Nervous System 11 to 20
Describe how pain is transmitted from the periphery to the brain.
Sensory organ or nerve ending
Transmission via 2 fibre types
- Small fast myelinated A-delta fibers
- Large slow unmyelinated C fibers
Spinal Cord: both fiber groups end in the doral horn of the spinal cord (“gate”)
- A delta on neurons in laminas 1 and 4
- C fibers on laminas 1&2
From spinal cord to brain via ventrolateral system – (second order neurons), lateral spinothalamic tract to the thalamus, then to the cerebral cortex (third order neurons)
Must mention dorsal horn of the spinal cord and at least 3 others in bold to pass
How can Acute pain be modulated?
- “Gate theory” eg stimulation of large touch/pressure afferents causing inhabitation of pain pathways in the dorsal horn of the spinal cord.
- Stress induced analgesia
- Drugs (eg opoids)
- Higher centre interpretation
Must get gate theory and 1 other.
What sites do opioid peptides act on?
- Receptors in the afferent nerve fibres
- Dorsal horn region of the spinal cord
- Periaqueductal grey matter in the brain
In the synapse, where can inhibition occur?
- Post synaptic: direct or indirect (refractory periods, after hyperpolarisations)
- Pre-synaptic: mediated by neurons that end on excitatory endings (axo-axonal synapses)
Must give presynaptic and post synaptic to pass
What are the mechanisms involved?
- Increased Cl- conductance – reduces Ca2+ influx and amount of excitatory transmitter released.
- Voltage gated K+ channels – K+ reduces Ca2+ entry
- Direct inhibition of excitatory transmitter release, independent of Ca2+ influx.
Must give reduction in Ca2+ influx.
Describe the structure and function of the sodium potassium ATPase pump.
- Antiport: catalyses hydrolysis of ATP to ADP to move 3 Na out cell in exchange for 2 K in.
- Maintains electrochemical gradient ECF (Electrogenic pump 3+ out / 2+ in = net 1+ out) and is large part of basal energy consumption – 33% energy use by cells (70% neurons)
- When Na binds to alpha subunit, ATP also binds. ATP is converted to ADP causing change in protein configuration extruding Na out of cell.
- Coupled to transport other substances (secondary active transport) e.g.glucose in SI mucosa,
- K then binds extracellularly dephosphorylating alpha subunit which returns to original configuration releasing K into cytoplasm
- alpha and ß subunits which pass through cell membrane
- Both heterogeneous
- alpha subunit intracellular binding sites for Na & ATP alpha subunit extracellular binding sites for K & ouabain ß subunit has no binding sites Na / K Variable distribution of alpha 1 + 2 and ß 1+2 subunits
What is the function of the reticular activating system?
- Network of centres in the brain that regulate respiratory, cardiovascular, vegetative and endocrine functions
- Non-specific activation from any modality
- Sends signals mostly to the thalamus
- Increases cortical electrical activity
- Increased consciousness, alert state, heightened sensory perception – Essential
Describe its location and structure.
- Complex polysynaptic network
- Mid ventral portion of medulla + midbrain
- Converging sensory fibres from long tracts and cranial nerves
Describe the sequence of events in the contraction of skeletal muscle.
- Discharge of the motor neurone
- Release of Ach at the motor endplate
- Ach binds nicotinic Ach receptors
- Increase in Naand K conductancein the end plate membrane
- End plate potential
- Muscle action potential
- Depolarization along T tubules
- Ca release at SR
- Ca binds Trop C and uncovers myosinbinding sites on actin
- Actin myosin cross links and thin filaments slide on each other.
How does tetanic contraction occur?
- Contractile mechanism has no refractory period
- Repeated stimulation before relaxation has occurred – summation of contractions
- Fast repeated stimulation causes a fused continuous tetanic contraction. Can be complete or incomplete.
- Series of maximal stimuli at a frequency just below tetanizing causes increasing tension between each twitch, due to increased calcium availability.
Can you give an example of a stretch reflex?
- Knee jerk
- Ankle jerk
Describe elements of a stretch reflex.
- Sensor from afferent limb (1a fibre from muscle spindle, monosynaptic neurone and excite the motor neurone, so the muscle contracts.
What is an inverse stretch reflex?
- Following prolonged stretch or muscle contraction, the muscle suddenly relaxes. This stimulates the Golgi tendon organ.
- Integrator (Synapse on the motor neurone for stretch and inhibitory interneuron for the inverse stretch)
- Efferent limb (ventral root for both)
- Effector (muscle fibers)
Describe the characteristics of nerve fibers responsible for transmission of 'fast pain'?
- myelinated A delta fibers
- 2-5 um diameter
- conduction rates 12-30 m/s
- end in dorsal horn (lamina 1 and 5)
- neurotransmitter is glutamate
Pass=myelinated plus 2/4 others
What differences are there between these nerve fibers and those responsible for transmission of “slow” or second pain?
- unmyelinated C fibers
- smaller 0.4 – 1.2 um diameter
- slower 0.5 – 2.0 m/s
- also dorsal horn but lamina 1 and 2
- neurotransmitter is substance P
- different sensation – dull / intense / diffuse
- different locations as less A delta fibers in deeper structures
Pass = 4 bolded points
What do you understand by the term referred pain?
- Same embryonic segment or dermatome
- Give an example
- eg diaphragmatic pain referred to shoulder tips.
How is the noradrenaline effect at the synapse terminated?
- Monoamine Oxidase
What types or noradrenergic receptors are there?
How is visual acuity measured? What does the fractions of a VA of 6/24 represent?
- Measurement from Snellen chart viewed at a distance of 6 metres or 20 feet
- 6/24 indicates reduced VA
- Numerator is the distance at which the chart is read
- The denominator is the smallest line that can be read
- 6/6 indicates normal vision
What factors influence visual acuity?
- Optical factors – the state of the image forming mechanisms/sharpness of focus
- Retinal factors – the state of the cones
- Stimulus factors – illumination, brightness of the stimulus, contrast between stimulus and background, legnth time exposed to stimulus
- Sensitivity and interpretative ability of the brain
- Resolving power of the eye, property of the cones
- 2/3 to pass
Why is the fovea important for visual acuity?
- Fovea is the point where visual acuity is greatest
- Fovea is the centre of the macula, a thinned out rod free portion of the retina where the cones are densely packed and each synapses on a single bipolar cell
- One of bold
What factors affect cerebral blood flow?
- Intracranial pressure
- Mean arterial pressure
- Mean venous pressure
- Local factors: pH, pCO2, cause constriction and dilatation of cerebral arteries
- Blood viscosity
- Bold + 1
Describe autoregulation of cerebral blood flow. You can draw a diagram if you wish.
- The process by which cerebral blood flow is maintained at a constant level despite variation in perfusion pressure.
- Average cerebral blood flow is 54mL/100g/min between MAP 65-140mmHg
- Able to draw a plateau region with a range for MAP of 50 – 150 mmHg
What is the Monroe-Kellie doctrine?
- Due to the fact that brain tissue and spinal fluid are essentially incompressible, the volume of blood, spinal fluid and brain tissue must be relatively constant. So when ICP rises, the cerebral vessels are compressed resulting in reduced cerebral blood flow.
- Need to pass 2/3 part to pass