Week 2

Question 1

What is the spinal cord

Answer 1

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

Question 2

On axonal fibres of the spinal cord

Answer 2

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

Question 3

Grey matter

Answer 3

Grey matter is anatomically divisible into 2 or 3 horns depending upon level
Dorsal horn
Lateral horn (sometimes absent)
Ventral horn

Question 4

Rexed laminae of grey matter of the cord

Answer 4

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

Question 5

Fasciculi and white matter of the cord

Answer 5

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

Question 6

Primary sensory neurons

Answer 6

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

Question 7

Primary afferents- different axon classifications

Answer 7

Aa, Ab, Adelta, C fibres
Aa- proprioceptors of skeletal muscle
Ab- mechanoreceptors of skin
A delta- pain temperature
C- temperature, pain, itch

Question 8

Axons have distinct receptive fields

Answer 8

Each nerve axon innervates a specific receptive field within its Dermatome
Receptive field sizes vary, determining precision of localisation
Meissners corpuscles, Pacinian corpuscles

Question 9

Receptive fields- size defined by 2 point discrimination

Answer 9

Precision of sensory localisation varies greatly across the body
Homunculus

Question 10

Sensory receptors- signal transduction

Answer 10

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)

Question 11

Adaptation of sensory receptors

Answer 11

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

Question 12

Cutaneous sensory receptors

Answer 12

Mechanoreceptors: touch, pressure, vibration
Thermoreceptors: hot, cold, temperature
Nociceptors: noxious stimulation (pain)

Question 13

Cutaneous mechanoreceptors

Answer 13

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

Question 14

Four major types of mechanoreceptors

Answer 14

More superficial layers skin:
Merkels receptors (disk)
Meissners corpuscles
Deeper layers of skin:
-ruffinis corpuscle (ending)
-Pacinian corpuscle

Question 15

Merkels receptor (disk)

Answer 15

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

Question 16

Meissners (or tactile) corpuscles

Answer 16

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

Question 17

Ruffinis (or bulbous) corpuscles

Answer 17

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

Question 18

Pacinian corpuscles

Answer 18

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

Question 19

Hair follicle receptor

Answer 19

Light touch but activation in dermis
Rapidly adapting
Constantly changing stimulus required
Nerve fibre wrapped around hair
Hair deflection detected

Question 20

Cutaneous thermoreceptors

Answer 20

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

Question 21

Thermoreceptors channels

Answer 21

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

Question 22

Cutaneous nociceptors

Answer 22

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)

Question 23

Proprioception, position sense

Answer 23

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

Question 24

The muscle spindle- detection of length and acceleration

Answer 24

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

Question 25

Golgi tendon organ- detection of muscle tension

Answer 25

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)

Question 26

Central pathways

Answer 26

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)

Question 27

The skin as an organ of sensation

Answer 27

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

Question 28

Muscle sense organs

Answer 28

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

Question 29

Key spinal neural levels

Answer 29

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)

Question 30

CNS mechanisms of sensation

Answer 30

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

Question 31

Ascending tracts

Answer 31

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

Question 32

General points

Answer 32

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

Question 33

The first stage of spinal sensation

Answer 33

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)

Question 34

The second stage of spinal sensation

Answer 34

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)

Question 35

Anterior white commissures

Answer 35

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

Question 36

The second stage of spinal sensation 2

Answer 36

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

Question 37

The third stage of spinal sensation

Answer 37

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

Question 38

Organisation of the spinothalamic tract

Answer 38

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

Question 39

Application of ascending tracts anatomy

Answer 39

Syringomyelia- fluid filled cyst (syrinx) forms in spinal cord
Brown sequard syndrome- neurologic syndrome from hemisection of the spinal cord

Question 40

Cortical representation of sensory and motor modalities

Answer 40

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

Question 41

The brain in the control of movements

Answer 41

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

Question 42

The brain in the control of movements 2

Answer 42

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

Question 43

Descending tracts

Answer 43

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

Question 44

The importance of descending tracts

Answer 44

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)

Question 45

Neurones of somatic motor system

Answer 45

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)

Question 46

Upper and lower motorneurones

Answer 46

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

Question 47

The 2 broad classes of upper motorneurones

Answer 47

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

Question 48

Cortical upper motorneurones

Answer 48

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

Question 49

Non cortical upper motorneurones

Answer 49

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

Question 50

Descending fibre tracts and the spinal cord

Answer 50

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