Week 2 Flashcards
What is the spinal cord
It’s an assembly of the neuronal cell bodies and axons of nerves collected as bundles or fibre tracts, all housed together within the vertebral column
The spinal cord has characteristic cross sectional presentation
On axonal fibres of the spinal cord
They have 2 main functions:
-carry sensory information from the surface of the body and muscles to the brain
They are collectively known as ascending tracts
They are many fasciculi of ascending tracts
-carry motor commands from the brain to cell bodies of spinal motorneurones
-they are collectively known as descending tracts
-they are many fasciculi of descending tract
Grey matter
Grey matter is anatomically divisible into 2 or 3 horns depending upon level
Dorsal horn
Lateral horn (sometimes absent)
Ventral horn
Rexed laminae of grey matter of the cord
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 ventral
They are also known as “rexed laminae”
Rexed was the neuroanatomist who first identified spinal 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 of the cord (in brown)
lamina VII is considered to be the intermediate nucleus of the cord (light chocolate/mocha)
Laminae Viii to X are found in the ventral horn
Fasciculi and white matter of the cord
Axonal fibres of the spinal cord with common origins and destination tend to travel together as a tight clump
These clumps are known as
-Fasciculi
Or
-funiculi
A single clump is known as either a fasciculus or funiculus
White matter divided into 3 funiculi
-dorsal funiculus is found between midline and medial edge of the dorsal horn
-lateral funiculus is that found between the lateral edge of the grey matter of the cord
-ventral funiculus is found between the midline and medial edge of the ventral horn
Primary sensory neurons
A receptive field (skin, dermis, cutaneous)
Action potential normally initiated here
Axon fibre to cell body in dorsal root ganglia, synapse
Into dorsal horn
To brain via dorsal column pathway
To brain via spinothalamic tract
Primary afferent always excitatory
Primary afferents- different axon classifications
Aa, Ab, Adelta, C fibres
Aa- proprioceptors of skeletal muscle
Ab- mechanoreceptors of skin
A delta- pain temperature
C- temperature, pain, itch
Axons have distinct receptive fields
Each nerve axon innervates a specific receptive field within its Dermatome
Receptive field sizes vary, determining precision of localisation
Meissners corpuscles, Pacinian corpuscles
Receptive fields- size defined by 2 point discrimination
Precision of sensory localisation varies greatly across the body
Homunculus
Sensory receptors- signal transduction
Adequate (preferred) stimulus depends on the nerve ending
Threshold for signal- depends on the nerve ending
Pain is high threshold
Touch is low threshold
Firing rate proportional to stimulus strength
Transduction channel opening -> graded receptor potential (membrane depolarisation)
Physical stimulus (energy)
Adaptation of sensory receptors
Slow adapting or tonic (non adapting) receptor
Slow or non-adapting, important when maintaining information about a stimulus is valuable eg amount of stretch or pain
Fast adapting or phasic receptor
Fast adapting constantly changing stimulus is required
-useful when change in stimulus important
-stop paying attention when stimulus no longer important eg tactile (touch) receptors
Slow adaptation detects strength of stimulus
Fast adaptation detects how fast it changes
Cutaneous sensory receptors
Mechanoreceptors: touch, pressure, vibration
Thermoreceptors: hot, cold, temperature
Nociceptors: noxious stimulation (pain)
Cutaneous mechanoreceptors
Tactile (touch) receptors at the end of Ab fibre
Nerve ending has specialised sensory apparatus (organ)
Apparatus comprises a specialised cell
Structure determines function
Function indicates location
Information about surface texture, pressure and vibration
Four major types of mechanoreceptors
More superficial layers skin:
Merkels receptors (disk)
Meissners corpuscles
Deeper layers of skin:
-ruffinis corpuscle (ending)
-Pacinian corpuscle
Merkels receptor (disk)
High density in epidermis of digits and around mouth (50 per mm2)
Lower density elsewhere on glabrous skin
Slowly adapting
Sustained light tough
Sometimes termed Merkel cell-neurite complex
Apparatus is specialised keratocyte
Respond to initial skin indentation
And also sustained pressure up to several seconds in duration
Perception of form and texture
Meissners (or tactile) corpuscles
Found in the papillary dermis
Rapidly adapting
Constantly changing stimulus required
Light touch
Vibration
Constantly changing stimulus required
Eg detect putting on clothes but not the wearing
Involved in adjustment of grip force when objects are lifted
Ruffinis (or bulbous) corpuscles
Respond to lateral movement or stretching of skin
Deep touch
Apparatus is a network of collagen fibres
Deeper touch and stretch
Involved in monitoring grasped object slippage
Reflex adjustment of grip force
Pacinian corpuscles
Found in deeper layers of dermis
Rapidly adapting
Stronger stimulus- ‘deep’ touch, poke
High frequency vibration
Fully encapsulated nerve ending
“Onion” structure deforms to take up distortion due to a mechanical stimulus (pressure)
Very sensitive to vibration
Sometimes referred to as a lamellar vibration receptor
Hair follicle receptor
Light touch but activation in dermis
Rapidly adapting
Constantly changing stimulus required
Nerve fibre wrapped around hair
Hair deflection detected
Cutaneous thermoreceptors
Bare nerve endings
Slowly adapting sensory receptors
