Reflex arcs and spinal tracts

Question 1

what are the different types of sensory nerve endings?

Answer 1

• free nerve endings: found in most tissues in the body, thermoreceptors, chemoreceptors, mechanoreceptors and nociceptors. INVOLVED IN proprioception and pain
• modified free nerve endings: found in epidermis, mechanoreceptors for light and pressure
• hair follicle receptors: mechanoreceptors for the movement of hairs
• tactile (Meissner’s) corpuscle: mechanoreceptors for light, pressure and some vibration
• lamellar (pacinian) corpuscle: found in tendons, ligaments and joint capsules. they are mechanoreceptors for deep pressure
• bulbous corpuscle (ruffini endings): found in skin and joint capsules, mechanoreceptors for deep pressure and stretch
• muscle spindles: specialised sensory organs in muscles. mechanoreceptors for muscle stretch and length
• tendon organs: found in tendons, mechanoreceptors of stretch and tension in tendons
• joint kinaesthetic receptors: found in joint capsules - mechanoreceptors and nociceptors

Question 2

what are the stages of a generic reflex arc?

Answer 2

1. receptor
2. sensory neuron (afferent impulse transmitted to CNS)
3. integration centre (interneuron or synapse within CNS)
4. motor neuron (efferent impulse transmitted to effector)
5. effector (organ or tissue that responds, usually a muscle)

Question 3

what is the purpose of the flexor reflex?

Answer 3

a reflex which causes automatic withdrawal of a limb in response to a painful stimulus. it stimulates flexors and inhibits extensors. it is ipsilateral and entirely unconscious but can be overridden by conscious thought (expectations of pain).

Question 4

what are the stages of the flexor reflex?

Answer 4

1. painful stimulus detected by nociceptor (free nerve endings)
2. afferent sensory neuron sends signal to spinal cord
3. signal is sent through a series of interneurons in the spinal cord
4. signal is sent to motor neurons. one of which activates the flexor muscles in the limb to remove it from the painful stimulus by bending. another motor neuron sends signals to inhibit the extensors to prevent further damage

Question 5

what is the crossed-extensor reflex?

Answer 5

This reflex often accompanies the flexor reflex, it allows the other leg to remain balanced when the flexor reflex happens. extensors within the contralateral side of the body are activated (in the leg, where it is most common, this means the leg extends and the body can remain balanced and standing)

Question 6

What is the purpose of the myotatic/stretch reflex?

Answer 6

This is a postural reflex that ensures muscles stay at a set length, ensuring that you stay upright. the most famous example is the knee-jerk reflex which is used by doctors to check that the spinal cord/motor or sensory nerves have not been damaged.

Question 7

What are the stages of the myotatic/stretch reflex? (using the knee-jerk reflex as an example)

Answer 7

1. tapping the patellar ligament rapidly stretches the quadriceps muscle. this is detected by muscle spindles in the quadriceps muscle.
2. afferent impulses from the muscle spindles travel to the spinal cord. synapses are formed directly with motor neurons and with some interneurons
   3a. motor neurons send signals to the quadricep muscles, causing them to contract (which extends the knee)
   3b. interneurons make inhibitory synapses with motor neurons which inhibit the hamstrings (the flexor muscles of the leg)

Question 8

what are muscle spindles?

Answer 8

These are specialised sensory organs embedded in muscles. they detect rapid stretch and communicate with the spinal cord via special afferent motor neurons called gamma motor neurons.

Question 9

what is the tendon reflex?

Answer 9

the tendon reflex is also known as the reverse myotatic reflex and acts to oppose the stretch reflex, promoting relaxation and protecting against excessive tension.

Question 10

what are the stages of the tendon reflex?

Answer 10

1. when tension in the muscle activates too quickly, the tendon organ sends this information to the spinal cord
2. signal is passed to interneurons in the spinal cord.
   3a. one interneuron forms a synapse with motor neurons which inhibit muscle contraction in the muscle that has been identified as contracting too quickly.
   3b. another interneuron forms synapses with motor neurons that excite the antagonist muscle or muscles to counteract the contraction

Question 11

what are golgi tendon organs?

