Case 2 Flashcards
describe the organisation of the spinal cord
- the arrangement of paired dorsal and ventral roots is repeated 31 times down the length of the spinal cord
- each spinal nerve, consisting of a dorsal root and ventral root axons, passes through a notch between the vertebrae - intervertebral foramen
- the 31 spinal segments are divided into 5 groups:
- cervical C1 - C8
- thoracic T1 - T12
- lumbar L1 - L5
- sacral S1 - S5
- coccygeal Cy1
what is a dermatome?
the area of skin innervated by the right and left dorsal roots of a single spinal segment, thus there is one-to-one correspondence between dermatomes and spinal segments
how do dermatomal maps vary?
among individuals
do the dermatomes overlap? why?
they overlap substantially, so that injury to an individual dorsal root does not lead to complete loss of sensation in the relevant skin region
what happens when a dorsal root is cut? what would have to happen to lose all sensation?
- when a dorsal root is cut, the corresponding dermatome on that side of the body does not lose all sensation
- the residual somatic sensation is explained by the fact that the adjacent dorsal roots innervate overlapping areas
- to lose all sensation in one dermatome, therefore, three adjacent dorsal roots must be cut
what is the overlap more extensive for? what does this mean?
touch, pressure, and vibration than for pain and temperature
thus, testing for pain sensation provides a more precise assessment of a segmental nerve injury than does testing for responses to touch, pressure, or vibration
what does the segmental distribution of proprioceptors follow and not follow?
it does not follow the dermatomal map but is more closely allied with the pattern of muscle innervation
what is knowledge of dermatomes essential for?
the clinical neurological evaluation of patients, particularly in determining the level of a spinal lesion
what are the white matter tracts often called?
columns
what is each half of the spinal grey matter divided into?
- dorsal horn
- intermediate zone (lateral horn)
- ventral horn
what are neurones that receive sensory input from primary afferents called? where do most of them lie?
- second-order sensory neurones
- most of the second-order sensory neurones of the spinal cord lie within the dorsal horns
what are two main functions of the spinal cord?
- to specify the characteristics of a stimulus in terms of its modality and position - function of grey matter
- to serve as a relay system for impulse propagation to and from the brain - sensorimotor information is relayed via the dorsal, lateral and ventral funiculi of the white matter
what are the two types of spinal cord cells?
- interneurones
- 97% of cells in spinal cord
- involved in modulating sensory input and motor output and make local connections with other cells in the spinal cord - projection neurones
- 3% of cells in spinal cord
- subdivded into:
- cells that give rise to axons of the ascending pathways, comprising of 1% population
- motor neurones, representing the remianing 2%, whose axons project from spinal cord to innervate skeletal muscles
how can spinal cord interneurones be further classified?
- inhibitory neurones
- these limit the RF (receptive field - area of skin where a stimulus can excite the sensory fibre) size or activity of other neurones
- these use inhibitory neurotransmitters such as Y-aminobutyric acid (GABA), glycine and enkephalin to regulate the activity of other neurones - excitatory neurones
- these cells use glutamate and various neuropeptides as neurotransmitters
- their stimulation evokes action potentials in other cells
describe the origination of the somatic sensation
- somatic sensation originates from the activity of afferent nerve fibres whose peripheral processes branch within the skin or in muscle
- the cell bodies of afferent fibres reside in a series of ganglia that lie alongside the spinal cord and brainstem and are considered art of the peripheral nervous system
- neurones in the dorsal root ganglia and in the cranial nerve ganglia are the critical links supplying the central nervous system circuits with information about sensory events that occur
- action potentials generated in afferent fibres propagate along the fibre and past the location of the cell body in the ganglia until they reach fibres’ synaptic terminals, which are located in various target structures of the central nervous system
- peripheral and central components of afferent fibres are continuous, attached to the cell body in the ganglia by a single process - for this reason, neurones in the dorsal root ganglia are often called pseudounipolar
what happens when afferent fibres do and don’t have receptors?
- they are often capsulated by specialised receptor cells (mechanoreceptors) that help tune the afferent fibre to particular features of somatic stimulation
- afferent fibres that lack specialised receptor potentials are referred to as free nerve endings and are especially important in the sensation of pain
what is sensory transduction?
process in which energy of a stimulus is converted into an electrical signal in the sensory neurone
what are the different types of afferent nerve fibres?
Ia/A-alpha fibres:
- largest diameter (13-20um)
- myelinated
- supply sensory receptors in muscles
- responsible for proprioception
A-beta fibres:
- smaller diameter than A-alpha (6-12um)
- myelinated
- most information for touch (light touch, deep touch), proprioception, heavy pressure, vibration, skin stretch
A-delta fibres:
- smaller diameter than A-beta (1-5um)
- myelinated
- information about pain (nociceptors) and temperature (thermoreceptors)
C fibres:
- smaller diameter than A-delta (0.2-1.5um)
- non-myelinated
- information about pain and temperature
how does diameter relate to speed of conduction? why is this useful?
- the largest, swiftest axons (A-alpha) are used to carry sensory information about the most rapidly changing stimuli over the longest distances (e.g., stretch receptors in muscle, mechanoreceptors in tendons and skin), or they are used to control finely coordinated contractions of muscles
- the thinnest, slowest C fibres are mainly sensory axons related to chronic pain and temperature, for which the speed of the message is not as critical
where do the different afferent fibres project into the dorsal horn?
A-beta fibres project mostly to the deep layers of the posterior horn, where as A-beta and C fibres project mostly to the superficial layers
what are rapidly and slowly adapting afferents? when are each useful?
some afferents fire rapidly when a stimulus is first presented, then fall silent in the presence of sustained stimulation; others generate a sustained discharge
- rapidly adapting afferents are thought to be particularly effective in conveying information about changes in ongoing stimulation such as those produced by stimulus movement
- slowly adapting afferents are better suited to provide information about the spatial attributes of the stimulus, such as size and shape
what is the threshold like for mechanoreceptors?
low-threshold
even weak mechanical stimuli of the skin induces them to produce an action potential
what type of afferent fibre are all low-threshold mechanoreceptors innervated by?
A-beta (relatively large myelinated axons), ensuring the rapid central transmission of tactile information
what are the different types of mechanoreceptors?
- Merkel’s disks
= highest spatial resolution, allows them to resolve tiny spatial details - ideal for processing information about form and texture - Meisenner corpuscle
= account for 40% of hand mechanosensory information - efficient in processing information about low-frequency vibration that occurs when objects move across the skin - Ruffini endings
= essential to internally generated stimuli (e.g. finger movements) - Pacinian corpuscle
= detecting vibrations transmitted through objects that contact the hand