Overview
The sensory system acquires information about the environment, converts it into action potentials, communicates it around the body, processes it and gives it meaning so that responses are stimulated. This lecture follows that chain for somatic sensation: the receptors that transduce and code a stimulus, the receptive fields and adaptation properties that shape what is felt, the two ascending spinal pathways (dorsal column/medial lemniscus for discriminative sensation, spinothalamic/anterolateral for affective sensation), the thalamic relay, and the columnar organisation of somatosensory cortex where modality, intensity and location are finally represented. It closes with three clinical case examples for working through.
The sensory system and its sub-systems
Functions of the sensory system:
- Acquires information about the environment
- Converts information into action potentials
- Communicates information around the body
- Processes the information
- Gives it meaning, stimulating responses
Three sensory sub-systems:
- Somatic: from the body, e.g. touch, pain
- Visceral: from internal organs, e.g. blood pressure
- Special senses: from specific organs, e.g. vision, hearing
Components of the sensory system, as a sequence: stimulus -> sensor -> communication network -> processor -> response.
Two classes of somatic sensation
Discriminative sensation (TVP)
- Identify and localise superficial and deep Touch
- Vibration
- Muscle length and tension
- Joint position sense (Proprioception)
Affective sensation (compels to act)
- Potentially damaging stimuli: pain
- Warm and cold
- Erotic touch
Sensory receptors
- Several types: mechanoreceptors, chemoreceptors, thermoreceptors, photoreceptors, nociceptors.
- Each type is sensitive to a specific stimulus, so coding depends on which receptors are stimulated, i.e. receptors are modality specific.
- Receptors transduce the stimulus into an action potential.
- Structure is one of two forms: a specialised ending of a sensory neuron, or a specialised cell associated with a sensory neuron.
- Intensity is coded either by increasing the frequency of activation of the receptors, or by activating more receptors.
- Receptors distinguish a wide range of intensities because the relationship between stimulus energy and intensity of sensation is logarithmic (Weber-Fechner): “10x stimulus -> 2x sensation”. Related to this is the “just noticeable difference”. Pain sensation is the exception to this logarithmic relationship.
Receptive fields
- One sensory neuron is usually associated with more than one receptor, i.e. each neuron has a receptive field.
- Small receptive fields improve localisation and discrimination.
- Surround inhibition also operates. [slide does not elaborate]
- Structure of the afferent neuron (Vander Fig 9-4): peripheral terminals bearing receptors, spread over the area that is the receptive field, feed into a peripheral process; the cell body lies between the peripheral and central processes, which together make up the afferent neuron axon; the central process ends in central terminals within the CNS.
Sensory adaptation
- Some types of receptor adapt to a constant stimulus with decreased neuronal output.
- Speed of adaptation varies between receptors.
- On a recording of action potential response, spikes fire densely when the stimulus goes on, then fall in frequency over time to a low sparse rate that persists past stimulus off.
Sensory axons and communication pathways
Sensory axons vary: big or small, myelinated or unmyelinated, conduction velocities 1 to 100 m/s.
Communication pathways run in stages:
- Peripheral nerves and dorsal horn: local processing (reflexes) and “ascending” signals in the spinal cord
- Spinal cord pathways to thalamus
- Thalamus to cortex
Pathways cross over, so right body maps to left brain and vice versa.
The two ascending tracts
Dorsal column
- Carries touch, vibration, joint position (TVP)
- Fast
- Crosses at the medulla, so it ascends ipsilaterally before crossing
Anterolateral, aka spinothalamic
- Carries pain and temperature
- Slow
- Crosses at the level of entry to the spinal cord
Dorsal column / medial lemniscus pathway
Carries TVP and ascends ipsilaterally. Route: dorsal horn -> dorsal column -> cuneate and gracile nuclei -> arcuate fasciculus -> VPL of thalamus -> primary somatosensory cortex.
