5 Spinal cord and basic reflexes

Question 1

Define a reflex

Answer 1

Reflex

- an automatic unlearned reaction to a stimulus

Question 2

Describe the structure of the spinal cord

Answer 2

• Gray matter on the inside
• White matter on the outside

The spinal cord does not run all the way down (vertebral canal)

Question 3

Describe the anatomy of the spinal cord

Answer 3

The spinal cord looks different when sectioned at different heights

• Cervical enlargements are for upper limbs
• Lumbar enlargements are for lower limbs

In the skull, there is a large hole for the brain stem and spinal cord to run down from
- Foramen Magnum

Question 4

Describe the differentiation of Grey matter on the spinal cord

Answer 4

Grey matter adopts a butterfly-shaped appearance at the centre of the spinal cord

3 horns:

• Dorsal horn - on the dorsal side (post.)
• Lateral horn - middle, on lateral sides
• Ventral horn - on ventral side (ant.)

Question 5

Describe the differentiation of white matter on the spinal cord

Answer 5

The white matter surrounds the grey matter

There are 3 funiculi:

• Posterior funiculus (on dorsal side)
• Lateral funiculus - to the lateral edges
• Anterior funiculus (on ventral side)

Question 6

Describe afferent vs efferent nerves

Answer 6

• Afferent means travelling towards the CNS (sensory)

- Efferent means travelling away from the CNS (motor)

Question 7

What is a spinal segment?

Answer 7

There is one for each vertebra, labelled T1, T2, etc.

• Each spinal segments have dorsal nerve rootlets
• which joint together to give a dorsal nerve root (afferent)

(same for ventral) - efferent

Question 8

Describe the structure of the spinal cord

Answer 8

• The tough outermost layer is the dura mater
• the delicate arachnoid lies in the middle
• and the innermost layer, the pia mater, surrounds the brain and spinal cord

There is space between the arachnoid and pia - Subarachnoid space
- It contains blood vessels and the Cerebro Spinal Fluid (CSF)

Question 9

Describe the meninges

Answer 9

The meninges

• cover the spinal cord, from inside out, pia, arachnoid and the dura
• The dura blends with the epineurium of the exiting spinal nerves
• The pia eventually continues down (after developing the spinal cord) to form a terminal strand - filium termalis
• The arachnoid lines the dura and ends within the sacrum

CSF extends down into the vertebral canal, where there is no spinal cord left
- this is where a lumbar puncture can be done

Question 10

Describe the vertebral column

Answer 10

The vertebral column
- made up of vertebrae

There are:

• 7 cervical vertebrae
• 12 thoracic vertebrae
• 5 Lumbar vertebrae
• 5 Sacral vertebrae (fused)
• 3-4 fused coccyx vertebrae

Question 11

Describe the formation of the spinal cord from a baby (and hence the length of the spinal cord)

Answer 11

In a baby, the spinal cord grows longer than the spine

- When they get older, the bones grow, and the spinal column then grows past the spinal cord

Question 12

Describe the significance of the Cauda Equina (horse’s tail)

Answer 12

The spinal cord ends here, but some nerves still come out

Special Drug Administration
- it is filled mostly with the subarachnoid space, and the CSF that is integrated into it

e. g.
- spinal anaesthesia is administered through here sometimes, for lower limb surgery
- into the epidural and extradural space - acting locally

Question 13

Describe the pathways for sensory input + motor control of muscle contraction

Answer 13

• Ascending tract = Sensory tract

- Descending tract = motor tract

Question 14

Describe the ascending tract of the white matter in the spinal column

Answer 14

These tracts are bundles of white matter, which carry signals from muscles to the cerebellum

2 main tracts here:

• Dorsal Columns (post. funiculus)
  > gracile (hindlimb) and cuneate (forelimb)
  > transmit information about light touch, vibration sense and proprioception
• Spinothalamic tract (ventral, ant.)
  > transmit information to thalamus about pain, temperature, crude touch (deep), and itch, tickle, and orgasm

Not as important

• spinocerebellar (in the middle, lateral)
• carries information from muscle and joint receptors to the cerebellum

Question 15

Describe how and why spinal cord structure varies with the level

Answer 15

(to do with the enlargements at cervical and lumbar levels)

The different arrangement of tracts varies to where in the body the spinal cord is located:

• Cervical - there are two gracile tracts, as it carrying information from both the upper limbs and the hind limbs
• Lumbar - there is only 1 gracile tract, as it carrying information only up from the hind limbs

