Neuroscience & Mental Health Flashcards
What are the 2 germinal layers from which the neural tube forms?
Endoderm
Mesoderm
Ectoderm
How does the neural tube form?
A strip of ectoderm proliferates and thickens to form the neural plate down the back of the embryo
The neural groove forms when the neural plate forms. The neural groove fuses at the midline to form the neural canal
What are neural crests?
Cells of the neural tube which do not fuse but instead form a separate source of neural tissue which are the neural crests
Which tissues form the CNS and PNS?
Neuroepithelium - CNS
Neural crests - PNS
Which types of principle cells does the neuroepithelium differentiate to form?
Neuroblasts - neural cells which form neurones with cell bodies in the CNS (motor
Glioblasts - supportive cells of the CNS
Ependymal cells - cells which line the ventricles and the central canal of the CNS
Give 2 examples of glioblasts
Astrocytes
Oligodendrocytes
Why are microglia not considered glioblasts?
They develop from the mesoderm and only then migrate to the CNS
Which principle cells does the neural crest cells form?
Sensory neurones
Post-ganglionic autonomic neurones
Schwann cells
Non-neuronal derivatives
What characteristic is shared by all neural crest cells?
Their ability to migrate long distances in order to reach their periphery
How does the neural tube differentiate into layers?
A cell contracts towards the inner membrane of the neuroepithelium and produces two daughter cells via mitosis
One daughter cells remains on the membrane and eventually returns to the cell cycle and forms the ependymal lining layer
The other daughter cell migrates from the membrane and begins to differentiate to form neuroblasts and glioblasts.
How is differentiation controlled?
By signalling molecules that surround the neural tube which interact with receptors on neuroblasts
They control migration and axonal growth by attraction and repulsion
What are the 3 layers of the neural tube?
Ependymal
Grey
White
With regards to the development of the spinal cord, what develops from the alar and basal plates?
Alar - neuroblasts develop into interneurons with sensory function
Basal - some neurons develop into interneurons and some into motor neurons
What are alar and basal plates called in the mature spinal cord?
Alar - dorsal horns
Basal - ventral horns
What is dorso-ventral patterning?
Signalling molecules derives fro the notochord spread out and induce neuroblasts in the ventral part to differentiate as motor neurons.
Signalling molecules from the ectoderm induce dorsal neuroblasts to develop into sensory cells
From where does the brain develop?
The most anterior tip of the neural tube
What are the 3 primary vesicles?
Prosencephalon - develops into the forebrain
Mesencephalon - develops into the midbrain
Rhombencephalon - develops into the hindbrain
At 5 weeks, what do the further divisions of the fore and hind brain form?
Forebrain:
Telencephalon - forms the two hemispheres
Diencephalon - consists of the thalamus and hypothalamus
Hind brain:
Pons
Medulla
What are the 3 folds during development of the brain?
Cephalic flexure
Pontine flexure
Cervical flexure
What folding of the neural tube occurs during development of the brain?
5 weeks - further differentiation and folding
8 weeks - cerebral hemispheres spread backwards to partially hide the diencephalon
Term- brain is recognisable as mature brain with ventricular system inside
When does myelination end?
Late teens
How does the 4th ventricle form?
Lateral proliferation of the roof plate enlarges the neural canal
How does the relationship of the basal and alar plates change when the 4th ventricle forms?
Basal plates now lie medially whilst alar plate lie laterally
How does the cerebral cortex develop?
A result of migration of neuroblasts towards the pial surface along radial glia
Successive waves of migration then form the cortical layers
What processes does normal neural development depend n?
Proliferation Differentiation Migration Axon growth Synapse formation
How can an understanding of developmental mechanisms lead to treatment of neurological disorders?
Use of stem cells to replace lost neurons
Use on axonal guidance mechanisms to induce regeneration in the CNS
What time of information do the dorsal horns receive?
Sensory information via spinal nerves and dorsal roots
What is the function of ventral horns?
Contain motor neurons whose axons control the muscles of the body via the ventral roots and spinal nerves
Describe the intermediate horns in the thoracic and upper lumbar region
They contain sympathetic preganglionic motorneurons whose axons control visceral function via the ventral roots and spinal nerves
What does the white matter of the spinal cord contain?
Short pathways which interconnect adjacent segments of the spinal cord
And longer pathways which convey information to and from the brain
What are the 3 meningeal coverings of spinal cord and brain?
Dura Mater
Arachnoid mater
Pia mater
How does the meningeal coverings of the spinal cord differ from the brain?
1) There is an extradural space which contains fat and a venous plexus
2) Lateral extensions of the pia mater called dentate ligaments extend to the dura which helps stabilise the spinal cord
3) The filum terminale (pial thread) anchors the low end of the spinal cord to the coccygeal vertebrae
4) The subarachnoid space below the end of the spinal cord is called the lumbar cisterns and contain lumbar and sacral spinal roots (cauda equina)
What does the degree of deficit following a spinal cord lesion depend on?
