Senses and Perceptions Flashcards
What is tactile information do we need to know?
Quality of stimulus; magnitude, intensity; duration, timing; location
How do we convert a stimulus into an electrical action potential and how is the information transmitted?
Specific sensory receptors
Specific sensory pathways
What type of neurotransmitter is glutamate?
Excitatory
Describe a typical sensory neuron
A pseudounipolar neuron
What is a neuron’s receptive field defined by?
The distribution of nerve endings/receptors
Which parts of the body are receptive fields large/small?
Small in the periphery
Large on the trunk
What does two-point discrimination refer to?
The ability to discern two separate mechanical stimuli
What is a neuron’s receptive field?
The area of skin innervated by a single nerve fibre
Areas with small 2-point discrimination have a high density of sensory input. Describe the cortical representation for areas with small 2-point discrimination
Large cortical representation
Describe the receptive fields and and 2 point values in the tips of the finger
Small receptive fields
Small 2-point discrimination
Describe the receptive fields and 2 point values in the trunk
Large receptive fields and large 2-point discrimination
Describe the transduction of a stimulus (e.g. mechanical stimuli changed to an electrial impulse)
- Stimulus
- Change in membrane permeability of the receptive ending of sensory neuron
- Influx of cations
- Depolarisation occurs in the sensory neuron
- Generation of an action potential
- Conveyed centrally to CNS
Describe the action potential in mechanotransduction
Once transmembrane pore is activated by mechanical stimuli, it opens. Na+ flows into cell. Depolarisation occurs. Once threshold is reached, an action potential is fired. K+ channels open -> repolarisation. Cell returns to resting membrane potential
Describe mechanoreceptors
Mechanoreceptors are tethered to the cell membrane. Pulling of subcellular tethers: membrane deforming/deflection of the cell membrane -> opens channel
Describe chemoreceptor activation
- Receptors in skin react to chemical stimuli e.g. acid on skin causes perception of burning
- Chemical binds to receptor. Causes conformation change in ion channel, causing it to open. Leads to passage of Na+ ions into cell -> AP generated
- Ligand-gated pore
What is a GPCR?
- G-protein coupled receptor
- Generally have a pore in the cell membrane
- When binding of a chemical to receptor protein occurs, chain of events occurs through various proteins, ultimately resulting in phosphorylation of G protein, causing the channel to open
What are the differences between ionotropic receptors and GPCRs?
- Easier to inactivate ionotropic receptors than GPCRs
- GPCRs stay ‘on’/activated for longer e.g. acid on skin, pain persists
How is the quality of a stimulus determined?
By the type of receptor being recruited e.g. thermoreceptors or mechanoreceptors
How is the intensity of a stimulus graded?
By the frequency of action potential firing or the number of neurons recrutied
How is the duration of a stimulus determined?
Duration of action potential firing - how long they fire for
How is the location of the stimulus coded?
By which receptive field has been stimulated. Allows us to perceive location of stimulus and respond to it
A high intensity stimulus will be coded by:
High action potential firing frequency
What is stimulus intensity coded by?
Action potential firing frequency and the number of neurons recruited/activated
Describe how duration of a stimulus is encoded by different types of neurons
- Stimulus duration is encoded in real time by the duration of AP firing
- Distinct neurons have distinct firing patterns
- When the membrane is first depolarised: high frequency APs
- The frequency diminished with time: slowly adapting neurons
What is topographical representation?
A body map that can be plotted in the sensory cortex that corresponds to particular body regions
What is the sensory homunculus?
- Distorted body map. Face, hands and feet have greater representation/larger
- Face, hands and feet have a larger region in the cortex => indicates high density of sensory receptors/sensory input
Where are cell bodies of sensory neurons located?
In the dorsal root ganglia
What is in a peripheral nerve?
- Some contain sensory axons only
- Other contain both motor and sensory axons
What does the brachial plexus innervate?
The upper limb
What are the 5 principle branches that make up the brachial plexus?
Radial, ulnar, axillary, mucsulocutanous, median
What is the perineurium?
