Session 3: Somatic Sensation and the Sensory Pathway Flashcards
Describe the receptors of afferent neurones
Afferent neurones have receptors at their peripheral endings, which continuously inform the CNS of the conditions within the external or internal environment.
A receptor may be the bare terminal of the afferent neurone, or a specialised structure at the nerve ending e.g. a Pacinian corpuscle or it may be a separate receptor cell such as a rod cell in the eye, which makes a synaptic connection with the afferent nerve.
These receptors generate action potentials which are conducted to the CNS.
Afferent information may a) enter consciousness to give rise to our perception of the world around us, b) lead to an efferent output altering motor behaviour, c) change our state of arousal and/or d) may be stored in memory for future reference.
Sensory receptors can be classified by structure (e.g. free nerve endings or encapsulated), by location (e.g. exteroreceptors and interoreceptors) or by stimulus type (e.g. thermoreceptors)
What is meant by Sensory Modality?
We are responsive to a variety of stimuli – the stimulus modalities – e.g. heat, light, chemical change, mechanical pressure etc. Qualities are the subdivisions of the modality such as taste can be divided into sweet, sour etc.
Sensory receptors respond preferentially to one modality e.g. mechanoreceptor, photoreceptor, chemoreceptor, nociceptor (pain), although exceptionally they may be activated by others.
The eye responds preferentially to light although with a blow to the head, a mechanical stimulus, we may “see stars”.
Sensation therefore is dependent on the type of receptor activated.
Adequate stimulus: the stimulus to which a receptor has the lowest threshold. Hence, light is the adequate stimulus from the eye.
Loss of specificity with strong stimuli.
What is meant by Sensory Transduction?
When a stimulus impinges upon a receptor, it causes a change in its membrane potential to change in the permeability to ions of the receptor membrane, which is proportional to stimulus intensity.
- Stimulus evokes change in the permeability to ions of the receptor membrane.
- Causing receptor potential (movement of ions across the receptor membrane)
- Transmitted into nerve fibre as a generator potential
- Action potentials (nerve impulse) propagated in primary afferent fibre to CNS.
This change affects the action potential generating regions of the nerve, to set off a series of action potentials, which encode information about the intensity, and duration of the stimulus.
How is Sensory Transduction achieved?
This is achieved by two mechanisms:
- Frequency coding whereby strength can be determined by rate of action potential stimulus (ie. Increased, higher frequency of action potential means a stronger stimulus).
- Activation of neighbouring cells, as a stronger stimulus will activate neighbouring cells (but to a lesser degree compared to frequency coding).
As all afferent nerves transmit information in the form of action potentials, knowledge of the nature and location of the stimulus depends upon the connections afferent nerve fibres make within the CNS.
What is meant by Receptor Adaptation?
Some receptors – slowly-adapting tonic receptors – respond continuously to the presence of an adequate stimulus e.g. joint and pain receptors
Others – phasic receptors – rapidly adapt - respond maximally and briefly so that the action potential frequency in the afferent nerve decreases during a maintained stimulus e.g. touch receptors.
Such receptors are sensitive to change in stimulus energy.
What is meant by Sensory Acuity?
where is the stimulus?
Each sensory neurone responds to a stimulus only if the stimulus falls within its receptive field. Acuity is the precision by which a stimulus can be located.
The size of the receptive field varies with receptor density. E.g. we have very few touch receptors on the trunk so each one has a large receptive field. On our fingertips however, we have a high density of receptors with small receptive fields. Receptive fields overlap with neighbouring receptive fields.
Sensory acuity can be determined by 3 factors (lateral inhibition in the CNS, two point discrimination, synaptic convergence and divergence):
How does lateral inhibition in the CNS enhance acuity?
