Download Neuroscience 5a – Touch and Proprioception

Neuroscience 5a – Touch and Proprioception
Anil Chopra
1. List the major somatosensory modalities
2. Define Proprioception and list the main types of receptor
3. Explain the terms: receptor, stimulus threshold and intensity, adaptation,
receptive field and lateral inhibition as applied to somatosensory systems.
4. Demonstrate on a diagram of model of the brain the 1° and 2° sensory cortex
and the parietal association cortex
5. Describe the pathway of touch/proprioception for information from the body
and face
6. Explain the somatotopic organisation of the touch pathway and sensory cortex
7. Outline the main circumstances in which somatosensory deficits may occur
8. Explain the clinical use of 2-point discrimination and why sensitivity caries
between regions of the body.
Somatosensory System:
involves the information coming
from the skin, muscles joints and
ligaments.
Touch: includes fine touch (light
discriminative), vibration and
pressure.
Proprioception – Joint position,
muscle length, muscle tension –
acts to provide position sense
(Mechanoreceptors, carried via
the dorsal columns pathway to
the somatosensory cortex)
» Proprioception provides a
position sense to the body by
measuring such things as
joint position, muscle length
and muscle tension
» All receptors for touch and
proprioception are
mechanoreceptors
Receptors
The receptors for touch are found as peripheral nerve terminals of axons of dorsal root
ganglion cells. They are all mechanoreceptors and they fire action potentials when
either nerve endings or connective tissue structures on the nerve endings become
deformed. As the endings become more and more deformed, depolarisation occurs
until the threshold level is reached. At this point the action potential is fired. Different
nerve endings and the structures that relate to them have different threshold levels.
There are 2 main types of receptors for touch and pressure:
Fast adapting receptors: these are mainly receptors
associated with touch, movement and vibration and fire
action potentials only when stimuli change. They include:
 Pacinian Corpuscle – pressure and vibration.
 Meissner’s Corpuscle – light touch
 Some free nerve endings
Slow adapting receptors: these are generally receptors
that are associated with pressure. They constantly fire
action potentials and change in frequency depending on
the strength of the stimulus. These include:
 Merkle’s Corpuscle – touch
 Ruffini Corpuscle – pressure
Receptor – Modified terminals of the peripheral axons of the primary sensory organs
which transduce a mechanical signal (deformation) into electrical signals. All
receptors for touch and proprioception are mechanreceptors
The function of mechanoreceptors depends on:
 The type of modified terminal is present. See above for different types.
 Degree of specialisation – from free nerve endings to elaborate accessory
structures
 Location – e.g. various layers of the skin, around hair shaft etc. Muscle spindle
detects changes in muscle length, Golgi organ tendons detect change in muscle
tension
 Physiological properties – activation threshold determines sensitivity (all touch
and proprioception receptors have a low threshold). May be slow or fast
 Generally the more complicated the receptor the more specific the stimulus
Stimulus Threshold and Intensity – The stimulus threshold dictates at what level of
stimulation the receptor fires, the lower the threshold the more sensitive the receptor.
The intensity of the signal is coded by the frequency of firing, i.e. the faster firing the
bigger the signal. The amplitude of the signal does not change. Usually firing
frequency is related to Log(stimulus intensity) i.e. if stimulus increases 10 fold, firing
frequency doubles
Adaptation – Dictates whether the receptor is constantly firing or only fires in bursts;
slow adapting or fast adapting. Mechanoreceptors used for touch and proprioception
are a mixture of both. The decrease in receptor sensitivity that occurs in the presence
of a maintained stimulus, getting rid of the noise.
Receptive Field – Is the number of receptors innervated by one sensory nerve. The
larger the receptive field size, i.e. more receptors on each nerve, the lower the acuity.
The density of receptive fields varies all over the body. The size of the field also
dictates the size of region around the neuron that will respond to a stimuli
Somatosensory System
Generally the somatosensory system is made up of 3
neurones.
(1) The first starts from the skin/muscles/joint and has
its cell body in the dorsal root ganglion. It then
goes from the dorsal root ganglion, up the dorsal
column (gracile and cuneate faciculus) of the
spinal cord and synapses in the brainstem
(specifically the gracile and cuneate nuclei in the
medulla). The inputs from the face come from the
trigeminal nerve (V).
(2) The second projects through the brain stem
anterolaterally (to the opposite side, passing up
the medial leminiscus) to synapse in the thalamus.
(3) The third goes from the thalamus to the appropriate
part of the somatosensory cortex.
NB: the pathway is somatotopic so the inputs are
arranged in a way that resembles the body map from
dermatome to cortex with information added laterally
as you go up.
Each limb has its own termination pathway in the
somatosensory cortex; here the information from
sensory stimuli becomes conscious sensation:
» Somatosensory I (SI) – is the primary somatosensory area. The body map is
distorted according to the relative density of input from different parts of the body,
i.e. the face and hands have the greatest area of processing. The response of
neurons in SI varies, and most only respond to a particular modality such as
pressure, vibration etc.
» Others respond to abstract properties such as movement or shape. SI has
projections to SII and the posterior parietal cortex
» Pain is processed in the SI
» Somatosensory II (SII) – receives intracortical projections from SI
» Posterior Parietal Cortex – combines different types of somatic sensation with
other modalities, this is necessary for interpretation of spatial relationships
Damage of the somatosensory system can lead to anaesthesia and parasthesia, but few
neurological diseases affect the somatosensory system specifically.
Lateral inhibition - enhances differences between adjacent inputs and so sharpens
the resolution of a signal. Collateral branches of neurons make inhibitory synapses
with surrounding relay neurons. E.g. if six receptor fields are stimulated, the middle
one will carry the highest discharge frequency due to lateral inhibition, will have a
higher chance of inhibiting its neighbours and so activating a relay neuron. This
increase in intensity of signal allows the cortex t more accurately pinpoint the location
of stimulation. Somatotopic organisation is maintained through the entire pathway
from dermatomes to cortex.