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Head and Neck Physiology > Spinal Cord Reflexes for Posture and Locomotion > Flashcards

Spinal Cord Reflexes for Posture and Locomotion Flashcards

1
Q

What is the function of the somatic nervous system?

A

controls the skeletal muscles for locomotion and the maintenance of posture

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1
Q

Name the reflexes that are coordinated by the spinal cord?

A

Simple posture reflexes
1. inverse stretch reflex
2. withdrawal reflexes
3. myotatic stretch reflex

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1
Q

Describe the role of the brainstem in the somatic motor system?

A

Maintenance of equilibrium and orientation in space

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1
Q

Describe the role of the cerebellum in the somatic motor system?

A

coordinates and smooths out all skeletal muscular activities throughout the body

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1
Q

Describe the role of the cerebral cortex in the somatic motor system?

A

initiates voluntary contractions for deliberate posture control and movement

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2
Q

Describe the two types of motor neurons in the ventral root of the spinal cord?

A
  1. Alpha: large type A alpha (Aα) nerve fibres that innervate skeletal muscle
  2. Gamma: small type A gamma (Aγ) nerve fibres that innervate small, special skeletal muscle fibres called intrafusal fibres
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3
Q

Proper control of muscle fibers requires?

A
  1. excitation of the muscle by alpha motor neurons
  2. continuous feedback of sensory information from each muscle to the spinal cord, indicating the functional status of each muscle at each instant
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3
Q

Muscles and their tendons are supplied abundantly with which two special types of sensory receptors?

A
  1. Muscle spindles are distributed throughout the belly of the muscle and send information to the nervous system about muscle length or rate of change of length
  2. Golgi tendon organs are located in the muscle tendons and transmit information about tendon tension or rate of change of tension
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4
Q

Describe muscle spindles?

A
  • Each spindle is 3 to 10 mm long.
  • It is built around 3 to 12 very small intrafusal muscle fibres
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5
Q

What are intrafusal muscle fibers?

A
  • Modified muscle fibres that lack myofibrils in their central portions.
  • Only the polar ends are contractile
  • Central portions act as sensory receptors
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6
Q

Describe the 2 types of intrafusal fibres?

A
  1. nuclear chain fibres
    - numerous (6-10/spindle), smaller in diameter (10-12 µm), shorter (3-4 µm)
    - have a single row of nuclei at the middle
  2. nuclear bag fibres
    - 2-5/spindle, 25 µm in diameter, 7-8 µm in length
    - have a dilated equatorial zone containing numerous nuclei
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7
Q

The muscle spindle receptor can be excited in what 2 ways?

A
  1. Lengthening the whole muscle stretches the midportion of the spindle and, therefore, excites the receptor.
  2. Even if the length of the entire muscle does not change, contraction of the end portions of the spindle’s intrafusal fibres stretches the midportion of the spindle and therefore excites the receptor
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8
Q

What are the two types of sensory nerve endings in each spindle?

A
  1. primary receptor endings
  2. secondary receptor endings
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9
Q

Describe the primary receptor endings?

A
  • are receptor origins of thick myelinated, fast conducting primary afferents/sensory neurons.
  • A single large sensory nerve fiber encircles the central portion of each intrafusal fiber, forming annulospiral endings
  • Type Ia fiber, 17µm wide, 70-120m/sec transmission speed
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10
Q

Describe the secondary receptor endings?

A
  • receptor origins of thinner secondary afferents
  • Type II fibers, 8µm wide
    Attached to one or both sides of the annulospiral terminal of nuclear chain (rarely also on the nuclear bag)
  • Often called flower-spray endings because the secondary afferents spread out like a bush on the intrafusal fibers
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11
Q

Describe the static response of the muscle spindle?

A

When the receptor portion of the muscle spindle is stretched slowly, the number of impulses fired from both primary and secondary endings increases directly in proportion to degree of stretch.
- both primary and secondary afferents fire impulses for several mins if the muscle spindle remains stretched

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12
Q

What is the dynamic response of the muscle spindle?