Lateral inhibition in the CNS involves the primary afferent fibre whose receptive field centre is closest to the point of stimulus will produce more action potentials than those on the periphery. This works by the primary afferent neurones synapsing with inhibitory interneurons as well as the second order neurones, which act with a negative impact on the adjacent second order neurone. As a consequence, action potentials in the second order neurones whose receptive fields are towards the periphery of the stimulus field are more strongly inhibited, and therefore produce fewer action potentials, than the cell with its receptive field in the centre. These differences in receptor density are reflected in the topographical map of the primary somatosensory cortex.
What is meant by two-point discrimination? And how can convergence and divergence be used to describe acuity?
The smaller the receptive field in a region the higher our acuity i.e. our ability to locate the stimulus accurately and to distinguish between two closely applied stimuli (two point discrimination).
Two point discrimination is the minimal distance required to perceive two simultaneously applied skin indentations; depending on the part of the body involved depends on the two point discrimination distance. This distance is determined by the density of the receptors (3-4 times greater in fingertips than other areas of hand, also very dense in the lips) and the size of the specific neuronal receptive field (which is the area in which the receptors from one area cover e.g. fingertips 1-2mm, palm 5-10mm). Two-point discrimination is also determined by psychological factors e.g. fatigue and stress.
Convergence and divergence can be used to describe acuity. Convergence (e.g of several first order neurones to one second order neurone) decreases acuity whereas divergence causes amplification (more actions projecting up to the second order neurone)
Describe the coding of Sensation
Property of Stimulus: Mechanism of coding
Stimulus Modality: Type of receptor stimulated and specific sensory pathway to the brain
Rate of change: Receptor adaptation
Location: Size of receptive field – enhanced by lateral inhibition and the projection to a particular area of the somatosensory cortex
Intensity: Frequency of action potentials and the number of receptors activated
Describe how we feel sensation at the thalamic level
Thalamic Level
- Crude localization and discrimination of stimuli (not precise)
- Highly organized, very precise projections to cortex (to post-central gyrus of the parietal lobe)
- Thalamic lesions e.g. stroke, can create thalamic overreaction (exaggerated response – much stronger than normal e.g. pain is much more exaggerated than normal). The projections can get confused or mixed up – some patients can have cross-contamination of their senses e.g. synaesthesia.
Describe how we feel sensation at the Somatosensory Cortex, including what is the Sensory Homunculus?
Sharp localisation and full recognition of qualities of modalities – very well structured pathways
Cortical columns
Somatotopic representation – every body area has specific cortical representation
The sensory homunculus (contralateral representation): the relative size of each area is reflective of the degree of sensory acuity associated with that body area
The cerebral cortex has full sensory representation of the body’s surface/skin
Most sensory signals will result in conscious sensory awareness of the signal and recognition of the part of the body from which it is arising.
The left side of the body is represented in the right cerebral cortex
The right side of the body is represented in the left cerebral cortex
The cerebral cortex can be said to house a distorted sensory clone of its respective individual. This the Sensory homunculus.
What is Perception?
Perception is our awareness of stimuli and our ability to discriminate between different types of stimuli – analysing the information
- Perception detection: what has changed?
- Magnitude estimation: how large is the change?
- Spatial discrimination: where is it?
- Feature abstraction: generally, what type of stimulus?
- Quality discrimination: specifically, what type of subtype of stimulus is it?
- Pattern recognition: is this familiar, unfamiliar, significant to me?
Somatosensory cortex:
- Somatosensory cortex relays to other cortical and sub-cortical areas.
- Choice to respond to stimulus taken at the cortical level e.g. is it a nice stimulus? Or is it a threat?
- Lesion of sensory cortex can lead to loss of submodalities: e.g. in repeated epileptic events, loss of two-point discrimination, astereognosis (unable to work out what they are touching if they have their eyes closed).
What is Sensation?
Sensation can be defined as a ‘conscious or sub-conscious awareness of an external or internal stimulus’. The senses can be divided down into general senses, of which can be somatic (from the body of tactile – (touch, pressure, vibration), thermal – (warm, cold), pain or proprioception). General senses can also be visceral (from the internal organs). General senses are different to special senses (smell, taste, vision, hearing, balance).
