Week 6 NEURO
How does the somatosensory cortex show somatotropy?
Area 1 - from medial to lateral - have a representation of the body - Those areas requiring sensitive touch (e.g. the hand) have a larger area of the cortex - important for our survival - Area 3 - area 1 blocks it off around the face
Given the organization of the pathways carrying somatosensory and proprioceptor information, what would be the nature of the losses in terms of perception of touch and proprioception from the right thigh if the dorsal column-medial-lemniscal were damaged on the left side of the thoracic region of the spinal cord? What is the damage is on the right side?
Damage on left - No loss Damage on right - All loss • Need to know where the pathway decussates relative to the site of the damage
What does it mean by cortical circuits showing plasticity?
Cortical circuits show plasticity- Reorganize in response to differential stimulation/training (increase in sensory input) • Area within primary somatosensory cortex responsible for finger 3 - if the monkey loses finger 3, the area within the cortex that was used for processing this information is lost - instead starts processing information from the fingers beside it (fingers 2 and 4) - the brain tissue isn't dormant, but changes function • E.g. greater amounts of stimulation to finger 2 than finger 5 - brain shows plasticity where the area within the cortex processing information from finger 2 increases at the expense of the area of finger 5
Where do the sensory inputs of the secondary somatosensory cortex come from and how do they all relate to one another?
Information does go further in terms of our conscious mind • Greatest contribution from the thalamus is to 3b which talks to the secondary somatosensory cortex • Parietal areas 5 and 7 provide cortex with information about the position of the body before skeletal muscle movements • Higher level processing Picture Explained: • area 3b receives bulk of input rom VP complex and provides particularly dense projection to areas 1 and 2 - establishes a functional hierarchy - area 3b serves as an obligatory first step in cortical processing of somatosensory information • Neurons in SI also project to parietal areas posterior to area 2 (especially areas 5a and 7b) - receive direct projections from area 2 - supply inputs to neurons in motor and premotor areas of the frontal lobe
Where are golgi tendon organs found and what is their role?
Proprioception • Golgi tendon organs sense the strain in the tendon - These both protect the muscle and provide a sense of the position of the muscle (and therefore, the limb) - Low-threshold mechanoreceptors in tendons - Help provide information on muscle tension - Formed by branches of group Ib afferents, distributed among the collagen fibers that form the tendons - Each GTO is arranged in series with a small number (10-20) of extrafusal muscle fibers
What nuclei in the thalamus are involved in relaying sensory information? What are the 2 main functions of the thalamus in regard to this input?
All sensory input to the cerebrum is relayed through the thalamus • From body and posterior head - VPL (ventral posterior lateral nucleus) • From face - VPM (ventral posterior medial nucleus) • Proprioceptors - other areas within VP complex - More diffuse, but still routes through the thalamus • A good fliter Filters sensory input - Filters out information that doesn't really matter (e.g. the feeling of clothes on your body) Relays sensory input - Sends information to the correct part of the cerebral cortex
What are the 4 different types of sensory receptors in the skin?
1. Meissner Corpuscle 2. Merkel Cells 3. Ruffini Corpuscle 4. Pascinian Corpuscle
What are 6 different ways in which somatic sensory afferent impulses differ?
1. Type of stimulus and threshold strength - May respond to vibration or pressure - May require lots of pressure to open - E.g. mosquito on the skin - stimulates receptors - there is a very strong to avoiding mosquito bites by the body 2. Receptive fields (two point discrimination) 3. Rate of action potential adaptation to a sustained stimulus - How quickly does the receptor stop producing AP's 4. Diameter 5. Myelination 6. Conduction velocity - depends on the type of tactile information we are taking in
What are the 4 different areas within the primary somatosensory cortex?
Inputs relayed to primary somatosensory cortex (S1) from thalamus • Where third order axons go from the thalamus to • Has 4 areas within it • Area 3 split into 3a and 3b - receive different types of somatosensory information • Secondary somatosensory cortex (S2) is located within the cerebral hemispheres
What pathway does unconscious proprioception take to the brain, how many neurone are involved and is there a decussation?
