sensory receptors and reflexes

Question 1

what are sensory receptors?

Answer 1

they are receptors that inform the brain about the internal and external environment, they are nerve endings with specialized non-neural endings.

They convert stimuli into frequency of action potentials, so they are transducers.

Question 2

what are the 3 types of sensory receptors?

Answer 2

Mechanoreceptors: are stimulated by mechanical stimuli - pressure, stretch, deformation - give us skin sensations of touch and pressure

Proprioceptors: are mechanoreceptors in joints and muscles, they signal information about body or limb position

Nociceptors: are receptors that respond to painful stimuli
(heat and tissue damage)

Question 3

what is sensory modality?

Answer 3

Sensory modality is the stimulus type that activates a particular receptor
e.g. touch, pressure, joint angle, pain.

An adequate stimulus is the form of energy to which a receptor normally responds.

Sensory receptors are highly sensitive to one energy form but can be activated by other intense stimuli.

Question 4

what does sensory receptor transduction involve?

Answer 4

opening or closing ion channels.

Question 5

what does an adequate stimulus cause?

Answer 5

a graded membrane potential change which is called a receptor potential or generator potenial, that is usually a membrane deformation in cutaneous mechanoreceptors or proprioceptors.

Question 6

describe how a stimulus leads to an action potential:

Answer 6

Stimulus → ion flow (ligand-gated) → hit threshold → depolarisation (voltage-gated) → action potential

So:

Low stimulus intensity → no AP

High stimulus intensity → AP

Question 7

what is the frequency coding of stimulus?

Answer 7

A larger stimulus → larger receptor potential → higher frequency of action potentials

Question 8

what is adaption?

Answer 8

In some mechanoreceptors that are going through continuous mechanical stimulation, you see a drop in AP frequency.

Stimulus → generator potential (suprathreshold graded potential)
→ triggers AP

Then it rapidly declines subthreshold and APs cease

It only responds to a change or a novel event

mechanoreceptors in this process only signal the onset of a stimulus

Question 9

what 2 groups are mechanoreceptors divided into based on their adapting capabilities?

Answer 9

1- rapidly/Moderately-adapting receptors
e.g. Pacinian corpuscles & Meissner’s corpuscles

2- Slowly-adapting receptors
e.g. Merkel’s discs & Ruffini endings

Question 10

describe the Pacinian corpuscle mechanoreceptor:

Answer 10

the best-understood mechanoreceptor

It’s comprised of a myelinated nerve with a naked nerve ending, enclosed by a connective tissue capsule of layered membrane lamellae, each layer is separated by fluid.

Question 11

Explain how a Pacinian corpuscle responds to a mechanical stimulus. and what happens as the stimulus is withdrawn?

Answer 11

1. A mechanical stimulus deforms the capsule and the nerve-ending
2. This stretches the nerve ending and opens ion channels
3. Na+ influx causes local depolarisation - triggers a receptor/generator potential
4. APs are generated and fire where myelination begins if the threshold is reached
   (because regenerative Na+ channels cluster at nodes of Ranvier)

Deformation → nerve ending stretch → ion channels open → local depolarization trigger GP → hits threshold → fire APs → Stimulus ON

Next:

5- Fluid rapidly redistributes within capsule lamellae (as you press down on it)→ stimulus impact spread laterally → minimizes downward deformation

Withdrawal of Stimulus:

6- Capsule lamellae spring back → AP fires again

Question 12

The function of a Pacinian corpuscle depends on a non-neural accessory structure called a
capsule lamellae

Explain why?

Answer 12

Capsule intact

• Normal, rapidly adapting ON/OFF response

Capsule removed

• Bare nerve ending loses much of adaptation
• So it continues to produce a receptor/generator potential

Question 13

what is a receptive field?

Answer 13

a specific area where a somatic sensory neuron is activated by a stimuli.

Touch-sensitive neurons have a more defined area to respond to pressure.

This allows you to tell where an itch is e.g.

Question 14

what is the 2-point discrimination test?

Answer 14

a test to measure our ability to tell two points apart on the skin.

You increase the distance between points adjusted on the skin until you just perceive 2 points rather than one.

Question 15

what 2 things does the 2-point discrimination test depend on?

Answer 15

Receptive field size - the size of the area of stimulus

Neuronal convergence - connections between the sensory neurons

Question 16

what do some sensory neurons with neighbouring receptive fields may exhibit neuronal convergence allow?

Answer 16

the sub-threshold stimuli coming from the different areas through primary sensory neurons summate at the secondary neuron, to form a larger secondary receptive field and initiate an AP.

