Spinal Reflexes

Question 1

What is a lower motor neurone?

Answer 1

• The motor neuron that has its cell body in the ventral horn of the spinal cord is a ‘lower motor neuron(e)*.

Question 2

What are upper motor neurons?

Answer 2

• 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’.

Question 3

Where are the cell bodies of upper motor neurons located?

Answer 3

• Some upper motor neurons have their cell bodies in the brainstem others in the motor cortex, basal ganglia or cerebellum.

Question 4

What is a motor unit? What happens when they contract? How can they vary based on size?

Answer 4

• 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.

Question 5

What happens when a motor unit contracts? How can contractions be kept smooth?

Answer 5

• 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!

Question 6

What is the tetanus fusion frequency? What effect does a higher frequency have? What is the frequency like in a healthy individual?

Answer 6

• 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.

Question 7

How do we increase or decrease the force of contraction in a muscle?

Answer 7

• 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.

Question 8

How are motor units used in muscles?

Answer 8

• 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.

Question 9

How are fine and large movements each created using motor units?

Answer 9

• 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.

Question 10

What happens if there is random death of motor neurons?

Answer 10

• 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

Question 11

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?

Answer 11

• 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!

Question 12

How are the lower motor neurons contained in the spinal cord?

Answer 12

• The Spinal cord contains lower motor neurons in the ventral horn that project out via a peripheral nerve to muscles.

Question 13

What two systems driven lower motor neurons? Why?

Answer 13

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

Question 14

What is a reflex? What are some classic examples? How are they modulated?

Answer 14

• 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.

Question 15

What type of reflex is the knee-jerk?

Answer 15

• 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

Question 16

What are monosynaptic reflexes? What are some examples?

Answer 16

• The myotactic knee, ankle, biceps triceps and supinator reflexes are sometimes called MONOSYNAPTIC reflexes as NO INTERNEURONES are involved in the reflex (only found in tendons, all other reflexes involve interneurons)

Question 17

What type of connections are there in monosynaptic reflexes? Where else do these occur?

Answer 17

• There is a direct synaptic connection between the input sensory nerve fibre (a muscle spindle afferent) and the motor neuron.
• This direct connection between sensory and motor nerves does not happen anywhere else in the nervous system; only in the tendon jerk reflexes.
• This direct connection makes it very hard for the brain to suppress or modulate this reflex
• Remember the names of the two neurons involved. The muscle spindle afferent is called a 1A afferent, the motor neuron is called an alpha (a) motor neuron.

Question 18

What does the tendon tap usually activate? What is this called?

Answer 18

• Normally the same tendon tap that activates the extensor muscle also inhibits the antagonist flexor muscle by the action of an inhibitory interneuron. Thus when the extensor contracts, the flexor relaxes. This is called reciprocal inhibition

Question 19

What is the muscle spindle? What does it consist of? How many are there in each muscle? What is the main sensory nerve fibre from the muscle spindle? What are special fibres?

Answer 19

• The muscle spindle is the receptor that mediates all the tendon reflexes.
• It is often called a proprioceptor as it responds to movements of the body rather than external stimuli
• It consists of a complex encapsulated stretch receptor inside a a connective tissue sheath. (There are normally about 4 to 20 muscle spindles in a muscle, more in muscles involved in fine control)
• Stretch of the muscle also stretches the muscle spindle and activates the muscle spindle sensory afferents.
• The main sensory nerve fibre from the muscle spindle is called a Ia afferent (pronounced “one-ay”)
• Muscle spindles contain a group of special muscle fibres inside them.
• These special fibres are known as intrafusal muscle fibres. (Normal muscle fibres are sometimes called extrafusal muscle fibres)

Question 20

What happens in the intrafusal fibres? When are muscle spindles active?

Answer 20

• In the intrafusal fibres all the contractile apparatus is at the ends, leaving the centre of the fibre without actin or myosin but full of cell bodies and mitochondria.
• This central region has no connective tissue and so is very easily stretched.
• The Ia sensory terminal branches coil around the centre region. When the whole muscle is stretched this central region of the intrafusal fibres is stretched and activates the 1A afferent
• Most muscle spindles are spontaneously active at resting muscle lengths. When a muscle is stretched they increase their firing rate. They are MUSCLE LENGTH DETECTORS. When a tendon is tapped the muscle is only stretched a very small amount; this however is sufficient to briefly increase the firing of the spindles which in enough to activate a reflex.

