Sensorimotor control Flashcards
What is a reflex?
Rapid, involuntary, stereotyped and coordinated response in response to a sensory stimulus
What are learned reflexes called?
Pavlovian reflexes
What are spinal reflexes?
Reflexes involving the somatic nervous system
What are somatic reflexes?
Same as spinal reflexes
What is the dorsal root ganglion (DRG)?
Collection of cell bodies of afferent sensory neurons
What is an example of a monosynaptic reflex?
Patellar reflex
How does the patellar reflex work?
- Sensory stimulus causes a signal to be sent down the primary afferent sensory neuron, travels through the DRG to the spinal chord
- Sensory neuron synapses directly onto a motor neuron
- Motor neuron stimulates the extensor muscle to contract, causing the knee to jerk
- Inhibitory interneurons inhibit innervation to the flexor muscle to make it relax (reciprocal inhibition)
What is the patellar reflex?
Knee jerk reflex
What are extensor muscles?
Muscles which cause limbs to extend when they contract and increase the angle between the members of a limb e.g. contraction of the triceps causes the arm to straighten
What are flexor muscles?
Muscles which cause limbs to flex (bend)
What is a monosynaptic reflex?
A reflex where only one synapse is involved meaning there is direct communication between the sensory and motor neurons
What is the patellar reflex an example of?
Stretch (myotatic) reflex
What is a myotatic reflex?
Stretch reflex
Why does the patellar reflex happen?
Tapping on the knee stretches the thigh extensor muscle and associated tendon which is corrected by the contraction of the extensor muscle which causes a knee jerk
What is the proprioceptive system?
The system responsible for maintaining posture/balance and awareness of limb positioning
What is reciprocal inhibition?
When an inhibitory interneuron inhibits firing of motor neurons connected to the antagonising muscle involved in the reflex, which forces it to relax so it can’t oppose the movement of the agonist muscle
What is the muscle spindle?
Sensory receptor which detects muscle stretch
How is muscle stretch detected?
By the muscle spindles
What are proprioceptors?
Sense organs that monitor the position and movement of body parts
What kind of receptor is the muscle spindle?
Proprioceptor
Where are muscle spindles found?
In most striated muscle
Where are muscle spindles especially abundant?
Muscles involved in fine motor control e.g. hand
Which class of sensory afferent are muscle spindles innervated by?
Ia
Which class of motor neurons do Ia sensory fibres provide feedback to?
Alpha
Which class of motor neurons are muscle fibres themselves innervated by?
Gamma
How do gamma motor neurons control muscle spindles?
Stimulate the muscle fibres within the muscle spindle to maintain tension so the spindle never slackens
What are the muscles within the muscle spindles called?
Intrafusal
What is the Golgi Tendon Organ (GTO)?
Sensory receptor which detects tension due to muscle contraction
What kind of receptor is the GTO?
Proprioceptor
Where is the GTO found?
In tendons
What is the difference between muscle spindles and GTOs?
GTOs detect tension due to contraction, not stretch which is detected by muscle spindles
Which class of sensory afferent are GTOs innervated by?
Ib
What does innervation of GTO sensory Ib afferents cause?
Activates inhibitory interneurons which inhibit alpha motor neurons which are innervating the same muscle
What is the purpose of the Golgi Tendon Reflex?
Negative feedback circuit which regulates muscle tension in order to protect the muscle/tendon from damage
What are tendons?
Fibrous connective tissue which attaches muscle to bone
What stimulus is the flexor (withdrawal) reflex a response to?
Painful/injurious stimulus
Which receptors are involved in the flexor reflex?
Nociceptors
What is the flexor (withdrawal) reflex?
Quick contraction of flexor muscles to withdraw a limb from an injurious stimulus
What is an example of a polysynaptic reflex?
Flexor (withdrawal) reflex
What do nociceptors detect?
Potentially damaging stimuli
What is a parallel after discharge circuit?
Where a neuron inputs onto several different chains of neurons which all converge onto the same output neuron
What is the purpose of a parallel after discharge circuit?
The result of the initial signal is sustained over an extended period
Is the patellar (stretch) reflex mono or polysynaptic?
Monosynaptic
Is the flexor reflex mono or polysynaptic?
Polysynaptic
How does the flexor reflex work?
- Flexor muscles contract to withdraw the limb
- Activation of multiple interneurons sustains the response (parallel after discharge circuit)
- Inhibitory interneurons inhibit innervation to the extensor muscles to make them relax (reciprocal inhibition)
- Crossed extensor reflex maintains balance
What is the crossed extensor reflex?
- Ipsilateral flexor contracts
- Signal sent via commissural axon to motor neurons innervating contralateral extensor, causes contraction to maintain balance
- Motor neurons to the contralateral flexor are inhibited (reciprocal inhibition)
What is a commissural axon?
Axon that crosses through the middle of the spinal chord
What does ipsilateral mean?
Occurring on the same side of the body
What does contralateral mean?
Occurring on opposite sides of the body
What are central pattern generators?
Local circuits which can generate patterns of alternating flexion and extension for movements such as walking, even without higher input from CNS
Which class of motor neurons are innervated to cause flexor contraction in the flexor reflex?
Alpha
Which class of motor neurons are innervated to cause extensor contraction in the stretch reflex?
Alpha
What are the 2 major input components of the somatic sensory system?
- Mechanical stimuli
- Painful stimuli (including temperature)
Where are Meissner corpuscles located?
