II. Neuroanatomy
Neuroanatomy
Central and Peripheral Nervous Systems
The Central Nervous System (CNS): the central nervous system consists of the brain and spinal cord
The Peripheral Nervous System (PNS): the peripheral nervous system consists of nerves joined to the brain and spinal cord (cranial and spinal nerves)
The Autonomic Nervous System (ANS): the autonomic nervous system consists of neurones detecting changes in and control the activity of the viscera.
- innervates smooth muscle, cardiac muscle and secretory glands
- important homeostatic regulator
- is composed of:
*The Sympathetic Nervous System (SNS): sympathetic nervous system activates our innate “fight or flight response”, "hyperarousal" or acute stress response", utilizing and activating the release of norepinephrine. It is a normal physiological reaction, in response to a perceived harmful event, attack or threat to survival, first described by Walter Bradford Cannon. The adrenal medulla produces the hormone cascade resulting in secretion of catecholamines, hormones and neurotransmitters reacting to acute stress.
*The Parasympathetic Nervous System (PaNS): parasympathetic nervous system, originating in the sacral spinal cord and medulla, works in concert with the SNS. It activates our innate “rest and digest response" to regain normal homeostasis following our “fight or flight response”. It utilizes and activates the release of an important neurotransmitter, acetylcholine.
*Functions of the Autonomic Nervous System*
Structure: Sympathetic Effect:
Iris of the Eye dilates pupil
Ciliary muscle of the Eye relaxes
Salivary glands reduces secretion
Lacrimal glands reduces secretion
Heart increases rate and force of contraction
Bronchi dilates
Gastrointestinal System *decreases motility*
Sweat glands increases secretion
Erector pili muscles contracts
Structure: Parasympathetic Effect:
Iris of the Eye constricts pupil
Ciliary muscle of the Eye contracts
Salivary glands increases secretion
Lacrimal glands increases secretion
Heart decreases rate and force of contraction
Bronchi constricts
GIT increases motility
Sweat glands
Erector pili muscles
*Functions of the Sympathetic Nervous System*
The Sympathetic Nervous System: somatic nervous system consists of neurons detecting changes in the external environment or control movements.
Specific Nerve Cells:
Afferent neurons (‘sensory neurons”): carry information from peripheral receptors to the CNS
Efferent neurons (“motor neurons”): carry impulses away from the CNS; cause movement.
Endoneurium: A layer of delicate connective tissue that encloses the myelin sheath of a nerve fiber within a fasciculus
- It serves to support capillary vessels, arranged so as to form a net-work with elongated meshes.
- Fibers are bundled together into groups known as fascicle
Perineurium: Connective tissue covering of fascicles making up nerves of the nervous system.
- A smooth, transparent tubular membrane which may be easily separated from the fibers it encloses.
Epineurium:
- the outermost layer of connective tissue surrounding a peripheral nerve
- encloses bundles of fascicles making up a nerve
- it includes the blood vessels supplying the nerve
- a tough and mechanically resistant tissue
**Regeneration: 1-2mm/day - distal to proximal**
External Features of the Spinal Cord: Topographical Anatomy, Spinal Nerves, Meninges
The spinal cord and associated spinal nerves function in order to:
- receive afferent fibres from sensory receptors of the trunk and limbs
- control movements of the trunk and limbs
- provide autonomic innervation for most of the viscera
**Many functions operate in an autonomic or reflex manner; Due to extensive connections with the brain, through various ascending and descending nerve fibre tracts/pathways, conveying specific information to higher centres, mediating their controlling influence over spinal mechanisms.**
General knowledge of Neuroanatomy is a prerequisite for clinical diagnosis of disorders of the nervous system. The diagnostic process involves: a thorough history, neurological examination, and confirmatory radiological imaging investigations. A pathological lesion acting on a specific location within the neuromuscular axis forms a recognisable syndrome, radiological imaging confirms diagnosis.
Extrinsic Disorders:
- leads to compression of the brain, spinal cord, nerve roots and peripheral nerves requiring neurosurgical interventions.
- the brain may be compressed due to the formation of haematomas (blood clots), abscesses and tumours arising within the skull and coverings of the cerebrum
- the fluid-filled ventricles may compress the brain from within when blockage to the flow of CSF leads to a rise in pressure and expansion (hydrocephalus)
- **Delay in decompressive neurosurgery leads to: permanent paralysis, sensory loss and incontinence.**
- the spinal cord may be compressed due to specific disease processes of the spine, eg. arthritis (spondylosis), prolapsed IVDs or bone or meninges tumours (meningiomas). *the central canal of the spinal cord may expand into a cavity, compressing the nerve fibres in the centre of the cord (syringomyelia).
