University:
American Career College
Course:
NURS-113 | Beginning Medical/Surgical Nursing
Academic year:

2018
Views:

37

Pages:

17
Author:

KillSteel

The somatosensory system • Somatic sensory information from the limbs ascends the nervous system in two anatomically discrete systems :A. Fibres from proprioceptive organs and those mediating well-localised touch (including vibration, JOINT POSITION) enter the spinal cord at the posterior horn and pass without synapsing into the ipsilateral posterior columns. B. Neural fibres conveying pain and temperature sensory information (nociceptive neurons) synapse with second-order neurons which cross the midline in the spinal cord before ascending in the contralateral anterolateral spinothalamic tract to the brain stem. The second-order neurons of the dorsal column sensory system cross the midline in the upper medulla to ascend through the brain stem. Here they lie just medial to the (already crossed) spinothalamic pathway. Both the dorsal column and spinothalamic tracts end in the thalamus, relaying from there to the parietal cortex. Brain-stem lesions can therefore cause sensory loss affecting all modalities of the contralateral side of the body. The main somatic sensory pathways. The sensory presentation Anatomy of the spinal cord. Anatomic Organization of ANS Preganglionic neurons of the parasympathetic nervous system leave the central nervous system (CNS) in the third, seventh, ninth, and tenth cranial nerves as well as the second and third sacral nerves. While the preganglionic neurons of the sympathetic nervous system exit the spinal cord between the first thoracic and the second lumbar segments . These are thinly myelinated. • The postganglionic neurons, located in ganglia outside the CNS, give rise to the postganglionic unmyelinated autonomic nerves that innervate organs and tissues throughout the body. • Responses to sympathetic and parasympathetic stimulation are frequently antagonistic reflecting highly coordinated interactions within the CNS; the resultant changes in parasympathetic and sympathetic activity provide more precise control of autonomic responses than could be achieved by the modulation of a single system. The Autonomic Nervous System (ANS) • The autonomic nervous system (ANS) innervates the entire neuraxis and permeates all organ systems. • It regulates blood pressure (BP), heart rate, sleep, and bladder and bowel function. • It operates automatically. • Its full importance becomes recognized only when ANS function is compromised, resulting in dysautonomia. Functional Consequences of Normal ANS Activation Parasympathetic Decreased Mildly decreased Voiding (decreased tone) Bowel motility Lung Sympathetic Increased Increased Increased sphincter tone Decreased motility Bronchodilation Sweat glands Pupils Adrenal glands Sweating Dilation Catecholamine release — Constriction — Sexual function Ejaculation, orgasm Erection Lacrimal glands Parotid glands — — Tearing Salivation Heart rate Blood pressure Bladder Increased Bronchoconstriction Visual loss • Fibres from ganglion cells in the retina pass to the optic disc and then backwards through the lamina cribrosa to the optic nerve. • Nasal optic nerve fibres (subserving the temporal visual field because the image on the retina is inverted) cross at the chiasm, but temporal fibres do not. • Hence, all fibres in the optic tract and further posteriorly subserve both eyes' representation of contralateral visual space. • From the lateral geniculate nucleus, lower fibres pass through the temporal lobes on their way to the primary visual area in the occipital cortex, while the upper fibres pass through the parietal lobe. Visual pathways and visual field defects Schematic representation of eyes and brain in transverse section • Transient visual loss is quite common and sudden visual loss lasting less than about 15 minutes is likely to have a vascular cause. • This can affect one eye (amaurosis fugax) or one visual hemifield. Whether the field loss was monocular (carotid circulation) or a homonymous hemianopia (vertebro-basilar circulation). • Transient visual loss lasting about 20-30 minutes suggests migraine, especially if accompanied by headache and/or positive visual phenomena. Abnormal pupillary responses • The pupillary response to light is due to a combination of parasympathetic and sympathetic activity. A. Parasympathetic fibres originate in the Edinger-Westphal subnucleus of the 3rd nerve, and pass with the 3rd nerve to synapse in the ciliary ganglion before supplying the constrictor pupillae of the iris. B. Sympathetic fibres originate in the hypothalamus, pass down the brain stem and cervical spinal cord to emerge at T1, return up to the eye in association with the internal carotid artery and supply the dilator pupillae. Lesions in the sympathetic pathway cause Horner's syndrome. • The pupils also constrict as part of the near reflex (in association with accommodation and convergence This three-neurone pathway projects from the hypothalamus to the intermediolateral column of the cervical spinal cord, then to the superior cervical ganglion and finally to the pupil, the levator palpebrae and the sweat glands of the face. A lesion at any site along the pathway can produce Horner's syndrome. • Lesions of the oculomotor nerve, ciliary ganglion and sympathetic supply produce characteristic 'efferent' disorders of pupillary function. 'Afferent' defects:- occur as a result of damage to an optic nerve, impairing the direct response of a pupil to light, although leaving the consensual response from stimulation of the normal eye intact. Structural damage to the iris itself can also result in pupillary abnormalities. Horner's syndrome • It results from interruption of the sympathetic innervation of the eye at any point. • This is the presence of 1) Partial ptosis (as sympathetic fibres supply the smooth muscle of both eyelids) 2) Constricted pupil (miosis) (unbalanced parasympathetic action) which reacts normally to light. 3) Anhydrosis:- difference (decrease) in the sweating over each eyebrow with the back of the finger (absence of this sign does not exclude the diagnosis).ii Horner's syndrome causes :1. Central (hypothalamus/brain stem). 2. Peripheral (lung apex, carotid artery pathology) 3. Idiopathic The cerebellum The cerebellum fine-tunes and coordinates movements initiated by the motor cortex. It also participates in the planning and learning of skilled movements through reciprocal connections with the thalamus and cortex, and in articulation of speech. A lesion in a cerebellar hemisphere causes lack of coordination on the same side of the body. Cerebellar dysfunction impairs the smoothness of eye movements, causing nystagmus and render speech dysarthric. In the limbs, the initial movement is normal, but as the target is approached, the accuracy of the movement deteriorates, producing an ‘intention tremor’. The distances of targets are misjudged (dysmetria), resulting in ‘past-pointing’. The ability to produce rapid, accurate, regularly alternating movements is also impaired (dysdiadochokinesis). The central vermis of the cerebellum is concerned with the coordination of gait and posture. Disorders of this therefore produce a characteristic ataxic gait