CNS Practical Physiology PDF

## Sensations ### Physiological classification: - **Somatic sensations:** - Pain - Temperature: cold & hot - Mechanoceptive (Mechanoreceptive) - **Tactile:** - **Proprioceptive (position):** - Static - Dynamic - **Touch:**...

## Sensations ### Physiological classification: - **Somatic sensations:** - Pain - Temperature: cold & hot - Mechanoceptive (Mechanoreceptive) - **Tactile:** - **Proprioceptive (position):** - Static - Dynamic - **Touch:** - Crude - Fine: tactile localization, tactile discrimination - two-point discrimination, texture of materials - Deep touch - Pressure (Fine & crude) - Vibration - Tickling & itching - Stereognosis - **Special senses:** - Vision - Hearing - Smell - Olfaction - Taste - **Organic sensations:** e.g. Thirst, hunger, sexual desire ... etc. ### Sensations | Somatic sensations | Epicritic sensations | Protopathic sensations | Deep sensations | Special sensations | |---|---|---|---|---| | Tactile sensation | Tactile sensation | Pressure sensation | Vibratory sensation | Vision | | Tactile localization | Tactile localization | Pain sensation | Visceral sensation | Hearing | | Tactile discrimination | Tactile discrimination | Temperature sensation | Kinesthetic sensation | Taste | | Temperature sensation (25°C to 40°C) | Temperature sensation (below 25°C and above 40°C) | | Conscious kinesthetic sensation | Smell | | | | | Subconscious kinesthetic sensation | | - **Epicritic sensations:** Mild or light sensations that are perceived more accurately. - **Protopathic sensations:** Crude & primitive sensations. - **Kinesthetic sensation or kinesthesia:** - Sensation of position & movements of different parts of the body. - This sensation arises from the proprioceptors present in muscles, tendons, joints & ligaments. - **Synthetic Senses:** - Sensations synthesized at cortical level, by integration of impulses of basic sensations. - Two or more basic sensations are combined in some of the synthetic senses. - Best examples of synthetic senses are vibratory sensation (combination of touch & pressure), stereognosis & two-point discrimination. ## Sensory Fibers of Trigeminal Nerve - Trigeminal nerve (Cr-V) carries somatosensory information from face, teeth, periodontal tissues (tissues around teeth), oral cavity, nasal cavity, cranial dura mater and major part of scalp to sensory cortex. It also conveys proprioceptive impulses from the extrinsic muscles of the eyeball. ### Origin - Sensory fibers of trigeminal nerve arise from the trigeminal ganglion (first order neuron) situated near temporal bone. - Peripheral processes of neurons in this ganglion form three divisions of trigeminal nerve, namely ophthalmic, mandibular, and maxillary divisions. - Central processes from these neurons of trigeminal ganglion enter pons in the form of sensory root of Cr-V. ### Termination - After reaching the pons, fibers of sensory root divide into two groups, namely descending fibers & ascending fibers. - Descending fibers terminate on primary sensory nucleus & spinal nucleus of trigeminal nerve: - Primary sensory nucleus is situated in pons. - Spinal nucleus of trigeminal nerve is situated below the primary sensory nucleus & extends up to the upper segments of spinal cord. - Ascending fibers of sensory root terminate in the mesencephalic nucleus of trigeminal nerve, situated in brainstem above the level of primary sensory nucleus. ## Central Connections - Majority of fibers from the primary sensory nucleus & spinal nucleus of trigeminal nerve ascend in the form of trigeminal lemniscus and terminate in ventral posteromedial nucleus of thalamus in the opposite side. - Remaining fibers from these two nuclei terminate on the thalamic nucleus of same side. - From thalamus, the fibers pass via superior thalamic radiation & reach the somatosensory areas of cerebral cortex. - Primary sensory nucleus & spinal nucleus of trigeminal nerve relay the sensations of touch, pressure, pain & temperature. - Fibers from mesencephalic nucleus