somatic sensation - Department of Physiology

SOMATIC SENSATION
Definition: Sensation from the skin, muscles, bones, tendons and joints.
Initiated due to activation of a number of distinct somatic receptors that respond
specifically to changes in heat, cold, touch, pressure, limb position, limb movement,
or pain.
Receptors for visceral sensations (i.e. from internal organs) are of same type as those
for somatic sensation. Some organs, e.g. liver have no sensory receptors.
Skin receptors
Information from somatic receptors enters the CNS and synapse on neurons that
ascend and go primarily to the somatosensory cortex via the brainstem and thalamus.
1
The Anterolateral Tract
Pathways cross over to opposite side of spinal cord to ascend.
e.g. from pain or temperature receptors.
The Dorsal Tract
Pathways ascend ipsilateral and then cross over in brainstem.
e.g. from vibration or joint position receptor.
Thus, somatic receptors on left side of body go to the right cerebral hemisphere
and vice-versa, both for anterolateral and dorsal tracts.
2
Suppression of Pain Transmission
3
Pathway terminations in Somatosensory cortex
Somatosensory cortex
In general, densely innervated areas of the body (e.g. fingers) are represented by the
largest areas of the somatosensory cortex. Although the areas in this topographical
map appear to be well defined, there is some anatomical overlap.
Interpretation of different stimuli
1) Touch-pressure
Interpretation of a mulititude of different stimuli (e.g. deep pressure, vibration) occurs
due to the processing of information from multiple receptor types located in the skin.
4
Receptor types
Pacinian corpuscles: found deep in the dermal layers of both hairy and glabrous skin,
and have relatively large endings that are widely spaced. Thus, relatively large
receptive field per cell. Upon stimulation pacinian corpuscles rapidly evoke action
potentials, they adapt and stop firing throughout the stimulus, and then fire action
potentials at the end of the stimulus. This rapid adaptation allows rate and pressure
detection of the stimulus.
Ruffini’s endings: found deep in the dermal layers of both hairy and glabrous skin, and
have relatively large endings that are widely spaced. Thus, relatively large receptive
field per cell. Continue to respond during stimulation, i.e. Slowly adapting.
The differences in fast and slow adaptation between Pacinian and Ruffini relate to the
absence of a capsule on the end organ of the Ruffini, not to differences in the
electrophysiological properties of the cells.
Meissner’s corpuscles: approx. 1/10 the size of Pacinian corpuscles. Located in
ridges of glabrous skin, e.g. raised parts of fingerprints. Small receptive field
compared with Pacinian and Ruffini. Rapidly adapting.
Merkel’s disk: located in the epidermis. Consist of a nerve terminal and a flattened
non-neural epithelial cell. Small receptive field. Slowly adapting.
Hair follicle receptors: hairy skin only. Can be either rapidly adapting or slowly
5
adapting.
2) Pain
Receptors, termed “nociceptors”, respond to stimuli that is about to cause tissue
damage, such as excessive mechanical strain, excessive heat, and chemicals released
from nearby cells and damaged tissue, such as neurotransmitters or prostaglandins.
These factors activate receptors on the nociceptor membrane.
Nociceptors are divided in to 4 classes: mechanoreceptors, thermal receptors,
chemoreceptors, and polymodal receptors (these respond to all 3 stimuli).
Nociceptors are similar to other receptor types but generally respond to higher levels
of stimulus. e.g. general thermal receptors respond to temperatures < 45 oC, whereas
nociceptive thermal receptors respond to temperatures > 45 oC.
Nociceptors have either free nerve endings or nonencapsulated end organs. They are
not found in bone or brain tissue.
Following tissue damage and nociceptor stimulation, the threshold for subsequent
stimulation is decreased and thus, sensitivity to that stimulus is increased. This
process is termed “hyperalgesia”. In some cases, hyperalgesia can induce a change in
nociceptor sensitivity from unimodal to bimodal, e.g. a mechanoreceptor can become
sensitive to thermal stimuli. Because hyperalgesia can modify the pain sensation, the
pain felt shortly after an injury can be very different to that felt initially.
3) Posture and movement
Proprioceptors provide information about body position. Generally, this information
is not sent to the higher centers in the brain, but is processed in the spinal cord via
reflexes. There are 2 classes of muscle proprioceptors:
i) Muscle spindles, that monitor length and rate of stretch of muscles.
ii) Golgi tendon organs, that adjust force by monitoring muscle tension.