In more detail on the pathway diagram, from below upwards: lumbosacral level -> cervical level -> gracile fascicle and cuneate fascicle in the spinal cord -> gracile nucleus and cuneate nucleus, where internal arcuate fibres cross -> medial lemniscus -> ventral posterior lateral (VPL) nucleus of thalamus -> postcentral gyrus.
Somatotopy is preserved in the medial lemniscus. The body-map order (head, arm, trunk, leg, foot, marked by the “HAL”/“LAH” fibre-order labels) is maintained at each relay: spinal cord -> dorsal column nuclei -> medial lemniscus in the medulla -> medial lemniscus in the midbrain -> VPL of thalamus -> somatosensory cortex.
Thalamic neurons in VPL project to primary somatosensory cortex. On a lateral brain view the central fissure separates SI from SII and the posterior parietal association cortex.
Unconscious proprioception
Slightly different from the conscious pathway:
- Lower body -> Clarke’s nucleus (nucleus dorsalis) -> spinocerebellar tract -> gracile nucleus, with branches to the cerebellum
- Upper body -> cuneate nucleus, with branches to the cerebellum
On the medulla/cord diagram: upper body proprioceptive fibres enter via the cuneate fasciculus, synapse in the cuneate nucleus, send branches to the cerebellum and continue in the medial lemniscus; lower leg and abdominal fibres enter via the gracile fasciculus, synapse in Clarke’s column, and branch to the cerebellum via the dorsal spinocerebellar tract.
Spinothalamic / anterolateral pathway
- Carries affective sensation
- Ascends contralaterally
- Route: dorsal root -> dorsal horn -> cross in the anterior commissure -> anterolateral tract -> VPL -> primary somatosensory cortex
- Plus branches to brainstem, cortex and cingulate gyrus, serving arousal and emotion
In cross-section, the dorsal root ganglion neuron enters the dorsal horn and its second-order fibres cross ventrally near the central canal before ascending contralaterally toward the medial lemniscus region. At brainstem level the tract passes through the medial lemniscus, relays in the thalamus and projects to S1. Some thalamic intralaminar nuclei neurons of the spinothalamic pathway project to the cingulate gyrus, giving the emotional component of sensation; the anterior cingulate gyrus and cingulate gyrus are labelled alongside primary somatosensory cortex.
Cortical processing
- The primary somatosensory cortex lies in the parietal lobe (Vander Fig 9-6, which also marks auditory, taste and visual cortices).
- Parts of the body map to specific parts of cortex (somatotopy), and more sensitive body parts have greater cortical representation. Location of sensation is determined by cortical mapping.
- Inputs are integrated to produce complex sensations. “Association cortex” combines sensory modalities and allows more complex interpretation, with links to emotion, memory and other CNS areas such as the cerebellum.
How the three attributes of a somatic sensation are coded:
| Attribute | Coded by |
|---|---|
| Type (modality) of sensation | Receptors activated |
| Intensity of sensation | Number / frequency of receptor activation |
| Location of sensation | Cortical mapping |
Somatosensory cortex organisation
- Hierarchy: thalamic nuclei -> S1 -> S2 -> posterior parietal cortex.
- S1 contains 4 areas: 3a and 3b -> 1 and 2. On the postcentral gyrus cross-section these run in sequence 3a, 3b, 1, 2, with areas 4, 5 and 7 (posterior parietal) adjacent; the lateral view labels the central sulcus, postcentral gyrus, postcentral sulcus, lateral sulcus, S-I, S-II and the posterior parietal lobe.
- S1 handles crude sensation, shows somatotopy (Penfield) and uses parallel processing.
- S2 handles complex sensations.
- Post-parietal cortex does integration and interpretation.
Cortical structure: layers and columns
- The cortex has an ordered vertical structure.
- Layers: inputs to outputs.
- Columns: modality specific. The column schematics label fast-adapting finger tip, slow-adapting finger tip, slow-adapting middle of finger and fast-adapting middle of finger; a further schematic labels superficial fast adapting, deep fast adapting (vibration), deep slow adapting (pressure), cold, warm, and a separate column for wetness.