form the trigeminocerebellar tract that enters spinocerebellum via the superior cerebellar peduncle of the same side. This nucleus relays proprioceptive impulses from facial muscles, mastication & ocular muscles. ## Pain - Pain: Unpleasant sensory or emotional experience associated with actual or potential tissue damage. - Pain is a protective sensation: - Pain gives warning signals about the existence of a problem or threat. It also creates awareness of injury. - Pain prevents further damage by causing reflex guard around injury. - Pain forces the person to rest or to minimize activities thus enabling rapid healing of injured part. - Pain urges the person to take the required treatment to prevent major damage. - Pain transduction by nociceptors: - Stimulation of pain receptors (nociceptors) opens specific transduction channels. - Opening of these channels → Na+ & Ca't inflow → DP. - Damaged tissues → proteolytic enzymes, K⁺, histamine, serotonin, ATP - Proteolytic enzymes → Kinins (e.g. bradykinin) & PGS. - All these substances →sensitivity of pain receptors (pain sensitizers). - Pain receptors (Nociceptors): "Free nerve endings" of Ad & C fibers - Types: - **Mechanical pain receptors:** Mechanical stimuli open transduction ion channels called "degenerin channels". - **Thermal pain receptors:** stimulated by extremes of temperature: - extreme heat opens TRPV12 channels. - extreme cold opens Anktm channels. - **Chemical pain receptors:** e.g. HCL in peptic ulcer → opens Acid Sensing Ion Channels (ASIC). - **Polymodal pain receptors:** respond to all type of stimuli - Adaptation: Slowly or non-adapting at all - Distribution: - More in: skin, periosteum, arterial walls & joints - Less in: deep tissues & internal viscera - Absent in: Liver parenchyma, Lung alveoli, Brain & Bones - Types of pain : - According to site - Cutaneous - Deep - Visceral - Neuropathic - According to quality - Fast (Acute): 1st pain - Slow (Chronic): 2nd or delayed pain ## Pathways of pain sensation: ### Lateral spinothalamic - **Paleospinothalamic pathway:** for slow pain (& thermal sensation especially heat) - 1st order neuron: Dorsal Root Ganglia (DRG) - Dentrites: - Unmyelinated: C-fibers (mainly) - Diameter → less than ₁μ - Velocity → ٠,٥ - ۲ m/sec - Release substance-P - Myelinated: Αδ fibers (few) - Diameter → ۱ - ٥μ - Velocity → ٣٠ - ٥ m/sec - Release glutamate - Axons: ascend or descend a few segments in Lissauer's tract - 2nd order neuron: Cells of Substantia Gelatinosa of Rolando (SGR) in laminae II & III. Axons cross to opposite side in front of central canal & ascend in the lateral column of the spinal cord. - Termination: - 1/5 of fibers (C-fibers) → Ventrobasal nucleus of thalamus - 4/5 of fibers (Ad fibers) → - Nuclei of reticular formation in brainstem → Non-specific thalamic nuclei - Tectum of midbrain - Gray matter surrounding the aqueduct of Sylvius. ### Neospinothalamic pathway: - for fast pain (& thermal sensation especially cold) - 1st order neuron: Dorsal Root Ganglia (DRG) - Dentrites: Ad fibers - Axons: ascend or descend a few segments in Lissauer's tract - 2nd order neuron: Cells of marginal nucleus in lamina I - Axons cross to opposite side in front of central canal & ascend in the lateral column of the spinal cord forming the spinal lemniscus together with ventral spinothalamic & paleospinothalamic fibers in the brain stem - 3rd order neurons of pain pathways are the neurons in: - Thalamic nuclei - Reticular formation - Tectum - Gray matter around the aqueduct of Sylvius. - Axons from these neurons reach the somatic sensory cortex via posterior limb of internal capsule. Some fibers from reticular formation reach hypothalamus. - Slow pain is perceived mainly in thalamus while fast pain is appreciated in sensory cortex & thalamus. - Perception of slow pain: in medial (non-specific) part of thalamus & limbi cortex (especially cingulate gyrus) ⇨ emotional reactions to pain (especially slow pain) - Perception of fast pain: in lateral part of thalamus & somatic