Question 17

what does high/ low neuronal converge with a large/ small secondary receptive field mean?

Answer 17

High neuronal converge + large secondary receptive field
→ insensitive area

e.g. back and limbs → 2 point threshold → 2 cm

Low neuronal converge + small secondary receptive field
→ sensitive area

e.g. lips and fingers → 2 point threshold → 2 mm

Question 18

what is the acuity? describe it’s relationship with convergence:

Answer 18

Acuity is the ability to locate a stimulus on the skin and differentiate it from another nearby stimulus.

It is inversely proportional to the level of neuronal convergence.

High convergence → low acuity

Low convergence → High acuity

Question 19

Why is lateral inhibition widespread in the spinal cord and pathways with high precision information?
e.g. touch and skin movement

Answer 19

Because lateral inhibition:

• Enhances the contrast between relevant and irrelevant information
• Sharpens sensory information

Question 20

what is lateral inhibition?

Answer 20

helps you locate the area of the stimulus precisely e.g. pin on your finger

The pathway closest to the stimulus inhibits neighbouring lateral neurons from enhancing the perception of the stimulus

Receptors that are at the edge of the stimulus are more strongly inhibited than the ones near the centre.

Question 21

what are proprioceptors? what 3 things do they include?

Answer 21

are mechanoreceptors that signal body or limb position.

They include:

1- Muscle spindles

→ muscle length & rate of change in muscle length

2- Golgi tendon organs

→ tendon tension which is produced by muscle contraction → so it monitors muscle tension too.

3 - Joint receptors

→ joint angle & rate of angular movement & tension on joint

Question 21

what does the most sensitive (high acuity) areas of the body innvervate?

Answer 21

the largest cortical space.

e.g. lips & fingers

Question 21

how do proprioceptors work?

Answer 21

1. Send sensory information to the spinal cord and on to the brain so that it can
   control our voluntary movements
2. Muscle Spindles and Golgi Tendon Organs provide the sensory information to spinal cord neurones which can generate spinal reflex movements
3. They provide sensory information to perceive limb and body position and movement in space = kinaesthesia.

Question 22

what is kinaesthesia?

Answer 22

awareness of the position and movement of the parts of the body through sensory organs (proprioceptors) in the muscles and joints.

Question 23

describe what it means by intrafusal and intrafusal muscle fibres:

Answer 23

Most skeletal muscle fibres are extrafusal muscle fibres, but some specialized few have their own sensory and motor innervation and are called Intrafusal muscle fibres.

These fibres are contained within a capsule and are called Muscle Spindles, which lie in parallel with muscle fibres.

Question 23

what do muscle spindles have?

Answer 23

sensory neurons in the middle of the intrafusal fibre and gamma motor neurons on the ends of the intrafusal fibre.

Question 24

what are extrafusal muscle fibres innervated by?

Answer 24

alpha motor neurons

Question 24

where are Golgi tendon organs located?

Answer 24

at the end extrafusal fibre near the tendons they are innervated by afferent sensory neurons

Question 25

Alpha motor neurons vs Gamma motor neurons

What are the differences?

Answer 25

Gamma motor neurons are smaller in diameter, and they innervate and cause contraction of the contractile ends of the intrafusal fibres.

Alpha motor neurons are larger in diameter, and they innervate and cause contraction of the extrafusal muscle fibres.

Question 26

How does muscle length indicate the spindle AP discharge?

i.e. agonist contracting, antagonist relaxing, joint moving

Answer 26

Muscle stretch stimulates the spindle stretch receptors:

Stretch sensitive ion channels open → trigger a suprathreshold generator potential → fires APs in 1a afferent neurons

Muscle spindle indicates muscle length and the rate of change in muscle length, so:

Increase in muscle length (stretching the agonist) → lengthens muscle spindle → increases spindle AP discharge

Decreases in muscle length (shortening the agonist) → shortens muscle spindle but doesn’t affect the central area → decreases or lack of spindle AP discharge

The opposite happens in the antagonist muscle.

Question 26

What happens to spindle AP discharge if a muscle lengthens from L0 to L1?

Answer 26

1. Resting AP frequency depends on the length L0
2. During a stretch from L0 to L1 (AP frequency is proportional to the velocity of stretch)
3. AP frequency remains higher at the new steady state when L1 is larger than L0

L1 - L0 = new muscle length

AP frequency at 2 = the rate of change in muscle length

Question 27

what does the Golgi tendon organ do?

Answer 27

they are proprioceptors that monitor muscle tension.

They initiate APs in Group Ib afferents

Nerve endings of GTO at the ends of muscles near tendons are stretch receptors and monitor the stretch of tendons.

