Nervous System Flashcards
How do astrocytes produce nutrition for neurons?
Glucose-lactate shuttle - they produce lactate which can be transferred to neurons to supplement their supply of glucose
Why is it important for astrocytes to maintain ionic environment in extracellular fluid?
Helps buffer K+ in brain extracellular fluid. K+ uptake prevents depolarisation of neuron due to increase in resting membrane potential
What is the resting membrane potential for a neuron?
-65mV
What is the resting membrane potential for an astrocyte?
-85mV
What is the role of oligodendrocytes?
Responsible for myelinating axons in CNS
What is the role of microglia?
Recognise foreign material and phagocytose Brain’s main defence mechanism
Explain immune response in the CNS
Alongside phagocytosis microglia act as antigen presenting cells to T-cells but CNS inhibit initiation of pro-inflammatory T-cell response. Rigid skull will not tolerate volume expansion
What forms the blood brain barrier?
- Tight junctions between endothelial cells 2. Basement membrane surrounding capillary 3. End-feet of astrocyte processes
Describe release of neurotransmitters
- Action potential causes depolarisation at axon terminal 2. Voltage-gated calcium channels open and calcium enters pre-synaptic terminal 3. Calcium causes vesicles to move to pre-synaptic membrane 4. Neurotransmitters are released via exocytosis 5. Neurotransmitters diffuse across synaptic cleft and bind to receptors on post-synaptic membrane
What are the two types of glutamate receptors?
Ionotropic - ion channels (e.g AMPA, kainate, NMDA) Metabotropic - GPCR
Name the major excitatory and inhibitory neurotransmitters in CNS and their actions
Excitatory: Glutamate - causes depolarisation Inhibitory: GABA, glycine - causes hyperpolarisation
Name some biogenic amine neurotransmitters
Acetylcholine, noradrenaline, dopamine, serotonin, histamine
Name some peptide neurotransmitters
Enkephalins, substance P, somatostatin, cholecystokinin, neuropeptide Y
Name some amino acid neurotransmitters
Glutamate, glycine, GABA
What does the basal ganglia consist of?
- Neostriatum (caudate nucleus and putamen)
- Globus pallidus (+ putamen = lenticular nucleus)
What is the internal capsule?
Tracts of axons going to the cortex that separate caudate nucleus and thalamus from putamen and globus pallidus
What are the 5 roles of astrocytes?
- Structural support 2. Nutrition for neurons 3. Removal of neurotransmitters 4. Maintain ionic environment 5. Help for blood-brain barrier
Which arteries supply the brain?
Internal carotid and vertebral arteries
What are the branches of the internal carotid and the vertebral arteries?
Internal carotid:
- Anterior cerebral artery
- Middle cerebral artery
- Ophthalmic artery
- Posterior communicating artery
Vertebral artery:
- Basilar artery which splits into L. and R. posterior cerebral arteries
What is the purpose of the circle of Willis?
Collateral supply in case of progressing occlusion
Why is cerebrovascular autoregulation needed for the brain?
Brain can only use glucose and cannot store it, so contant perfusion pressure must be maintained.
A change in cerebral perfusion pressure causes compensatory changes in cerebral blood vessels
- Decreases perfusion causes vasodilation
- Increase perfusion causes vasoconstriction
Explain how chemoregulation of brain perfusion occurs
Build up of metabolic by-products (decrease pH, decrease O2, increase CO2) causes vasodilation to increase blood flow
Decrease of metabolic by-products (increase pH, increase O2, decrease CO2) causes vasoconstriction to decrease blood flow
What is cerebral perfusion pressure? What is the equation?
Net pressure gradient causing cerebral blood flow
CPP = mean arterial pressure - intracranial pressure
What reflex occurs if ICP increases?
Cushing’s reflex
- Increased BP
- Decreased pulse
- Irregular RR
Define a UMN
Motor efferent fibres with cell body within the motor region of the cerebral cortex or brainstem and axon that remains within the CNS
They synapse with LMNs
Define a LMN
Somatic motor fibres with cell body either in lamina IX (anterior horn) of spinal cord or in cranial nerve nuclei
They leave the CNS to supply the skeletal muscles of the body
There are two types of LMNs: alpha and gamma
Describe the function of alpha-motor neurone
Innervate extrafusal muscle fibres of skeletal muscle. Responsible for initiating skeletal muscle contraction
Describe the function of gamma-motor neurones
Innervate intrafusal muscle fibres of muscle spindles. They detect amount and rate of change in length of muscle (proprioception) and keep muscle taut.
