Sensation and reflexes Flashcards
(36 cards)
1
Q
What are cutaneous mechanoreceptors?
A
Receptors in skin that relay sensory information, specifically touch from skin surface receptors and pressure from receptors in deep tissue
2
Q
What is glabrous skin?
A
Skin surfaces that do not have hair
3
Q
Describe pacini’s corpuscle
A
- Largest mechanoreceptor - 2mm long
- Onion like encapsulation of nerve endings
- Found in deep layers of dermis
- Detects high frequency (40-500Hz) vibration
- A-beta fibres - glabrous & hairy skin
- Rapidly adapting due to a slick viscous fluid between the layers
- Has a low activation threshold i.e is sensitive
4
Q
Describe meissner’s corpuscle
A
- Encapsulated nerve endings similar to pacini’s but much smaller
- Stacks of discs interspersed with nerve branch endings
- Found between dermal papillae - detects touch, flutter & low frequency vibration (2-40Hz)
- A-beta fibres - glabrous skin types
- Rapidly adapting - low activation threshold (sensitive)
5
Q
Describe merkel discs
A
- Non-encapsulated nerve endings
- Consist of a specialised epithelial cell + nerve fibre
- Found just under the skin surface in, for example, the finger tips - good discrimination - detects static touch and light pressure
- A-beta fibres - all skin types
- Slowly adapting - low activation threshold (sensitive)
- Work with meissner’s corpuscles to help determine texture
6
Q
Describe hair follicles
A
- Embedded in skin - innervated by nerve ending wrapped around its follicle
- Detect muscular movements of the hair (erector muscle) and external displacements of hair
7
Q
Describe ruffini corpuscles
A
- Encapsulated nerve ending
- Nerve ending weave between collagen fibres which activate the nerve when they are pulled longitudinally
- Responds to skin stretch and is located in the deeper layers of the skin as well as tendons and ligaments
- A-beta fibres - all skin types but especially abundant in hands and fingers as well as soles of feet
- Slowly adapting - low threshold activation (sensitive)
8
Q
Describe muscle spindles
A
- Main proprioceptors that provide information about the state of musculature
- Muscle spindles lie within muscles in parallel with skeletal muscle fibres
- Innervated by gamma-motoneurons (efferents) and group Ia and II afferent fibres
- Afferents respond to muscle stretch while gamma-efferent activity regulates the sensitivity of the spindle
9
Q
Describe golgi tendon organs
A
- Main proprioceptors that provide information about the state of musculature
- Golgi tendon organs lie within tendons in series with contractile fibres
- Respond to degree of tension within the muscle
- Group Ib afferent fibres relay information to CNS (particularly spinal cord and cerebellum)
10
Q
Describe generator potential
A
- Potential caused by a stimulus to a nerve ending
- Generates action potentials in a sensory neuron
11
Q
Describe receptor potential
A
- Potential caused by a stimulus to a receptor cell
- Affects amount of neurotransmitter released by receptor cell onto sensory neuron
12
Q
Receptor potential generation in a pacini corpuscle
A
- Tip unmyelinated - nerve fibre myelinated before leaving corpuscle
- Compression anywhere on outside of corpuscle elongates and indents/deforms central fibre
- Receptor potential induces local current flow (Na+ current) which spreads along nerve fibre
- At first node of ranvier local current flow depolarizes fiber membrane at this node which sets off action potentials to CNS
13
Q
Explain the relationship between receptor potential and action potential generation
A
- When receptor potential (from receptor) rises above threshold in nerve fibre - action potentials fire
- Amplitude of receptor potential increases rapidly at first then less rapidly at high stimulus strength
- The more receptor potential rises above threshold level, the greater the action potential frequency
- APs generated in a sensory nerve at a frequency directly related to stimulus size
14
Q
How is the precision localisation of a particular stimulus determined?
A
- Size of individual nerve fibre receptive field
- Density of sensory units
- Amount of overlap in nearby receptive fields
15
Q
What is two-point discrimination?
A
- Minimum distance at which two points can be perceived as distinct
- It is a result of receptive field size and receptor density in the are
16
Q
Where is the somatosensory cortex located?
A
In a strip posterior to the post central sulcus of the brain
17
Q
What happens at the somatosensory cortex?
A
- Sensory information projected in a topographical manor to this area
- Areas of higher discrimination/senses have a larger proportion of the space
17
Q
What happens at the somatosensory cortex?
A
- Sensory information projected in a topographical manor to this area
- Areas of higher discrimination/senses have a larger proportion of the space
18
Q
What is a reflex?
A
A specific, involuntary, unpremeditated, built-in response to a particular stimulus
19
Q
What are some examples of reflexes?