1) Loss of neural tissue eg. metastases, degenerative disease
2) Vertical level- the higher the level, the more severe the disability
3) Transverse plane- damage to the white matter tends to be more important that damage to the grey matter
What can damage to the ascending of descending spinal tracts produce?
Motor and/or sensor loss
eg. paralysis, anaesthesia
What can develop as a result of raised sensitivity of spinal motorneurons?
Hyperreflexia and/or spasticity
Why might there be functional improvement after damage to spinal cord tracts?
If the tracts are only compressed
What is syringomyelia?
A disorder in which a cyst of cavity forms within the spinal cord, usually occurs in the cervical region and so upper limbs are affected
What is the brainstem?
The part of the CNS, excluding the cerebellum that lies between the cerebrum and the spinal cord
What are the 2 major divisions of the brainstem?
Midbrain- most rostral & anterior
Pons- bulbous structure
Medulla- most caudal and posterior
How is the medulla connected to the spinal cord
Via to foramen magnum
What is the purpose of colliculi?
Super colliculi- important in head and neck reflexes related to vision
Inferior colliculi- involved in auditory reflexes
What does the trochlear nerve (IV) supply?
Superior oblique muscle of the eye
What are the main ascending pathways seen in the brainstem?
Dorsal columns (cuneate fasciculus & gracile fasciculus) to the thalamus, carrying fine touch and proprioceptive information to the brain
How is the cerebellum held onto the back of the brainstem?
Cerebellar peduncles
They are the superior, middle and inferior, connecting the cerebellum to the midbrain, pons and medulla respectively
Where does the optic nerve (II) originate?
Diencephalon
Where do 50% of optic nerve fibres from the retina cross over?
Optic chiasm
Why can the olfactory nerve (I) no be seen in the anterior view?
Its origin is in the cerebrum
Where does the occulumotor nerve (III) arise from?
Interpendicular fossa at the midbrain level
What does the oculumotor nerve (III) supply?
Most of the extrinsic muscles of the eye
Which nerve is responsible for the muscles of mastication?
Trigeminal nerve (V)
Which nerves arise from the pontine-medullaly junction?
Abducens nerve (VI) Facial nerve (VII) Vestibulocochlear nerve (VIII)
What are the pyramids of the medulla?
Motor fibres run down the corticospinal tract through the cerebral peduncle and disappear from view in the pons, then they reappear as the pyramids
What is pyramidal decussation?
95% of the pyramidal fibres cross over to the other side at the base of the medulla
Which cranial nerves arise from the medulla?
(Superior to inferior) Glossopharygeal nerve (IX) Vagus nerve (X) Accesory nerve (XI)
How can cranial nerves be classified by general functions?
1) General somatic afferent- fibres provide sensation from the skin and mucous membranes
2) General visceral afferent- fibres provide sensation from the GI tract, heart vessels and lungs
3) General somatic efferent- fibres supply muscles of the eye and tongue movements
4) General visceral efferent- fibres are preganglionic parasympathetic fibres
How can cranial nerves be classified by special functions?
1) Special somatic afferent- fibres are vision, hearing & equilibrium
2) Special visceral afferent- fibres are for smell and taste
3) Special visceral efferent- fibres supply the muscles involves in chewing, facial expression, swallowing, vocal sounds and turning the head
What are the characteristics of the midbrain level?
‘Mickey mouse’ shaped, defined by cerebral peduncles which are the main fibres coming down from the cortex, into the spinal cord and forming the corticospinal tract
Cerebral aqueduct
Inferior colliculi
Substansia niagra
What is the space between the two cerebral peduncles of the midbrain called?
Interpendicular fossa
What are the characteristics of the cerebral aqueduct, inferior colliculi and substantia nigra?
Cells are full of neuromelanin therefore appear to be black
Neuromelanin is a by-product of dopamine metabolism
What are the distinguishing features of the pons?
Transverse fibres which run between the two middle cerebellar peduncles
Pons forms floor of fourth ventricle
What are the distinguishing features of the upper medulla?
Inferior olivary nucleus (wiggly structure) is important in motor function
Re-emergence of the corticospinal tract as the pyramids
Hypoglossal nerve nucleus
What are the distinguishing features of the hypoglossal nerve?
Has a round shape nucleus
The gracilis and cuneatus dorsal columns are visible
Central canal
Pyramidal decussation
What is lateral medullary syndrome?
A set of symptoms seen where there is thrombosis of the vertebral artery or the posterior inferior cerebellar artery
What will a patient with lateral medullary syndrome present with?
Vertigo
Ipsilateral cerebellar ataxia
Ipsilateral loss of pain/thermal sense in the face
Signs of Horner’s syndrome (droopy eyelids, lack of sweating and miosis)
Hoarseness
Difficulty in swallowing
Contralateral loss of pain/thermal sense in the trunk and limbs
What are the sensory cranial nerve nuclei?