Describe the Somatosensory System
The somatosensory system can be described by 3 neurones that synapse, described as primary, secondary and tertiary neurones.
First order neurones (or the primary afferent neurones) pass the electrical stimulus from the action potential, synapse with the second order neurone in the spinal cord, when then ascends to the brain to synapse with the third order neuron. Second order neurones can decussate therefore some pathways are contralateral.
The initial sensory receptors of first order neurones can vary yet all act to pick up stimuli and send to the CNS; the most common type is the free nerve ending, yet others include encapsulated nerve endings or those with additional specialised cells for activation. Primary afferent fibres have cell bodies in the trigeminal or dorsal root ganglion.
Descibe the types of stimulus modalities and senosry receptors
Stimulus modalities include light, touch, temperature, chemical changes (e.g. taste) etc. Qualities are a subdivision of modality e.g. taste can be dvided into sweet, sour, salt etc. Sensory receptors are modality specific (to a point e.g. trauma to the eye can lead to you seeing light – because the stimulus is so strong to the rods cones).
Those found in the skin used for sensation are Merkel discs (vibration, pressure and texture) found in the epidermis, Meissner’s Corpuscle (light touch and vibration), Riffini Corpuscle (temperature) and Pacinian Corpuscle (vibration and pressure).
The skin contains a whole variety of specialised receptors for different types of stimuli, most of which are found in the dermis therefore any third degree burn will destroy all of these pain receptors such that the patient will not feel any pain initially with a full thickness burn.
What are Proprioceptors
Proprioceptors are used to determine where the body is in space and allows the body to be able to function properly. Two main types of receptors are present for proprioception to function effectively:
Muscle spindle tells about the length of the muscle. Receptor density is low in large muscle for coarse movement and high in muscles for fine movement e.g. hand and extraoccular muscles.
Golgi tendon organ measures the tension in the tendons
What happens in Shingles?
Herpes zoster, the virus which normally causes chicken pox, infects neurones of the peripheral nervous system particularly cells in the dorsal root ganglia.
After an initial infection with chicken pox, the virus may remain dormant, often for many years, before it is reactivated in some way to produce the condition known as shingles.
Shingles increases the sensitivity of dorsal root neurones triggering burning, tingling sensations, which are extremely painful – the skin becomes scaly and then blisters.
As the virus is usually restricted to only one or two dorsal root ganglia, the body areas affected by shingles reflect the dermatomal distribution of these dorsal roots.
What is required to ‘feel’ the sensation?
‘Feeling’ the Sensation: for a sensation to be felt once the action potential has reached the CNS, there are two major regions of the brain involved:
Thalamic level is used for crude localisation and distribution of the stimuli and produces highly organised projections to the cortex. Any thalamic lesions (e.g. following a stroke) can result in ‘thalamic overreaction’.
Somatosensory cortex (post-central gyrus) receives projections from the thalamus and acts to localise and recognise the qualities of the modalities received; this is found in the post-central gyrus.
The somatosensory cortex will then relay the information to other areas and allow further choice to respond to the stimulus to be taken.
What are the two main functions of neuronal cell bodies?
The spinal cord is an assembly of neuronal cell bodies and axons of nerves collected as bundles or fibre tracts all housed together within the vertebral column.
Neuronal cell bodies have two main functions:
Serving local functions of a neuronal segment (i.e. neural level of the spinal cord)
- Local reflexes (brain not involved)
- Sensory functions of the neural segment to which they belong (dermatome)
- Supplying muscles of their local neuronal segment (myotome)
- They also receive and carry out commands to enact movements through supplying myotomes
Relaying Sensory Information to the Brain
- Some cell bodies receive and in turn send sensory signals to the brain
- Such neurones have long axons that collect as fibre bundles that travel together to higher centres of the brain
- Collections of axons with similar origins and destinations are known as fibre tracts or fasciculi
- Such axons are said to “ascend” because their signals are sent to higher levels of the neuraxis (e.g. Cerebral Cortex), hence they are also known as “ascending fibre tracts”
What are the functions of the axonal fibres of the spinal cord?