Pathways: Unconscious Proprioception: Spinocerebellar • Doesn't arise through the thalamus into the cerebral cortex - Cerebellum is important in judging whether the movements we are trying to make is what the brain wants to happen • Sense of body position travels via first order afferent fibres from periphery to synapse with the second order sensory afferents. The signal then ascends to the cerebellum (for motor coordination) and to dorsal column nuclei (for motor reflexes) • Two orders only in this pathway - only 2 neurons in the chain • No decussation - Ipsilateral - stays on the same side as the receptors - Second order neuron cell body in Clarke's neuron - Doesn't enter cerebral cortex - subconscious information • Provides quality control for movements already initiated by the cortex • First-order neuron from muscle spindle • IMAGE - Left side goes to left side of cerebellum. There is also another part to the pathway which decussates which goes to the other half of the cerebellum - so both halves know what is going on in the body for quality control and to keep track of unconscious proprioception • from muscle spindles, golgi tendon organs and joint capsules
What sensory pathway takes tactile sensation from the body to the brain? Where are its first, second and third order neurone located and where does decussation occur?
Pathways: tactile sensation from body Dorsal column-medial lemniscal • First order afferents from peripheral receptors with cell bodies in the spinal ganglia - Sense vibration, etc. - Ascends through dorsal roots of spinal column to medulla • Second order afferents from synapse in the caudal medulla cross over (decussation) before ascending to the thalamus - Go from being ipsilateral to being contralateral • Third order afferents from the synapses in the thalamus ascend to the primary somatosensory cortex, S1 - Everything is routed through the thalamus in terms of sensory information - Information from left side of body goes to right side of cerebral hemisphere and vice versa - Everything but the face goes through this pathway
Which sensory pathway takes tactile sensation from the face to the brain? Where are the first, second and third order neurone located and where does decussation occur?
Pathways: tactile sensation from the face Trigeminothalamic • Also 3 orders in pathway - First order neuron has axons in skin of face - Cell body in trigeminal ganglion • Also crosses over, but at the level of the mid-pons - axon arises on contralateral side of the brain - decussation isn't in the medulla (unlike the rest of the body) • third order neuron arises from thalamus • Afferent fibres are carried by the trigeminal nerve, a cranial rather than spinal nerve • Trigeminal ganglion rather than a dorsal root ganglion
Where are muscle spindles found, what is their role and what are they made up of?
Proprioception • Muscle spindles sense the degree of tension across the skeletal muscle - Monitors tension - Individual muscle fibers are contractile, innervated by alpha motor neuron - extrafusal muscle fibers - Spindle is encapsulated - what the 'fusal' refers to - the muscle fibers outside th capsule - Intrafusal muscle fibers - within the capsule - innervated by motor neurons and afferents (group Ia and 2) - the fibers associated with proprioception - thick - bring information in quickly - Brain knows what the skeletal is up to - Ib afferents wind around the collagen of the tendon of the muscle - makes sure the tension in the tendon isn't so great as to cause damage - prevents muscle from elongating too much - Strain may tear tendon right off the bone if it is very severe -will inhibit muscle contraction before the damage occurs - Up to 120m/s conduction velocity - position of limb is immediately recognized by the brain - Found in all but a few striated (skeletal) muscles - 4-8 specialised intrafusal muscle fibers surrounded by a capsule of CT - intrafusal fibers are distributed among and in parallel with extrafusal fibers of skeletal muscle (true force-producing fibers) - sensory afferents coiled around central part of intrafusal spindle - when muscle is stretched, tension in inrafusal fibers activated mechanically gated ion channels in nerve endings, triggering action potentials - primary and secondary endings ϖ Primary endings - arise from the largest myelinated sensory axons (group Ia afferents) - have rapidly adapting responses to changes in muscles lneghts - transmit info about limb dynamics (velocity and direction of movement) ϖ Secondary endings - group II afferents - produce sustained esponses to constant muscle lengths - provide info about static position of limbs - Intrafusal muscle fibers are contractile - innervated by gamma motor neurons that lie in ventral horn of spinal cord ϖ Changes in tension of these fibers have significant impact on the sensitivity of the spindle afferents to changes in muscle length - the level of gamma neuron activity must be taken into account when reading the information from the proprioceptors - Large muscles generating coarse movements have smaller numbers of spindles - Relationship between receptor density and muscle size - Signal changes in muscle length
One "higher order" ability is that of distinguishing spatial and temporal sequences of stimuli. Blindfold the subject. Ask the subject to identify common objects placed in the hand by manipulating them rather than viewing them. Using an instrument with a narrow, blunt tip (e.g. a ball point pen with the lid on), trace out letters or numbers of the same size on a finger tip, on the palm of the hand and on the back, asking them to draw on paper the spatial pattern sensed. Note your findings. Q. Which types of tactile receptors would be activated by these activities? Q. Which parts of the sensory cortex enable identification of objects by touch alone?