- Complex columns: higher order analysis, with inputs from several modalities, giving integration.
- Perception arises from this organisation.
- Selective attention involves suppression of afferent input by descending inhibition.
Case examples
The slides pose three cases without giving answers, so work them through against the pathways above.
- Hemisection injury. What are the likely sensory (and motor) changes arising from a traumatic injury that damages one side of the spinal cord and leaves the other half intact, e.g. a stabbing injury cutting through one side of the cord? [slide gives no answer]
- Syringomyelia, a rare disease in which the central canal of the spinal cord enlarges, typically at lower cervical or upper thoracic levels; the enlarging cyst (syrinx) compresses and damages adjacent parts of the cord. What sensory changes might the patient experience as the cyst enlarges? [slide gives no answer]
- Stroke case. A 67 year old man suffered a stroke. He could not speak and his right arm and leg were paralysed. After a few days his speech returned, though he had considerable difficulty using his tongue. Examination 6 weeks later showed spastic paralysis of the right arm and leg with increased muscle tone and brisk tendon reflexes. On protrusion the tongue turned to the left and its left half showed atrophy. There was no paralysis of the soft palate, pharynx or larynx. Pain and temperature sensation were normal over the entire body, but there was loss of the sense of posture and passive movement and impairment of tactile sensation over all of the right side of the body except the head. Which part of the CNS was damaged, i.e. what is the most likely site of the infarct, and why? [slide gives no answer]
A labelled cross-section of the medulla is supplied as a reference aid for case 3, with structures including the hypoglossal nucleus, dorsal vagal nucleus, nucleus tractus solitarius, reticular formation, spinal tract and nucleus of the trigeminal nerve, nucleus ambiguus, anterior spinocerebellar tract, lateral spinothalamic tract, tectospinal tract, medial lemniscus, olive and inferior olivary nucleus, arcuate nuclei, medial longitudinal fasciculus, vestibular and cochlear nuclei, inferior cerebellar peduncle, vagus and hypoglossal nerves, and the pyramid. The slide does not explain it further.
Self-test
- List the five functions of the sensory system.
- Name the three sensory sub-systems and give an example of each.
- Distinguish discriminative from affective somatic sensation, listing what each covers.
- Draw out the components of the sensory system as a sequence from stimulus to response.
- List the receptor types named in the lecture and explain what “modality specific” means for coding.
- Describe the two possible structures of a sensory receptor.
- Explain the two ways intensity of sensation is coded.
- State the Weber-Fechner relationship and name the exception to it.
- Define a receptive field and explain the effect of receptive field size on sensation.
- Describe what happens to neuronal output during sensory adaptation to a constant stimulus.
- Give the range of sensory axon conduction velocities and the axon properties that vary.
- Distinguish the dorsal column from the anterolateral tract by modality, speed and site of crossing.
- Describe the dorsal column / medial lemniscus route in order from dorsal horn to cortex.
- Explain what is meant by somatotopy being preserved in the medial lemniscus, and name the relay levels at which it is preserved.
- Describe the two routes of unconscious proprioception for upper and lower body.
- Describe the spinothalamic route in order, and state which branches account for arousal and emotion.
- Predict which sensations would be affected on which side if the anterolateral tract were interrupted on one side of the cord, based on where each tract crosses.
- For each of modality, intensity and location of a sensation, state what codes it.
- Describe the cortical hierarchy from thalamic nuclei onwards and the four areas of S1.
- Distinguish the functions of S1, S2 and posterior parietal cortex.
- Explain the difference between cortical layers and cortical columns, and what complex columns add.
- Explain the mechanism by which selective attention alters sensory input.
- Integrative: a patient can feel pain and temperature normally in one leg but has lost vibration and joint position sense in that leg. Using the two ascending pathways, explain which pathway is involved and on which side of the cord the lesion must be.
