Sensation and reflexes Flashcards
What are cutaneous mechanoreceptors?
Receptors in skin that relay sensory information, specifically touch from skin surface receptors and pressure from receptors in deep tissue
What is glabrous skin?
Skin surfaces that do not have hair
Describe pacini’s corpuscle
- 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
Describe meissner’s corpuscle
- 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)
Describe merkel discs
- 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
Describe hair follicles
- Embedded in skin - innervated by nerve ending wrapped around its follicle
- Detect muscular movements of the hair (erector muscle) and external displacements of hair
Describe ruffini corpuscles
- 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)
Describe muscle spindles
- 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
Describe golgi tendon organs
- 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)
Describe generator potential
- Potential caused by a stimulus to a nerve ending
- Generates action potentials in a sensory neuron
Describe receptor potential
- Potential caused by a stimulus to a receptor cell
- Affects amount of neurotransmitter released by receptor cell onto sensory neuron
Receptor potential generation in a pacini corpuscle
- 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
Explain the relationship between receptor potential and action potential generation
- 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
How is the precision localisation of a particular stimulus determined?
- Size of individual nerve fibre receptive field
- Density of sensory units
- Amount of overlap in nearby receptive fields
What is two-point discrimination?
- 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
Where is the somatosensory cortex located?
In a strip posterior to the post central sulcus of the brain
What happens at the somatosensory cortex?
- Sensory information projected in a topographical manor to this area
- Areas of higher discrimination/senses have a larger proportion of the space
What happens at the somatosensory cortex?
- Sensory information projected in a topographical manor to this area
- Areas of higher discrimination/senses have a larger proportion of the space
What is a reflex?
A specific, involuntary, unpremeditated, built-in response to a particular stimulus
What are some examples of reflexes?
- Remove hand from hot object (spinal reflex)
- Lift foot off sharp object (spinal reflex)
- Shutting eyes as object rapidly approaches face (cranial nerve)
Name and describe some reflexes involving special senses
- 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
What is an example of a baroreceptor reflex?
Blood pressure regulation
Describe the reflex arc
- 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
Describe the reflex arc
- 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
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
What spinal cord root do sensation neurons come in from?
Dorsal root
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)
Lamina VII (lateral horn): -preganglionic sympathetic (T1-L2) & parasympathetic (sacral)
Lamina VIII & IX (ventral horn):
-cell bodies of motor neurons
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)
Lamina VII (lateral horn): -preganglionic sympathetic (T1-L2) & parasympathetic (sacral)
Lamina VIII & IX (ventral horn):
-cell bodies of motor neurons
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
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
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
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
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
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
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
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
