M2 Topic 9: Sensation & Senses Flashcards
(27 cards)
Special senses
Main 5 senses, special as they have dedicated organ and area of brain responsible for them
- Vision
- Hearing
- Taste
- Smell
- Equilibrium
General (somatic) senses
Mainly relate to ‘body’, distributed throughout body, has receptor cells within structures of other organs
- Touch (4 kinds of receptors)
- Temperature
- Nociception (pain, itch)
- Proprioception (perception of body’s position)
Somatic stimuli
Stimulus processing usually subconscious
- Muscle length and tension
- Proprioception
Visceral stimuli
Stimulus processing usually subconscious
- Blood pressure
- Distension of GI tract
- Blood glucose concentration
- Internal body temperature
- Lung inflation
- pH of CSF
- pH and oxygen content of blood
Sensory pathways
Primary stimulus is physical energy
- Receptor acts as transducer (change one kind of energy into another)
Intracellular signal produced is usually change in membrane potential called a receptor potential, causes release of neurotransmitter
Stimulus > threshold > AP to CNS > Integration in CNS > cerebral cortex
- Conscious from sensory neurons
- Or acted on by subconscious - afferent neurons
What is the job of sensory pathways?
Process sensory inputs by sending signals to correct parts of the brain
How does sensory info travel to the CNS?
Via…
- Spinal cord to brain by ascending pathways
- Directly to brain stem via cranial nerves
Most pathways coming into CNS first synapse is at the thalamus
- Thalamic neurons then project to cortex
What are the exceptions to the normal pathway?
- Olfactory = enters CNS and goes directly to olfactory complex
- Equilibrium = enters CNS and synapses at thalamus, however, it also branches and projects directly at the cerebellum
Sensory nerves
Responsible for brining info to CNS
- Carries sensations such as pain, touch, temperature, proprioception
- Also involved in spinal reflexes (hand from hot stove)
Somatic sensory pathways
First-order neurons conduct impulses to brainstem or spinal cord
- Either via spinal or cranial nerves
Second-order neurons conducts impulses from spinal cord or brainstem to thalamus
- Cross to side before reaching thalamus
Third-order neurons conducts impulses from thalamus to primary somatosensory cortex
Mapping in the primary somatosensory cortex
Sizes of cortical areas are proportional to…
- Number of sensory receptors
- The sensitivity of the body part
On the opposite side of the body
Sizes can be modified with learning
- Learning braille will have larger area representing fingertips
Temperature receptors
- Free nerve endings (no specialisations)
- Terminate in subcutaneous layers
- Cold receptors activated at temp lower than body
- Hot receptors activate above body temperature to about 45 deg
- Hot/pain receptors activated above 45 deg
Touch receptors
Mechanoreceptors
- Pacinian corpuscles (vibration)
- Ruffini endings (stretch)
- Meissner’s corpuscles (flutter, stroking movements)
- Merkel receptors (steady pressure and texture)
Nociceptors
Pain
- Subjective perception
- Fast pain (‘first’ pain), sharp, localized, by delta fibres (myelinated)
- Slow pain (‘second’ pain), duller, not localised, by C fibres (unmyelinated)
Itch
- Histamine + other chemicals sensitise and activate C fibres, causing itch
Proprioception
Perception of body position
- Receptors in muscles, tendons, joints
- E.g. close eyes and touch nose
- E.g. sensing body movement, not causing it, close eyes and someone moves arm, you still know where they put it
Olfaction (smell)
Allows perception of smell
- Allows avoiding hazards, detect tasty food (closely linked with gustation)
- Odorants (molecules of smell) bind to odorant receptors and activate olfactory sensory neurons
- Olfactory sensory neurons are in the olfactory epithelium in nasal cavity (behind nose)
Olfactory pathways
- Olfactory epithelium lies high within nasal cavity (brown pathway)
- Olfactory neurons project to the olfactory bulb (via cranial nerve I)
- Smell info then bypasses the thalamus and goes straight to the olfactory cortex
- Further pathways lead to cortex and limbic system (not shown in diagram)
- Smell does not get filtered by thalamus
Gustation (taste)
Tongue covered in taste buds, each composed of taste cells specific for one kind of taste
- Sweet
- Sour
- Salty
- Bitter
- Umami (savoury)
Additional taste sensations may be linked with thermoreceptors and nociceptors (minty/cool, spicy/hot)
Gustatory pathways
- Tongue lies within oral cavity
- Taste cells activate gustatory neurons which project to brain stem (purple pathway)
- Taste info then carried to thalamus and then to the gustatory cortex
- Taste info carried by cranial nerves VII, IX, and X
- All taste info is filtered by thalamus
Auditory (hearing)
Allows perception of sound wave energy
- Frequency of waves translated into pitch
- Loudness is an interpretation of intensity - a function of wave amplitude
Sound in the cochlear
- Sound waves turned into mechanical vibration when striking the tympanic membrane (ear drum)
- Three middle bones vibrate and transfer vibrations to membrane in oval window
- Vibrations generate fluid waves in the cochlea
- Hair cells (sensory receptors) bend in the cochlea and activate sensory neurons
- AP’s travel to brain via cranial nerve VIII
Auditory pathways
- Primary auditory neurons project via cranial nerve VIII to brain stem
- Secondary sensory neurons synapse in nuclei in the thalamus and then to the auditory complex
- All auditory info is filtered by thalamus
Equilibrium and vestibular apparatus
- Vestibular apparatus is a series of interconnected fluid-filled chambers
- Semi-circular canals for detecting rotational acceleration
- Can also detect linear acceleration (speeding up in a car)
Equilibrium pathways
- Cranial nerve VIII carries equilibrium info
- Projects to thalamus (and then to cortex) and cerebellum
- Cerebellum keeps you standing upright