Describe a motor unit
LMN plus all the muscle fibres that it innervates
Define a spinal reflex
A involuntary, unlearned, repeatable autonomic reaction to a specific stimulus that does not require the brain
What are the 5 parts of a reflex arc?
- A receptor
- Afferent fibre
- Integration centre
- Efferent fibre
- An effector
What is the role of muscle spindles? How does this occur?
Connective tissue capsule surrounding intrafusal fibres. Middle portion innervated by afferent senory neurones and end portions innervated by efferent gamma-motorneurones.
Detect changes in length of the muscle. Stretching of muscle spindle detected by afferent neurones increases firing at excitatory synapse at LMN causing contraction of muscle.
What is the role of gamma-motor neurones?
Innervates intrafusal muscles to prevent muscle spindle from becoming slack when extrafusal fibres contract.
What is the role of a golgi tendon organ?
Found at the junction between muscle and tendon, innervated by sensory afferent neurones.
Increased firing occurs in response to stretching of tendon
Increased firing into interneurones = synapse onto inhibitory alpha-LMN = decreased firing = decreased contraction of muscle
What is a stretch reflex? Why is it important?
A stretch activated contraction of skeletal muscle.
It is the minimal neural circuit that underlies all movements of muscles in the body and sets all motor tone of the body.
What is muscle tone?
- A muscle’s resistance to passive stretch in resting state
- Continous, passive, partial contraction of all skeletal muscles.
- Allows us to maintain body posture and hold our heads up.
When is muscle tone not present?
Absent in new born, returns a few months after birth
Inhibited during REM sleep, except for muscles of breathing, extra-ocular muscles, urinary sphincter and anal sphincter
What are the two types of pyramidal tracts, what do they do and where do they decussate?
Corticospinal tract
- Lateral - voluntary movement of limb muscles
- Decussate at medualla
- Anterior - voluntary movement of axial muscles
- Decussate at spinal level
Corticobulbar tract
- Supplies muscularture of head and neck
- Many innervate bilaterally (except facial and hypoglossal nerve)
What are the 4 extra-pyramidal tracts and what do they do?
- Vestibulospinal - recieve information about balance and relay it to control balance and posture
- Reticulospinal:
- Medial - faciliates voluntary movements and increases muscle tone
- Lateral - inhibits voluntary movements and decreases muscle tone
- Rubrospinal - function unclear ?fine control of hand movements
- Tectospinal - Coordinates movements of head in relation to visual stimulus
What is pain?
A coomplex, unpleasant awareness of sensation modified by experience, expectation, immediate context, culture etc
What is the difference between pain stimulus threshold and pain tolerance?
Pain stimulus threshold - The range at which tissue damage occurs e.g. temperature activated nociceptive pathways between 44-46oC
Pain tolerance - variable reaction to a painful stimulus that is different in individuals e.g. environment, situation, psychological/emotional factors, increases with age, on-going pain
What type of pain is relayed by A-delta fibres and where do the fibres synapse within the spinal cord?
Sharp stabbing pain
Laminae I, V
What type of pain is relayed by C fibres and where do the fibres synapse within the spinal cord?
Dull-nagging pain
Laminae I, II, V
Explain the modulation of pain information
Pain information can be inhibited or amplified by a combination of local neuronal circuits and descending tracts from high brain centres
Constitutes gate-control mechanism
Which structure is affected to cause a chorea?
Indirect pathway in the basal ganglia - subthalamic nucleus
What is meant by the term hemiballismus?
- Unilateral chorea
At what level does the spinal cord end? And at what level would you perform a lumbar puncture?
- L1/L2
- L3/L4
What is the role of basal ganglia?
To regulate the amplitude and velocity of planned movement
Describe transduction of pain
- Noxious stimuli triggers firing of primary afferent fibres through activation of nociceptors
- Tissue damage causes release of histamine, prostaglandin, serotonin, K+, bradykinin and H+
- Lowers threshold of nociceptors
- Substance P also released causing:
- increases capillary permeability
- contributes to inflammation
- mast cells to release histamine - further sensitises nociceptors
How does visceral pain occur?
- Visceral sensory afferent fibres converge on 2nd order neurones in spinal cord (lamina V) of somatic nociceptor fibres