Midbrain: Mesencephalic trigeminal (V) Pons: Pontine trigeminal (V) Vestibulocochlear (VIII) Medulla: Spinal trigeminal (V) Solitarius (VII, IX, X)
What are the motor cranial nuclei?
Midbrain: Edinger-westphal (III) Oculomotor (III) Trochlear (IV) Pons: Trigeminal Motor (V) Abducens (VI) Facial (VII) Salivatory (VII) Medulla: Salivatory (IX) Dorsal Motor (X) Ambiguus (IX, X, XI) Hypoglossal (XII) Cervical spinal cord: Spinal accessory (XI)
What are the characteristics of the olfactory nerve (I)?
Component fibres: Sensory Structures innervated: Olfactory epithelium via olfactory bulb Function: Olfaction
What are the characteristics of the optic nerve (II)
Component fibres: Sensory Structures innervated: Retina Functions: Vision
What are the characteristics of the oculomotor nerve (III)?
Component fibres: Motor Structures innervated: Superior, inferior and medial rectus, inferior oblique, levator palpebrae muscles Functions: Movement of the eyeball
Component fibres Parasympathetic Structures innervated: Pupillary constrictor and cilliary muscle of the eyeball via cilliary ganglion Function: Pupillary constriction and accommodation
What are the characteristics of the trochlear nerve (IV)?
Component fibres: Motor Structures innervated: Superior oblique muscle Function: Movement of the eyeball
What are the characteristics of the trigeminal nerve (V)?
Component fibres: Sensory Structures innervated: Face, scalp, cornea, nasal and oral cavities, cranial, dura mater Function: General sensation
Component fibres: Motor Structures innervated: Muscles of mastication Tensor tympani muscle Functions: Opening and closing the mouth Tension of tympanic membrane
What are the characteristics of the abducens nerve (VI)?
Component fibres: Motor Structures innervated: Lateral rectus muscle Functions: Movement of the eyeball
What are the characteristics of the facial nerve (VII)?
Component fibres: Sensory Structures innervated: Anterior two-thirds of tongue Functions: Taste
Component fibres: Motor Structures innervated: Muscles of facial expression Stapedius muscle Functions: Tension on bones of middle ear
Component fibres: Parasympathetic Structures innervated: Salivary and lacrimal glands via submandibular and pteryglopalatine glanglia Functions: Salivation and lacrimation
What are the characteristics of the vestibulocochlear nerve (VIII)?
Component fibres: Sensory Structures innervated: Vestibular apparatus Cochlea Functions: Vestibular sensation Hearing
What are the characteristics of the glossopharygeal nerve (IX)?
Component fibres:
Sensory
Structures innervated:
Pharynx, posterior third of tongue, eustachian tube, middle ear, carotid body and carotid sinus
Functions:
General sensation and taste, Chemo- and baroreception
Component fibres: Motor Structures innervated: Stylopharygeus muscle Function: Swallowing
Component fibres: Parasympathetic Structures innervated: Parotid salivary gland via otic ganglion Function; Salivation
What are the characteristics of the vagus nerve (X)?
Component fibres: Sensory Structures innervated: Pharynx, larynx, oesophagus, external ear Aortic bodies, aortic arch Thoracic and abdominal viscera Function: General sensation Chemo-and barreception Visceral sensation
Component fibres: Motor Structures innervated: Soft palate, pharynx, larynx, upper oesophagus Function: Speech, swallowing
Component fibres:
Parasympathetic
Structures innervated: Thoracid and abdominal viscera
Function:
Control of cardiovascular system, respiratory and gastro-intestinal tracts
What are the characteristics of the accessory nerve (XI)?
Component fibres: Motor Structures innervated: Sternomastoid and trapezius muscles Function: Movement of head and shoulder
What are the characteristics of the hypoglossal nerve (XII)?
Component fibres: Motor Structures innervated: Intrinsic and extrinsic muscles of the tongue Function: Movement of the tongue
Which two sets of arteries form the circuit of the Circle of Willis?
Internal carotid arteries (anteriorly) Vertebral arteries (posteriorly)
With regards to the circle of Wills, how is impairment of the blood supply to the brain prevents?
3 main pairs of cerebral arteries from the Circle of Willis supply the anterior, middle and posterior cerebrum which allows for compensation if one of the arteries is occluded
What does the external carotid artery supply?
Structures of the face
What is the vertebral artery a branch of?
Subclavian artery
What do two vertebral arteries come together to form?
Basilar artery
Describe the branching of the Circle of Willis
Internal carotid- branches into a large middle cerebral artery and a smaller anterior cerebral artery
Basilar artery- bifurcates to form 2 posterior cerebral arteries
2 posterior communicating and one anterior communicating artery complete the circle
How are venous sinuses formed?
