Sensory-motor system Flashcards
What is the somatosensory pathway?
Sensor–> Sensory input > integration (CNS)—> motor output (PNS) –> Effector
What is a sensory receptor
encodes the nature and the localization, intensity and duration of a stimuli
What happens when a sensory receptor is stimulated?
- Stimuli applied to the skin changes the chemistry of the receptor à changes in permeability of the receptor’s cell membrane à neural signal
- Cell bodies are located in the dorsal root ganglion
What are some types of receptors ?
- Receptors respond to specific type of stimuli (e.g. pressure, temperature) – main types of sensory receptors are classified by modality; specificity is determined by the nerve endings; size of the receptive field ( the extend of the body are that elicits receptor response); rate of adaptation (fast vs slow) – response of a receptor decreases with stimulus repetition
* Mechanoreceptors – mechanical pressure, touch * Thermoreceptors – temperature * Nociceptors – pain
What are fast adapting receptors?
Fast adapting receptors (FA) – responds when stimulus appears/disappears but not during steady state
What are slow adapting receptors?
Slow-adapting receptors (SA) – active during the period when the stimulus is in contact with the receptive field
What is sensory threshold and what are the types?
- Absolute threshold – minimal intensity to perceive a stimulus
- Differential threshold – minimal difference in intensity between two stimuli to be perceived (Weber’s Lwas: Delta stimuli = KS or minimal intensity difference = constantintensity of the stimulus)
Sensory (afferent) neurons in the periphery (e.g. skin)
Sensory cells within the central nervous system
Includes touch, pressure, pain, body senses, muscle tension and joint position, visceral state
What are some examples of sensory receptors?
- Free end nerves –(no specialized ending structure) – sensitive to pain and temperature change
- Pacinian corpuscles – largest an deepest (looks like onions); adapt rapidly and respond to sudden displacements of the skin; DO NOT respond to constants pressure
- Markel’s disks and Ruffini’s endings – adapt slowly; respond to gradual skin indentation and skin stretch respectively
How does the information travels from the sensory receptors to the brain?
- Information travels from receptors to spinal cord via dorsal root ganglion (bundles of nerve fibers from the peripheral nerves and become spinal nerves when they approach the spinal cord); contains cell bodies of the somatosensory receptors;
* Speed depends on the diameter of the fibres and has functional significance * High speed – large myelinated fibres; small myelinated fibres * Low speed – smaller unmyelinated fibers
What is the structure of the spinal cord?
- pinal cord – peripheral white matter and central gray matter; WM (columns/funiculi) and GM into horns; relay point for sensory info, contains ascending afferent pathways and descending motor tracts; contains interneurons
* GM – interneurons (association neurons), cell body of motor neurons, ganglia cells, unmyelinated axons; shape and size depends on the size of the spinal nerve roots *Association neurons connect sensory and motor neurons; small and locally projecting * WM – fibre tracts (ascending and descending); divided in three funiculi (columns) * Posterior (dorsal) * Lateral * Anterior (ventral)
What is the longest neuron in the human body?
The longest neuron in the human body is dorsal column neurons that originate in the toes
What is the structure of the spinal cord?
- 31 pairs of spinal nerves, where each vertebrae has:
* Corresponding segment of the spinal cord * Segments with a name and number that indicate their position in the corresponding spinal nerve * Cervical (neck) region * Thoracic (chest) region * Lumbar (middle back) region * Sacral (lower back) region * Each receives peripheral sensory innervation through the dorsal root
What is the dermatome?
nnervated by peripheral nerves that enters the spinal cord at specific segments; ordered to create a map that is also used as a diagnostic tool for localizing injuries at the spinal cord or dorsal roots;
What are the main sensory myelinated tracts in the white matter?
Dorsal-column medial lemniscla (MLS) and anterolateral system (AS)
Describe the role and structures of the dorsal column-medial lemniscal (MLS) system?
- Dorsal column – medial lemniscal (MLS)– touch, body position and movement
* Wide diameter axons and few synapses * Information is conveyed quickly to the brain * Tactile and kinesthetic information carried out is used for planning and executing rapid movements * The dorsal tracts * Ascend ipsilaterally to the medulla * Decussate in medulla * To ventral posterior nucleus of the thalamus through medial lemniscus * To primary somatosensory cortex through internal capsule
Describe the role and structures on the anterolateral system?
