Sensory and Motor Physiology Flashcards
Describe the two types of transduction
- Receptor cells are stimulated and release neurotransmitter to stimulate a generator potential in a sensory neuron. 2. The receptor ending of a specialised sensory neuron is stimulated, causing a receptor potential.
How does a sensory neuron code the characteristics of a stimulus?
A stronger stimulus will produce a greater depolarisation of the receptor above threshold potential (and stimulate more nerve endings), causing increased rate of AP and more neurotransmitter release at the synapse. A longer-lasting stimulus will cause APs to fire for longer, having the same effect.
What is the receptive field of a sensory cell or neuron?
This is the area over which changing stimuli modulate the rate of firing in a given sensory neuron. Primary sensory neurons converge on a secondary, which as a result has a wide receptive field. If primary neuron receptive fields overlap this decrease location acuity but increases sensitivity to the intensity of the stimulus - thigh rather than hands.
How can a very specific location of stimulus be focused in on?
E.g. pin prick is detected by three primary sensory neurons, but the stimulus is most intense for the middle one. When the signal is communicated to secondary neurons, the very strong middle signal (lots of neurotransmitter to secondary) will inhibit neighbouring neurons through axoaxonic synapses to give tertiary neurons a more localised signal.
What is the range of AP firing frequencies available to code for stimulus intensity?
A momentary stimulus will stimulate one action potential. A threshold stimulus, sustained, will stimulate an action potential following the absolute+relative refractory period. A very intense suprathreshold stimulus will produce a maximal response where an AP is produced again immediately following the absolutely refractory period of the previous one. (Not linear, plateaus)
Phasic vs tonic receptors
Tonic receptors are slow adapting and APs are generated for the duration of the stimulus. Phasic receptors rapidly adapt to a constant stimulus and stop generating APs, but start again when the stimulus is removed (e.g. Pacinian corpuscle)
Type I vs type II receptors
Type one have a small, focal receptive field (e.g. Merkel cells) while type two have a more diffuse, wide receptive field (e.g. Ruffini endings, Pacinian corpuscle)
Describe the dorsal column system
Large diameter - most rapid conducting sensory system. Ascending pathway: 1st order neuron travels into the spinal cord up to the brainstem and decussates. 2nd order neuron travels to the thalamus. 3rd order travels to the relevant part of the somatosensory cortex. Processes fine touch, proprioception and vibration - small stimuli
Describe the Anterolateral System (Spinothalamic System)
1st order neuron travels into the spinal cord, 2nd order decussates in the spinal cord and travels to the thalamus, 3rd order travels to the somatosensory cortex. Processes nociception, temperature and coarse touch.
Describe the features of the Dorsal Column System
Large diameter afferents so fast conduction velocity. Important for localising inputs and perceiving their intensity. High degree of discrimination
Describe the features of the Anterolateral System
Smaller diameter afferents so slower conduction velocity. Touch and pressure perception but with poor stimulus discrimination. Involved in perception of temperature and pain.
What is the spinocerebellar system?
Afferent neurons conveying proprioceptive information from the spinal cord to the cerebellum.
Where do the extrapyramidal tracts originate?
The brain stem and medulla: from the reticular formation, the vestibular nuclei, the red nucleus and the tectum
Where does the pyramidal (corticospinal tract) originate?
the cerebral cortex
Describe the descending tracts of the reticular system
two important tracts within the cord: the lateral and medial reticulospinal tracts. These are largely uncrossed, terminate on interneurons rather than motor neurons and influence muscles of the trunk and proximal parts of the extremities. Thought to control posture and the startle reflex - jumping.
Describe the vestibulospinal tract
Originates in the medial and lateral vestibular nuclei. Most neurons involved synapse with interneurons on the same side of the body. concerned mostly with extensor muscle activity and important for posture.
Describe the role of the red nucleus
One of the most important extrapyramidal structures. Represent upper limbs dorsomedially and lower ventrolaterally. Receives input from the cortex, cerebellum and globus pallidus (of the basal ganglia). It gives rise to the rubrospinal tract.
Describe the rubrospinal tract
fibres from the red nucleus decussate and travel to the spine, terminating in the lateral grey matter. Some synapse with motor neurons but most terminate on interneurons that excite flexor and extensor muscles of the contralateral limbs. Damage impairs voluntary movement but not posture.
Describe the tectospinal tract
Originates in the tectum. Projects to the cervical spinal cord. Crossed fibres end on interneurons influencing head and eye movement
Describe the pyramidal (corticospinal) tract
Originates in the cerebral cortex and runs down into the spinal cord with long axons. ~80% of fibres cross as they pass through the medullary pyramids and travel down the lateral corticospinal tract. The rest descend as the ventral corticospinal tract and cross later. Fine motor skills of the extremities.