sensory area I ⇨ information about location & intensity of pain. - **Reactions to pain:** - In fast pain : - Motor → Withdrawal reflex (to remove injured part from painful stimulus) - Autonomic → ↑ HR & ↑АВР - Emotional → Screaming & anxiety - In slow pain : - Motor → Guarding rigidity of overlying muscle as in cases inflamed appendix (↑ muscle tone to protect inflamed area → limits movement of the injured part → gives it a better chance for healing) - Autonomic → ↓ HR & ↓ ABP, sweating, nausea & vomiting - Emotional → Depression - Unlike other types of pain, cutaneous (superficial) pain is accurately localized because: - High density of pain receptors in skin. - Fast pain fibers reach the sensory cortex. - Touch & vision help in localization of pain - **Deep pain** - C - fibers pain - Def.: Musculoskeletal pain produced from muscles, tendons, ligaments, joints &periosteum of bones. - **Causes:** - Inflammation - Mechanical trauma - Ischemia ### Ischemic pain - **Cause:** - Narrowing or compression of an artery → Ischemia → activate pain receptors - **Mechanism:** - Accumulation of metabolites & release of proteolytic enzymes - **Examples:** - ischemia intermittent claudication ### Visceral pain - c - fibers pain - **Sharp cut** in viscera doesn't cause pain, but diffuse stimulation of pain nerve endings causes severe pain (Localized injury e.g. knife cut doesn't produce pain). This is because viscera have few pain receptors. - Pain from viscera is carried along C-fibers "slow dull poorly localized pain". - while pain from peritoneum, pleura or pericardium is carried along Að fibers "sharp well localized pain". - **Causes:** - Ischemia↑↑ H+, K+, bradykinin & PGs - Inflammation: products of inflammation → activation of pain receptors - Irritation e.g. chemical irritation by HCL in peptic ulcer - Overdistension of a hollow viscus e.g. urinary bladder & gall bladder - Spasm of a hollow viscus e.g. gut, ureters & gall bladder - (4) & (5) cause: (a)Compression of blood vessels → Ischemic pain - (b)Mechanical stimulation of pain receptors - **Characters:** Dull aching (except when peritoneum or pleura are involved) - **Diffuse** (poorly localized) due to - pain receptors in viscera are small in number. - less accurate topographic representation of viscera in cerebral cortex. - Depressor autonomic changes: ↓ HR & ↓ ABP, nausea, vomiting & sweating - Rigidity of overlying muscle (Guarding rigidity) - Referred to skin surface ### Referred pain - It is a painful sensation at a site other than the injured one. The pain is not localized to the site of its cause (visceral organ) but instead is perceived at a distant site (usually a skin area). - Dermatomal rule: - Pain is referred to a structure, which is developed from the same dermatome from which the pain producing structure is developed. - A dermatome includes all the structures or parts of the body, which are innervated by afferent nerve fibers of one dorsal root. For example, the heart and inner aspect of left arm originate from the same dermatome. So, the pain in heart is referred to left arm. - **Mechanism** - Convergence-projection theory: - Afferent pain fibers from the diseased viscus & skin arising from the same dermatome converge on the same neuron of SGR & activate the same cortical neuron. - The brain projects the pain sensation as if it is coming from the skin. - The reason for this misinterpretation may be: - Skin: - is well represented (topographically represented) in cerebral cortex while viscera are not. - is more rich in pain receptors. - is more exposed to stimulation. - Brain: is more accustomed to receiving pain from skin than from viscera. - **Facilitation theory:** - Afferents from diseased viscus give collaterals to SGR cells which also receive pain from skin, producing EPSPs on them → Facilitation. - **Examples:** - Cardiac pain: is felt retrosternal, and referred to left shoulder, left arm, root of the neck, lower jaw or epigastrium - Gall bladder pain: is referred to right shoulder, tip of