But because tendons are inelastic, passive stretch does not affect them as it does in muscle spindle.

So muscles have to develop tension by contracting to stretch the tendons

Question 27

Why do GTOs fire APs in an isometric contraction but muscle spindles do not fire APs?

Answer 27

Because in an isometric contraction:

muscle contraction increases tension in the GTOs and their Ib afferent neurons fire action potentials.

muscle length does not change in the muscle spindles so the Ia afferent neurons do not fire.

Question 27

How does the Golgi tendon organ (GTO) work?

Answer 27

GTOs monitor muscle tension, so:

As you increase tension, the muscle is contracts - increasing in firing rate of action potential

GTOs lie in series with the muscle fibres, so when the muscle contracts, they pull on the tendon

Deforms/stretches sensory endings of 1b afferent → triggers suprathreshold generator potential →
fire APs

Question 28

What happens when alpha motor neurons fire without gamma motor neurons?

Answer 28

1- Extrafusal muscle contracts and shortens

2- Intrafusal muscle stays the same length - spindle will go slack

3- 1a spindle sensory firing would decrease during contraction

Question 28

what is alpha gamma activation?

Answer 28

the process of excitation of gamma motor neurons and alpha motor neurons at the same time.

Alpha motor neurons fire causing extrafusal fibre contraction and simultaneously gamma motor neurons fire causing intrafusal fibre ends to contract, this leads to:

1- Stretching the central sensory elements in the muscle spindle

2- Restoring tension in the spindle

3- Resetting the sensitivity of the central sensory part of the spindle, to match new muscle length

Question 29

What happens when alpha motor neurons and gamma motor neurons fire together?

Answer 29

1- The extrafusal muscle and the muscle spindle shorten together

2- Spindle stretch is reset to match muscle stretch

3- There is no drop off in 1a spindle sensory firing during contraction

Question 30

what are annulospiral and flower spray endings?

Answer 30

Annulospiral endings are Ia afferent nerves or primary endings that wrap around the centre in the muscle spindle.

Flower-spray endings are II afferent fibres or secondary endings that connect to the chain in the muscle spindle.

Question 30

what are spindle 1a afferents and gamma motor neurons?

Answer 30

Spindle 1a afferents are neurons that report muscle shortening to the brain in the muscle spindle.

Gamma motor neurons are neurons that fire to maintain spindle sensitivity to stretch.

Question 31

what are mono and polysynaptic reflexes?

Answer 31

monosynaptic reflex is a reflex that has a single synapse between the afferent and efferent neurons

A polysynaptic reflex is a reflex that has two or more synapses

Question 32

what is the simplest spinal reflex?

Answer 32

strech reflex i.e. patellar tendon (knee jerk) monosynaptic reflex

Question 33

why do extrafusal fibers maintain a certain level of tension even at rest?

Answer 33

Spindles are tonically active and firing even when muscle is relaxed,

So alpha neurons receive tonic input from muscle spindle and fire continously

Question 33

what 3 types of connections can spindle sensory afferents make? (2 in spinal cord + one in brain)

Answer 33

[1] Direct contact to alpha motor neuron in stretched muscles → rapid contraction of agonist

• one synapse, no interneurons → monosynaptic

[2] Indirectly connects with the antagonist muscle

• when the agonist contracts (shortens), the antagonist relaxes (stretches)
• Reciprocal Inhibition → activation of interneuron which inhibits alpha motor neurons in antagonist

[3] Travels up the spinal cord to thalamus and somatosensory cortex

• informs brain about muscle length

Question 34

describe the stretch reflex in the patellar tendon and arm muscle:

Answer 34

Stretch reflex:

e.g. patellar tendon

Sharp tap to patella → stretches muscle spindle → fires AP through Ia afferent neuron

→ Alpha motor neuron receives signal in spinal cord → muscle contraction

e.g. arm muscle

Addition of load → stretches muscle spindle → fires AP through Ia afferent neuron

→ Alpha motor neuron receives signal in spinal cord → muscle contraction

Question 35

what is reciprocal inhibition?

Answer 35

when spindle afferents activate interneurons in the spinal cord which inhibits alpha motor neurons in the antagonist muscle.

Question 36

what is the inverse stretch/ golgi-tendon organ/ clasp knife reflex?

Answer 36

a reflex caused by 1b afferent from the Golgi Tendon Organs (GTO) that monitor muscle tension.

It is a polysynaptic and a protective reflex - avoids muscle contracting so hard that it rips tendon off the bone.