Carry sensory information from the surface of the body and muscles to the brain. (There are many fasciculi of ascending tracts)
Carry motor commands from the brain to cell bodies of spinal motoneurones
They are collectively known as descending tracts
There are many fasciculi of descending tracts
Describe the locaiton of the grey matter and white matter
The grey matter is always found in the centre of cord
- H-shaped configuration
- The precise shape of the H changes rostrocaudally
White matter is always found more superficial than grey matter. It fully surrounds the grey matter – makes the remainder of the surface area of the spinal cord.
Describe the 3 horns of the grey matter
Grey Matter is anatomically divisible into 2 or 3 horns depending upon level:
- Dorsal Horn
- Lateral Horn (sometimes absent)
- Ventral Horn
What is the Grey Matter made of?
Grey Matter is made from collections of neuronal cell bodies of the spinal cord. Cell bodies of neurones with similar functions occur together as layers or clusters.
A single layer stretches the full horizontal extent of the grey matter
Some clusters do not stretch fully in the horizontal plane
Neuronal layers or clusters are arranged from dorsally to ventrally
There are 10 discrete layers of cell bodies making up the grey matter. They are labelled I to X, Lamina I being dorsal and Lamina X being ventral. Aka “Rexed Laminae”
What are the Rexed Laminae?
Each lamina of the grey matter of the cord is equivalent to a neuronal nucleus. Each lamina contains cell bodies of neurones with common functions. Some have discrete names
The dorsal horn contains Rexed Laminae I-VI
Lamina VII is considered to be the intermediate nucleus of the cord
Laminae VIII to X are found in the ventral horn.
What are the funiculi of the spinal cord
The white matter of the spinal cord is divisible into 3 funiculi (snglr. Funiculus). Spinal funiculi are conveniently divided by the grey matter of the spinal cord.
Dorsal Funiculus is found between the midline and medial edge of the dorsal horn
Lateral Funiculus is found between the lateral edge of the grey matter of the cord
Ventral Funiculus is found between the midline and the medial edge of the ventral horn.
Describe the two systems through which somatosensory information is carried from body to brain
Somatosensory information is conveyed from the body to the brain through one of two systems:
- Spinal sensory system: sensory signals are carried by the sensory division of spinal (or segmental nerves). There are 31 pairs of segmental nerves carrying sensory signals from the body surface to the brain.
- Cranial Sensory System: sensory signals are carried by sensory roots of cranial nerves (the trigeminal nerve carries the majority of somatosensory signals).
- NB: visceral sensation is not carried out by the somatosensory system.
How are the somatic senses percieved? How is unconconscious proprioception percieved?
Sensory stimuli in the environment generate afferent impulses in peripheral sensory nerves, which are transmitted into the spinal cord or brainstem.
Touch, pain, temperature and proprioception (position sense) are the general or somatic senses.
Sight, hearing taste and smell are the special senses.
With the exception of some forms of proprioception, the somatic senses are perceived consciously. For this to take place, information has to pass beyond the spinal cord or brainstem to reach the “highest” level of the brain, the cerebral cortex.
Unconscious proprioception is a function of sub-cortical structures
Each tract carries a specific sensory modality and as they ascend most tracts will decussate from one side of the CNS to the other, meaning that each side of the body sends sensory information to the opposite side.
What are the ascending tracts/
The ascending tracts are the fibre tracts by which sensory information is conveyed to the brain. They are the pathways through which impulses are passed from neurone to neurone until they reach the cortex.
