brainstem (I) Flashcards
Brainstem
The brainstem is the conduit for information flow between the cortex and the spinal cord.
Fiber tract
Refers to a bundle of axons in the CNS.
Nerve
Refers to a bundle of axons in the PNS.
Projections
Refer to axons that extend from one region of the nervous system to another.
Brain sections
Three different planes:
1. coronal
2. horizontal
3. sagittal
Brainstem components
- Medulla
- Pons
- Midbrain
Important structures of the brainstem
+ Fill in the blanks!
- Fourth ventricle
- Cerebral aqueduct
- Superior colliculi
- Inferior colliculi
- Pineal gland
Fiber tracts: caudal medulla
- Dorsal column
→fasciculus gracilis (legs)
→fasciculus cuneatus (arms) - Anterolateral pathway
→spinothalamic tract
grey matter starting to lose its shape
Fiber tracts: mid-rostral medulla
(this is now the level where we are slicing through 2nd order neurons these axons are crossing over to the other side)
- Dorsal column
→nucleus gracilis (legs & lower body)
→nucleus cuneatus (arms & upper body)
dorsal column axons synapse with 2nd order neurons in the dorsal column nuclei of the medulla - Anterolateral pathway
→spinothalamic tract
Fiber tracts: rostral medulla
The axons of the dorsal column nuclei have crossed to the contralateral side of the brainstem to form a fiber tract called the medial lemniscus.
- Dorsal column
→medial lemniscus - Anterolateral pathway
→spinothalamic tract
at the junction between the medulla and the pons
Fiber tracts: pons
Ascending somatic sensory tracts
- Dorsal column
→medial lemniscus - Anterolateral pathway
→spinothalamic tract
the two fiber tracts straighten out
a lot of axons
Fiber tracts: midbrain
Ascending somatic sensory tracts
- Dorsal column
→medial lemniscus - Anterolateral pathway
→spinothalamic tract - Cerebral aqueduct
Cerebral aqueduct
Connects the 4th ventricle to the other ventricles of the brain
Fourth ventricle
Ventricle (cavity) filled with CSF which protects the brain from trauma.
Cranial nerves table!
☆slide 62☆
General organization of cranial nerve nuclei in the brainstem
Lateral → medial
- Somatic sensory
- Visceral sensory
- Visceral motor
- Somatic motor
☆have fun, read slide 63 for an example!☆
Cranial nerve nuclei in the brainstem
Lateral → medial
- Special
- Somatic sensory
- Autonomic sensory (visceral in)
- Autonomic motor (visceral out)
- Somatic motor: Brachial
- Somatic motor
Cranial nerve nuclei in the brainstem: Special
-carrying info from inner ear
-corresponds to vestibular and auditory systems
Cranial nerve nuclei in the brainstem: Somatic sensory
-spread across entire length of the brainstem
-trigeminal nerve carries sensory input from head
Cranial nerve nuclei in the brainstem: Visceral in
-visceral sensory has only one nuclei (nucleus of the solitary tract)
-most autonomic input comes from the vagus nerve
Cranial nerve nuclei in the brainstem: Visceral out
-visceral autonomic nuclei (out) emerge and form cranial nerves necessary for output to parasympathetic innervation of organs
Cranial nerve nuclei in the brainstem: Brachial
-brachial somatic motor neurons
-essentially control voluntary movements in head
Nucleus of the solitary tract
-important for autonomic function
-brainstem integration center
→receives sensory inputs coming in from the visceral organs / cranial nerves, so this autonomic information provides info about what’s going in the body, this is being processed at the level of the brainstem to generate outputs that control the autonomic nervous system
Which level of the brainstem contains the most nuclei?
Medulla
Reticular formation
Central core of grey matter in the brainstem
→a “meshwork”
→neurons necessary for more complex functions/circuitry are found in the reticular formation
Reticular formation function examples
Involved in numerous integrative functions of the brainstem…
- Stereotyped motor responses
→breathing rhythms, chewing, oscillating movement when walking - Autonomic functions
→cardiac function - Ascending arousal (reticular activating system)
→sleep/wake patterns
PAG
Periaqueductal grey.