Process:

Muscle greatly contracts and shortens → pulls hard on tendon → 1b afferent from GTO fires AP rapidly

1- Activation of inhibitory interneuron to the agonist muscle → ↓ contraction strength

2- Activation of excitatory interneuron to the antagonist muscle

3- info about muscle tension ascends up the spinal cord to somatosensory cortex

Question 37

what are flexor/ withdrawal reflexes?

Answer 37

are reflexes that use info from nociceptors (pain receptors) in skin, muscles and joints.

They are polysynaptic and protective reflexes - move body away from painful stimulus

Causes ipsilateral flexion in response to pain

Process: (flex one side and extend other side - to prevent falling over)

Increased APs from nociceptors nerves due to a painful stimulus

1- Activation of excitatory interneurons → increase activity in the flexor muscle (posterior thigh)

• A-delta nociceptive fibres enter spinal cord → branching in segments → activates alpha motor neurone controlling flexor muscle

2- Activation of inhibitory interneurons → decrease activity in the extensor muscle (anterior thigh)

3- Excitation of contralateral extensors

4- Inhibition of contralateral flexors

• maintain upright posture on other side to shift body weight

5- Sensory information ascends to the brain in the contralateral spinothalamic tract

Question 38

Why is the flexor and crossed extensor reflex slower than the stretch reflex?

Answer 38

Because nociceptive sensory fibres have a smaller diameter than muscle spindle afferents and so they conduct more slowly

Question 39

Reflexes can be over-ridden by voluntary input from the CNS

e.g. if you’re holding a child you can override Golgi tendon reflex

How does that work?

Answer 39

Voluntary excitation of alpha motor neurons overrides the inhibition of the agonist muscle from the GTOs and maintains muscle contraction – preventing the GTO reflex

Question 40

how is the stretch reflex overridden?

Answer 40

e.g. patellar reflex can be hard to evoke in anxious patients because :

strong descending inhibition hyperpolarizes alpha motor neurons (moves subthreshold)

That’s why neurologists uses a technique called Jendrassik manoevure where you interlock your fingers in front of your chest and pull hard, then they test the patellar reflex.

it causes high activity in upper motor neurons to spread and depolarise lower motor neurons, overcoming the descending inhibiton

Question 41

how is the withdrawal reflex overridden?

Answer 41

Withdrawal reflex can be overridden

e.g. holding on hot soup for brief amount of time to move leg away so you don’t burn your leg

Activating neurons voluntarily to inhibit the reflex briefly.

Anticipating pain can increase the vigour of the withdrawal reflex when a painful stimulus arrives.

Question 42

What is the clinical relevance of reflexes?

Answer 42

1- Important in assessing the integrity of the whole spinal cord circuit :(includes - afferent nerves, balance of synaptic inputs to the motoneurones, motoneurones, neuro-muscular junction and muscles).

2- Reflexes can help spinal level localisation of a problem
e.g. localise damage in the spinal core

3- Reflexes evoked above, but NOT below a given level may localise a problem.
e.g. segmental trauma to the spine.

Question 43

describe the effects of stretch and withdrawal reflex on spinal segements:

Answer 43

in withdrawal spindle input is highly localised and affects alpha motor neurons at one or two spinal segments.

In the Withdrawal reflex, a painful stimulus spreads over several spinal segments.

i.e. more powerful pain, causes greater segmental spread, more muscles are recruited and the response gets larger.

e.g.

one finger touches hot plate → move hand away (weak response)

hand touches hot plate → move arm away & or even jump away! (strong response)

Question 44

what is a facilitation?

Answer 44

a process through which a more powerful pain stimuli, causes greater segmental spread, more muscles are recruited and the response gets larger.

It can occur between the same stimuli, pain fibres and between different stimuli (cross-modality)

e.g.

one finger touches hot plate → move hand away (weak response)

hand touches hot plate → move arm away & or even jump away! (strong response - facilitated response)

Question 45

Why would the biceps stretch reflex become greatly exaggerated after you burn your hand on a hot plate?

Answer 45

Because pain fibre activity (nociceptors) facilitates the muscle spindle activity by maintaining nearby alpha motor neurons in a more depolarised state.

Question 46

how does facilitation work?

Answer 46

Both input A & B are not enough to threshold area C and trigger an AP.

Area C is in a subliminal zone (slightly depolarized - not enough to hit threshold)

If we add them together, they bring Area C to threshold and it triggers an AP

A + B = 12 motor neurons activated

This is called Faciliation

This is what happens to neurons in the spinal cord, when you touch a hot plate with your hand
(on a larger scale of course) it’s why you get an exaggerated response afterwards.