right scapula & epigastrium - Gastric pain: is referred to area between xiphoid process & umbilicus - Pancreatic pain: is referred to back - Renal pain: is referred to flank (loin), inguinal region (groin) & also to testicles in * - Appendicular pain: is referred to area around umbilicus - Ovarian pain: is referred to umbilicus - Testicular pain: is referred to pelvis & abdomen - Diaphragmatic pain: is referred to shoulder - Headache: - **Test of pain sensation:** - The sharp and dull end of any objects like a safety pin, a reshaped paperclip, or neurological pin is used. - The sharp and dull end is randomly applied perpendicular to the skin, should not be applied too close to each other or in a too rapid manner to avoid the summation of impulses. - The patient is asked verbally to indicate sharp/dull when a stimulus is felt. - All areas of the body should be tested. - After testing the instrument should be sterilized or disposed. ## Deep reflexes = Tendon jerks - Dynamic stretch reflex: is the basis of deep reflexes - Stimulus Sudden & quick stretch - Receptors Nuclear bag fibers - Afferent Primary (la) - Center α-AHCs (monosynaptic) - Efferent Αα - fibers - Response Sudden strong contraction followed by sudden relaxation - **How to test:** - Examined muscle should be slightly stretched - Use the reflex hammer - Strike the tendon suddenly & strongly, this will make the muscle more stretched - Compare both sides - Reinforcement may be used if the reflex is weak e.g clenching the teeth (in U.L jerks) ▪ asking the patient to grasp his/her hands together & exert max. effort to pull them apart "Jendrassik's maneuver" (in L.L jerks) - These maneuvers act by the following mechanisms: - ↑ discharge of y - efferent neurons - distracting patient's attention to prevent any voluntary inhibition of the reflex - **Important tendon jerks:** - **Biceps reflex:** - Center C₅,₆ - Elbow is flexed 120° & examiner's thumb is put on biceps tendon & thumb is stroked by the hammer - Response contraction of biceps & flexion of the elbow - **Triceps reflex:** - Center C₆,₇,₈ - Elbow is flexed 90° & triceps tendon above its insertion on the olecranon is stroked by the hammer - Response contraction of triceps & extension of the elbow - **Knee jerk** - Center L₂,₃,₄ - Seat the subject so that knees become semiflexed - Strike the patellar tendon below the patella by the hammer - Response contraction of quadriceps & extension of the knee - **Ankle jerk** - Center → Lo & S₁,₂ - Foot is slightly dorsiflexed - Strike tendoachilles by the hammer - Response contraction of the calf muscles (gastrocnemius & soleus) & planta flexion of the ankle - **Jaw jerk** - Center → Trigeminal nerve nuclei - Place the tip of your index finger on a relaxed jaw, that is about one-third open - Tap on your index finger → Flexion of the mandible. - **Significance of tendon jerks:** - Localization of spinal cord lesions - Assessment of stretch reflex & muscle tone:- - Areflexia & atonia - due to interruption of the reflex arc - Causes: - Peripheral neuritis - Poliomyelitis - Tabes dorsalis - Complete T.S of spinal cord (stage of spinal shock) - Hyperreflexia & hypertonia - due to interruption of supraspinal inhibitory impulses - Causes: - Anxiety - Hyperthyroidism - U.M.N.L Σ - Tetany - Hyporeflexia & hypotonia - due to interruption of supraspinal facilitatory impulses - Causes: - Sleep & anesthesia - Hypothyroidism - L.M.N.LE - Neocerebellar syndrome - **Findings associated with hypertonia:** There is a background of facilitation (e.g. in lesion of internal capsule) - **Clasp knife rigidity (Lengthening reaction)** - If the elbow is passively flexed → moderate stretch of triceps → contraction of triceps (↑↑ tone) → ↑↑ resistance. (Stretch reflex) - Triceps becomes markedly stretched → relaxation (disappearance of tone) → sudden loss of resistance. (Inverse stretch reflex) - Lengthening reaction: Reaction of hypertonic muscles to passive stretch. - Clasp knife rigidity: when resistance suddenly disappears, this