Physiology 14 Flashcards
Where do preganglionic sympathetic fibres travel? What type are they?
Lateral cord - B fibres
Where do sympathetic preganglionic fibres synapse?
What type of fibres are the postganglionic neurones?
Sympathetic chain - C fibres
From where does the sympathetic supply to the head and neck arise?
Superior, middle and stellate ganglia
What is different about the autonomic supply to the adrenal gland?
What is the adrenal response to sympathetic stimulation?
Receives terminal preganglionic fibres
Stimulation causes release of adrenaline and noradrenaline (30:70)
Which cranial nerves contain preganglionic parasympathetic fibres?
CN III, VII, IX, X
What type of fibre are parasympathetic preganglionic neurones?
B fibres
What is the distribution of nicotinic AChRs?
- All autonomic ganglia
- NMJ
What is the distribution of muscarinic AChRs?
- Parasympathetic postganglionic synapses
- Sympathetic postganglionic fibres to sweat glands and skeletal vascular smooth muscle
What subtypes of nicotinic receptor have been identified?
- Skeletal muscle
- Autonomic ganglia
- CNS pain pathways
- CNS movement and cognition pathways
Outline the regulation of transmitters at the autonomic ganglion
Release of ACh stimulates the postganglionic nicotinic receptor but also activates a muscarinic interneurone which releases dopamine, acting on the presynaptic nerve terminal - providing a feedback mechanism
Contrast the structures and functions of nicotinic and muscarinic AChRs
Nicotinic:
-Ligand gated ion channel permeable to Na+, K+ and for some subtypes Ca2+
Muscarinic:
-G-protein coupled receptors classified according to associated second messenger
What is the neurotransmitter at postganglionic sympathetic nerve endings?
Noradrenaline
What are the types of adrenergic receptor?
α receptors:
- α1 : postsynaptic
- α2: usually presynaptic
β receptors:
-β1 + β2 both postsynaptic but also evidence for a presynaptic β2R
What are the excitatory/inhibitory properties of different adrenergic receptors?
α: excitatory via increased Na+ permeability
β: usually inhibitory via increased potassium efflux causing hyperpolarisation. Excitatory in the heart.
How does the ANS govern pupillary tone?
Symp -> mydriasis (dilatation)
Para -> miosis (constriction)
How does ANS tone affect sweat and salivary gland activity?
Symp -> sweating, dry mouth
Para -> no effect on sweating, salivation
How does ANS tone affect hepatobiliary activity?
Symp -> Glycogenolysis, gallbladder relaxation
Para -> Glycogenesis, biliary constriction
How does ANS tone affect stomach activity?
Symp -> Decreased peristalsis and tone (except sphincters - increased)
Para -> Increased peristalsis and tone, sphincter relaxation
How does ANS tone affect lung activity?
Symp -> Bronchodilatation, resultant mild increase in PVR. Equalises flow across lung zones improving V/Q
Para -> Bronchoconstriction, no effect on PVR/flow
How does ANS tone affect cardiac activity?
Symp -> Increased HR + contractility, dilated coronary circulation
Para -> Decreased HR + contractility (esp atrial), no effect on coronary flow
Aside from the effects of NA, what other mediators of vascular tone are released by sympathetic terminals?
Neuropeptides (eg. Y1)
Purines
ATP
Summarise the baroreceptor reflex
^BP -> stretch receptors in carotid sinus and aortic arch -> CN IX + X -> NTS - glutamate release and activation of GABAergic neurones to vasomotor centre -> inhibition of vasomotor stimulation -> vasodilatation
From what levels does the sympathetic outflow occur?
T1 - L4
Which systems are involved in co-ordination of movement?
Cerebrum - complicated movement
Brainstem and cerebellum - Postural control and balance
Spinal cord - spinal reflexes
What are the components of the spinal reflex system?
- Sensory neurones:
- Type Ia/Ib fibres (60-80m/s). Cell body in dorsal root ganglion. Enter cord via dorsal horn. - Motor neurones:
- Aα fibres (60-80m/s). Exit cord via ventral horn - Interneurones:
- Most reflex arcs include an interneurone within the grey matter of the cord. These conduct signals between sensory and motor fibres and interact with descending tracts and other interneurones. Enables integrative control.
How are skeletal muscle fibres classified?
Extrafusal / Intrafusal
Extrafusal:
-Main group, comprising the contractile unit. Innervated by α-motor neurones.