Q. Which types of tactile receptors would be activated by these activities? Mostly Merkel, some Ruffini Q. Which parts of the sensory cortex enable identification of objects by touch alone? Areas 1, 2, 3a and 3b
What are receptive fields and how do they vary? How does this affect receptor density and sensitivity? What happens if there is overlap?
The area of the skin surface over which stimulation results in a significant change in rate of action potential production in a single afferent receptor • Vary in size, reflecting receptor density - Depending on how sensitive the area of the body needs to be - Thumb has a lot - Shoulder is very insensitive - lower density of sensory afferents - can't tell as correctly on the shoulder where the stimulus is coming from • Considerable overlap with neighbouring sensory afferents • AP frequency is highest when the stimulus occurs toward centre of field • Two-point discrimination - Is it one thing or 2 things stimulating the receptive field? - IMAGE - Green calipers - very close to centre of receptive field of B (where it has the highest stimulation) - will cause lots of AP's in B, whereas it is on the periphery of A and C (they produce not as many Ap's) - RED - brain won't be able to tell whether it is one stimulus or two - BLUE - A and C are receiving lots of action potentials and a low frequency in B - Brain can take this information in overlapping fields and determine that it is 2 stimuli - Wrist - have to separate 35mm before the brain can tell there are 2 stimuli • Single afferent sends out branches so it has an area in which it is responsible for any tactile stimulation in that area - Some areas can overlap - Depends on where the stimulus acts in the field - image - stimulus will cause a greater action if it happens towards the centre
What is adaptation and what are the 2 types of information it can communicate about stimulus? How does it differ between slowly adapting and rapidly adapting receptors?
• Ability of a nerve to continue or not continue to produce AP's when you have a sustained stimulus - E.g. putting your clothes on in the morning) • Communicates the static (sustained) nature of the stimulus (size, shape) Pain, proprioception; some tactile receptors • Communicates the dynamic nature of the stimulus - when it begins (and often, when it ends) - to provide a sense of stimulus movement Some tactile receptors • Burst of AP's when the change first occurs - Rapidly adapting only sense the change - look at when the stimulus starts and ends, but not in between, whereas the slowly adaptive continue producing AP's - With pain, need a continuing stimulus for the brain to respond • Proprioception - don't want to lose track of where your limbs are - need slowly adapting receptors - the neurons need to constantly let the brain know what is happening - Rapidly acting receptors will only fire when you move your arm, not when you are holding it behind your back
Somatotopically, what areas are 3a, 3b, 1 and 2 responsible for in the somatosensory cortex? What happens if there are lesions here?
• Areas 3b and 1 = from cutaneous receptors (meisner, merkle and persinnian corpusles - have sens of touch • Area 3a = from proprioceptors (raffini corpusles, spindles and golgi tendon organs) • Area 2 = from mixed tactile and proprioceptors • Areas 3b and 1 = from cutaneous receptors - Lesions here will cause loss of all tactile sense or loss of texture discrimination • Area 3a = from proprioceptors • Area 2 = from mixed tactile and proprioceptors • Lesion area 3b = loss of all tactile sensation from mechanoreceptors • Lesion area 1 = loss of texture discrimination • Lesion area 2 = loss of size and shape discrimination
What are Ruffini Corpuscle receptors sensitive to and where are they located in the skin?