resembles sudden closure of a pocketknife. - **Clonus** Def.: regular rhythmic contractions & relaxations of the muscle when subjected to sudden maintained stretch. - Example: Ankle clonus (initiated by maintained dorsiflexion of foot → rhythmic planter flexion at the ankle). - Mechanism: Stretch reflex & Inverse stretch reflex. ## Polysynaptic spinal reflexes - **Inverse stretch reflex** - **Superficial reflexes:** ### Flexor withdrawal reflex: - Stimulus Injurious painful stimulus - Receptors Free nerve endings - Afferent Ad - Center Spinal cord (polysynaptic) - Efferent Αα - Response Flexion & withdrawal of limb away from injurious stimulus - Properties of flexor withdrawal reflex: - Reciprocal inhibition: contraction of flexors & relaxation of extensors. - Crossed extensor reflex: Extension of opposite limb if stimulus is severe enough. To allow opposite limb to support body weight when the other limb withdrawn. ### Planter reflex: - It is a polysynaptic spinal nociceptive reflex. It is strongly modified by corticospinal tract. - **Procedure:** - Stroke the lateral part of the sole of the foot with a fairly sharp object (starting from the heel towards the little toe, then along the bases of the toes medially) → planter flexion of the big toe and flexion & adduction of the other toes "flexor planter reflex". - In some patients, stroking the sole → dorsiflexion (extension) of the big toe and extension & abduction (fanning) of the other toes "extensor planter reflex = Babinski reflex". - The center of this reflex lies in Lo & S₁,₂. - **Babinski reflex** indicates the presence of dysfunction of the corticospinal tract such as in: - Cerebral hemorrhage or thrombosis. - Brain or spinal cord tumors. - Coma. - General anesthesia. - Deep sleep. - Children up to the age of one year. ### Positive supporting reaction: - **Stimulus** Deep pressure on sole of foot - **Response** Contraction of flexors & extensors to convert limb into rigid column to support body weight ### Superficial abdominal reflexes: - Stimulus Scratching skin of abdomen with blunt object - Response Contraction of abdominal muscles: - Upper abdominal muscles: center → T₁-T₄ - Lower abdominal muscles: center → T₁-T₁₂ ### Cremasteric reflex: - Stimulus scratching skin of upper inner aspect of thigh - Center → L₁-L₂ - Response contraction of cremasteric muscle → elevation of testis ## Near Response (Near Reflex) - Def.: Changes which occur in the eye when looking at a near object within the near point. ### Near point (Punctum proximum): - It is the shortest distance from the eye at which an object can be seen clearly using maximum accommodation. - In young adults ٧ - १० cm & at ١ years > १० cm due to presbyopia i.e. Near point increases with age. ### Far point: - It is the furthest point from the eye at which an object can be seen clearly without accommodation. - Normally, far point = ≥ 1 m or २० feet. ### Range of accommodation: - It is the distance between far & near points (Far point - Near point). - This distance decreases with age. - **Changes:** - **Accommodation** - dioptric power of lens (from ૨ DAT rest to ૩૪ D at full accommodation) to bring divergent light rays to a focus on the retina. - It is involuntarily controlled. - Light rays from near objects are divergent, they would form an image behind the retina & this image would be blurred if the power of the lens remains unchanged. - **Mechanism:** Young-Helmholtz theory - **Contraction of ciliary muscle** → tension in suspensory ligaments → allows elastic lens to recoil to a more spherical shape (radius of curvature becomes ৬ mm instead of १० mm). - Circular fibers: Make circle of attachment of suspensory ligaments narrower → decreases tension of the ligaments. - Radial fibers: Pull insertion of suspensory ligaments to ciliary body forwards → decreases tension on the lens. - Suspensory ligaments lax - Lens More spherical & convex (especially the anterior surface) - **Amplitude (Power) of accommodation:** - Refractive power during far