is the role of the hypothalamus?
= Control centre for regulating ANS - controls pre-ganglionic spinal neurons - activating symp system
-Also important in emotion regulation/control - experiments on cats
1. Full lesion of hypothalamus = elicits strong emotion - “sham rage” - i.e., shows constant activation of symp NS
2. Partial lesion of cortex - not hypothalamus = no sham rage
What is the Papez circuit?
= proposed mechanism of emotional expression = by hippocampus - encompassing SOME of the components of the limbic system
–> shows the interaction between emotion & consciousness
-Used dyes injected into limbic system - he saw axon tracks going around limbus to hypothalamus
= He was the 1st person to suggest the limbic system had a role in emotion (not just olfaction)
Purpose of the Papez circuit?
Gives emotional significance to episodic memories - for survival
Give the process of the Papez circuit.
Hippocampus –> fibres travel through C-shaped fornix –> connect to mammillary bodies (posterior hypothalamus) –> fibres then travel in mammillothalamic tract (= a tract in between mammillary body & ant. nucleus of thalamus) –> anterior thalamic nucleus –> fibres project to cingulate gyrus & cingulum (cingulate cortex) –> then fibres go to entorhinal cortex (specific area of parahippocampal gyrus) –> then fibres return back to hippocampus
*Via axon projections
Hippocampus axons went around to contact ant. diencephalon - linked up directly to hypothalamus
What does the Papez circuit start & end with?
Hippocampus = why it is a circuit
What is the fornix?
C-shaped axons/bundles of nerve fibres leading from hippocampus to mammillary bodies
-So it = an important output tract from hippocampus
-Is considered the main connecting tract of limbic system
-It is a part of the hippocampus - that is the white matter connection to the hypothalamus
Role of the cingulate cortex?
Emotion formation and processing, learning, and memory
Where is the hippocampus?
Deep in temporal lobe
-Next to entorhinal & perirhinal cortex
–> entorhinal cortex = pyriform olfactory cortex!! = where Broca got the idea that emotions are all related to smell
-Parahippocampal gyrus = surrounds hippocampus & primary olfactory cortex (entorhinal cortex) …………………………
Role of hippocampus?
Helps us process & retrieve 2 kinds of memory: declarative memories & spatial relationships
–> so is crucial in memory formation***
Label the components of the Papez circuit.
Where is the amygdala?
Deep & medial in temporal lobes
Label this ventral view of the brain.
What is the entorhinal cortex? XXXXXXXX
-Rhinal = relating to the nose or olfactory part of brain
-Entorhinal cortex makes up various parts of the olfactory cortex
Describe what is meant by the hippocampus having reciprocal communications to all association areas of the cortex?
Hippocampus receives sensory afferents from all association areas of the cortex -> via the parahippocampal & rhinal cortical areas - these fibres sent to hippocampus (hippocampus will then also send axons back to cortical association areas) -> then fibres from hippocampus to fornix -> to
–> so as part of this circuit - hippocampus sends & receives nerve fibres to fornix -> to thalamus -> to hypothalamus
–»» i.e., hippocampus sends axons/fibres to cortical association areas (after receiving signals from these) & also sends axons/fibres out to thalamus & hypothalamus (via the fornix)
**Inputs/afferents to hippocampus
Major inputs come from entorhinal cortex, which in turn communicate inputs from cingulate, temporal, orbital & olfactory cortices & amygdala to hippocampus.
???
What is procedural & what is declarative memory?
-Non-declarative/procedural = long-term memory that doesn’t require conscious thought or recollection to access. Examples = riding a bike & knowing how to do maths
-Declarative = storage of facts & events we have personally experienced
How does making lesions of the hippocampus affect rats?
Problems with declarative memory - memory of facts
–> so hippocampus is associated with memory formation - shown by being unable to remember where food had been placed in a maze test
What are the 2 types of synaptic plasticity?
-Long term potentiation - left –> amount of electrical potential in post-synaptic cell after stimulation (due to neurotransmitter release) –> so after depolarising currents = inc in potential/beh of post-synaptic neurone changes forever more - remains high once have stopped firing current - this is what learning is! = creating new synapses & making them bigger - so they respond more
-Long term depression - right –> when have stimulation entering post-synaptic neurone = opposite effect - over-use of synapses makes them weaker
-SO with use of synapases - can get weaker or stronger = basis of plasticity - i.e., learning
Summarise long term potentiation.
After large stimulation = inc in size of synapse (synaptic boutons) physical & also after stimulation - get more current initiated in synapse = greater response to same signal
How do inputs & outputs of the limbic system relate to other areas of the cortex?
-Can have multiple inputs into amygdala - e.g., auditory, visual, sensory (pain, touch, taste) = act as stimuli
-Influence learning - because these simultaneous action potentials create long term potentiation of synapses –> meaning size & firing rate = increased - i.e., neurons ‘learn’ their firing pattern
-Outputs from amygdala to circuits to control somatic & visceral motor activity (get motor actions as a result) - because
-Amygdala output neurons project to somatic & visceral motor activity
BUT
-For hippocampus - directs these fibres (as output) back to same neocortical areas
–> hippocampal neurons have connections to whole cerebral cortex!
Amygdala receives inputs from many cortical regions but outputs only to somatic & visceral regions
WHEREAS
Hippocampus receives inputs from all neocortex association areas & outputs to these same areas
What makes up the basal ganglia?
-Caudata nucleus
-Putamen
-Globus pallidus (int & ext nucleus)
-Subthalamic nucleus
-Substantia nigra (of midbrain)
–> are both functionally related to basal ganglia but not actually a component of BG
Role of basal ganglia?
-Motor control = direct & indirect pathways!!
-Limbic control = the limbic loop
What modulates the motor control by the basal ganglia?
Motor basal ganglia circuits = modulated by the substantia nigra of the midbrain
–> these are the neurons which die in Parkinson’s disease
What modulated the limbic loop of the basal ganglia?
The ventral tegmental area (the mesolimbic reward pathway) - which is dopaminergic
–> modulation of this area is pathological (underlies conditions…) in people with drug addiction, alcohol addiction, anorexia…
What is the entire limbic system made up of?
-Limbic lobe
-Orbitofrontal cortices
-Amygdala
-Ventral striatum
-Ventral pallidum
= all interconnected