-exemplifies the integrative functions of the reticular formation
-connected to regions of the brainstem involved in autonomic, somatic and behavioral responses and to higher brain regions involved in fear
-does not need to be triggered by fear → more of a controller
PAG stimulation
-in cats: increased respiration, hissing, arching of back
-in mice: freezing or escape responses
-in humans: stereotyped defensive behaviours, anxiety
Possible clinical relevance of PAG
PAG stimulation can act as an analgesic
→makes sense, because you don’t want to focus on pain when in a fearful situation
→regulates pain signals
→a person with chronic pain could be treated with PAG stimulation
→risk of high anxiety
Ascending arousal system: Experiment in cats
In cats, lesions at the level of the caudal medulla did not affect arousal; however lesions in the rostral pons/caudal midbrain resulted in a persistent sleep-like state.
→suggested an ascending arousal system that started in the brainstem and “activated” the cerebral cortex
EEG signals
Measurement of electrical activity in cerebral cortex
EEG signals: awake
-low amplitude
-no defined pattern
-combination of all signals somewhat cancel out
EEG signals: asleep
-large amplitude that oscillates
-neurons in cerebral cortex fire in unison, resulting in an additive effect of these signals
-neurons fire rhythmically (phasic bursts of activity)
Ascending arousal system: Experiment in cats - severing caudal medulla
no effect on awake EEG
Ascending arousal system: Experiment in cats - severing caudal midbrain
resulted in asleep state EEG
Ascending projections from the brainstem arousal system
Ascending projections from the brainstem arousal system comprise a dorsal pathway that goes through the thalamus and a ventral pathway that project directly to the cortex.
→these two pathways are very important for the maintenance of an awake and alert state
Norepinephrine neurons
-originate in the locus coeruleus
-project to virtually the entire brain as well as the spinal cord
-the ascending projections are involved in sleep-wake cycles, arousal and attention
-fire during wakefulness
-firing decreases during slow wave sleep
-silent during REM sleep
Locus coeruleus
Small cluster of neurons from which NE (neuromodulator) originates from
REM sleep characteristics
Dramatic physiological changes occur
→eye movements
→dreams
→changes in muscle tone
NE neuron firing correlates with _____
Levels of attention.
→inattentive/non-alert: regular AP pattern
→novel event: burst of APs
→scanning: constant increase of AP firing
Which medication often works by modulating NE levels?
ADHD meds!
Serotonin neurons
-originate in the raphe nuclei
-project through the brain and spinal cord
Which medication often works by modulating serotonin levels?
Drugs used to treat depression/anxiety (SSRIs)!
→suggests that serotonin is correlated with being a neuromodulator that regulates mood
Dopamine neurons
-originate in the substantia niagra & ventral tegmental area
-project to the basal ganglia & cerebral cortex
-dopamine signalling is involved in cognition, movement and learning
-addictive drugs increase dopamine release
Substantia niagra
-projects dopamine neurons to the basal ganglia
-death of these neurons is associated with loss of movement in PD
Ventral tegmental area
-projects dopamine neurons to the cerebral cortex & subcortical structures involved in salience, reward, and learning
Parkinson’s disease (PD)
Parkinson’s disease results from loss of dopamine neurons.
Acetylcholine neurons
-originate in the basal forebrain
-project to the cerebral cortex, hippocampus and amygdala
-ACh in the brain acts trough ionotropic nicotinic receptors and metabotropic muscarinic receptors
-ACh neurons are among the first to die in Alzheimer’s disease
-involved in sleep-wake cycles
Sensory information throughout the different synaptic levels
Sensory information is transformed as it moves through each synaptic level
→the receptive fields of output neurons are different (generally more complex) than the receptive fields of input neurons
Why do higher order neurons in the sensory pathway have more complex receptive fields than primary afferents?
Because of convergence and divergence of lower order inputs
Convergence
Integration of information
→a single neuron will receive synaptic inputs from many other primary afferents
Divergence
A single neuron will branch to spread information
Lateral inhibition
-allows a strong central signal to pass through but filters out weak surrounding signals
-sharpens the contrast of of sensory signals
→a central excitatory neuron can relay excitatory information, but it can also inhibit surrounding neurons
→in addition to excitatory synapses made by the center, there are excitatory synapses made on inhibitory neurons that go towards neighbouring neurons, thus inhibiting the surrounding/lateral part of the pathway this sharpens the signal by dampening surrounding noise
→EX: this can be done through excitation of inhibitory GABA-releasing neurons (which tend to be shorter to quickly access nearby neurons)
What is the most common excitatory neurotransmitter of the nervous system?
Glutamate
What is the most common inhibitory neurotransmitter of the nervous system?
GABA
How does sensory information from the brainstem reach the cerebral cortex?
It must pass through the thalamus!