Intrafusal:
-Part of muscle spindle. Innervated by γ-motor neurones (2-30m/s)
How are skeletal muscle sensory units classified?
Outline their function
Muscle spindles / Golgi tendon organs
Muscle spindles:
-Respond to length/rate of change. Stimulate reflex contraction to reduce stretch
Golgi tendon organs:
-Adjacent to myotendinous junction. Sense stretch. Each organ innervated by single afferent sensory Ib axon
What is the structure of a muscle spindle?
Up to 10 intrafusal muscle fibres (parallel to extrafusal fibres) enclosed in a connective tissue capsule.
2 types of intrafusal fibre in the spindle:
Nuclear bag fibre:
- Multiple nuclei in a dilated central part of the spindle
- Involved in dynamic and static response to stretch
Nuclear chain fibre:
- Thinner, no bag.
- Involved only in static response to stretch
What is the contractile capacity of the muscle spindle?
How are they innervated?
The ends of intrafusal fibres are striated and contractile.
Central portions are non-contractile
Spindles are supplied by γ-motor neurones which regulate the sensitivity of the stretch receptor reflex through contraction/relaxation of the peripheral spindle. Contraction stretches the central portion and increases sensory output.
From where do activatory γ-motor neurone impulses originate?
What is the significance of this system?
Facilitatory reticular formation
This is the main mechanism for central control of skeletal muscle tone
What types of sensory endings are present in the muscle spindle?
Primary / Secondary endings
Primary (annulospiral) endings:
- large fibres encircling central portion of nuclear bag and nuclear chain fibres
- Type Ia (70-120m/s)
- Respond to actual length and rate of change
Secondary endings:
- Smaller Type II fibres encircling ends of nuclear chain fibres
- Slower conduction speeds
- Only sensitive to length, not rate of change
Outline the tonic control of muscle tension provided by spindle fibres
Muscle tone primarily dependent on rate of α-motor neurone discharge
Main influence on the above is rate of discharge of Ia spindle afferents (which are influenced by γ-motor neurone tone)
Outline the reflex response to external stretching of a skeletal muscle
Increased tone of primary/annulospiral nerve endings dependent on rate of change in length.
Above triggers extrafusal contraction proportional to rate of stretching, which then reduces spindle output.
Following release of stretch -> short period of inhibition of primary nerve endings due to preceding spindle relaxation
Describe the structure and function of the Golgi tendon organs
- In series with extrafusal muscle fibres within tendon.
- Sensory ends lie in between tendon fibres and are squeezed when tendon is stretched - increasing discharge rate.
- Afferent fibres from Golgi organs synapse with inhibitory interneurones in the cord which synapse with the motor neurone supplying the same muscle.
- Increased tonic discharge inhibits contraction, preventing damage from overstretching
Summarise the stretch reflex
Stretch/myotatic reflex:
-Monosynaptic response to stretching of muscle spindle causing muscular contraction
Define reciprocal inhibition and explain its role in the stretch reflex
Stretch sensory neurones simultaneously synapse with synergistic muscle but also with inhibitory interneurones supplying the antagonistic muscle, causing relaxation. This is reciprocal inhibition
Summarise the withdrawal/nociceptive/flexor reflex
Polysynaptic
Noxious stimulus -> sensory excitation -> synapse with interneurone -> synapse with motor neurone -> contraction of flexor muscle, withrawing from stimulus
Summarise the crossed extensor reflex
Polysynaptic. Occurs in conjunction with flexor reflex
Noxious stimulus -> sensory excitation -> synapse with interneurone -> interaction with contralateral interneurones supplying motor neurones of contralateral limb -> contraction of extensors and relaxation of flexors of opposite limb
Outline the pathway for conscious movement
Initiation in motor cortex -> corticospinal tract -> Spinal interneurones -> integration with cerebellar and spinal reflexes to produce signal to LMN -> movement
Outline the hierarchy of perception in postural control
Ophthalmic/oculomotor > Vestibular > Neck proprioceptors > Lower body proprioceptors
Explain spastic hypertonia following CVA
Loss of central modulation of spinal reflexes results in hyperreactivity -> spasticity
Describe the nature and course of afferent visceral nerves
Unmyelinated fibres travelling from viscera through pre- and paravertebral ganglia (passing rostrally or caudally) on their way to the cord, terminating mainly in superficial dorsal horn
What types of sensory afferents innervate the viscera
spinal afferents generally signal visceral pain
vagal afferents signal non-painful sensations (eg. hunger)
Why is visceral pain poorly localised?