The folds of the dura mater
Where does most venous blood of the brain drain?
Into the superior saggital sinus via the cerebral veins
Venous blood then circulates to the back of the head and moves laterally through the lateral sinus and sigmoid sinus to become continuous with the internal jugular vein
What is a stroke?
A cerebrovascular accident
Definition: a rapidly developing focal disturbance of brain function of presumed vascular origin which lasts for more that 24 hours
What are the proportions of strokes caused by infarctions and haemorrhages?
Infarction (85%)
Haemorrhage (15%)
What is a transient ishaemic attack?
A rapidly developing focal disturbance of brain function of presumed vascular origin that resolves completely within 24 hours
What is an infarction?
Degenerative changed which occur in tissue following occlusion of an artery
What is cerebral ischaemia?
A lack of sufficient blood supply to nervous tissue resulting in permanent damage if blood flow is not restored quickly
What are the risk factors of stroke?
Age Hypertension Cardiac disease Smoking Diabetes mellitus
What does interruption of flow to the anterior cerebral artery present with?
Paralysis of the contralateral leg more than the arm and the face.
Frontal lobe affected so disturbance of intellect and judgement (abulia)
Loss of appropriate social behaviour
What does interruption of flow to the middle cerebral artery present with?
'a classic stroke' Contralateral hemiplegia in the arm more than the leg Hemisensory deficits Hemianopia Aphasia (left side lesion)
What does interruption of flow to the posterior cerebral artery present with?
Visual deficits
Homonymous hemianopia
Visual agnosia
What are the types of haemorrhagic stroke?
Extradural- trauma, immediate effects
Subdural- trauma, delayed effects
Subarachnoid- ruptured aneurysms
Intracerebral- spontaneous hypertensive
How long does cerebral blood flow need to be interrupted for in order for unconsciousness to result?
4 seconds
Irreversible damage results after a few minutes
Why is provision of glucose to the brain vital?
The brain cannot synthesise or utilise any other source of energy
Ketones can only be metabolised to a very limited extent
Under which glucose concentration will unconsciousness result?
2mM
How is cerebral blood flow regulated?
Systemically- mechanisms which affect the total cerebral blood flow
Locally- mechanism which relate activity or requirement in specific brain regions by altered localised blood flow
How is total cerebral blood flow autoregulated?
Between mean arterial blood pressures of 60 and 160 mmHg
What are the two mechanisms by which cerebral blood flow is regulated?
Neural control
Chemical control
Describe neural control of cerebral blood flow
When increased blood flow is required in active areas of the brain, neural control involved diverting blood flow to those active areas by means of vasoconstriction/vasodilation of the cerebral arteries
Parasympathetic innervation of which nerve produces slight vasodilation of the cerebral arteries?
Facial
What do central cortical neurones release?
A variety of vasoconstrictor neurotransmitters eg. Catecholamines
What effect do dopaminergic neurones produce?
Vasoconstriction
What do dopaminergic neurones innervate?
Penetrating arterioles and pericytes around capillaries
What are pericytes?
A form of brain macrophages with diverse activities eg. immune function, transport properties, contractile
With regards to chemical control of cerebral blood flow, what chemical factors are released which effect blood flow?
1) CO2 (indirect)
2) pH (i.e. H+, lactic acid)
3) nitric oxide
4) K+
5) adenosine
6) anoxia
7) kinine, prostaglandins, histamine, endothelins
Explain the relationship between increased carbon dioxide and cerebral blood flow
Sinusoidal relationship
Carbon dioxide itself is not responsible for the vasodilatory effect
Carbon dioxide diffuses across the blood brain barrier.
H+ ions are produced due to the presence of carbonic anhydrase in the neural tissue of the brain
The H+ ions lead to smooth muscle dilation
How does NO synthesis increase cerebral blood flow?
Nitric oxide conversion from arginine is catalysed by NO synthase
NO activates guanylyl cyclase
GTP is converted to cyclic GMP which leads to vasodilation of the cerebral arterioles
Where is cerebrospinal fluid found?
In intracerebral ventricles
What is the choroid plexus
Forms the cerebrospinal fluid
Is a collection of capillaries surrounded by ependymal cells with tight junctions between the cells
How much cerebrospinal fluid is formed by the choroid plexus?
80-150 ml
How does the choroid plexus secrete cerebrospinal fluid?
Into the ventricles
lateral ventricles > 3rd ventricle via intraventricular foramina > down cerebral aqueduct > into 4th ventricle > into subarachnoid space via medial and lateral apertures
What are the functions of cerebrospinal fluid?
Protection (physical and chemical)
Nutrition of neurones
Transport of molecules
What is the function of the blood-brain barrier?
Protects
- from certain toxins and circulating transmitters such as catecholamines
- from wide variations in ion concentrations
What is the structure of the blood-brain barrier?