A

Occurs when the length of the spindle receptor increases suddenly, even a fraction of a micrometer for only a fraction of a second.
- only primary afferents fire impulses, muscle is stretched suddenly and released

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13
Q

What are gamma efferents/motor neurons?

A

The contractile polar ends of the intrafusal fibres by small diameter motor neurons

14
Q

Describe the 2 types of gamma efferents/motor neurons?

A
  1. the motor end-plate or plate endings on the nuclear bag fibres
    - enhance the dynamic response of the nuclear bag fibres and are also called gamma-dynamic (gamma-d) efferents
  2. the trail endings which form extensive network on the nuclear chains
    - enhance the static response of the nuclear chains and are also called gamma-static (gamma-s) efferents
15
Q

What is a reflex action?

A

An involuntary stereotyped motor response to particular sensory stimulus
- Most are based on inherent properties of the organism and are demonstrable right from birth
- Also called unconditioned reflexes

16
Q

Give examples of a relfex action?

A
  1. Change of pupil size in response to light (pupillary light reflex, coma checklist)
  2. Sudden jerky withdrawal of hand or leg when pricked by a pin.
  3. Coughing or sneezing because of irritants in the respiratory and nasal passages.
  4. Knee jerk in response to a blow
  5. Sudden blinking when an object comes very near to the eyes
17
Q

What is the basic structural unit of the nervous system?

A

neuron

18
Q

What is the functional unit of the nervous system?

A

reflex arc

19
Q

The neural circuit on which a reflex action is based has which following components?

A

Receptor > Afferent neuron > One or more synapses in the CNS (interneurons) > Efferent Neuron > Effector

20
Q

Describe the types of reflex arcs?

A
  1. Monosynaptic - sensory neuron directly synapses with motor neuron
  2. Polysynaptic - several synapses, comprises majority of reflex arcs
21
Q

What is the skeletal muscle stretch reflex (myotatic reflex)?

A

the reflex contraction of a stretched muscle
- When an intact muscle is stretched, it reflexively contracts and its antagonist muscle is inhibited
- The sensory receptor for stretch reflex is the muscle spindle
- It is the simplest reflex (monosynaptic)

22
Q

What is reciprocal inhibition?

A

the reflex inhibition of the antagonist muscle

23
Q

Describe how the skeletal muscle stretch reflex (myotatic reflex) comes about?

A
  • The stretch stimulates the sensory neurons that innervate the centre of the muscle spindles, which conduct APs to the spinal cord.
  • Sensory neurons synapse with alpha motor neurons (glutamate is the excitatory neurotransmitter) that conduct APs to the muscle (NMJ), causing it to contract and resist stretch.
24
Q

Describe the mechanism for strecth reflex and reciprocal inhibition?

A
  1. Stretching stimulates sensory receptor (muscle spindle)
  2. Sensory neuron excited
  3. Within integrating center (spinal cord) sensory neuron activates motor neurons
  4. Motor neuron excited
  5. Effector (same muscle) contracts and relieves the stretching
  6. Motor neuron to antagonistic muscles is inhibited
  7. Antagonistic muscles relax
25
Q

What are Golgi bottle interneurons?

A

Some collateral branches of the sensory neurons from the spindle synapse with inhibitory interneurons known as Golgi bottle neurons
- synapse with alpha motor neurons supplying the antagonistic muscle and inhibit them (glycine is the inhibitory neurotransmitter)

26
Q

Describe the role of the stretch reflex in the control of posture and movement?

A
  1. Monitor and regulate muscle length, especially when the muscle is stretched (resist attempt to stretch muscle)
    - Resistance to muscle stretch is called muscle tone
    - In the upright position, the weight tends to flex the lower limbs and stretch its extensors
    - This provokes reflex contraction of the extensors, thereby maintaining posture against gravity
    - For this reason, extensors of the lower limbs are called antigravity muscles
    - In humans, the flexors (biceps) of the upper limbs are extended by gravity and are the antigravity muscles
  2. Some collaterals from spindles make connections with pathways to the cerebellum and cerebral cortex which contribute towards proprioception
27
Q

Describe the clinical applications of the stretch reflex?

