Spinal Reflexes Flashcards
What is a lower motor neurone?
- The motor neuron that has its cell body in the ventral horn of the spinal cord is a ‘lower motor neuron(e)*.
What are upper motor neurons?
- Nerve cells from ‘higher up’ the brain send axons down the spinal cord in descending tracts to synapse on the cell bodies and dendrites of the lower motor neurons.
- These neurons with cell bodies in the brain which project down to the lower motor neurons are called ‘upper motor neurons’.
Where are the cell bodies of upper motor neurons located?
- Some upper motor neurons have their cell bodies in the brainstem others in the motor cortex, basal ganglia or cerebellum.
What is a motor unit? What happens when they contract? How can they vary based on size?
- A motor unit is a motor neurone together with its cell body in the dorsal horn, its motor axon and the set of muscle fibres it innervates
- Contraction of one motor unit is obviously the smallest unit of contraction we can produce.
- Motor units can vary in the number of muscle fibres. Some motor units in extraocular muscles may have <10 muscle fibres, some units in large extensor muscles may have >1000 muscle fibres.
- Activation of a small motor unit produces a small force, activation of a large motor unit produces a large force.
What happens when a motor unit contracts? How can contractions be kept smooth?
- Contraction of one motor unit by a single action potential generates a twitch. To produce a smooth contraction the motor unit has to be activated by a train of action potentials at a frequency high enough to produce a smooth fused contraction; this is a tetanus or tetanic contraction.
- We do not produce muscle contraction in a series of jerks but as a smooth force. Physiological tetanus is NOT a pathological condition, but how healthy muscles normally work!
What is the tetanus fusion frequency? What effect does a higher frequency have? What is the frequency like in a healthy individual?
- The tetanus fusion frequency (usually about 10 s-1) is the minimum frequency of action potentials in the muscle to produce a smooth sustained contraction. If the motor neuron fires at a higher frequency, there will be no increase in tension. If it fires at a lower frequency, the contraction will not be smooth but jerky. In a healthy person, motor neurons fire at their fusion frequency or not at all: there is an all-or-none frequency code in motoneurons.
How do we increase or decrease the force of contraction in a muscle?
- We increase or decrease the force of contraction in a muscle by recruiting more or less motor units, each one firing at its tetanus fusion frequency. This is in contrast to sensory nerve fibres, where frequency in a single nerve fibre codes for intensity of stimulation.
How are motor units used in muscles?
- Most muscles contain a mixture of small and large motor units. For fine control of force we activate the small motor units in a muscle. For more power we recruit the large motor units.
How are fine and large movements each created using motor units?
- For fine movements we use only small motor units which gives us small increments in force but weak total force. For greater force we recruit larger motor units but with less fine control.
What happens if there is random death of motor neurons?
- If there is random death of motoneurons, the remaining motor axons sprout peripherally to innervate the denervated muscle fibres. However this leads to an overall increase in motor unit size and decreased fine control
Where are the cell bodies of the lower motor neurons? What are its three main synaptic inputs?
What percentage of lower motor neurons are interneurons?
- The lower motor neuron (with its cell body in the spinal cord) has three main types of synaptic inputs
o Descending tracts in the spinal cord from upper motor neurones
o Input from local interneurones (cells with all their processes inside the CNS)
o Input from local sensory nerve fibres via reflexes - More than 80% of neurons in the spinal cord are interneurons!
How are the lower motor neurons contained in the spinal cord?
- The Spinal cord contains lower motor neurons in the ventral horn that project out via a peripheral nerve to muscles.
What two systems driven lower motor neurons? Why?
Lower motor neurons are driven by two systems of upper motor neurons
o 1) Pyramidal system: Upper motor neuron cell bodies in cortical frontal lobe (mainly motor cortex). Axons travel to spinal cord. They travel via corticospinal tract (aka “pyramidal tract”) to synapse on lower motor neurons
o 2) Extrapyramidal system: Upper motor neuron cell bodies which are in brainstem project to spinal cord. These brainstem neurons are regulated by input from motor cortex.
- Why two systems? EPS is a more primitive system, regulating reflexes, found in all animals. PS is more recent evolutionary development, for fine motor control
What is a reflex? What are some classic examples? How are they modulated?
- It is an involuntary motor action triggered by a sensory input.
- Classic examples of reflexes are the tendon jerk reflexes elicited by tapping the patella or achilles tendon. These are also known as myotactic reflexes
- One can think of reflexes as tiny ‘motor programs’ stored in the synaptic connections between sensory inputs, outputs and interneurones in the spinal cord (or brainstem for cranial nerve reflexes). Most reflexes, although their connections are in the spinal cord, can be modulated from the cortex and brainstem. For example, flexion reflexes can be partially or completely inhibited by the forebrain acting via the pyramidal tract.
What type of reflex is the knee-jerk?
- The knee-jerk (patella) reflex is the classic myotactic reflex. Note that the stretch of the quadriceps muscle activates the same (quadriceps) muscle to contract. This is why it is called a homonymous reflex