Two types (in general) adequate stimulus is either warmth of cooling
Poor indicators of absolute temperature
But very sensitive to changes in temp
Sense of temp comes from the comparison of the signals from warm and cold receptors
Thermoreceptors channels
The transient receptor potential family
-non specific cation channels, nerve ending sensitivity dependent on what transducer channels are expressed
-TRPM8: cold channels open 10-38 degree celcius max at 25, also opened by menthol
TRPV3/4: warm channels open at 29-45 degrees max 45 to different transducer channels
Cutaneous nociceptors
Bare nerve endings
Non adapting sensory receptor; high threshold
Adequate stimulus must be capable of damaging tissue
2 types:
-high threshold mechanoreceptors (Adelta fibre)
-well localised ‘pricking’ pain
-polymodal nociceptor- sensitive to mechanical stimulus, damaging heat (46C) and noxious chemicals (C fibre)
-poorly localised burning pain
-TRPV1 an identified transducer channel (also opened by capsaicin in chillies)
Proprioception, position sense
Muscle spindle
Golgi tendon organ
Detects the mechanical status of the musculoskeletal system
Proprioceptors provide information about:
-joint position
-muscle length
-muscle movement
-acceleration
-tension/force
The muscle spindle- detection of length and acceleration
Specialised muscle fibres in a fibrous capsule
Termed intrafusal fibres (cf extrafusal, power generating fibres)
Group 1a afferents wrap around central (sensory) portion
Firing contributes to muscle tone
Stretch sensitive —> increasing firing
Golgi tendon organ- detection of muscle tension
Located at the junction of muscle and tendon
Innervation by group 1b sensory afferents
Position in series with muscle
Sensitive to tension generated by contraction
(Cf spindle in parallel, sensitive to length)
Central pathways
Lemniscal pathway (dorsal columns)
Large sensory Ab fibres
Touch vibration, two point discrimination, proprioception
Spinothalamic pathway (anterolateral tracts)
-small sensory Adelta and C fibres
-pain and temperature (some touch)
The skin as an organ of sensation
It’s the largest organ of the body
It forms the body’s universal external envelope
It has an average surface area of 2 SQ M
It’s a major sensory apparatus of the body
Both low and high threshold afferents
It’s sensory territories are organised Dermatomal slices
Muscle sense organs
Muscles are richly supplied with millions of tiny muscle sense organs
Proprioceptive organs
Proprioception is a special sense in its own right
It’s defined as the sense of awareness of body position in 3 dimensional space
Proprioceptive afferents: provide for sensory awareness of limb position in 360 space of the body
Muscle sense organs include
-muscle spindles
-Golgi tendons organ
-joint receptors
General sensory afferents of muscles
-free nerve endings: general sensation such as pain and temperature
Key spinal neural levels
Diaphragm C3-5
Biceps C5-6
Wrist C8-T1
Nipple T4
Umbilicus T10
Hip flexion L1-2
Knee jerk (or quadriceps contraction) L3-4
Knee flexion S1
Great toe L5 (foot Dorsiflexion)
Foot plantar flexion S1-2
Bladder L1-2 or S2-4
Anal sphincteric tone S2-4 (onuffs nucleus)
CNS mechanisms of sensation
The brain has full sensory representation of the bodys surfaces (or skin) and muscles
The left side of the bodys sensations are represented in the right brain
The bodys surfaces map on the brain is not isometric with body surfaces from which signals arise
Ascending tracts
They are fibre tracts of the spinal sensory system information is conveyed to the brain
They are divisible into 3 categories
Lateral spinothalamic tract
Dorsal column
Dorsal spinocerebellar tracts
Conscious sensation: processed in the sensory cortex, dorsal columns, spinothalamic tract
Non conscious sensation: processed in the cerebellum, clarke’s column (from dorsal column), DSCT, VSCT
General points
Sensory signals that are detected by the body are conveyed to the brain via a longitudinal sequence of three separate stages
The last stage gives rise to a projection to the sensory cortex of the cerebrum
- it can also give rise to awareness of sensation by the brain
-sensation is said to be conscious sensation if we are directly aware of the information
Sensation is said to be unconscious if the signals do not reach the cerebrum
Sensations giving rise to conscious sensation are conveyed differently from those not giving rise to conscious sensation
The first stage of spinal sensation
On reaching the spinal cord a primary sensory neurone then divides:
-1 axonal process terminates in the dorsal horn of its respective spinal segment
-another axonal process is sent to the dorsal horn of the spinal segment above
-another axonal process is sent to the dorsal horn of spinal segment below
- the axonal segments travelling to the spinal segments below and above travel in the posterolateral tract of lissauer
-the axon intended for its respective spinal neural segment then enters the respective dorsal horn
-it then terminates in the dorsal horn
-the lamina upon which it terminates is determined by the modality it represents
-pain fibres terminate in laminae I and II 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)
The second stage of spinal sensation
The secondary sensory neurone then sends its axon as follows:
-towards the midline
-towards the central canal of the spinal cord
-it then divides under the central canal
-it crosses the midline to emerge on the opposite side of the
The axon of the secondary sensory neurone then travels into the white matter of the opposite side
It joins the lateral funiculus