- Mainly tight junctions between endothelial cells (capillaries ‘non-fenestrated’)
- Pericyte end-feet close to the capillary walls
What substances are allowed through the blood-brain barrier?
- Lipophilic molecules are allowed access to the brain CSF and ECF
- Certain hydrophilic substances can enter the brain CSF and ECF but only with the help of transport mechanisms e.g.
1) water via aquaporin (AQP1,AQP4) channels
2) glucose via GLUT1 proteins
3) amino acids via 3 different transporters
4) electrolytes via specific transporter systems
What are circumventricular organs?
Lie outside the blood brain barrier
Have fenestrated capillaries and respond directly to changes in the blood but have neural connections with the blood brain barrier
e.g.
- median eminence region of the hypothalamas
- subfornical organ (SFO)
- organum vasculosum of the lamina terminalis (OVLT)
Which has a high pH, plasma or cerebrospinal fluid?
Cerebrospinal fluid
What is the location of the thalamus?
Occupying most of the diencephalon
How is the thalamus organised?
It is divided into left and right thalamus by the third ventricle
Each is a collection of individual nuclei with separate functions and connections with ipsilateral forebrain structures
What is the function of the thalamus?
Acts as a relay centre between the cerebral cortex and other parts of the CNS
Integrates/modulates information en route
Integrates all functions except olfaction
Some nuclei form part of the Reticular Activating System
Which thalamic nuclei connect with the motor cortices?
Ventral lateral
Ventral anterior
Which thalamic nuclei connect with the body of the somatosensory cortex?
Ventral posterolateral
Which thalamic nuclei connect with the head of the somatosensory cortex?
Ventral posteromedial
Which thalamic nuclei connect with the visual cortex?
Lateral geniculate
Which thalamic nuclei connect with the auditory cortex?
Medial geniculate
What are specific nuclei?
Nuclei with reciprocal connections with a primary cortical area
What are association nuclei?
Thalamic nuclei with more diffuse reciprocal connections with association cortex
From where do the intralaminar nuclei receive inputs?
The Reticular Formation of the brainstem and project diffusely to all cortical areas
From where do the reticular nucleus receive inputs?
The Reticular Formation and projects to the other thalamic nuclei which regulated the flow of information through these to the cortex
What forms the Reticular Activating System?
Reticular Formation
Intralaminar Nuclei
Reticular Nuclei
What is the location of the hypothalamus?
In the diencephalon, below the thalamus
How is the hypothalamus organised?
Divided into the left and right hypothalamus by the third ventricle
Each is a collection of individual nuclei with separate functions and largely ipsilateral connection
What is the function of the hypothalamus?
It co-ordinated homeostatic mechanisms by:
- Regulating the autonomic nervous system via connections with the brainstem and spinal cord
- Acting as an endocrine organ, via the pituitary
- Controlling behaviour via connections with forebrain structures
In what instances is there hypothalamic involvement in the control of certain behaviours?
1) Eating and drinking
2) Expression of emotion
3) Sexual behaviour
4) Circadian rhythm
4) Memory
What structures are associated with the hypothalamus?
Olfactory system
Limbic system- hippocampus, amygdala, cingulate cortex, septal nuclei
Behaviour directed towards well-being triggers the reward system in the hypothalamic-limbic circuitry, leading to reinforcement of that behaviour
What is the somatosensory system?
It is concerned with sensory information coming from the skin, muscles, joints and ligaments
Touch and proprioception are carried via the dorsal columns/medial lemniscus pathway to the somatosensory cortex
What type of receptor are the receptors for touch and proprioception?
Mechanoreceptors
They are modified terminal of the peripheral axons of primary sensory neurones
What is the mechanism of touch and proprioception receptors?
They transduce a mechanical stimulus (deformation) into electrical signals
What does the function of mechanoreceptors depend on?
1) Degree of specialisation- from free nerve endings to elaborate accessory structures
2) Location- e.g. in various layers of skin, around hair shaft, in muscles, in tendons
3) Physiological properties- activation threshold determines sensitivity (all touch and proprioception receptors are low threshold) May be slow or fast adapting
Where are the cell bodies of sensory nurones?
In the PNS (DRG or trigeminal ganglia)
What types of axons do sensory neurones have?
Muscle spindles and tendon organs are innervated by type 1 axons.
Most other mechanoreceptors are innervated by type 2/alphabeta axons
All are fast conducting
Describe the receptive field of sensory neurones
It is the number of receptors that are innervated by one sensory neurone.
The larger the receptive field, the lower the resolution.
Density of receptive fields varies over the body
Describe the intensity of the stimulus from a sensory neurone
It is coded by the frequency of firing of the neurone.
Amplitude of action potential does NOT change.
If stimulus increases 10 fold, firing frequency doubles
Where does decussation of the central pathway occur?
