Sensation & Ascending Tracts Flashcards
What are some of the different types of sensory receptors of first order neurones?
Free nerve ending e.g. cold stimuli
Encapsulated nerve ending e.g. pressure
Specialised cell e.g. gustatory (taste) receptors sense glucose molecules
What is the difference between stimulus modality and the quality of a sensation? How specific are sensory receptors to stimulus modalities?
Stimulus modality e.g. light touch, temp., chemical changes
Quality of a sensation is a subdivision of stimulus modality
e.g. taste (modality) can be subdivided into sweet, sour, salt, etc.
Sensory receptors are modality specific up to a point (strong enough stimulus may activate a different specificity receptor e.g. seeing light when punched in the eye)
Contrast muscle spindles and golgi tendon organs.
Muscle spindle = receptor embedded between and parallel to fibres of striated muscle that is sensitive to stretch
Golgi tendon organ = receptor within tendon that responds to tension/stretching
Both are involved in proprioception
How is the strength of a stimulus determined? How can the strength of a stimulus affect sensory neurones?
Strength of stimulus determined by rate of action potential stimulus (frequency coding)
Stronger stimuli:
- activate sensory neurone with different sensory modalities
- activate neighbouring neurones
Contrast tonic and phasic receptors.
Tonic (slowly adapting receptors) = keep firing as long as a stimulus lasts
e.g. joint and pain receptors
Phasic (rapidly adapting receptors) = respond maximally and briefly to a stimulus e.g. light touch receptors (hence why we stop feeling our clothes whilst wearing them, unless movement causes a change in position of the clothing on our skin)
Define acuity. Briefly, what is it determined by?
Precision by which a stimulus can be located
Determined by:
- lateral inhibition (CNS)
- two-point discrimination
- convergence & divergence
How does lateral inhibition determine acuity of sensation?
Points of skin which are stimulated —> first order neurone —> second order neurone —> stimulus detected by CNS
Points of the skin which are not stimulated —> first order neurone —-> inhibitory interneurones inhibit second order neurone —> no stimulus sensed by CNS
This allows precise points of stimulus to be sensed by contrasting areas of stimulation to areas not being stimulated
How does two-point discrimination determine acuity of sensation?
Two-point discrimination = minimal interstimulus distance required to perceive two simultaneously applied skin indentations
e.g. fingertips = 2mm apart, forearm = 40mm apart, trunk = 6cm apart
Determined by:
- density of sensory receptors
- size & overlap of neuronal receptive fields (the larger the field, the more likely it is to overlap with another field, therefore there is reduced two-point discrimination)
How does convergence and divergence of neurones determine acuity of sensation?
Convergence of multiple sensory first order neurones onto a single second order neurone —> decreased acuity
Divergence of multiple sensory second order neurones from a single first order neurone —> increased acuity (amplifies signal, as more axons are projecting to the CNS)
How is the thalamus involved in sensation? How can sensation be affected by thalamic lesions?
Thalamus involved in crude localisation and discrimination of stimuli via highly organised projections to the cortex
Thalamic lesions (e.g. stroke) —> thalamic syndrome (severe chronic pain not in proportion to the stimulus)
What is synaesthesia?
Secondary subjective sensation (e.g. colour) is experienced at the same time as the sensory response normally evoked by the stimulus.
Caused by confusion between nerve bundles
What is the somatosensory cortex? Where is it? What can a lesion of the somatosensory cortex result in?
Post-central gyrus
Required for precise localisation and discrimination of stimuli (somatotrophic representation) - each body area has specific cortical representation (and specific modalities)
e. g. sensory homunculus
note: relative size of each area is reflective of the degree of sensory acuity associated with the body area
Relays to other cortical and subcortical areas —> choice to respond to stimulus taken at cortical level (based on previous memories of stimulus, whether the stimulus is pleasant/unpleasant, whether action is needed, + input from limbic system & amygdala)
Lesion of somatosensory cortex (e.g. repeated epileptic events) =
- loss of two-point discrimination
- astereognosis = retains normal sensation but cannot recognise 3D objects by touch alone
What is ICU psychosis? Contrast this with institutionalised sensory deprivation and delirium.
ICU psychosis = patients in ICU experiencing anxiety, paranoia, auditory & visual hallucinations, disorientation, agitation etc. (contributed to by dehydration, hypoxia, heart failure, infection, drugs, etc.)
Delirium refers to an acute brain syndrome.
People who are institutionalised (e.g. in care homes) can be affected by the resultant sensory deprivation.
Define perception.
Awareness of a stimuli and our ability to discriminate between different types of stimuli
Contrast the central process and peripheral process of the ascending tracts.
CENTRAL PROCESS = conduct impulses to the spinal cord
- terminate in the grey matter OR
- ascend into the white matter without terminating
PERIPHERAL PROCESS = convert sensory signals to electrical impulses and conduct to main axonal segment of neurone
