Brain and spinal cord Flashcards
Describe the tendon stretch reflex (myotatic reflex).
When the muscle is stretched, this causes the intrafusal fibres of the muscle spindle to increase in length, which causes the 1a afferent to stimulate the alpha motor neurone (monosynaptic reflex) and gamma motor neurone in the spinal cord. The alpha motor neurone causes the extrafusal muscle fibres to contract, and the gamma motor neurone causes the intrafusal muscle fibres to contract so the muscle spindle stays sensitive. There is also reciprocal inhibition (via an inhibitory interneurone) of the antagonist muscle.
Describe the inverse stretch reflex
The muscle is over-stretched, which causes the collagen fibrils of the golgi tendon organ to come close together so the 1b afferent is stimulated. The 1b afferent activates inhibitory interneurones in the spinal cord, which inhibit the alpha motor neurone so that the muscle relaxes. There is also reciprocal contraction of the antagonist muscle.
What is the flexor withdrawal/crossed extensor reflex?
You step on a tac. Cutaneous nociceptors are stimulated and stimulate an interneurone in the spinal cord which activated the alpha motor neurone of the flexor in the affected leg (so you remove your foot from the tac), and the extensor in the other leg (so you don’t fall over). At the same time there is reciprocal inhibition of the antagonist muscles on both sides.
What are the neurotransmitters for nociception?
Glutamte and Substance P.
What do Aalpha fibres carry?
Conscious proprioception.
What do Abeta fibres carry?
Fine touch.
What do Adelta fibres carry?
First pain.
What do C fibres carry?
Second pain.
What is Gate theory?
If the Aalpha and Abeta fibres from the injured area are stimulated, they activate an inhibitory interneurone which prevents the C fibre activating the projection neurone, so the painful stimulus is suppressed.
What is hyperalgesia?
A state where there is a heightened pain response to a stimulus that would normally cause pain (lowered pain threshold).
What is allodynia?
A person experiences pain from a stimulus that would not normally cause pain (e.g light touch).
What causes Huntingdon’s disease?
A mutation on chromosome 4 which means the output neurones of the striatum are lost so the globes pallidus external segment can’t be inhibited and the cortical motor areas are overstimulated. This means there are excessive uncontrollable jerky movements (chorea).
It is treated pharmacologically by depleting dopamine.
What causes hemiballismus?
Damage to the subthalamus nucleus which causes rapid, flinging, violent movements of limbs on only one side.
What is the name of the white matter in the cerebellum?
Arbor vitae.
What are the two major functions of the cerebellum?
Comparative functions - detecting motor error (difference between intended movement and actual movement).
Motor memory - storing learned movements (implicit memory).
Name the three major divisions of the cerebellum.
Spinocerebellum (vermis and medial bits), cerebrocerebellum (lateral bits of lobes), vestibulocerebellum (nodulo-floccular lobe).
What are the functions of the cerebrocerebellum?
Receive inputs from the cerebral cortex, regulate highly skilled movements involving complex sequence of movements (speech).
What are the functions of the spinocerebellum?
Receive direct input from the spinal cord. Lateral part involved in movements of the distal muscles (limbs). Central part (vermis) involved in movement of the axial muscles (trunk).
Which cerebellar peduncle is entirely efferent?
Superior.
Which cerebellar peduncle is entirely afferent?
Middle.
Which cerebellar peduncle is a mix of efferent and afferent?
Inferior.
Where does the cerebellum get motor inputs?
Contralaterally from the cerebral hemisphere to the pontine nuclei, then through the transverse fibres of the pons and the middle cerebellar peduncles.
Where does the cerebellum get its sensory inputs?
Proprioception from muscle spindles, vestibular nuclei, visual and auditory inputs, ipsilaterally entering the cerebellum through the inferior cerebellar peduncle.
Where does the cerebellum receive inputs from modulation on timing, learning and memory.
The inferior olive in the medulla, conveying information to the contralateral cerebellar hemisphere through the inferior cerebellar peduncle.
Where do the deep cerebellar nuclei send information?