In the brainstem
What enhances the difference between adjacent inputs in the central pathway?
Lateral inhibition
When do touch and proprioceptive stimuli become a conscious localised sensation?
When it reaches the cortex
What is the somatotopic arrangement of the somatosensory cortex known as?
The homunculus
Describe cortical analysis
Somatosensory I is where the body map is distorted according to relative density input from different parts of the body
The response of neurones in SI varies depending on stimulus or abstracted properties
Information is analysed by SI and goes to SII and the posterior parietal cortex for further analysis
What is the posterior parietal cortex necessary for?
Interpretation of spatial relationships
What can injury to the pathway anywhere from the periphery to the cortex result in?
Anaesthesia or paraesthesia
What is nociception?
Provides information about noxious (unpleasant or harmful) stimuli.
This information is processed by the brain but it is percieved as pain.
How is visceral pain carried?
It is carried peripherally by autonomic nerves and centrally in the spinothalamic and dorsal columns pathways.
How is somatic pain and temperature information carried?
By the spinothalamic tract to the brain from the skin, muscles, joints and ligaments
What are the features of nociceptors?
Polymodal (triggered by mechanical, thermal or chemical stimulus)
Free nerve endings
High threshold
Slow adapting
What are the types of axon for sensory neurones that are involved in nociception?
Aδ: mechano-or thermoreceptor, faster, produces sharp pain, leads to avoidance
C: chemoreceptor (eg. bradykinin, histamine) produces dull aching pain, leads to guarding to allow recovery
What are the features of sensory neurones that are involved in nociception?
Cell bodies are in PNS
Receptive fields usually large
Intensity coded by frequency of firing
Which pathway is responsible for carrying information about nociception and temperature?
Spinothalamic pathway
Via which structure does the basic pathway convey information about nociception and to which destination?
Via the VPL and VPM nuclei of the thalamus to the SI and SII cortex
The information is sent for analysis of localisation and intensity of the noxious stimulus
Which structures convey information to forebrain structures for perception of pain?
Collateral branches to the brainstem
affective pathway
What is the central inhibition pathway?
Collateral branches to the periaqueductal grey of the midbrain inhibit pain
Where does decussation of the central pathway for information coming from the body occur?
In the spinal cord
Where does decussation of the central pathway for information coming from the head occur?
In the brainstem
What is the affective pain pathway?
Where axons of the spinothalamic tract send collateral branches to the brainstem (reticular formation) thalamus (intralaminar nuclei) hypothalamus and and some cortex which triggers an increase in awareness and registered the unpleasantness of the stimulus such as pain
How does central (descending) inhibition work?
Cerebral activity triggers a descending pathway in the branstem which inhibits the nociceptive pathway in the dorsal horn
The pathway uses endogenous opioids and other transmitters
What is peripheral inhibition?
- Concerns the gate theory
- Takes place in superficial levels of the dorsal horn (substantia gelatinosa)
- Stimulation through non-nocioceptive inputs inhibits projection of nociceptive stimilus to spinothalamic tract
What is nociceptive dysfunction?
- Disruption of pathway may reduce pain but predisposes to increased injury
- Some changes may exacerbate pain. eg/ windup in dorsal horn, thalamic syndrome, phantom pain
What is volition?
Where the motor system produces movements that are adaptive and accomplish a certain goal
What is functional segregration?
How the motor system is organised in a number of different areas that controls different aspects of movement
What is hierarchal organisation?
High order areas are involves in more complex tasks (programme and decide on movements, coordinate muscle activity)
Lower level of hierarchy performs lower level tasks (execution of movement)
What do primary sensory areas include?
Primary visual cortex
Primary auditory cortex
Primary somatosensory cortex
What do principle motor areas of the cerebral cortex include?
Primary motor cortex (M1: executes voluntary movements)
Motor association cortex (selects voluntary movements)
What is the function of association areas within the cerebral cortex?
To produce a meaningful perceptual experience of the world.
Enable us to interact effectively and carry out abstract thinking and language
Where is the primary motor cortex located?
In the frontal lobe, on the precentral gyrus just anterior to the central sulcus
What does stimulation of the primary motor cortex result in?
Muscle movement at stimulus intensities far lower than any other part of the cerebral cortex
How is primary motor cortex control specific?
Small populations of cortical neurons in M1 control small groups of motor neurons in the spinal cord
This related to the most delicate and precise movements
How are the somatotopically related areas of each of the 3 motor areas of the cerebral cortex interconnected?
Via intracortical motor pathways
What is the function of the supplementary motor area?
It is involved in motor planning of internally driven voluntary movements
What is the result of stimulating the supplementary motor area neurons?
Complex movements involving many muscle groups as opposed to highly specific movements generated by M1 stimulation
Which type of scans show supplementary motor area activity?
PET scans
How much larger than the primary motor cortex is the premotor area?