It joins the fibres of the lateral spinothalamic tract
The spinothalamic tract is in the lateral funiculus
Fibres of the spinothalamic tract travel to the thalamus (opposite side to that from which sensory signals rose)
Anterior white commissures
Axons of second order sensory neurones crossing the midline comprise the “spinal decussation”
Spinal decussation fibres are known as anterior commissures
They are also known as spinal Arcuate fibres
Spinal sensory decussation occurs for all 31 neural levels of spinal cord
The second stage of spinal sensation 2
Secondary sensory neurones of the spinothalamic tract ascend the medulla, pons, midbrain and end in the thalamus
In particular they travel to the ventral posterolateral VPL nucleus of the thalamus
In the VPL, spinothalamic tract fibres terminate on 3rd order sensory neurones
The third stage of spinal sensation
Axons of 3rd order sensory neurones of the VPL ascend further to terminate in the post central gyrus of the cerebral cortex
Axons of 3rd order sensory neurones of the VPL travel via the internal capsule
Organisation of the spinothalamic tract
The spinothalamic tract is somatotopically organised throughout its full extent
This means that the spatial mapping of the bodys surface is preserved within the tract
Fibres arising from the lowest part of the body ascend dorsolaterally within the spinothalamic tract
Fibres arising from cervical cord ascend ventromedially
Application of ascending tracts anatomy
Syringomyelia- fluid filled cyst (syrinx) forms in spinal cord
Brown sequard syndrome- neurologic syndrome from hemisection of the spinal cord
Cortical representation of sensory and motor modalities
The cerebral cortex represent that highest centre for processing of sensory information and for commanding of motor commands
Sensory info from the body is first represented in the cerebral cortex in the primary sensory cortex
Voluntary motor commands of muscles are issued through the motor cortex
The primary motor cortex is separated from the primary sensory cortex by the central sulcus of the cerebral cortex
The brain in the control of movements
Normally movements of skeletal/somatic muscles are initiated voluntarily
Most movements originate in the cerebral motor cortex
Somatic muscles of the body are controlled by dedicated neurones of the “motor strip” of the frontal cortex
Neurones of the “motor strip” are grouped according to skeletal muscles they command
The motor map of cortical neurones can be reconstructed into a motor homonculus
The brain in the control of movements 2
The brain commands muscles to either:
-rest or contract
-levels of contraction vary depending on the requirements of the body
-resting muscle tone: maintenance of posture (minimal contraction)
-displacement of joints: movements (large forces required)
The brains motor commands of muscles of the body are decussated
Irrespective of site of origin of motor commands, skeletal muscles of the left side of the body are commanded to move or rest by the right cerebral motor cortex vice versa
Descending tracts
The brain commands muscles through descending motor tracts
Descending tracts are formed from axons of neurones with cell bodies in motor nucleus of the brain
Cortical descending tracts originates in cerebral cortex: primary motor cortex (in pre central gyrus)
Other in subcortical areas- non cortical descending tracts
-midbrain, brainstem, pons, medulla
The importance of descending tracts
The brain gives its only commands either to glands, smooth muscle or skeletal muscle
The brains outputs are conveyed through a group of neurones collectively known as efferents
Motorneurones are divisible into two broad categories: motorneurones with cell bodies in spinal cord (cerebral cortex, diencephalon, brainstem)
Neurones of somatic motor system
Motorneurones with cell bodies in the brain (upper motorneurones) are in hierarchically higher centres of the CNS (eg cerebral cortex or brainstem)
Lower motorneurones with cell bodies in the spinal cord or cranial nerve motor nuclei (lower motorneurones) are in hierarchically lower centres of the CNS (eg facial nerve motor nucleus or Rexed lamina IX of the spinal cord)
Upper and lower motorneurones
An upper motor neurone is that motor efferent whose entire axon resides within the central nervous system
A lower motor neurone is that motor efferent whose axon travels in peripheral nerves
The 2 broad classes of upper motorneurones
Upper motor neurones with cell bodies in the cerebral cortex are called cortical efferents
Upper motorneurones with cell bodies in subcortical brain areas such as the brainstem are called brainstem efferents or bulbar efferents
Cortical upper motorneurones
These are also called pyramidal upper motorneurones
They comprise what is known as pyramidal system of the brain (motor)
Lesions of the pyramidal motor system impair movements in a characteristic way that distinguishes this particular system from
Non cortical upper motorneurones
Extrapyramidal upper motorneurones
Extarpyramidal motor system of the brain
Lesions of the extrapyramidal motor system impair movements in a characteristic way that is different from that arising from injuries/defects arising from dysfunction of the pyramidal system
Descending fibre tracts and the spinal cord
Axons of upper motor neurones travelling in the spinal cord are collectively known as descending fibre tracts
Descending tracts are found either anterior (ventral) or lateral funiculus of the spinal cord
Ascending and descending tracts are mixed within the ventral or lateral funiculi