vision is called static refraction (R). - Refractive power during near vision is called dynamic refraction (P). - Amplitude of accommodation (in D) = P - R. - Amplitude of accommodation in normal eye = १०-० १० D. - It decreases with age due to gradual loss of lens elasticity (presbyopia):- ૧૪ D in young adults, ૧२ D at the age of ૪૦ years, ૧ D at the age of ૬૦ years & less than ૧ D over the age of ٦૦ years (It decreases by about २ D every १० years). - **Miosis (Pupilloconstriction)** Involuntarily controlled - Aim: - ↑ visual acuity by preventing spherical & chromatic aberrations. - amount of light entering the eye. - depth of focus. - **Convergence of both eyes** It is voluntary controlled. Produced by contraction of medial recti muscles so that the images of near object fall on the fovea centralis of both retinae. - Aim: to prevent double vision (diplopia). - **Neural pathway of near reflex:** - Stimulus: Near object - Receptors: Pholoreceplors (rods & cones) - Afferent: Visual pathway - 1st O.N :- Bipolar cells - 2nd O.N:- Ganglion cells - 3rd O.N:- LGB in thalamus - Center: Superior colliculus in midbrain - Efferent: Occulomolor (3rd cranial) nerve - Temporal fibers - Nasal fibers ## Errors of refraction - Emmetropic eye: It is the normal eye in which parallel rays converge to focus on retina without accommodation. - **Myopia** - (Short sightedness) - Abnormally LONG eyeball - Inc. lens or corneal curvature - INFRONT of retina - DIVERGENCE - Biconcave divergent lens - LESS than १० cm - LESS than १ m - Worsens the condition - **Hypermetropia** - (Far sightedness) = Hyperopia - Abnormally SHORT eyeball - Dec. lens or corneal curvature - BEHIND retina - CONVERGENCE - Biconvex convergent lens - MORE than १० cm - Normal - Patient uses accommodation in far vision & needs more accommodation in near vision - Headache - **Astigmatism:** (Stigma = point) - Cause: Asymmetrical corneal (or less common lens) curvatures⇨ curvatures aren't the same in all meridia➡ rays passing in normal meridian has a focus while those passing in abnormal meridian has a focus in front or behind retina. - Effects: A point is seen as a line or a line having a halo on either side. - Types: - REGULAR - It is the condition in which there are two principle meridians separated by 90 degree - This is the most common form of astigmatism - IRREGULAR - It is the condition in which there are more than twoO principle meridians which are not separated by 90 degree - This is less common form of astigmatism - TTT: Cylindrical lens its longitudinal axis perpendicular to abnormal meridian. ### Presbyopia: - Def.: Gradual loss of lens elasticity by aging → Deficient accommodation. - Cause: Protein denaturation (Lens sclerosis). It is not due to weakness of ciliary muscle. - Optical changes: Far point doesn't change. Near point moves away from the eye → Image of near object lies behind retina (Decreased range of accommodation). - TTT: Convex lens for near vision e.g. for reading. - **Miosis** - Near vision (with accommodation) - Light adaptation - Horner's syndrome: Cutting of sympathetic supply to head & neck - Sleep: due to parasympathetic predominance - General anesthesia: Stage 1 (Surgical stage) - Drugs: - Parasympathomimetics e.g. pilocarpine, eserine - Morphine poisoning - Histamine - **Mydriasis** - Far vision - Dark adaptation - Lesion in oculomotor nerve or its nucleus - Sympathetic stimulation e.g. emotions, fear, pain, asphyxia - General anesthesia : - Stage 1: Dilated reactive pupil - Stage 2: Dilated fixed pupil (Stage of medullary paralysis) - Drugs: - Passive mydriasis (No LR): Parasympatholytics e.g. atropine - Active mydriasis (LR present): Sympathomometics e.g. adrenaline - Alcohol intoxication - Cocaine ## Pupillary light reflex (LR):- - When one eye is exposed to light, pupilloconstriction occurs in this eye (= direct reflex) as well as in the other eye (= indirect "consensual" reflex). - **Light-Near Dissociation (LND):** - It is a pupil that doesn't constrict in response to light but