6x larger
What is the purpose of the premotor area?
Prepares the primary motor cortex for an impending motor act
Under what circumstances will electrical stimulation of the premotor area produce muscle movement?
If the stimuli are much more intense than the effective stimuli for the primary motor cortex
What 3 stages are needed to achieve motor control?
Strategy
Tactics
Execution
What do the descending motor pathways consist of?
The corticospinal (pyramidal tracts The subcorticospinal (extrapyramidal) tracts: - Rubrospinal tracts - Reticulospinal tracts - Vestibulospinal tracts
What is the function of the descending motor pathways?
They act as a direct connection between neurons in the motor cortex and neurones in the spinal cord
What is the largest component of the descending motor pathway?
Corticospinal tract
From where does the corticospinal tract originate?
The pyramidal cells in layer V of the primary motor cortex (grey matter)
From where does the corticospinal tract recieve inputs from?
The somatosensory cortex & up to layer IV of the primary motor cortex
How do fibres of the corticospinal tract travel?
They travel via cerebral peduncles into the medulla
In the medulla they come together and form two column-like structures on the ventral surface of the medulla (pyramids)
How do the lateral corticospinal tracts and anterior corticospinal tracts form?
At the medulla 80% of the corticospinal tract fibres decussate and form the lateral corticospinal tracts
The remaining 20% uncrossed fibres form the anterior corticospinal tract
How is motor innervation for voluntary muscles provided?
In the corticospinal tract, axons of the upper motor neurones project to the ventral horns of the spinal cord.
Here, they connect with the lower motor neurones which provide the motor innervation
What is the function of subcortical (extrapyramidal) tracts?
Support voluntary movement and help control posture, locomotion and stereotypes automatic movements
From where do the subcortical (extrapyramidal) tracts receive inputs?
Brain stem nuclei & cerebellum
Indirect input comes from the basal ganglia
What is the purpose of the rubrospinal tract?
Provides information for supporting somatic motor & skeletal muscle control and regulation of muscle tone for posture
What is the vestibulospinal tract involved in?
Balance
What does the reticulospinal tract play a role in?
Involved in somatic motor control
Plays a roll in the control of autonomic functions
What are the clinical features of an upper motor neuron lesion?
Weakness
Hyperreflexia
Spasticity
Babinski Sign
What are the component structures of the basal ganglia?
Striatum
Globus pallidus
Subthalamic nucleus
Substantia nigra
What is the basal ganglia?
A group of nuclei situated deep in the white matter of the forebrain (cerebral cortex)
They are associated with a variety of functions which include motor control, procedural learning, eye movements, cognitive and emotional function
What is striatum?
2 distinct masses of grey matter which are separated by a large tract of white matter (the internal capsule)
The 2 masses are caudate nucleus and the lentiform nucleus
The lentiform nucleus is putamen and the globus pallidus
How is the globus pallidus divided?
GPe: receives input from the striatum and projects to the subthalmic nucleus
GPi: receives signals from the striatum via the direct pathway and the indirect pathway
Why does the globus pallidus have largely inhibitory effects on targets?
The segments it is composed of, primarily contain GABAergic neurons
What is the substantia nigra and how is it divided?
It is a mesencephalic grey matter portion of the basal ganglia
Divided into-
SNr (Reticulate): works with GPi to inhibit the thalamus
SNc (Compacta): produces dopamine, significant in the maintenance of striatal pathway balance
What is the largest component of the basal ganglia?
Striatum
What is the output of the globus pallidus?
Recieves most important inout from the striatum and sends inhibitory output to a number of motor-related areas
What is the pathway of the basal ganglia circuit
From the cerebral cortex (SMA, PMA, primary motor cortex, somatosensory and parietal cortex) projections to the striatum (caudate and putamen nuclei)
From the putamen, there are projections to
i) GPi and SNr (direct pathway)
ii) GPe (via SNT involving GABA)
From GPi and SNr, there are projections to the thalamus
From the thalamus back to the cortex - the SMA and PMS (two areas of motor cortex involved in the planning of movements)
How is the pathway of the basal ganglia circuit inhibited/excited?
GPi and SNr inhibit the thalamus (involves GABA)
The putamen inhibits GPi and SNr which releases the thalamus from inhibition
The thalamus released the selected movement through its projections to the cortex
How is the correct balance of excitation/inhibition of the thalamus maintained?
SNc which provides excitory inputs to the caudate and putamen
This involves dopamine
What is Parkinson’s disease?
A progressive degenerative disorder of the CNS
It is neuronal degeneration of dopaminergic cells in the substantia nigra which results in loss of dopaminergic terminals in the putamen and to a lesser extent- the caudate
What are the signs of Parkinsons disease?
Bradykinesia Hypomimic face Akinesia Tremor at rest Lead pipe rigidity Parkinson gait Stooped posture
Why are the initial symptoms of Parkinson’s disease movement related?