Dermal papillae in the palms, eyelids, lips, tongue etc.
What do Meissner corpuscles detect?
Light touch and texture
What frequency are Meissner corpuscles sensitive to?
30 - 50 Hz
Where are Pacinian corpuscles located? (3)
- Dermis
- Joint capsules
- Viscera
What do Pacinian corpuscles detect? (4)
- Deep pressure
- Stretch
- Tickle
- Vibration
What frequency are Pacinian corpuscles sensitive to?
250 - 350 Hz
Which sensory receptors are encapsulated nerve endings? (2)
- Meissner corpuscles
- Pacinian corpuscles
Where are Ruffini corpuscles located? (3)
- Dermis
- Subcutaneous tissue
- Joint capsules
What do Ruffini corpuscles detect? (4)
- Heavy touch
- Pressure
- Skin stretch
- Joint movements
Where are Merkel discs located?
Superficial skin (epidermis)
What do Merkel discs detect? (4)
- Light touch
- Texture
- Edges
- Shapes
Which sensory receptors are rapidly adapting? (2)
- Meissner corpuscles
- Pacinian corpuscles
Which sensory receptors are slowly adapting? (2)
- Ruffini corpuscles
- Merkel discs
Where are free nerve endings located?
Widespread in epithelia and connective tissues
What do free nerve endings detect? (3)
- Pain
- Heat
- Cold
Which sensory receptors are unencapsulated nerve endings? (2)
- Merkel discs
- Free nerve endings
What are the sensory receptors? (8)
- Golgi Tendon Organ (GTO)
- Muscle spindle
- Pacinian corpuscle
- Meissner corpuscle
- Hair follicle
- Free nerve endings
- Ruffini corpuscle
- Merkel disc
What are the 2 different classes of mechanoreceptor response?
- Rapidly adapting (phasic)
What are the 2 different classes of mechanoreceptor response?
- Rapidly adapting (phasic)
- Slowly adapting (tonic)
What kind of information do phasic receptors give?
Changes in the stimulus i.e. they respond at the start of a stimulus and don’t respond again until the stimulus changes
Are Pacinian corpuscles phasic or tonic receptors?
Phasic
Are Meissner corpuscles phasic or tonic receptors?
Phasic
Are Ruffini corpuscles phasic or tonic receptors?
Tonic
Are Merkel discs phasic or tonic receptors?
Tonic
What kind of information do tonic receptors give?
Persistence of a stimulus i.e. continue to respond for as long as the stimulus is present
Which diameter of axon conducts signals more rapidly?
Larger the diameter, the faster the signal
Which class of axon subtype are mechanoreceptors of the skin?
A beta
What is the system of classifying axons coming from the skin?
- A, B, C etc. with A being the fastest and largest and C being the slowest and smallest
- A is further broken down into alpha, beta, delta, fastest to slowest again
What is the system of classifying axons coming from the muscles?
- Roman numerals I, II, III, IV going from largest to smallest
- I group further broken down into Ia, Ib etc. with a being the fastest
Which class of axon subtype are GTOs and muscle spindles in?
Group I
What is the dorsal horn?
The dorsal section of the spinal chord where afferent neurons enter with sensory information to be processed
What is the ventral horn
The ventral section of the spinal chord where efferent neurons leave and take signals to skeletal muscle
What are the 2 main routes via which sensory information is relayed to the brain?
- Medial Lemniscal tracts
- Spinothalamic tract
Which sensory inputs are carried to the brain via the Medial Lemniscal tracts? (2)
Mechanoreceptive and proprioceptive signals
Which sensory inputs are carried to the brain via the Spinothalamic tract? (2)
Pain and temperature signals
Where do the Medial Lemniscal tracts carry signals to?
Thalamus
Where does the Spinothalamic tract carry signals to?
Thalamus
What are the 3 neurons which a sensory signal travels through to reach higher centres?
- First order DRG neurons detect the stimulus and relay it to the dorsal horn
- Second order neurons relay the signal to the thalamus
- Third order neurons carry the signal from the thalamus to the cortex
Which order neuron is commissural in the route of sensory signals travelling to the brain?
Second order neuron (in both pathways)
What is topological organisation?
The spatial arrangement of objects relative to one another
What are the dorsal column nuclei? (2)
- Cuneate nucleus
- Gracile nucleus
Which nucleus contains the second order neurons which first order neurons synapse onto from the upper body in the Medial Lemniscal system?
Cuneate nucleus
Which nucleus contains the second order neurons which first order neurons synapse onto from the lower body in the Medial Lemniscal system?
Gracile nucleus
Is lower body sensory information sent to a medial or lateral location in the cortex?
Medial
Is upper body sensory information sent to a medial or lateral location in the cortex?
Lateral
What are dermatomes?
- Body segments which are each innervated by a separate DRG
- Arise from somites
What determines the size of the area of the sensory cortex dedicated to sensory input of each body part?
Areas of the body where there are more receptive fields i.e. more neurons, have larger areas in the cortex as there is more sensory information being transmitted e.g. hands and face
What are Brodmann areas?
The sensory map of the cortex
How is the cortical map plastic?
- If one body part is lost, its area in the cortex will gradually disappear and the neighbouring segments will increase in size
- If one body part is trained, its area in the cortex will increase at the expense of other, less stimulated areas
Where are first order neurons located?
DRG
Where are second order neurons located?
Dorsal column nuclei
Where are third order neurons located?
Thalamus
