Spinal Cord Reflexes for Posture and Locomotion

Question 1

What is the function of the somatic nervous system?

Answer 1

controls the skeletal muscles for locomotion and the maintenance of posture

Question 1

Name the reflexes that are coordinated by the spinal cord?

Answer 1

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

Question 1

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

Answer 1

Maintenance of equilibrium and orientation in space

Question 1

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

Answer 1

coordinates and smooths out all skeletal muscular activities throughout the body

Question 1

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

Answer 1

initiates voluntary contractions for deliberate posture control and movement

Question 2

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

Answer 2

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

Question 3

Proper control of muscle fibers requires?

Answer 3

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

Question 3

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

Answer 3

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

Question 4

Describe muscle spindles?

Answer 4

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

Question 5

What are intrafusal muscle fibers?

Answer 5

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

Question 6

Describe the 2 types of intrafusal fibres?

Answer 6

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

Question 7

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

Answer 7

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

Question 8

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

Answer 8

1. primary receptor endings
2. secondary receptor endings

Question 9

Describe the primary receptor endings?

Answer 9

• 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

Question 10

Describe the secondary receptor endings?

Answer 10

• 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

Question 11

Describe the static response of the muscle spindle?

Answer 11

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

Question 12

What is the dynamic response of the muscle spindle?

Answer 12

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

Question 13

What are gamma efferents/motor neurons?

Answer 13

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

Question 14

Describe the 2 types of gamma efferents/motor neurons?

Answer 14

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

Question 15

What is a reflex action?

Answer 15

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

Question 16

Give examples of a relfex action?

Answer 16

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

Question 17

What is the basic structural unit of the nervous system?

Answer 17

neuron

Question 18

What is the functional unit of the nervous system?

Answer 18

reflex arc

Question 19

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

Answer 19

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

Question 20

Describe the types of reflex arcs?

Answer 20

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

Question 21

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

Answer 21

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)

Question 22

What is reciprocal inhibition?

Answer 22

the reflex inhibition of the antagonist muscle

Question 23

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

Answer 23

- 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.

Question 24

Describe the mechanism for strecth reflex and reciprocal inhibition?

Answer 24

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

Question 25

What are Golgi bottle interneurons?

Answer 25

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)

Question 26

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

Answer 26

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

Question 27

Describe the clinical applications of the stretch reflex?

Answer 27

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

Question 28

What is the Golgi tendon organ?

Answer 28

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.

Question 29

Describe the Golgi tendon reflex?

Answer 29

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

Question 30

Describe the mechanism behind the Golgi tendon reflex?

Answer 30

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

Question 31

What is spinal shock?

Answer 31

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.

Question 32

What are the signs and symptoms of spinal shock?

Answer 32

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

Question 33

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

Answer 33

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

Question 34

Describe phase I of spinal shock?

Question 35

Describe phase II of spinal shock?

Question 36

Describe phase III of spinal shock?

Question 37

Describe phase IV of spinal shock?

Question 38

What are the permanent consequences of spinal transection?

Answer 38

Spastic paraplegia Anaesthesia Lower motor neuron paralysis at region of transection

Question 39

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

Answer 39

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