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BRAIN AND BEHAVIOUR 1 > SPINAL REFLEXES > Flashcards

SPINAL REFLEXES Flashcards

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1
Q

Where do upper motor neurons synapse?

A

Synapse on the cell bodies dendrites of the lower motor neurons

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2
Q

Where is the cell body of a lower motor neuron?

A

In the ventral horn of the spinal cord

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3
Q

What is a motor unit?

A

Motor neuron together with its cell body in lamina IX of the ventral horn, its motor axon and the set of muscle fibres it innervates

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4
Q

What is tetanus?

A

Sustained muscle contraction evoked when the motor nerve that innervates a skeletal muscle emits action potentials at a very high rate

  • Contraction of one motor unit by a single action potential generates a twitch.
  • To produce 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 contracition
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5
Q

All-or-none frequency

A

In a healthy person, motor neurons fire at their fusion frequency of bot at

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6
Q

How is the force of contraction in muscle changed?

A

By recruiting more or fewer motor neurons

  • Each motor neuron will fire at its tetanus fusion frequency or not at all
  • Contrast to sensory nerve fibres where frequency in a single nerve fibre codes for intensity
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7
Q

What are the effects of neuronal loss?

A

If there is a death of motor neurons, the remaining motor axons sprout peripherally to innervate the denervated muscle fibres

  • This leads to overall increase in motor unit size and decreased fine control
  • Similar to changes in receptive field size in sensory neurons
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8
Q

What are the three types of synaptic inputs of the lower motor neuron?

A
  1. Descending tracts in the spinal cord from upper motor neurons
  2. Input from local interneurons (cells with processes inside the CNS)
  3. Input from local sensory nerve fibres via reflexes
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9
Q

What are the outputs from the motor cortex?

A
  1. Pyramidal system:
    - UMN cell bodies project directly from the frontal lobe (mainly the motor cortex) to spinal cord.
    - Travels via the corticospinal tract to LMN
  2. Extrapyramidal system:
    - UMN cell bodies in the brainstem project to the spinal cord
  3. Spinal cord:
    - Has LMN in the ventral horn that projects out via peripheral nerve to muscles
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10
Q

What is a reflex?

A

Involuntary muscle action triggered by a sensory input

Eg. Tendon jerk reflex elicited by tapping the patella

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11
Q

What are myotatic reflexes?

A

Stretch reflexes that causes muscle contraction after a muscle is stretched

  • No interneurons involved
  • Direct connection between synaptic connection between the input sensory nerve fibre and the motor neuron
  • The direct connection makes it hard for the brain to suppress this reflex
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12
Q

What are interneurons?

A

Interneurons are the central nodes of neural circuits, enabling communication between sensory or motor neurons and the central nervous system (CNS). They play vital roles in reflexes, neuronal oscillations, and neurogenesis in the adult mammalian brain.

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13
Q

Where are reflexes stored?

A

In the dorsal and ventral grey matter of the spinal cord in the forms of patterns of synaptic connections between inputs, outputs and interneurons

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14
Q

What is the patellar reflex?

A

The knee jerk (patella) reflex is when the stretch of the quadriceps muscles activates the same muscle to contract
-AKA a homonymous reflex

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15
Q

What is reciprocal inhibition?

A

Relaxation of muscles on one side of the joint, accommodating contraction on the other side.

  • The same tendon tap that activates the extensor reflex also inhibits the antagonist flexor muscle
  • When extensor contracts, flexor relaxes
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16
Q

What are flexion reflexes?

A

Reflex withdrawal from painful stimulus

  • Mediated by activation of small myelinated nociceptive afferents
  • These act on interneurons in the spinal cord and not directly on motor neurons
  • Involves at least 1-3 excitatory interneurons between the pain afferent input and the flexor motor neuron. POLYSYNAPTIC REFLEX
  • Because a flexion reflex involves several interneurons,, it can be suppressed by the brain (overcoming pain in extreme circumstances)
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17
Q

What is polysynaptic reflex?

A

A reflex action that involves an electrical impulse being transferred from a sensory neuron to a motor neuron via at least one connecting neuron (interneuron) in the spinal cord.

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18
Q

What is the crossed extensor reflex?

A

Contracted reflex that allows the body to compensate on one side for a stimulus on the other

  • Activation of extensors in the other leg while one flexes
  • Eg. Stepping on a nail, a flexion reflex withdraws the foot from injury and the extensors relax
  • To produce this reflex, branches of the afferent nerve fibres cross from the stimulated side of the body to the contralateral side of the spinal cord, there it synapses with interneurons which, in turn excites or inhibits alpha motor neurons to the muscles of the contralateral limb
19
Q

What is tone?

A

Muscle tone is the continuous and passive partial contraction of the muscles, or the muscle’s resistance to passive stretch during resting state. It helps to maintain posture and declines during REM sleep.

20
Q

What are antigravity muscles?

A

A muscle that acts, often through the stretch reflex, to counterbalance the pull of gravity and to maintain an upright posture. Many antigravity muscles have a high proportion of slow-twitch muscle fibres and are often called tonic muscles.

21
Q

How is posture maintained?

A

Muscle tone is constantly finely adjusted unconsciously by muscle fibres contracting in relays
-Requires input from higher levels of the nervous system

22
Q

Why is the monosynaptic reflex easiest to elicit in leg extensor muscles?

A

Fatigue in the muscles active during standing means that we would fall over if not for the continuous feedback input from muscle spindles

  • Continuous feedback from muscle spindles 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 the awareness of falling)
  • The faster the feedback response, the quicker the corrective muscles action, improving balance and agility
  • The need for maximum postural stability and agility is why monosynaptic reflexes are particularly active in postural and leg extensor muscles
  • Damage to these reflex arcs makes a patient more likely to fall or trip
23
Q

What is a monosynaptic reflex?