Primary visceral afferents are less numerous than somatic afferents.
Several primary visceral afferents may stimulate a single second order spinal afferent.
Thoracic and abdominal viscera also have vagal afferents.
This results in poorly localised (and usually midline) pain.
What is visceral afferent convergence?
This is the mechanism responsible for referred pain.
Second order neurones receive equal stimulatory input from both somatic and visceral afferents. Stimulation from one can be perceived as the other.
Visceral pain is generally localised to the embryological dermatome of the organ
Contrast somatic and visceral pain
Somatic: Highly localised, highly differentiated, intensiy proportional to potential tissue damage
Visceral: Poorly localised + differentiated, poor correlation between intensity and potential damage. Association with autonomic Sx
What is the phenomenon of pain sensitisation?
An increase in magnitude of response to a particular level of stimulation.
Why does visceral hypersensitivity occur?
Three mechanisms:
- Sensitisation of primary afferents
- Hyperexcitability of second order neurones (ie. central sensitisation
- Dysregulation of descending modulatory neural pathways
Discuss central sensitisation syndrome
Sensitisation causing allodynia/hyperalgesia
Genetic factors: COMT, 5HTT, ADRB2 polymorphisms
Associated with activation of NMDA receptors in the spinal cord
Describe the structure of the NMDA glutamate receptor
Ligand-gated Na+/Ca2+ channel
Four subunits (2xN1, 2xN2), each containing four helical domains
N1: each has 1x glutamate binding site
N2: each has 1x glycine binding site
What are the different types of visceral pain?
True / Referred / Functional
True:
-Vague, diffuse, midline, +-autonomic, allodynia unusual
Referred:
- Convergence of somatic/visceral neurones
- Better localised
- May be associated hyperalgesia
Functional:
- Pain in absence of structural or biochemical explanation
- Often features CSS
Outline the management options for acute visceral pain
Pharmacological / Psychological / Interventional
Pharm:
- WHO ladder
- Treat specific cause eg. cardiac pain - nitrates etc. H2 antagonists/PPIs for gastritis. Spasmolytics for IBS
Psych:
- Physchotherapy
- CBT
- Hypnotherapy
- Good counselling re condition
Interventional:
- Percutaneous cervical cordotomy (eg. C1/2 lateral spinothalamic tract in mesothelioma)
- Sympathetic blocks (eg. coeliac plexus, superior hypogastric
How may treatment of chronic visceral pain differ from acute visceral pain
Evidence for antidepressants in chronic pain.
TCAs / SNRIs / SSRIs
Mechanism not clear but probably involves activation of descending inhibitory pathways. TCAs have multiple possible mechanisms
Define neuropathic pain
IASP definition:
Pain arising as a direct consequence of a lesion or disease affecting the somatosensory system
What are the possible features of neuropathic pain?
Positive symptoms:
- Hyperalgesia
- Hyperaesthesia
- Allodynia
- Dysaesthesia
- Paraesthesia
- Hyperpathia
Negative symptoms:
- Hypoalgesia
- Hypoaesthesia
- Hyperpathia
Physical signs:
- Tropic changes (skin thickening, hair changes)
- Vasomotor
- Sudomotor
- MSK (muscle wasting, osteopaenia)
How may neuropathic pain be classified?
Central / Peripheral
What are possible causes of central neuropathic pain?
Spinal cord injury (66%)
MS (30%)
Stroke (30%)
What are the possible mechanisms of central neuropathic pain?
- Disinhibition
- Sensitisation
- Neuroplastic maladaptive changes to abnormal sensory input
What are possible causes for peripheral neuropathic pain?
Metabolic: Diabetic neuropathy, Thiamine deficiency
Autoimmune: GBS
Trauma
Ischaemia: CLI
Toxins: chemo, alcohol
Physical compression: TGN, radicular pain
What are pharmacological treatments for neuropathic pain?
- Local anaesthetic
- Capsaicin
- Baclofen
- Ketamine
- Opioids (controversial)
- TCAs
- SNRIs
- Antiepileptics
- Gabapentin/pregabalin
What are some interventional treatments for neuropathic pain?
For trigeminal neuralgia:
- RFA of gasserian ganglion
- Balloon compression rhizolysis
- Stereotactic radiosurgery
- Microvascular decompression (MVD)