• Contribute to proprioception (sense of where the body is in space) • In dermis
What are Merkel cells receptors sensitive to and where are they located in the skin?
• Highest spatial resolution with sensitivity to points, edges, and curvature and textures • In epidermis - E.g. feeling around in mud
Where are join capsule fibres located and what is their role?
• Joint capsule fibers: in joint capsules (proprioception of joint position, damaged in arthritic joints) - So we know what our fingers, for example, are doing in relation to one another - Damage in joints also affects joint capsule fibers - can be more likely to hyperextend a joint than a person with a normal proprioception of their joint
What types of stimuli can activate a sensory afferent fibre inside a receptor? What causes the depolarisation of the receptor and what determines whether an AP is generated?
• Mechanical stimuli such as touch influence mechanosensitive cation channels (here a Na+ channel) - Nerve fiber within a mechanoreceptor - has mechanically activated ion channels - when pressure or vibration is applied across the skin, the membrane will be stretched - will open ion channel - Gated by the stretch across the membrane - mechanically gated - Sodium will enter when stretched - receptor potentials (are graded potentials - relate to amount of stretch there is) - A stronger stretch causes a larger receptor potential - threshold for generation of AP needs to be reached - all or none AP - cause information to travel up the sensory pathway without the message running down - Non-degnerative - good for transmitting information across a long distance - Frequency of action potentials that a stimululs provokes gives an idea of its intensity • Mechanical deformation thereby depolarises the sensory receptor • If the resultant "receptor potential" is strong enough, an AP is generated
Outline a general sensory processign pathway in the body. Where is the input from and what structures does it travel through and to?
• Need to get information from the periphery of the body - mechanosensory receptors - Mechanoreceptors in skin to dorsal part of spinal cord and up to the thalamus, then the cortex - First-order neuron interacts with second-order neuron in spinal cord, third-order neuron extends to cortex • Input from receptors in the skin and muscles • Travels through pathways to thalamus • Processed in sensory cortex or cerebellum
What are Pascinian Corpuscle receptors sensitive to and where are they located in the skin?
• Respond to transient, high frequency stimulation • Thought to detect vibrations across the skin • Deepest - in subdermis/subcutaneous layer • In the complex layers of membranes
What are Meissner Corpuscle receptors sensitive to and where are they located in the skin?
• Sensitive to low frequency mechanical stimulation • Detects slippage between skin and an object held, to control grip • At junction between epidermis and dermis in the folds of the fingerprints - useful for holding something (if it starts to slip, they let the brain know that something is slipping out of the hand)
For the different fibres involved in proprioception, touch, pain and temperature and pain, temperature and itch, what are the: a) receptor types b) afferent axon types c) axon diameters d) conduction velocity
• The larger the diameter, the faster the conduction velocity • The greater the myelination, the faster the conduction velocity • The sense of touch is carried by Aβ fibres - Differ in terms of size (Ia and II are the thickest - are the fastest in conduction) - Not just diameter that is important, but myelination also - Salutatory conduction - AP jumping from node to node - increases conduction velocity - Touch - information travelling at 35-75m/s - very fast • Smaller somatic sensory afferents carry the senses of pain, temperature and itch • Proprioception is carried by the fastest afferents, types Ia & II
What types of information do the 3 trigeminal nuclei in in the brainstem relay?
• There are 3 nuclei - mesencephalic, sensory and spinal - Sensory and spinal (which is within the brainstem) nuclei carry touch, etc. - Sensory input through the trigeminal nerve is all cranial - Spinal nucleus - site of processing pain and temperature information - Mesencephalic nucleus - proprioception - Sensory nucleus - touch, pressure and vibration