- the cranial nerves (emerging from the brain stem) may be compressed/“trapped”, by tumours or swollen arteries (aneurysms)
- the spinal nerve roots may be compressed by tumours or prolapsed IVDs leading to pain, weakness and/or sensory loss in their specific region of distribution (radiculopathy).
- the peripheral nerves may be compressed/“trapped” at vulnerable pressure sites in the limbs by ribs and tough fibrous bands causing pain, weakness and sensory loss in their specific distributions (entrapment neuropathy).
Systemic Disorders:
- primarily disorders of organs other than the nervous system that disrupt neuromuscular function due to abnormal metabolism.
- you have have symptoms of a neurological condition but the attributing cause lies elsewhere!!
- causes: intoxication (eg. drugs, alcohol), dietary deficiencies (eg. vitamin B12), cardiorespiratory system failure, liver/kidney/hormonal (endocrine) dysfunction
- investigations: haematology/biochemical testing, specific measures of cardiorespiratory, liver, renal and endocrine function.
Vascular Disorders:
- primarily due to: damages of the circulation to the nervous system e.g. occlusion of the vessels (thrombosis), restriction of the blood and oxygen supply (infarct) or bleeding into the nervous tissues (haemorrhage).
- rapid development of a vascular lesion = stroke (in these situations, Act FAST: Face.Arms. Speech. Time)
- *congenital swellings of arteries (aneurysms) or tumours of blood vessels (angiomas) can compress cranial nerves and the brain itself.
- investigations: *exclude abnormal clotting disorders of the circulating blood, echocardiography, electrocardiography, and cardiac angiography.
Intrinsic Disorders:
- primarily due to: disorders of the nervous system itself.
- uncommon, chronic & irreversible!
- such as: innate metabolic dysfunction (enzymatic factors), paroxysmal disorders (episodic LOC, epilepsy, narcolepsy, migranes/CGH).
- system degenerations (youth): muscular dystrophies, hereditary sensorimotor neuropathies, hereditary spastic paraparesis, cerebellar ataxias, Huntington’s disease.
- system degenerations (adults): motor neurone disease, Parkinson’s disease, dementia, Alzheimer’s disease.
- leads to: premature death (atrophy) of the neuromuscular system
Neoplasias: excessive, uncontrolled growth of tissues forming a benign or malignant tumour.
primary: within neuromuscular tissues
secondary: spreading in the circulation from other primary organ sites (eg. lung, breast)
non-metastatic/paraneoplastic: tumours at distant sites that damage the nervous system by humeral or immune mechanisms.
Inflammation:
resulting from infection: by microorganisms - meninges (meningococcal meningitis), the brain (viral encephalitis, neurosyphilis) or peripheral nerves (leprosy)
resulting from immune disorders, in the absence of infection: multiple sclerosis (CNS), acute inflammatory neuropathy - GBS (peripheral nerves), myasthenia graves (NMJ), polymyositis (muscle tissue).
investigations: microbiological, serological testing of the blood and CSF
Rx: antimicrobial agents, suppression of immune responses (CSTs).
General Time Course of Disease (pic)
The Prototypical Figure Illustrating Major Syndromes of the neuromuscular system.
Major Sensory Pathways
Sensation in the trunk and limbs is conducted via sensory receptors in the periphery (by peripheral nerves and spinal nerve roots), to the dorsal root ganglia, then to the spinal cord.
Within the spinal cord are divergent sensory pathways for pain and temperature (spinothalamic tract) and touch and proprioception (dorsal columns).
**Clinically: due to the decussation (“crossing”) of ALL ascending sensory pathways in either the spinal cord or lower brainstem, lesions in the upper brainstem or cerebral hemisphere lead to loss of ALL sensation on the opposite side of the body (contralateral).**
Major Motor Pathways:
Lower Motor Neurones: motor neurones originating in the cranial nerve nuclei of the brainstem or the ventral horn of the spinal cord.
- innervate specific groups of skeletal muscle fibres
- damage: weakness (paresis), paralysis, muscle wasting, depresses stretch reflex (DTR: hyporeflexia), lose muscle tone (hypotonia), fasciculations (spontaneous contractions of muscle fibres/ “ripple-like movements of the muscle beneath the skin” due to denervation.
Upper Motor Neurons: descending motor pathways controlling activity of lower motor neurons.
- arise from: cerebral cortex and brainstem
- tracts/pathways: corticospinal (pyramidal) and corticonuclear/corticobulbar) *descending motor pathways*