Sensory signals of the body give rise to 2 categories of sensation – so ascending tracts can divisible in this way:
- Conscious: pain, temperature, crude touch
- Non-Conscious Sensation: tactile sensation, muscle length, muscle tension, joint position, joint angle etc
- The successive, sequential neurones are referred to as a first order neurone, second order neurone etc.
- Sensation is said to be conscious sensation if we are directly aware of the information
Sensation is said to non-conscious if we are not directly aware of the information.
Sensations giving rise to conscious sensation are carried to the brain separately and differently from those not giving rise to conscious sensation.
Describe the first order neurones
The neurone of the first stage transduces and encodes external stimuli into electrical impulses
- First order neurones have their cell bodies in the dorsal root ganglion (or trigeminal ganglion for cranial nerves).
Peripheral segment of pseudo-unipolar axons of Primary sensory neurones convert sensory signals from source into electrical impulses and also conduct these to the main axonal segment of the neurone.
These sensory signals are conducted through the dorsal root of the spinal segment of interest.
The central segment of the pseudo-unipolar axons of Primary sensory neurones conduct the impulses into the spinal cord.
The last stage (third order neurones) gives rise to awareness of sensation by the brain
Where do the ascending tracts reach in the brain? And what sensory modalities do they carry?
Each tract carries a specific sensory modality e.g. the dorsal columns convey information about fine touch and proprioception.
As they ascend most tracts cross (decussate) from one side of the CNS to the other, so that each side of the body sends sensory information to the opposite (contralateral) side of the brain.
The destination of the ascending tracts for conscious sensations is the postcentral gyrus – the primary sensory cortex or somatosensory cortex – in the parietal lobe. On the way to the cortex, most ascending tracts (there are some exceptions) pass through the thalamus.
- Dorsal Columns: Fine touch, proprioception
- Lateral Spinothalamic: pain, temperature
- Anterior Spinothalamic: crude touch, pressure
- Spinocerebellar: unconscious proprioception
- Cuneocerebellar: unconscious proprioception
Describe the beginning of pain and temperature inputs travelling via the ascending tracts
On entering the spinal cord, a primary sensory neurone divides, sending branches to adjacent spinal segments. Its branches will reach a) 3-4 spinal segments rostrally, b) 3-4 spinal segments caudally and c) the main axon will terminate in the dorsal horn of its respective spinal segment (e.g. Nerve of T6 Dermatome in T6 neural segment of cord).
Axonal branches travelling to adjacent spinal segments rostral and caudal to the neural level of entry travel in the Posterolateral Tract of Lissauer. AKA Funiculus of Lissauer
Describe the first stage of spinal sensation (pain and temperature)
The First Stage of Spinal Sensation
- The unbranched main axon enters the grey matter of its respective dorsal horn and then terminates in the dorsal horn.
- The lamina upon which it terminates is determined by the modality it represents.
- Pain fibres terminate in Laminae II and III (Substantia Gelatinosa) of dorsal horn or in Laminae III and IV of the dorsal horn. Laminae III and IV are known as Nucleus Proprius.
- The in-coming axon terminates on a cell body of a secondary sensory neurone (in nucleus proprius).
What happens to the axonal branch not terminating in the cord level of its ermatome?
The axon branch stays on the same side as its origin.
It ascends still as primary sensory neurones (without terminating). It particular it ascends in either the fasciculus gracilis or fasciculus cuneatus of the dorsal columns (of funiculus)
There are 2 distinct columns. A medial column is known as the Fasciculus Gracilis and a lateral column is known as the Fasciculus Cuneatus.
Proprioceptive signals arising from neural segments above T6 travel in the dorsal funiculus as Fasciculus Cuneatus.
Proprioceptive signals arising from neural segments below T6 travel in the dorsal funiculus as Fasciculus Gracilis
These two fasciculi terminate in the medulla on the same side as the origin of proprioceptive signals. Fasciculus Gracilis terminates in nucleus Gracilis of the medulla. Fasciculus Cuneatus terminates in nucleus Cuneatus of the medulla.