constricts during accommodation. - Accommodation remains intact as its pathway is not affected. - Occurs in cases where there is a lesion in the pretectal nucleus (center of light reflex) e.g. Neurosyphilis → Argyll Robertson pupil. - Adie's tonic pupil: The affected pupil is significantly larger than the normal pupil; the direct & indirect reflection of light from the affected pupil disappears & the affected pupil is sensitive to low concentrations of pilocarpine. - **Pathway of pupillary light reflex:** - Stimulus: Light - Receptors: Photoreceptors - Afferent: - 1st O.N Bipolar cells - 2nd O.N Ganglionic cells - Optic nerve Optic chiasma (nasal fibers cross while temporal fibers pass uncrossed) - Optic tract collaterals to center - Center: - Pretectal nucleus (in mid-brain) EWN of oculomotor n. - Efferent: From EWN⇨ ciliary ganglion postganglionic fibers (Short ciliary nerve) - Response: - Constriction of both pupils - There is no relay in LGB. - Consensual light reflex occurs because: - Crossing of fibers at optic chiasma - Pretectal nucleus supplies both EWN ## Corneal Reflex - Steps: - Ask the subject to look at the far wall. - Twist a small piece of cotton to a fine hair. - Approaching from the side, touch the lateral aspect of the cornea. - Repeat the same test on the other eye. - Bilateral blinking is the normal response. - This indicates that the ophthalmic division of the trigeminal nerve and the motor fibers of the facial nerve are intact. It is a protective reflex. - **Neural pathway:** - Receptors: Free nerve endings - Afferent: Ophthalmic division of trigeminal nerve - Center: Main sensory nucleus of trigeminal nerve - Efferent: Motor nuclei of facial neve of both sides (in pons) → Temporal branch of facial nerve - N.B Supraoptic nerve also conducts afferent impulses through the descending spinal tract of trigeminal nerve in medulla oblongata to the caudal spinal trigeminal nucleus. Then efferent impulses are conducted via the medullary pathway that ascends bilaterally to connect the facial nuclei in the pons. - Response: Contraction of orbicularis oculi muscle of both eyes → bilateral blinking ## Clinical significance of examining corneal reflex: - Dysfunction of corneal (blink) reflex can occur from: - Lesion to trigeminal nerve → No bilateral blinking. - Lesion of spinal nucleus of trigeminal nerve → No bilateral blinking. - Lesion of facial nerve → No blinking in the eye supplied by the affected facial nerve. - Confirmation of brain stem death: - It will be lost together with other brain stem reflexes e.g. pupillary light reflex, gag reflex ... etc. - Assessment of depth of anesthesia: Not preferred to be performed routinely because of the risk of corneal damage. ## **Color vision** - It is the ability of the retina to distinguish colors. - It is the function of CONES. - **Characteristics of colors :** - Hue: is the actual color e.g. red, green ... etc. - Primary colors: red, blue & green. Mixing them in different proportions ➡white or any other spectral color. - Complementary colors: are colors when mixed → white (i.e. They cancel each other) e.g. yellow & blue, red & green. - Extra spectral colors: are the colors other than those present in visible spectrum. These colors are formed by the combination of two or more spectral colors e.g. purple is the combination of violet & red. Pink is the combination of red & white. - **Mixing light of different wavelengths differs from mixing pigments:** - e.g. mixture of blue & yellow lights sensation of white. - mixture of blue & yellow pigments → green color. - **The normal eye:** can distinguish the colors of the spectrum & about १०० different intermediate colors. - Color perception is a retinal phenomenon while color translation is a cortical phenomenon. ## Color blindness: - A disease characterized by the inability to distinguish between colors. - Causes:- Hereditary sex-linked recessive. It affects ٨ % of ♂ & ٠,٤ % of - Types:- - Color anopia: Complete loss of function of one or more types of