Question 21

What has intracellular recording of lower motor neurons shown?

Answer 21

• Intracellular recording from lower motor neurons has shown that a single action potential in a Ia sensory nerve fibre does not trigger an action potential in the motor neuron. Instead, it produces a small depolarisation (5-15 mV) which is called an excitatory post-synaptic potential or EPSP.

Question 22

What happens if you have 2 different synaptic inputs on a motor neuron?

Answer 22

• Suppose we have two different synaptic inputs E1 and E2 on a motor neuron. If both are active at the same time the two EPSPs add together and may trigger an action potential.. This is called spatial summation

Question 23

What is spatial summation? What is ‘noise’ and how does this affect the motor neuron?

Answer 23

• Lower motor neuron dendrites have many dozens or even hundreds of synaptic contacts from muscle spindle Ia nerve fibres.
• Activity at a single synapse appears to count as ‘noise’ and does not activate the motor neuron. There has to be at least two synapses simultaneously active to make the motor neuron fire. In actual life there may be many synapses more or less simultaneously active and spatial summation occurs over all of them.

Question 24

What is temporal summation and when does it occur?

Answer 24

• Temporal summation can also activate a motor neurone. This occurs when a single Ia nerve fibre fires a high frequency burst of action potentials so that the EPSs sum together to trigger an action potential.
• Motor neurons only fire action potentials if there is sufficient spatial and/or temporal summation at their inputs.

Question 25

What is efferent control and how does it happen in the muscle spindle?

Answer 25

• Efferent control = controlled by the brain
• As well as the Ia afferent fibre, the muscle spindle is also supplied with a special motor neuron. This is the gamma motor neuron. The gamma motor neurone stimulates contraction of the intrafusal fibres and ‘prestretches’ their centres. This increases the response of Ia afferents to muscle stretch.
• In other words the gamma motor neuron sets the ‘gain’ or sensitivity of the 1A afferent. If you want to keep your arm very steady, the gain can be set high so small movements give a high rate of action potentials. Or the gain can be set low so large movements can still be detected accurately

Question 26

What does NOT activate the gamma motor neurons? What happens to these neurons at high frequency? What about pathological increases in activity?

Answer 26

• Gamma motor neurons are NOT activated by the muscle spindle 1A afferents; They are driven from descending motor pathways such as the extrapyramidal and pyramidal pathways.
• If the gamma motor neurons become tonically active at a high frequency, the spindle becomes oversensitive to stretch and the tendon tap reflex is pathologically enhanced.
• Pathological Increases in gamma motor neuron activity lead to hyperactive tendon reflexes and spasticity: the patient shows signs of an upper motor neuron lesion

Question 27

What happens during muscle fatigue?

Answer 27

• This means that despite a steady ‘drive’ from the brain via the descending motor pathways, the force of contraction per muscle action potential decreases, the longer the muscle is contracting.
• So without feedback from muscle spindles if you tried to keep an arm or leg at a steady position (without looking at it), it will droop or collapse.
• The monosynaptic reflex is used by the brain as a form of negative feedback to maintain a constant muscle length despite ongoing muscle fatigue.

Question 28

Give a step by step account of what would happen during muscle fatigue from holding a beer glass.

Answer 28

1) Imagine you are holding a beer glass. Initially there is an input to the motor neurons from both the descending axons from the forebrain and also from the muscle spindle Ia afferents. The combined synaptic input keeps the arm in the desired position
2) However after a while fatigue in the biceps means it contracts less strongly and thus starts to stretch. The arm starts to droop. This stretch of the muscle increases the activity in the muscle spindles.
3) Increased stretch of muscle means stretched muscle spindles send increased frequency of action potentials to spinal cord. This Increases synaptic input to motor neurones which fire at higher frequency to increase force of contraction. Result: Arm moves back to desired (initial) position.
- Increased firing in muscle spindles increases action potentials in muscle. Muscle contracts more strongly & restores limb to desired position

Question 29

What does the brain do (via the corticospinal tract) during any voluntary movement of a muscle?