A
  1. Used by neurologists to assess the degree of facilitation of spinal cord centres
  2. Testing many stretch reflexes helps determine integrity of peripheral nerves and predictable spinal cord areas
    - hypoactive or absent with peripheral nerve damage or ventral horn injury of a certain body area
    - absent in people with chronic diabetes mellitus or neurosyphilis, and in comatose individuals.
    - hyperactive when corticospinal tract lesions reduce inhibitory effect of the brain upon the spinal cord, as in stroke patients
28
Q

What is the Golgi tendon organ?

A

an encapsulated sensory receptor through which muscle tendon fibres pass
- About 10 to 15 muscle fibres are usually connected to each Golgi tendon organ, and the organ is stimulated when this small bundle of muscle fibres is “tensed” by contracting or stretching the muscle.

29
Q

Describe the Golgi tendon reflex?

A

When Golgi tendon organs are stimulated by increased tension in the connecting muscle, signals are transmitted to the spinal cord to cause inhibitory reflex effects in the respective muscle.
- this reflex provides a negative feedback mechanism that prevents the development of too much tension on the muscle

30
Q

Describe the mechanism behind the Golgi tendon reflex?

A
  1. Increased tension stimulates sensory receptor (tendon organ)
  2. Sensory Neuron excited
  3. Within integrating center (spinal cord) sensory neuron activates inhibitory interneuron
  4. Motor neuron inhibited
  5. Effector (muscle attached to same tendon) relaxes and relieves excess tension
  6. Motor neuron to antagonistic muscles is excited
  7. Antagonistic muscles contract
31
Q

What is spinal shock?

A

Spinal shock occurs after a spinal cord injury (SCI) and presents as reversible loss of all neurological and associated sensorimotor function below the level of the injury
- Caused by trauma
> transection, compression, bruising, haemorrhage, or ischemia of the cord or by injury to blood vessels supplying it
Note: Apart from suppression of spinal reflex, there is also fall in arterial blood pressure (loss of sympathetic tone) which returns eventually.

32
Q

What are the signs and symptoms of spinal shock?

A

Flaccid, areflexic paralysis of skeletal and smooth muscles
Autonomic dysfunction
loss of urinary bladder tone and paralytic ileus (lack of intestinal muscle contraction)
Sweating and piloerection are diminished
Loss of temperature regulation
Loss of genital reflexes
Loss of vasomotion which may lead to deep venous thrombosis and pulmonary embolism
Autonomic dysfunction is worse with injuries above T6 level
Patients require mechanical ventilation due to paralysis of abdominal and intercostal muscles

33
Q

Describe spinal shock in a patient with upper motor neuron lesions?

A
  1. reflexes return with hyperexcitability so that the muscles increasingly resist stretch (muscle tone is high)
    - The increased resistance is called spasticity
    - Patients are predisposed to pain, contractures and pressure ulcers
  2. Even a minor noxious stimulus applied to the skin of a limb provokes not only a withdrawal reflex, but also a spread of impulses through the cord, recruiting several autonomic reflexes so that the patient:
    - Defecates, urinates, sweats, and arterial blood pressure may rise or fall
    > This is called mass reflex
34
Q

Describe phase I of spinal shock?

A
35
Q

Describe phase II of spinal shock?

A
36
Q

Describe phase III of spinal shock?

A
37
Q

Describe phase IV of spinal shock?

A
38
Q

What are the permanent consequences of spinal transection?

A

Spastic paraplegia
Anaesthesia
Lower motor neuron paralysis at region of transection

39
Q

Describe the relation between the stretch reflex and ascending plus descending tracts?

A

Collateral axons from sensory neurons of the muscle spindles also synapse with neurons in ascending nerve tracts.
- Enables the brain to perceive that a muscle has been stretched
Descending neurons in the spinal cord synapse with neurons of the stretch reflex to modify their activity
- Maintaining posture, coordinating muscle actions

Head and Neck Physiology (18 decks)

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