- Anterolateral system (AS) – pain, temperature, light touch, tickle and itch
* More synapses * Information is slower to reach the brain * Information is mostly though thermos receptors and nocicepros, poorly localized * The dorsal spinal tract *Decussate in spinal cord (spino-thalamic tract) ventrally * To posterior nucleus of thalamus but also brain stem (spino-reticular, spino-encephalic) * To primary somatosensory cortex
Compare and contrasy the differences between the MLS and AS systems
1. Type of information conveyed: • tactile & kinaesthetic vs temperature & pain 2. Where they decussate: • medulla vs spinal cord 3. Type of fibers: • MLS: large, myelinated • AS: thinly myelinated and unmyelinated 4. Where do they travel: • MLS: mainly to thalamus then cortex; • AS: also brain stem (spinothalamic, spinoreticular, spino-mesnecefalic paths) 5. Laterality: • MLS: controlateral only; • AS: also ipsilateral
What do lesions to the somatosensory paths do to patients?
if both are transected by a spinal injury the patient will have no body sensation from below the level of the cut
How does the information from the face skin travel?
Information from face skin and head travels through trigeminal cranial nerve and it does not enter the spinal cord as it happens for sensory information from the rest of the body
What is the role of the thalamus in the sensory system?
- Processing station in the centre of the brain
- Major sensory relay station
- Nearly all pathways projecting to the cerebral cortex do so via the thalamus
- Distinguished: medial, ventral and anterior nuclei separated by a y-shaped white matter structure, internal medullary lamina
Describe the organisation and structures of the somatosensory cortex
- Primary somatosensory cortex (post central gyrus) (S1) – BA1, BA2, and BA3 – contralateral input from thalamus
- Secondary somatosensory cortex (S2)– pre insular – part of BA2 – ipsilateral input from S1
- Somatic sensory association – posterior-parietal cortex – BA5 and 7 – ipsilateral and contralateral from S1 and S2
- The primary and secondary somatosensory cortex are somatotopically organized
- Somatosensory cortex is organised in columns (perpendicular to the surface of the cortex)
- Neurons in one column are activated by the same type of receptor (e.g. pressure)
- Neurons in one column respond to activation of the same skin areas
- A particular type of stimulus is dominant in different areas
What does it mean that the somatosensory cortecies are somatotopically organised
- Different places of the cortex represent different parts of the body
- Different parts of the body are represented in the cortex in different sizes
- Greatest proportion of the somatosensory cortex is dedicated to receiving input from the parts of the body used for finer tactile discrimination (lips, tongue, hands)
What are the afferent aand efferent connections to and from the primary somatosensory cortex?
- Thalamus à layer IV
- Associative connection within the same hemisphere with layers II and III
- Amongs BA1,2,3a and 3b
- Between S1 and S2 * Between S1, S2 abd posterior parietal areas (BA5 and 7)
- Between S1 and S2 and M1 (BA4)
- Callosal connections mainly with layers II and II to the same cortical areas
- Projections mainly layers V and VI
- To basal ganglia
- To thalamus from pyramidal cells of VI layer
- To nuclei of dorsal column – from pyramidal cells of V layer
- To dorsal horn of spinal cord – from pyramidal cells of V layer
what does damage to the somatosensory system does?
- Sensory loss and spinal cord syndromes
* Phantom limb phenomenon - occurs when nerves that would normally innervate the missing limb cause pain
What does the motor system consist of?
- Consists of muscles and motor neurons as well as pyramidal and extrapyramidal systems.
- Responsible for voluntary movements, reflex and rhythmic movements such as walking.
What are the three types of muscles?
- Skeletal – bundles of stripped muscle cells; Attached to the bone and work in opposition; Made out of muscle fibres which consists of myofibrils; Myofibrils can contract thanks to flaments of actin and myosin
Smootin muscles - in the walls of internal structures and blood vessels
Cardiac muscles - heart
What receptors do muscles and joints contain?
- Length (via spindles) – composed of small muscle fibres innervated by gamma motor neurons
- Tension via golgi tendon organs – when the muscle fibres contract they cause the axon to fire
What are the two types of motor neurons?