Fasciculus Gracilis and Nucleus Gracilis process proprioceptive information equivalent to that carried and also processed by the Cuneate system.
As such they can be lumped into one as representative of each other’s attributes.
Describe the Dorsal Columns
The dorsal white column of the spinal cord consists of the fasciculi gracilis and cuneatus.
- The fasciculi gracilis exists at all levels of the spinal cord, containing long fibres from the lower limbs and is located medially.
- The fasciculi cuneatus exists only at C1-C8 and T1-T6 spinal levels, containing fibres from the upper limb.
- These neurones send information regarding fine touch and conscious proprioception to the CNS.
- The neuronal axons’ cell bodies are located in the dorsal root ganglion and the axons then pass on the ipsilateral side in either the fasciculi gracilis or cuneatus, with information of fine touch and any proprioceptors in the joints.
- The first order neurones then synapse with the second order neurones in the nucleus gracilis or cuneatus (in the medulla) on the ipsilateral side.
- Second order neurones then decussate and run in the medial lemniscus. This tract then ascends through the pons and midbrain to synapse with the third order neurone located in the thalamus.
- Axons of third order neurones terminate in the medial aspect of the somatosensory cortex.
- Damage to this column results in symptoms appearing on the ipsilateral side at and below the level of the spinal cord region. Symptoms include loss tactile sense, proprioception and an ability to identify objects placed in their hand.
Describe the Lateral Spinothalamic Tract
The lateral spinothalamic tract transmits pain and temperature sensations.
Neurones entering this tract have their cell bodies located in the dorsal root ganglion and synapse with the second order neurone in the dorsal horn.
The axons then decussate via the ventral white commissure ascend in the contralateral tract.
From medial to lateral, fibres ascend cervical, thoracic, lumbar and sacral.
Damage to the lateral spinothalamic pathway results in a complete loss of pain and temperature sensation on the contralateral side of the body, at and below the damage.
Loss of pain however occurs about one or two levels below the lesion because the first-order afferent fibres in the zone of Lissauer ascend or descend one or two levels before making their synaptic link with the second order neurone.
Due to the sacral and lumbar fibres lying dorsolateral to the thoracic and cervical fibres, any expanding tumour or lesion in the grey matter will affect the thoracic and cervical fibres first, resulting in the sacral and lumbar fibres having intact pain and temperature sensation at least initially, known as sacral sparing.
Describe the Anterior Spinothalamic Tract
The anterior spinothalamic tract is used for transmission of crude touch and pressure; its pathway is exactly the same as that of the lateral spinothalamic tract.
As both the lateral and anterior spinothalamic pathways ascend in the anterolateral system of the ascending tracts, they can be collectively known as the anterolateral system of ascending tracts.
Dscribe the Anterior Spinocerebellar tract
The spinocerebellar tracts are used for unconscious proprioception, with the neurones peripheral processes acting on muscles and tendons (muscle spindle fibres and Golgi spindle organs respectively).
The cell bodies of the first order neurones for the anterior spinocerebellar tract are located in the dorsal root ganglion and synapse with the second order neurones in the dorsal horn. The second order neurones decussate and ascend in the spinocerebellar tracts, ascending through the medulla to the pons. The fibres then decussate again in the pons before terminating in the cerebellum. Consequently there is no third order neurone.
This tract acts to convey information about whole limb movements and postural adjustments to the cerebellum.
Damage to this tract results in loss of proprioception and coordination to the contra-lateral lower limb.
Describe the Posterior Spinocerebellar Tract
The first order neurones for the posterior spinocerebellar tract synapse in the dorsal horn and carry information on unconscious proprioception for the upper limb. Second order nerve fibres then ascend on the ipsilateral side and terminate in the medulla. As with the anterior spinocerebellar tract, there is no third order neurone.
Damage to this tract results in loss of proprioception and co-ordinated movement to the ipsilateral side of the lesion.
Summarise the ascending tracts?