Contralaterally through the superior cerebellar peduncle to the thalamus and the cerebral cortex.
What are the functions of the vestibulocerebellum?
Receiving nuclei from the vestibular nuclei in the brainstem, involved in movements controlling posture and balance, and eye movements.
Which of the deep cerebellar nuclei does the vestibulocerebellum send information to?
Fastigial nucleus or direct to the spinal cord.
Which of the deep cerebellar nuclei does the spinocerebellum (lateral) send information to?
Interposed (and fastigial) nucleus.
Which of the deep cerebellar nuclei does the spinocerebellum (vermis) send information to?
Fastigial nucleus.
Which of the deep cerebellar nuclei does the cerebrocerebellum send information to?
Dentate nucleus.
Name the three layers of the cerebellar cortex.
Granule layer, Purkinje layer, molecular layer.
What are the input cells of the cerebellar cortex?
Climbing fibre (input from inferior olive). Mossy fibre (from all other inputs).
What are the output cells of the cerebellar cortex?
Purkinje cells (have large cell bodies in the Purkinje layer, and highly branched dendrites in molecular layer).
What are the interneurones in the cerebellar cortex?
Granule cells (branch into parallel fibres in the molecular layer). Also basket cells, stellate cells, Golgi cells.
Where do granule cells receive input?
Mossy fibres synapsing with granule cell bodies in the granule layer.
Which fibres modify the outputs of the Purkinje cell, by altering the effectiveness of the parallel inputs to the Purkinje cell, and so adjust and correct movements (comparative function).
Climbing fibre from inferior olive in the medulla oblongata.
Where are the mossy fibres carrying input from?
Input from the cerebral cortex via the pontine nuclei and middle cerebellar peduncle. Input from the vestibular nuclei and spinal cord via the inferior cerebellar peduncle.
Which neurotransmitter is produced by Purkinje fibres at the synapse with the deep cerebellar nuclei?
Gamma amino butyric acid (GABA). Purkinje fibres modify movements by inhibiting for learning, memory and timing.
Does damage to the cerebellum lead to movement disorders ipsilaterally or contalaterally?
Ipsilaterally (because that’s how the input from the muscle proprioception, vestibular nuclei, auditory + visual, is represented).
What does damage to the vestibulocerebellum lead to?
Disturbances of balance and eye movements.
What does damage to the spinocerebellum lead to?
Central = postural and gait ataxia (wide based with small shuffling movements). Lateral = upper limb ataxia and dysarthria.
What does damage to the cerebrocerebellum lead to?
Impairment in highly skilled sequences of learned movements.
Damage to cerebellum leads to cerebellar ataxia. What is dyssynergia, dysmetria, days dysdiadochokinesis, intention tremor, ataxic dysarthria?
Dyssynergia = loss of synergistic multi-joint movement, so joints have to be moved one at a time.
Dysmetria = inability to judge distances, so movements overshoot.
Dysdiadochokinesis = inability to perform rapid alternating movements.
Intention tremor = shaking when trying to move but no tremor when at rest.
Ataxic dysarthria = slurred speech.
Which part of the cerebellum does alcohol particularly degenerate?
Anterior cerebellum - affecting movements of the lower limbs especially (so wide staggering gait, but little impairment of hands or arms).
Where do you find medium spiny neurones?
In the striatum.
What are the chief symptoms of Parkinson’s?
TRAP - tremor, rigidity, akinesia, postural problems.
Which cells are lost in Parkinson’s?
Dopaminergic neurones of substantia nigra.
How much dopamine needs to be depleted before symptoms of Parkinson’s will appear?
80%
What type of disorder is Parkinson’s?
Hypokinetic movement disorder.
Why is levodopa, not dopamine, used to treat Parkinson’s?
Levodopa can cross the blood-brain barrier.
Which basal ganglia pathway is responsible for most of the symptoms of Parkinson’s?
Indirect pathway.
Which enzyme breaks down dopamine?
Dopa decarboxylase. An inhibitor for this enzyme is given with levodopa to Parkinson’s patients to decrease the breakdown in the circulation and increase the amount making it across the blood-brain barrier.