A

A reflex that only contains one space for an action potential to travel between a sensory and motor neuron.

24
Q

What is a muscle spindle?

A

The receptor that mediates all the tendon reflexes

  • 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 connective tissue sheath
  • There are around 4-20 muscle spindles in a muscle
  • Stretch of the muscle also stretches the muscle spindle, and activates the muscle spindle sensory afferents
25
Q

What is the main sensory nerve fibre of the muscle spindle?

A

1a afferent
-A type Ia sensory fiber, or a primary afferent fiber is a type of afferent nerve fiber. It is the sensory fiber of a stretch receptor found in muscles called the muscle spindle, which constantly monitors how fast a muscle stretch changes.

26
Q

What are intrafusal muscle fibres?

A

Intrafusal muscle fibers are skeletal muscle fibers that serve as specialized sensory organs (proprioceptors). They detect the amount and rate of change in length of a muscle. They constitute the muscle spindle, and are innervated by both sensory (afferent) and motor (efferent) fibers.

27
Q

How is the muscle spindle activated?

A

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 by a very small amount; this is sufficient to briefly increase the firing of the spindles
28
Q

What is EPSP?

A

An excitatory postsynaptic potential (EPSP) is the change in membrane voltage of a postsynaptic cell following the influx of positively charged ions into a cell (typically Na+) as a result of the activation of ligand-sensitive channels.

29
Q

What is subthreshold EPSP?

A

A single action potential in a 1a sensory nerve fibre is not enough to produce an action potential, instead there is a small depolarization, the EPSP

30
Q

What is temporal summation?

A

Temporal summation occurs when a high frequency of action potentials in the presynaptic neuron elicits postsynaptic potentials that summate with each other. The duration of a postsynaptic potential is longer than the interval between action potentials.

  • Activates a motor neuron
  • Occurs when a single 1a nerve fibre fires a high frequency burst of action potentials
31
Q

What is spatial summation?

A

Spatial summation is the effect of triggering an action potential in a neuron from one or more presynaptic neurons. This occurs when more than one excitatory postsynaptic potential (EPSP) originates simultaneously and a different part of the neuron.

  • Activity at a single synapse will not activate the motor neuron
  • Two separate synaptic inputs adding together
32
Q

When does a motor neuron fire action potentials?

A

When there is sufficient spatial and temporal summation

33
Q

What is the central region of muscle spindles?

A

The central region has no connective tissue and is very easily stretched
-The sensory 1a sensory afferents coil around the centre region, when it is stretched, they are deformed and fire action potentials

34
Q

What motor neuron innervates the muscle spindle?

A

The gamma motor neuron

  • Stimulates contraction of the intrafusal fibres and “pre-stretches” their centres
  • This increases the response of 1a afferents to muscle stretch
35
Q

What are gamma motor neurons?

A

A gamma motor neuron (γ motor neuron), is a type of lower motor neuron that takes part in the process of muscle contraction, and represents about 30% of ( Aγ ) fibers going to the muscle.

  • NOT reflexively activated by muscle spindle afferents
  • They are driven from descending motor pathways such as corticospinal tracts
  • They act to contract the ends of the muscle spindles
  • If the gamma motor neuron increase their activity, the spindle becomes more sensitive to stretch
  • Pathology increases gamma activity, leading to HYPERactive tendon reflexes
36
Q

What is the initial motor function of muscle spindles?

A

Combined synaptic input keeps desired position
-initially, there is an input to the motor neurons from both the descending axons from the forebrain and also from the muscle spindle 1a afferents

37
Q

What is the effect of fatigue on muscle spindle?

A

Fatigue in the muscle means weaker contraction and it starts to stretch
-This stretch increases the activity of muscle spindles

38
Q

What is compensation in muscle spindles?

A

Increased stretch of muscles means stretched muscle spindles send increased frequency of action potentials to the spinal cord.
-This increases synaptic input to motor neurons which fire at higher frequency to increase force of contraction

39
Q

How is muscle tension maintained?

A

When a muscle is shortened, muscle spindles become slack and insensitive to small changes in muscle length, in order to keep sensitivity of muscle spindles, gamma motor neurons innervating the intrafusal muscle fibres fire action potentials

40
Q

What is alpha and gamma coactivation?

A

During any voluntary movement of the muscles the brain activates both

  • This keeps the muscle spindle feedback at right levels during movement
  • When the muscle reaches the desired length, the gamma motor neuron output adjusts to a level appropriate for the new length
41
Q

What is the golgi tendon organ?

A

The Golgi tendon organ is neurotendinous organ that is a proprioceptive sensory receptor organ that senses changes in muscle tension. It lies at the origins and insertion of skeletal muscle fibers into the tendons of skeletal muscle.
-Activated by muscle tension not length

42
Q

What are the connections of the golgi tendon organ?

A

Disynaptic interneuron connection to its own motor neurons

  • Signals through 1b afferent
  • The interneuron is a glycinergic inhibitory neuron
  • 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 it might damage the muscle, the GTO switches it off by a powerful inhibitory action
43
Q

UMN lesion symptoms

A
  • Increased muscle tone
  • Hyperactive/exaggerated reflexes
  • Babinski sign
  • Slight atrophy
  • Spastic paralysis
44
Q

LMN lesion symptoms

A
  • Decreased muscle tone
  • Decreased/absent reflexes
  • Pronounced muscle atrophy
  • Flaccid paralysis

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