Answer 29

• During any voluntary movement of a muscle the brain (via the corticospinal tract) activates both the alpha and gamma motor neurones. This co-activation serves to keep the muscle spindle feedback at the right level during movement. When the muscle reaches the desired length the gamma motor neurone output adjusts to a level appropriate for this new length.

Question 30

Why is the monosynaptic reflex easiest to elicit in leg extensor muscles (quadriceps and gastrocnemius)?

Answer 30

• Fatigue in the muscles active during standing means that we would fall over were it not for the continuous feedback input from muscle spindles.
• Continuous feedback from muscle spindles in the spinal cord plus the high speed of action potential conduction in the afferent and efferent nerves gives the tendon jerk reflex the shortest possible delay time. (quicker than your awareness of falling)
• The faster the feedback response, the quicker the corrective muscle action. And the better the person’s balance and agility.
• The need for the maximum postural stability and agility is why monosynaptic reflexes are particularly active in postural muscles and leg extensor muscles.
• Damage to these reflex arcs makes patient more likely to fall or trip.
• (for example, falls are common in people with MS or diabetic neuropathy)

Question 31

What is the Golgi Tendon Organ (GTO) and what does it do? How is it activated? What type of connection does it have and what to? What is its interneurone? What do Golgi organs mediate?

Answer 31

• The Golgi Tendon organ (GTO) is the second major proprioceptor in muscle. It is found in muscle tendons.
• It is activated by muscle tension (not length). Both the spindle and GTO are activated when a muscle is passively stretched but the spindle is switched off if the muscle shortens back to its original length.
• Golgi organ has a disynaptic (interneurone) connection to its own motoneurones. The interneurone is a glycinergic inhibitory neurone
• Golgi organs mediate the reflex relaxation in the ‘clasp knife’ reflex; if a muscle contracts so strongly that it increases the tension in the tendon to a level where might damage the muscle or joint tissue, the GTO ‘switches it off’ by a powerful inhibitory action.

Question 32

What are flexion reflexes? How are they mediated? What type of neurones do they act on? How many of these neurones are involved? What name does this give these types of reflexes?

Answer 32

• Reflex withdrawal from a painful stimulus is mediated by activation of small myelinated nociceptor afferents (Ad). These cells act on interneurones in the spinal cord, not directly on motor neurones. The pathway involves at least one and usually two or three EXCITATORY interneurones between the pain afferent input and the flexor motor neurone. This flexion reflex is therefore known as a polysynaptic reflex
• Because a flexion reflex involves several interneurones (which may be excitatory or inhibitory) it can be suppressed by the brain. We can overcome pain in extreme circumstances

Question 33

What is the crossed extensor reflex? (e.g. if you stepped on s nail) Where do the signals travel for this to happen?

Answer 33

• Suppose you step on a nail. A flexion reflex withdraws your foot from the injury. When this occurs the flexors in the withdrawing limb contract and the extensors relax.
• Normally the extensors in the opposite leg will also contract, to take the extra weight of your body when you stand on one limb. (At the same time, signals travel up the spinal cord and cause contraction of the contralateral muscles of the hip and abdomen to shift the body’s center of gravity over the extended leg).
• Activation of extensors in the other leg during a flexion reflex is the crossed extensor reflex
• It is again a polysynaptic reflex.
• To produce this reflex, branches of the afferent nerve fibers cross from the stimulated side of the body to the contralateral side of the spinal cord. There, they synapse with interneurons, which, in turn, excite or inhibit alpha motor neurons to the muscles of the contralateral limb.

Question 34

What is muscle tone? What causes pathological reflexes to do with muscle tone? What are some examples of these pathological reflexes?

Answer 34

• Normal muscles have a certain slight resistance to passive movement, even if the patient is co-operating fully. This is due to small amounts of contraction in muscles when they are passively moved. This slight resistance is known as muscle tone.
• When lower motor neurons are damaged tone is reduced or becomes completely absent. In the latter case the muscle is said to have flaccid paralysis.
• Lesions of upper motor neurons produce syndromes where there are exaggerated reflexes and pathological increases in muscle tone. This condition is known as spasticity.
• Pathological types of reflexes indicating upper motor neuron injury include; cog wheel rigidity, lead pipe rigidity, exaggerated tendon reflexes, clonic reflexes, pendular reflexes etc.

Question 35

What are the differences between lower and upper motor neuron lesions?

Answer 35

See table on lecture notes