Answer 11

specialised sensory nerve endings which detect changes in stretch within a tendon.

Question 12

Name the ascending/sensory spinal tracts

Answer 12

• spinothalamic (spine to thalamus) includes direct and indirect pathways
• dorsal column (spine to thalamus but via the dorsal white column)
• spino-cerebellar (spine to cerebellum)

Question 13

describe the purpose of a sensory pathway

Answer 13

To carry information from sensory receptors, through the spinal cord and to the brain. conscious information ultimately reaches the cerebral cortex (the somatosensory cortex) whereas unconscious sensory information does not reach this area of the brain.

Question 14

Describe a generic sensory pathway

Answer 14

1. sensory receptor: receives sensory input
2. first order neuron: cell body is found in the dorsal root ganglion and its axon extends up into the grey matter of the spinal cord.
3. second order neuron: interneuron with its cell body in the dorsal horn, or the brain stem if it is further up the spine
4. third order neuron: cell body of this neuron is found in the thalamus (the distribution centre of the brain) and its axon extends to the somatosensory cortex (if it’s a conscious pathway)

Question 15

what is the purpose of the spinothalamic pathway?

Answer 15

the primary sensory pathway in the spine. it carries information about touch as well as pain and temperature.

Question 16

what are the subdivisions of the spinothalamic pathway?

Answer 16

ventral spinothalamic pathway = found on the anterior aspect of the spinal cord, this carries information about touch
lateral spinothalamic pathway = found on the lateral aspect of the spinal cord and carries information about pain and temperature
There is also:
direct spinothalamic tract = information carried straight to the thalamus for conscious appreciation of pain
indirect spinothalamic tract = information is passed to other areas of the brain, for example the spino-mesencephalo-limbic branch which is involved in the emotional response to pain.

Question 17

where does the spinothalamic pathway decussate?

Answer 17

in the spinal cord

Question 18

what are the potential effects of injury to the spinothalamic pathway?

Answer 18

injury to the spinothalamic tract would result in contralateral loss of function. i.e. injury on the right of the spinal cord would result in loss of feeling/sensation in the left hand side of the body.

Question 19

What is the purpose of the dorsal column pathway?

Answer 19

this is a sensory pathway which carries information about fine touch (for example two-point spatial resolution) and proprioceptive information from the body to the brain via the dorsal white column in the spinal cord.

Question 20

describe the structure of the dorsal column pathway

Answer 20

1st order neuron: cell body in the dorsal root ganglion
2nd order neuron: cell body in the medulla (brain stem) this is where the dorsal column pathway decussates
3rd order neuron: cell body in the thalamus

the dorsal column pathway splits into two in the dorsal white column, information from the upper limbs is carried laterally whereas information from the lower limbs is carried medially. this is the start of somatotopic organisation of sensory information within the brain.

Question 21

what are the potential effects of injury to the dorsal column pathway?

Answer 21

• loss of sense of touch and proprioception, experienced on the same side of the body to the injury if the damage is below the point of decussation in the brain stem.
  loss of proprioception may lead to:
• sensory ataxia
• staggering/falling/abnormal gait.

Question 22

what is the purpose of the spinocerebellar pathway?

Answer 22

this pathway carries unconscious proprioceptive information to the cerebellum. it never reaches the cerebellum so is entirely unconscious

Question 23

describe the structure of the spinocerebellar pathway

Answer 23

1st order neuron: cell body is found in the dorsal root ganglion
2nd order neuron: cell body is found in the spine and the axon of this neuron extends to the cerebellum. there is no 3rd order neuron in this pathway and it is ipsilateral (does not decussate)

Question 24

what are the potential effects of damage to the spinocerebellar pathway?

Answer 24

• cerebellar ataxia
• clumsy movements
• incoordination of limbs (intention tremor)
• wide-based, reeling gait
  interestingly, alcohol produces similar effects as alcohol effects the signalling of the cerebellum.