A
- Remove hand from hot object (spinal reflex)
- Lift foot off sharp object (spinal reflex)
- Shutting eyes as object rapidly approaches face (cranial nerve)
20
Q
Name and describe some reflexes involving special senses
A
- Rotatory nystagmus - Eye movements driven by moving visual images
- Port rotatory nystagmus - Eye movements driven by the movement of fluid in the semi-circular canals of the inner ear
- Interpretation of movement within images on the retina - used to make postural adjustments to preserve balance
21
Q
What is an example of a baroreceptor reflex?
A
Blood pressure regulation
22
Q
Describe the reflex arc
A
- Stimulus - a detectable change in internal/external environment
- Receptor - detects the change
- Integrating centre - signal received (along with those from other stimulus/receptor interactions)
- Effector
- Response
- Sometimes response gives negative feedback
22
Q
Describe the reflex arc
A
- Stimulus - a detectable change in internal/external environment
- Receptor - detects the change
- Integrating centre - signal received (along with those from other stimulus/receptor interactions)
- Effector
- Response
- Sometimes response gives negative feedback
23
What are the different classes of movement?
Voluntary:
- complex actions (reading, writing)
- purposeful goal directed
- learned
Reflexes:
-involuntary, rapid, stereotyped (knee jerk, eye blink)
Rhythmic motor patterns:
- combines voluntary and reflex acts (chewing, walking, running)
- initiation and termination voluntary
- once initiated repetitive and reflexive
24
What spinal cord root do sensation neurons come in from?
Dorsal root
25
Explain spinal cord laminae
Lamina I-VI (dorsal horn):
- terminations for primary afferent sensory neurons & neurons of reflex circuits
- sensory input from joints and muscle (lamina VI)
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Lamina VII (lateral horn):
-preganglionic sympathetic (T1-L2) & parasympathetic (sacral)
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Lamina VIII & IX (ventral horn):
-cell bodies of motor neurons
25
Explain spinal cord laminae
Lamina I-VI (dorsal horn):
- terminations for primary afferent sensory neurons & neurons of reflex circuits
- sensory input from joints and muscle (lamina VI)
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Lamina VII (lateral horn):
-preganglionic sympathetic (T1-L2) & parasympathetic (sacral)
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Lamina VIII & IX (ventral horn):
-cell bodies of motor neurons
26
Organisation of the spinal cord
- Alpha-motor neuron cell bodies lie in clumps within ventral horn of spinal cord (lower motor neurons)
- Each motor neuron activates a motor unit (6-1500 skeletal muscle fibres)
- Some axons branch back into cord and synapse with interneurons called Renshaw cells (recurrent or feedback inhibition)
- Suppresses weakly firing motor neurons and dampening strong firing ones
- Produces economical movement
- Importance reflected in strychine poisoning - disables Renshaw cell inhibition - leads to convulsions
27
Explain a myotatic (knee jerk) reflex
- Example of a monosynaptic stretch reflex
- Tap of patellar tendon stretches quadriceps muscle
- Stimulates dynamic nuclear bag receptors of muscle spindle
- Increase in rate of firing of group Ia afferents leads to contraction of quadriceps muscle
- Ia fibres also stimulate inhibitory interneurons which inhibit antagonistic (flexor) muscle of knee joint
28
Muscle spindles and voluntary activity
- Muscle spindles play important role as comparators for maintenance of muscle length during foal directed voluntary movements
- Voluntary changes in muscle length initiated by motor areas of brain, orders include changes to set-point of muscle spindle
- Simultaneous activation of extrafusal fibres (alpha-motor neurons) and intrafusal fibres (gamma-motor neurons) is called alpha-gamma-co-activation
29
Explain the inverse myotatic (golgi tendon) reflex
- -Protective to prevent tearing of muscle/detachment of tendon
- Contributes to maintenance of posture
- GTO stimulated by increasing tension in muscle
- Signals transmitted to spinal cord - reflex entirely inhibitory
- Negative feedback mechanism which prevents development of excessive tension in muscle
- Inhibitory effect from GTO can be so great leads to sudden unloading of muscle
30
Explain the withdrawal reflex and crossed extensor reflex
- Protective reflex of rapidly removing limb from damaging stimuli
- Stimulation of withdrawal reflex, frequently elicits extension of the contralateral limb 250ms later
- Helps maintain posture and balance
31
Describe the process of the withdrawal reflex and crossed extensor reflex
- In the leg that feels the pain, the reflex inhibits -in the spinal cord - the motor neurons to the extensor muscle and stimulates the motor neurons to the flexor muscle
- In the opposite leg, the reflex stimulates - in the spinal cord - the motor neurons to the extensor muscle and inhibits the motor neurons to the flexor muscle
32
What is the central pattern generator?
- Located in spinal cord - capable of autonomous signals
- Modulated by proprioception input
- Thought to be inflated by mesencephalic locomotor region - output through reticular nuclei and reticulospinal tracts
33
Central pattern generators for walking
- 2 half centres which activate flexors and extensors respectively and which mutually inhibit each other
- Can be modelled using inhibitory 1a interneurons and renshaw cells