Due to the loss of dopamine to the striatum leads to less inhibition of GPiSNr which results in increased inhibitory output from GPi/SNr to the thalamus.
Too much inhibition of the thalamus produces a decreased facilitation to the motor cortex- especially the SMA
What is Huntington’s disease?
A neurodegenerative disorder which involves the degeneration of spiny GABA neurons in the striatum (mainly caudate)
What causes cognitive decline and dementia in Huntington’s disease?
The degeneration of spiny GABA neurons in the striatum results in reduced GABAergic inhibition of GPe and thus increased inhibitory output from GPe to STN
- The facilitatory output from STN to GPi/SNr is consequently reduced leading to less inhibitory output from GPi/SNr to the thalamus
- The lack of inhibitoru control of the thalamus on the motor cortex affects muscle coordination
What is chorea?
Rapid jerky movements most commonly caused by Huntington’s
The movements tend to affect the hands and face first
Gradually increase over time until a patient becomes totally incapacitated
Symptoms can be described as hyperkinesia
What is the cerebellum?
A region of the brain which plays an important role in motor control.
Contributes to coordination, precision and accurate timing
From where does the cerebellum receive inputs?
Sensory systems and other parts of the brain and spinal cord
These inputs are integrated into fine motor activity
How is the cerebellum separated from the overlying cerebrum?
A layer of the dura mater
What are the 3 layers of the cerebellum?
From top to bottom:
- A molecular layer
- A layer of Purkinje cells
- A granular cell layer
Within the layers of the cerebellum, what are the types of fibres?
Mossy fibres
Climbing fibres
What are the three highways into and out of the cerebellum?
Superior, middle and inferior cerebellar peduncle
What is the narrow midline zone between the two hemispheres of the cerebellum?
The vermis
What are the folds of grey matter on the surface of the cerebellum?
Folia
What are the three deep cerebellar nuclei embedded within the white matter?
Fastigial nucleus- involved in balance and has connections with the vestibular system and reticular nuclei
Interposed nucleus and Dentate nucleus- both involved with voluntary movement
What are the three sources of input into the cerebellar nuclei?
1) Mossy fibres from spinocerebellar pathways
2) Climbing fibres from inferior olive
3) Mossy fibres from pons bringing information from cerebral cortex
What are the 3 main divisions of the cerebellum?
1) Vestibulocerebellum
2) Spinocerebellum
3) Cerebrocerebellum
What is the function of the vestibulocerebellum?
Tunes balance (stance and gait) and vestibulo-oculo-reflex (VOR)
What are the disorders of the vestibulocerebellum?
1) Ataxic gait- wide based stance (looks drunk)
2) Imbalance when eyes closed (Romberg sign)
3) Nystagmus
What is the function of the spinocerebellum?
Tuns motor execution by adjusting movements and muscle tone
What is a disorder of the spinocerebellum?
Hypotonia
What is the function of the cerebrocerebellum?
Initiation of skilled movements
What are the disorders of the cerebrocerebellum?
1) Tremor and clumsy movements: movements overshoot or undershoot target (dysmetria)
2) Coordination problems (dysdiadochokinesia)
What are the contact ratios for synapses?
Ranges from 1:1 for muscles to 10,000:1 in the CNS
How can membrane potential of the post synaptic neurone be altered?
1) Be made less negative
i. e. be brought closer to threshold for firing; excitory post synaptic potential (EPSP)
2) Be made more negative
i. e. be brought further away from threshold for firing; inhibitory post synaptic potential (IPSP)
What is a neuromuscular junction?
A specialised synapse between the motor neuron and motor end plate, the muscle fibre cell membrane
What occurs when a neuromuscular junction is activated?
Depolarisation leads to calcium ion channels opening and the influx of ions in the pre-synaptic terminal
Vesicle fusion occurs with the pre-synaptic membrane and Ach is released into the synaptic cleft
Ach binds to receptors on the motor end plate and sodium ion channels open.
There is a sodium ion influx leading to an action potential being generated
What are alpha motor neurons?
Lower motor neurons of the brainstem and spinal cord. They innervate the (extrafusal) muscle fibres of the skeletal muscles. Their activation causes muscle contraction. The motor neuron pool contains all alpha motor neurons innervating a single muscle
Where are alpha motor neurons situated?
In either side of the ventral horns (anterior grey matter)
What is the motor unit?
The name given to a single motor neuron, together with all the muscle fibres that it innervates. It is the smallest functional unit with which to produce force. Stimulation of one motor unit causes contraction of all muscle fibres in that unit
On average, how many muscle fibres does each motor neuron supply?
600
What are the different types of motor unit?
Type I: Slow twitch, low tension, fatigue resistant
Type IIa: Fast twitch, moderate tension, fatigue resistant
Type IIb: Fast twitch, high tension, high fatigue