Question 25

describe the generic structure of a descending/motor tract

Answer 25

1. upper motor neuron: sits in the primary motor cortex, it receives information from other areas of the brain and sends it down an axon into the spinal column
2. small interneuron/series of interneurons found in the spine
3. lower motor neuron: cell body is in the ventral grey matter of the spinal cord, the axon leaves the spine via the ventral nerve root.
   most motor pathways do not have interneurons so consist of only an upper and lower motor neuron
   there are two types of motor pathway - pyramidal (direct) and extrapyramidal (indirect)

Question 26

what is the difference between direct and indirect motor pathways?

Answer 26

direct pathways (also known as pyramidal as they pass through the pyramids of the brain stem) carry conscious information.
indirect pathways (extrapyramidal as they do not go through the pyramids) carry mostly subconscious information

Question 27

describe the structure of the corticospinal tract

Answer 27

• fibres originate from the primary motor cortex (mostly)
• tract continues down the brainstem where it forms ‘the pyramids’ in the medulla, which are just triangular shaped bundles of axons
• at the end of the pyramids (start of the spinal cord) 90% of fibres decussate. these control movement of the limbs
• 10% of fibres continue ipsilaterally and control the movements of the trunk, neck and shoulders.

Question 28

what are the potential effects of damage to the corticospinal tract?

Answer 28

this is the main tract by which conscious motor information is carried, damage may result in loss of movement control (likely contralaterally). some damage may be reversible i.e. people may regain the ability to move limbs or walk, but full recovery of fine motor dexterity after injury to the corticospinal tract is rare

Question 29

list the descending/motor tracts of the spine

Answer 29

• corticospinal (conscious and direct)
  unconscious and indirect:
• rubrospinal
• tectospinal
• vestibulospinal
• reticulospinal

Question 30

briefly describe the rubrospinal tract

Answer 30

controls fine motor coordination of upper limb. it also helps to modulate reflexes by facilitating flexors and inhibiting extensors.
it is called rubrospinal as rubro = red. and the upper motor neuron is found in the red nucleus within the midbrain

Question 31

briefly describe the tectospinal tract

Answer 31

controls neck muscles in the form of head-eye coordination and visual/auditory feedback. the upper motor neuron is located in the midbrain and decussation occurs here almost immediately

Question 32

briefly describe the vestibulospinal tract

Answer 32

responsible for modulating extensor muscles in the lower limb to control balance. upper motor neuron is found in the pons within the vestibular nuclei

Question 33

briefly describe the reticulospinal tract

Answer 33

responsible for some autonomic functions such as heart rate and pain modulation. upper motor neuron originates in the reticular formation. composed of one ipsilateral and one contralateral tract

Question 34

describe the structure, function and sensory modality of a free nerve ending

Answer 34

free nerve endings are unencapsulated dendrites of sensory neurons. they are sensitive to painful stimuli, hot and cold and light touch.

Question 35

describe the structure, function and sensory modality of golgi tendon organs

Answer 35

a golgi tendon organ is a tree-like sensory ending enclosed in a connective tissue capsule that lies near the junction of a muscle and tendon. golgi tendon organs are sensitive to tension in a muscle. they assist in proprioception

Question 36

describe the structure, function and sensory modality of muscle spindles

Answer 36

an encapsulated sensory nerve endings within muscles, they consist of afferent coils around non-contractile intrafusal fibres. muscle spindles respond to stretch and muscle length. they assist in proprioception

Question 37

describe the structure, function and sensory modality of pacinian corpuscles (also known as lamellar corpuscles)

Answer 37

pacinian corpuscles have a singular axonal fibre at the centre surrounded by 15-20 lamella in a concentric pattern. they are found in the skin and are highly sensitive to changes in pressure and vibration (mechanoreceptors).

Question 38

describe the structure, function and sensory modality of Meissner’s corpuscles

Answer 38

An encapsulated sensory nerve ending in the skin. it is composed of a myelinated sheath made by schwann cells. it is sensitive to light touch and low frequency vibration

Question 39

describe the structure, function and sensory modality of Merkel cells

Answer 39

they are cells found in the epidermis . their role is not very well understood but they may be mechanoreceptors sensitive to touch.

Question 40

describe the structure, function and sensory modality of ruffini corpuscles (also known as bulbous corpuscles)

Answer 40

enlarged dendritic endings with an elongated capsule. found in the skin. it is sensitive to skin stretch.