WEEK 5 Flashcards
What is a neural reflex?
A stereotyped, involuntary reaction of the CNS to specific sensory input
What are the three general functions of reflexes?
Protective-limb withdrawal, cough reflex
Postural control-walking
Homeostasis-BP
Describe the characteristic chain of components of a reflex arc
sensory receptor-afferent neurone-(integration centre)-efferent neurone-effector
What is the role of sensory receptors in the reflex arc?
Where information comes into reflex arc, detect sensory stimuli-can be part of sensory neurone/can be afferent neurone
What is the role of afferent/sensory neurones in the reflex arc?
Convey AP from sensory receptor site towards CNS
What is the role of the integration centre in the reflex arc?
It contains interneurones (not relay neurones) in CNS/enteric NS which cause modulation, allowing adjustment of reflex arc activity via brain involvement
What is the role of efferent/motor neurones in the reflex arc?
Convery AP from CNS to effector
What is the role of the effector in the reflex arc?
causes reflex action-muscle/gland
What is the role of simple stretch reflexes?
Posture control, adjusting degree of skeletal muscle contraction to allow postural change
What sensory receptors are responsible for simple stretch reflexes?
Proprioceptors
Give two examples of proprioceptors
Muscle spindles
Golgi tendon organs (GTO)
What are muscle spindles?
Proprioceptors consistent of intrafusal muscle fibres enclosed in a sheath (spindle), providing information on muscle length/stretch and the rate of stretch (myotatic reflex)
Process of muscle spindle action in knee jerk reflex
Spindles stretched when quadricep muscles lengthen, depolarisation causing AP, sensory neurone carries AP into spinal cord via dorsal root and synapses directly to motor neurone (MONOSYNAPTIC)/inhibitory interneurone, motor neurone carries/doesn’t carry AP to quadricep/hamstring muscles, quadricep muscles contract/hamstring muscles relax, leg extension
What are golgi tendon organs (GTO)?
Sensory nerve endings found within tendons that are activated when muscle contracts due to collagen fibre stretching in tendons
Process of GTO action
muscle contraction, depolarisation of GTO nerve endings, AP transmitted to interneurone via sensory neurone, synapse with excitatory interneurone (POLYSYNAPTIC), synapses with inhibitory interneurone, synapses with motor neurone, motor neurone lacks AP, less muscle contraction (reverse myotatic reflex)
What is a monosynapse?
When there is only one synapse between afferent and efferent neurones (no interneurone involvement)
What is a polysynapse?
When there is more than one synapse between afferent and efferent neurones (interneurone involvement)
Describe the cross extensor reflex
nociceptors receive pain in right foot, depolarisation causes AP, sensory neurone carries AP to spinal cord via right dorsal root, synapse with ascending pathways to brain for pain and postural adjustment, synapse with interneurones which cause inhibition of AP in quadriceps/activation of AP in hamstring in right side of spinal cord (right leg), synapse with interneurones which cause activation of AP in quadriceps/inhibition of AP in hamstring in left side of spinal cord (left leg), causes inhibition of extension in right leg and inhibition of flexion in left leg
What are the structural features that may be affected by neuronal injury?
Epi/Peri/Endoneurium, Axon and Myelin sheath
What are the three Sedon classification of nerve injuries?
Neuropraxia, Axonotmesis, Neurotmesis (->increasing in severity)
What is Neuropraxia?
disturbance (compression) to neuronal connective tissue and myelin sheath
What is Axonotmesis?
damage to the axon and its myelin sheath
What is Neurotmesis?
complete transection of nerve
What is the cause and result of Neuropraxia?
temporary loss of neuronal function, most likely due to disturbance of myelination, complete restoration of function upon recovery
What is the cause and result of Axonotmesis?
usually result of stretch of a nerve/its severe crush, very good chance of function recovery due to connective tissue remaining intact-guides nerve sprouts
What is the cause and result of Neurotmesis?
entire nerve fibre severed, damaging axon and connective tissue, complete recovery doesn’t occur
How does the local environment change in response to neuronal damage?
Based on behaviour of 3 classes of glial cells:
Oligodendrocytes/Schwann Cells (myelination)
Astrocytes (provide neurones with nutrients)
Microglia (provide immune response of NS)
Signs of neuronal reaction minutes after inury
no AP conduction beyond site of injury
two ends of axon exposed and start leaking intracellular fluid
cut ends soon pull apart, sealing themselves and swelling
Signs of neuronal reaction an hour or so after injury
synaptic terminal degenerates
astroglia surround terminal-pulling apart Pre- and Post-S neurones
Signs of neuronal reaction days/weeks after injury
fate of proximal segment-cell body undergoes chromatolysis, injured nerve seals wounded stump forming neuroma, nerve segment doesn’t die
fate of distal segment-Wallerian degeneration, soon dies due to loss of nutritional support from cell body, digestion by phagocytosis (myelin sheath and connective tissue preserved for regrowth of proximal end)
What is chromatolysis?
Dissolution of Nissl bodies, occurring at proximal segment. Cell body becomes very active producing lots of proteins for cell repair, volume of cell body increases and it becomes bloated displacing nucleus to peripheral margins of soma
Acute phase of denervated muscle that isn’t reinnervated
Muscle immediately paralysed, becomes areflexic, fasciculations start but subside if muscle not reinnervated
Chronic phase of denervated muscle that isn’t reinnervated
bulk loss due to denervation/lack of use (denervation/disuse atrophy), muscle death, muscle replaced by connective tissue incl. fat (fibrosis)
Process of neuronal axon regeneration in PNS
1) macrophages clean up debris
2) macrophages release mitogens inducing Schwann cell division
3) repopulation of nerve sheaths with myelin
4) Schwann cells produce laminin
5) macrophages make interlukin, inducing Schwann cells to make Nerve Growth Factor
6) axons sprout, and some enter new Schwann cell tubes
What is Crush Syndrome?
a severe, often fatal condition following a severe crushing injury characterised by fluid and blood loss, shock, haematuria and renal failure->traumatic rhabdomyolysis
What occurs to neurones adjacent to damaged neurone?
Transneuronal degeneration (retrograde->Pre-S neurone, anterograde->Post-S neurone)