Answers
Reveal answers
- Acquires information about the environment; converts it into action potentials; communicates it around the body; processes it; gives it meaning and stimulates responses.
- Somatic, from the body, e.g. touch and pain; visceral, from internal organs, e.g. blood pressure; special senses, from specific organs, e.g. vision and hearing.
- Discriminative (TVP) covers identifying and localising superficial and deep touch, vibration, muscle length and tension, and joint position sense (proprioception). Affective sensation compels to act and covers potentially damaging stimuli (pain), warm and cold, and erotic touch.
- Stimulus -> sensor -> communication network -> processor -> response.
- Mechanoreceptors, chemoreceptors, thermoreceptors, photoreceptors, nociceptors. Each type is sensitive to a specific stimulus, so what is coded depends on which receptors are stimulated.
- Either a specialised ending of a sensory neuron, or a specialised cell associated with a sensory neuron.
- By increasing the frequency of activation of the receptors, or by activating more receptors.
- The relationship between stimulus energy and intensity of sensation is logarithmic: 10x stimulus gives 2x sensation. Pain sensation is the exception.
- One sensory neuron is usually associated with more than one receptor, and the area those receptors cover is its receptive field. Small receptive fields improve localisation and discrimination.
- Some receptor types respond to a constant stimulus with decreased neuronal output: firing starts dense at stimulus onset and falls in frequency to a low sparse rate. Speed of adaptation varies between receptor types.
- 1 to 100 m/s. Axons vary in size (big or small) and in whether they are myelinated.
- Dorsal column: touch, vibration, joint position; fast; crosses at the medulla. Anterolateral/spinothalamic: pain and temperature; slow; crosses at the level of entry to the spinal cord.
- Dorsal horn -> dorsal column (gracile and cuneate fascicles) -> cuneate and gracile nuclei, where internal arcuate fibres cross -> arcuate fasciculus / medial lemniscus -> VPL of thalamus -> primary somatosensory cortex (postcentral gyrus). It ascends ipsilaterally before crossing.
- The head/arm/trunk/leg/foot body-map order of fibres is maintained at every relay: spinal cord, dorsal column nuclei, medial lemniscus in the medulla, medial lemniscus in the midbrain, VPL of thalamus, and somatosensory cortex.
- Lower body: to Clarke’s nucleus (nucleus dorsalis), then spinocerebellar tract to gracile nucleus with branches to the cerebellum, entering via the gracile fasciculus. Upper body: to the cuneate nucleus with branches to the cerebellum, entering via the cuneate fasciculus and continuing in the medial lemniscus.
- Dorsal root -> dorsal horn -> cross in the anterior commissure -> anterolateral tract -> VPL -> primary somatosensory cortex. Branches to the brainstem, cortex and cingulate gyrus give arousal and emotion; some thalamic intralaminar nuclei neurons project to the cingulate gyrus for emotion.
- Pain and temperature would be affected, and because the tract crosses at the level of entry to the cord and then ascends contralaterally, the loss would be on the side of the body opposite the lesion.
- Modality is coded by which receptors are activated; intensity by the number and frequency of receptor activation; location by cortical mapping.
- Thalamic nuclei -> S1 -> S2 -> posterior parietal cortex. S1 has four areas: 3a and 3b projecting to 1 and 2.
- S1 handles crude sensation with somatotopy (Penfield) and parallel processing; S2 handles complex sensations; posterior parietal cortex does integration and interpretation.
- Layers run inputs to outputs in the vertical structure; columns are modality specific (e.g. fast-adapting and slow-adapting finger tip columns, cold, warm, wetness). Complex columns perform higher order analysis with inputs from several modalities, giving integration.
- Selective attention suppresses afferent input through descending inhibition.
- Vibration and joint position sense travel in the dorsal column, which ascends ipsilaterally and crosses only at the medulla, so a cord lesion causes loss on the same side as the lesion; pain and temperature are spared because the spinothalamic tract has already crossed at the level of entry and ascends on the other side.