They rule the activity of the skeletal muscles and are divided by upper motor neurons and lower motor neurons
Describe the characteristics of upper motor neurons
Part of the CNS; cell body in the brain or spinal cord; voluntary motor activity through descendent motor pathways send to lower motor neurons; involved in planning, initiating movements and postural control
Describe the characteristics of lower motor neurons
Part of the PNS, cell body is in the enterior horn, axon in the anterior root, spinal nerve, axon terminal – neuromuscular junction; effector – the skeletal muscle; link between CNS and skeletal muscles; classified based on the type of muscles they innervate
How are motor neurons classified?
They are classified based on the muscle they innervate and there are two distinct types:
- Alpha motor neutons – extrafusal muscle fibers (muscle contraction)
- Gamma motor neurons – intrafusal muscle fibres (muscle spindels) – body position
What does damage to the upper motor neurons leads to?
Upper motor neuron syndrome (motor cortex or descending axon) - paralysis; hyperreflexia, spasticity
What does damage to the lower motor neurons leads to?
Lower motor neuron syndrome –(grey matter of spinal cord or brainstem or pheripheral nerve or cranial nerve) – paralysis; weak or lack of reflexes; reduced muscle tone
What are monosynaptic and polysynaptic reflexes?
Monosynaptic reflexes - single synapse between afferent and efferent neurons
Polysynaptic reflex - have two or more synapses - flexion and crossed extensor reflexes
What are the cortical descending motor pathways?
Pyramidal system (cortico-spinal tract) extrapyramidal system
Describe the pyramidal system pathway and features?
- Pyramidal system (cortico-spinal tract)
Originates in the cortex;
mainly contralateral at the pyramid decussation;
main link to voluntary movement
Starts at the cortex at layer 5 and ends at alpha motor neurons of the spinal cord and brain stem;
executes isolated voluntary movements; lesions cause upper motor neuron paralysis
Pathway is trhough the:
motor cortex –>internal capsule–>
midbrain via cereberal peduncles–>pons–>medulla—>in lateral WM column of the spinal cord—>axon of lateral CS tract enter the GM to synapse the anterior horn
Describe the function and structures of the extrapyramidal system
- mainly in subcortical structures; contralateral and ipsilateral regions of the spinal cord; links to reflexive and stereotypical movements and posture
* Strats at the basal ganglia and the cerebellum and links to the pyramidal system via the thalamus and cerebellum; responsible for initiation and planning of movements (BG) and monitoring, smoothing and terminating of movements (cerebellum); lesions to BG causes bradykinesia in the cerebellum ataxia (balance, coordination and speech disorder)
Describe the architecture of the motor cortex
- Primary motor area (BA4)
- Premotor area (BA6- lateral)
- Supplementory motor area (BA6 superior)
- Sends information to the spinal cord via the corticospinal tract; also premotor and SMA project to M1
- Premotor and SMA – planning and coordination
- Motor areas – somatotopically organised
What is the cytoarchitecture of the motor cortex?
- No layer IV
- Layer 5 – giant pyramidal cells (Betz cells) – largest neurons in the human CNS and form the corticospinal tract; synapse directly onto motor neurons in the ventral horn of spinal cord and spinal interneurons
- Corticospinal tracts originates from M1 (50%) and BA6, while small amount of fibres in the somatosensory primary arears
- Afferent information from
- Periphery (via thalamus)
- Cerebellum (via thalamus)
- Globus pallidus (via thalamus)
- Somatosensory cortex and associative areas
What is the role of the basal ganglia in the motor system?
- BG output ascends towards the cortex via the thalamus
* Descending fibres towards the brainstem from the extrapyramidal tract
What is the role of the cerebellum in the motor system?
organized in two hemispheres; outer surface is called cerebellar cortex and consists of Purkinje cells
Compare the somatic vs the autonomic sensory system
- Somatic – voluntary and other movements
- Autonomic – controls automatic and visceral function
- Symphatetic -à fight or flight
- Parasympathetic -à rest and digest
- Autonomic – controls automatic and visceral function
What happenes following damage to the association motor cortex
no severe deficit in basic movements but in higher order sensory analysis or motor planning (e.g. apraxia – inability to execute learned movements, despite the desire and ability to execute them)
What syndromes does damage to the spinal cord causes?
- total section (paralysis and anaesthesia below that section)
* Over the cervical spine - Quadriplegia – upper and lower limbs * Over the lumbar – paraphlegia (lower limbs) * Over the sacral section – incontinence * Flaccidity – marked loss of muscle tone * Analgesia – lack of pain information * Anaesthesia – lack of thermos information