cones: - **Monochromatism:** Total color blindness = Achromatopsia - Rod monochromatism: Cones are functionless & the vision depends purely on rods. Their visual acuity is lowered, and foveal vision is absent → central scotoma. - Cone monochromatism: Vision depends upon one single type of cone. Central scotoma does not occur in this condition. The patients match their spectrum by varying intensity of one color. - Dichromatism: - Two cones are present. - Patients match their spectrum by mixing primary colors. - Subtypes: - Protanopia➡ red blindness. - Deuteranopia⇨ green blindness. - Tritanopia blue blindness. - Color anomaly (Trichromatism): - Patient can perceive the primary colors but one of them is weak. - One cone is weak: - Protanomaly weakness of red-sensitive cones. - Tritanomaly weakness of blue-sensitive cones. - Deuteranomaly⇨ weakness of green-sensitive cones (most common). - The most common test for color blindness uses ३८ Ishihara's color plates: - Each plate shows a number composed of various sizes circular spots that are of different color than the surrounding background spots. - A person with normal color vision can correctly identify the number written on all plates. - However, for each plate a person with a certain color vision defect can see different number than seen by a normal person. - **Ishihara test mainly detects red-green color blindness.** - The passing score is १२ correct of १४ red/green test plates (not including the demonstration plate). Scores below twelve indicate color vision deficiency, and twelve or more correct indicate normal color vision. ## Visual acuity - Def.: It is the ability to perceive the details of objects seen by the eye (It is the ability to see "points as separate points") - **Requirements:** - **Visual angle:** (angle subtended by the light rays from separate points at the nodal point of the eye) must be at least minute - **Light rays from each of the points must stimulate cones separated by at least one unstimulated cone.** - **These conditions allow the retinal images to be micrometers from each other & allow the receptors to have separate connection lines to the brain** - **Factors affecting:** - **Illumination & contrast:** in low illumination, visual acuity ↓ as cones aren't stimulated - **Pupilloconstriction:** visual acuity by ↑ depth of focus - **Max. at fovea centralis:** where cones are closely packed - **Errors of refraction & eye diseases:** ↓ visual acuity - **Clinical determination:** - Charts: - Snellen's alphabetical chart - Broken C-circle of Landolt - **How to test:** - Person sits ٦ meters from the chart to avoid accommodation. - The chart contains letters or broken circles which can be seen by a normal person at ٦,٩١٢١٨٢٤٣٦٦٠ meters & are of varying sizes (big at the top row and smaller in each next row) & placed in rows. - The ends of the letters or broken circles of the smallest row make an angle of minute at a distance of meters & the ends of the letters or broken circles of the biggest row make an angle of minute at a distance of ٦٠ meters. - Test each eye separately, then test both eyes together. - Visual acuity is expressed as a ratio bet. one's acuity to normal e.g. - if the person can see all mark, visual acuity is 6/6 - if the person can't see except the 1st row, visual acuity is ٦/٦٠ ## VISUAL FIELD - Def. part of the world seen by one fixed eye. - **Measurement:** - Perimeter (Goldmann perimetry) - Bjerrum (Tangent) screen - Confrontation method - Humphrey field analyzer (useful to test the central portion of visual field) - **Shape:** Not circular. - It extends to more than ٩٠º at temporal side (١٠٠º). - It is cut off to : - ٦٠º at nasal side by nose. - ٦٠º at upper side by eye brows. - ٧٥º at lower side by cheeks. - Horizontal meridian: ١٠٠º + ١٦٠º = ٦٠º - Vertical meridian: ١٣٥º = ٧٥º + ٦٠º - **Area for colors:** - Widest for white targets. - Becomes successively smaller for blue, red & green targets. - **Importance of measurement of visual field:** - (۱) Helps in diagnosis