Sheath of connective tissues that surrounds a fascicle within a nerve
What is a fascicle?
Bundle of nerve fibres
What is the epineurium
Most outermost sheath of connective tissue that surrounds a nerve
What is the endoneurium
Sheath of connective tissue that surrounds each axon
Axons can be subdivided based on…
Their morphology: axon diameter and degree of myelination
Describe the morphology of A-alpha axons
Diameter = 13-20 um
Thick myelin sheath
Speed = 80-120 m/s
Sensory receptors: proprioceptors of skeletal muscle
Describe the morphology of A-beta axons
Diameter = 6-12 um
Relatively thick myelin sheath
Speed = 35-75 m/s
Sensory receptors = mechanoreceptors of skin
Decrease the morphology of A-delta axons
Diameter = 1-5 um
Thin myelin sheath
Speed = 5-30 m/s
Sensory receptors = pain, temperature
Describe the morphology of C axons
Diameter = 0.2-1.5 um
No myelin sheath
Speed = 0.5 - 2 um
Sensory receptors = pain, temperature, itch
Where are motor axons derived from?
Cells that sit in the grey matter of the ventral horn
What are motor neurons important for?
- The recruitment of muscle cells for muscle contraction
- Each motor neuron innervates a distinct muscle group
The spinal cord is divided into how many segments?
31
What are dermatome maps important for?
- Localising where stimuli are applied
- Important clinically for determining the extent of spinal cord injury
Which nerve innervates the muscles of mastication?
V3 (mandibular branch of the trigeminal nerve)
What is the trigeminal nerve important for? Discuss the different branches
- Innervation of the face
- V1 (opthalmic branch) and V2 (maxillary branch) - sensory
- V3 (mandibular branch) - mixed motor and sensory
Describe a sensory pathway and the location of neurons
- Primary sensory neuron in the periphery
- Secondary sensory neuron in the spinal cord/brainstem
- Tertiary sensory neuron in the thalamus
What is the posterior (dorsal) trigeminothalamic tract
- For mechanoreception
- Primary neuron from the face to the brainstem
- Secondary neuron from the brainstem to the thalamus
- Tertiary neuron from the thalamus to the somatosensory cortex
What is the spinothalamic pathway?
- For noiception (pain)
- Pain fibres project into spinal cord
- Synapses onto 2nd sensory neuron in the dorsal horn of the spinal cord
- 2nd sensory neuron sends axons to opposite side of the spinal cord before projecting all the way up to the thalamus
- 3rd sensory neuron in thalamus sends axons up to somatosensory cortex
What is the anterior (ventral) trigeminothalamic tract?
- For noiception (pain)
What is the dorsal column-medial lemniscal pathway?
- For mechanoreception
- Innocuous tactile stimulus in the periphery
- Abeta fibres travel along periphery nerve to the dorsal horn of spinal cord
- Axon has side branch in the dorsal horn but also continues up and terminates in the brainstem
- Activates secondary sensory neuron at the level of the brainstem. Crosses over to the opposite side of the brainstem before projecting up to the thalamus
How does the dorsal column-medial lemniscal pathway differ from the posterior (dorsal) trigeminothalamic tract?
Dorsal trigeminothalamic tract doesn’t go through the dorsal horn of the spinal cord, whereas the dorsal column-medial lemniscal pathway does
Which nerve fibre is responsible for dull aching pain?
C fibres
Which nerve fibre is responsible for sharp stabbing pain?
Adelta fibres
Why is the brain image distorted in the homunculus?
- Every peripheral nerve in represented in the cortex
- The more sensory neurons that supply an area, the more ‘brain space’ allocated in the sensory cortex
- Areas with high densities of receptors (small receptive fields, e.g. fingers, lips) are represented more widely in the cortex
What is stereognosis?
- The ability to identify objects without visual cues e.g. by touch alone
- Involves more than just ‘touch’ sense
- Requires a 3-D mental image (memory)
- Co-ordinated cortical processing