Question 16

Describe how and why all sensory information decussate

Answer 16

The concept that all sensory information is flipped

• so the other side of the brain receives the information to where it is coming from
• not understood why this occurs

2 types of decussation:
- Ipsilateral (same side)
> e.g. Dorsal column transit (light tough/proprioception)
- Contralateral (opposite side)
> e.g. Spinothalmic tract (pain)
> decussate at point of entry, and ascends on contralateral side

Question 17

Name the descending tracts of white matter

Answer 17

1. Lateral Corticospinal
2. Rubrospinal
3. Lateral reticulospinal
4. Vestibulospinal
5. Medial reticulospinal
6. Tectospinal
7. Anterior corticospinal

Question 18

Describe the roles of the descending tracts

Answer 18

• Tectospinal: though to mediate reflex movements in response to visual stimuli
• Reticulospinal: comes from the reticular system influences voluntary movement, reflex activity and muscle tone by controlling the activity of both alpha and gamma motor neurons
• Rubrospinal: large muscle of limb
• Vestibulospinal: posture and balance, input from vestibular nuclei
• Corticospinal - controls voluntary skilled movements, especially those of the distal parts of the limbs (main motor tracts)

Question 19

Describe the corticospinal tracts

Answer 19

It is the primary motor tract

• consists of two neurons
• upper and lower motor neuron

Question 20

Describe the two main types of motor neurons

Answer 20

2 types:

• Alpha: innervate extrafusal muscle fibres; responsible for contraction
• Gamma: spinal motor neurons innervate intrafusal muscle fibres in the muscle spindles

Question 21

Describe alpha motor neurons

Answer 21

They innervate the Neuromuscular Junction (NMJ)

• these are referred to generally as Lower motor neurons (LMN)
• These are the only connection between the CNS and skeletal contraction units, the extrafusal muscle fibres (standard muscle fibres)

Acetylcholine (ACh) is the neurotransmitter at these sites
- these alpha motor neurons are multipolar neurons (they receive inputs from many sources)

Question 22

Describe somatotopic organisation (of alpha spinal motor neurons)

Answer 22

Alpha spinal motor neuron arranged in ventral horn in relationship to the muscles they innervate

• The motor neuron for medial muscles are placed medially, and vice versa
• Or in the anatomical position, proximal limb muscles = medial

Question 23

Describe gamma motor neurons

Answer 23

(30% of total)

• Gamma (much smaller diameter axon) innervate muscle stretch receptors (intrafusal muscle fibres)
• to change the sensitivity of that muscle spindle to stretch (these convey length information to the central nervous system via afferent nerve fibres)
• They also plan an important role in muscle tone

Question 24

Describe reflexes

Answer 24

They are an automatic unlearned reaction to a stimulus

Reflexes are needed for:

• Escape/avoidance/withdrawal from a noxious stimulus (pulling hand from a hot surface)
• Postural control
• Locomotion – co-ordinated movement

Question 25

Describe the differences between reflex activity vs volitional control

Answer 25

Reflex

• stimulus-response (triggered by input)
• Innate/Hard wired

Volitional control

• internal desire
• learned
• conditioned/programmed

Question 26

Describe the circuitry underlying the monosynaptic reflexes and reciprocal inhibition

Answer 26

Myotatic reflex (tendon, stretch reflex)

• The myotatic reflex is the contraction of a muscle when it is stretched
• Reflex is initiated by stretching the muscle (or its connecting tendon), which can be done with a hammer or tendon
• Ia afferent fibres and their annulospiral stretch receptors located at the centre of muscle spindles
• The Ia fibres will also excite interneurons that will synaptically inhibit LMN innervating antagonist muscles (the basis for reciprocal innervation)

Question 27

Describe the schematic of a myotatic reflex arc (monosynaptic)

Answer 27

Advantage of this reflex pathway from the experimenter’s point of view

• is that the afferent fibres are readily accessible in the dorsal root for controlled stimulation
• while the postsynaptic cell bodies are large enough to be punctured easily by a microelectrode

Muscle stretch receptor

• Sensory neuron
• Through dorsal root ganglion
• Through relay neuron/to motor neuron
• then to effector muscle

Question 28

Describe the central modulation of spinal reflexes

Answer 28

The influence of higher centres is what stops reflex micturition
- inhibitory signal from higher centres prevent the signal from being passed straight to the motor neuron

Question 29

What is reciprocal inhibition?

Answer 29

It prevents antagonist muscle activity

• once signal passes from sensory neuron to motor neuron (then to effector’s muscle)
• there is an inhibitory signal being sent to the motor neuron of the antagonistic muscle