Additional information is provided by mechanoreceptors that are located in the
connective tissue of joints.
NOTE *Skeletal motor control is covered in detail later in this series of lectures*
4) Temperature
2 types of receptors in skin:
i) Warmth receptors: respond to temperatures between 30 oC and 43 oC. Action
potential discharge rate increases with elevation of temperature.
ii) Cold receptors: action potentials are stimulated by small decreases in temperature.
6
HEARING
Principles of Sound
Sound energy is transmitted via vibration of molecules.
Disturbance of air molecules causes a zone of compression, where molecules are
closer together and pressure increases, and a zone of rarefaction, where molecules are
further apart, and pressure becomes lower (Fig. a, b, and c).
A sound wave consists of rapidly alternating high and low pressures (Fig. d).
Sound waves/vibrating tuning fork
Amplitude: difference between low and high pressures. Related to the loudness of the
sound.
Frequency: speed at which the high and low pressures interchange. Determines pitch.
Human ears hear sound frequencies between 20 – 20,000 Hz, but are most sensitive to
those between 1000 Hz – 4000 Hz. 1 Hz = 1 per second.
Humans can distinguish 400,000 different sounds, and can filter unwanted noise in
order to distinguish important sounds.
7
Transmission of sound to the inner ear (cochlea)
The human ear
Complex shapes within the pinna help to amplify and direct sound waves into the
external auditory canal.
Air molecules push against the tympanic membrane, which is stretched across the end
of the external auditory canal, causing it to vibrate at the same frequency as the sound
wave. A zone of compression (i.e. high pressure) pushes the membrane inwards, and
vice versa. The distance the membrane moves is related to the loudness of the sound.
The tympanic membrane separates the external auditory canal from the middle ear
cavity, which is maintained at atmospheric pressure through the eustachian tube,
which ends in the pharynx.
3 small bones in the middle ear: malleus, incus and stapes, amplify sound waves to
vibrate the oval window of the fluid filled inner ear, or cochlea. Amplification (~ 15
– 20 times) occurs primarily because the oval window is smaller than the tympanic
membrane, but also due to the lever action of the bones. 2 small skeletal muscles can
modify amplification by: i) adjusting the tension of the tympanic membrane and, ii)
moving the position of the stapes.
8
The cochlea
The middle ear bones
and the cochlea
Receptor cells are located in the cochlea, which is a fluid filled, spiral-shaped passage.
It is almost completely divided lengthwise by the cochlear duct. The oval window
separates the middle ear from the scala vestibuli. The scala tympani is separated from
the middle ear by a second membrane called the round window. The scala vestubuli
and the scala tympani meet at the helicotrema, which is at the end of the cochlear
duct.
Movement of the oval window by the stapes creates waves of pressure in the fluid of
the scala vestibuli, vibrating the cochlear duct.
Transmission of sound vibrations
9
Membranes and
compartments of
inner ear
On one side of the basilar membrane of the cochlear duct is the organ of Corti, which
contains the receptor cells for the ear. The basilar membrane vibrates with pressure
changes across the cochlear duct. Sounds of a particular pitches vibrate the basilar
membrane at different distances from the oval window. High frequency sounds
vibrate the basilar membrane nearest to the oval window, low frequency sounds
vibrate the membrane furthest away from the oval window.
10
Hair cells of the organ of corti
The receptor cells are called hair cells, and they transform pressure waves into
membrane potentials. Hairlike stereocilia protruding from one end of the receptor cell
are in contact with the tectorial membrane.
Movement of the basilar membrane in response to a sound, bends the sterocilia which
activates ion channels in the plasma membrane, inducing membrane depolarization of
the hair cell. Depolarization releases glutamate onto adjacent neurons, whose axons
form the cochlear nerve. Glutamate activates action potentials in these cells. The
louder the sound, the higher the frequency of action potentials generated by the nerve
fibers.
Efferent fibers from the brainstem can dampen the hair cells to protect them from loud
sounds.
11
Neural pathways in hearing
The cochlear nerve is a component of cranial nerve VIII. Neurons of the cochlear
nerve enter the brainstem and synapse with interneurons. Different arrival times
and intensities of inputs from each ear determine the direction of the sound
source.
Information from the brainstem goes to the thalamus and auditory cortex for
processing.
12