Anatomy of spinal cord

          It starts at the foramen magnum and ends at the L1-L2.
Spinal Cord is 2cm in diameter and 43 – 45cm long.
It has greater diameter  at cervical and lumbar region due to cervical and lumbar plexus respectively. There are 21 pairs of spinal nerves which are classified below.
8 pairs of cervical spinal nerves
12 pairs of thoracic spinal nerves
5 pairs of lumbar spinal nerves
5 pairs of sacral spinal nerves
1 pair of coccygeal spinal nerves

Sensory tracts of spinal cord
Anterior spinothalamic and Lateral spinothalamic tract
          These tracts conduct nerve impulses responsible for pain, warmth, cold, itching, tickling, deep pressure, and crude poorly localized sensation of touch.
Posterior column
          This tract perceives the following sensations.
Propioception: sensation of joint, muscle and tendon positions.
Discriminative touch: to localized exactly which part of the body is touched.
2-point discrimination: two parts fairly close to each other can be differentiated.
Light pressure
Vibration
Motor tracts of spinal cord
Direct pathway (pyramidal)
Indirect (extra-pyramidal)
Direct tracts

• Lateral corticospinal tract

          At the junction of medulla oblongata and spinal cord 90%of motor tracts cross over to the opposite side. They control skillfull and fine movements of distal part.
Anterior corticospinal tract
          This tract is formed by 10% motor tracts and they cross-over at their exiting level from spinal cord. They control movements other than skillful.

• Cortico-bulbar tract

          They start in brain and also ends in the brain. They help in positioning, equilibrium, balance in response to visual stimulus. They also maintain muscle tone.

Indirect tracts

• Vestibulo-spinal tract: Present in internal ear responsible for equilibrium.
• Rubro-spinal tract: Present in the red nucleus of mid brain responsible for trunk muscles( movements).
• Lateral-reticulo-spinal tract: Present in reticular formation of brain stem. It inhibits flexor reflexes and facilitates extensor reflexes.
• Medial reticulo-spinal tract: Present in reticular formation of brain stem. It inhibits flexor reflexes and facilitates extensor reflexes.

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Anatomy of Eye

Accessory structures


• Extrinsic muscles of eye

         

          The following are six extrinsic muscles of the eye.

Levator palpebrae superioris muscle (Oculomotor nerve)

Superior rectus muscle (Oculomotor nerve)

Inferior rectus muscle (Oculomotor nerve)

Medial rectus muscle (Oculomotor nerve)

Inferior oblique muscle (Oculomotor nerve)

Superior oblique muscle (Trochlear nerve)

Lateral rectus muscle (Abducent nerve)

• Eyelids (palpebrae)

         There are two eyelids: upper and lower. Upper eyelid is more mobile due to presence of levator palpebrae superioris muscle. The angles (2) at which the two eyelids meet are called commisures (medial and lateral). The medial commisure contains an elevation called as commissural carbuncle which secretes whitish color material when we are asleep.

Layers of eyelids

          From superficial to deep: skin→ subcutaneous tissue→ fibers of levator palpebrae superioris (upper eyelid only) muscle→ tarsal plate→ tarsal (meiobiam) glands→ conjunctiva (Palpebral)

          Reflection of the conjunctiva onto the surface of the eye is called bulbar conjunctiva. The portion where the Palpebral conjunctiva transforms into bulbar conjunctiva is called fornix of the eye.

• Eye lashes

          They are located at the margins of both the eyelids.

• Eye brows

          Each eye brow is located above an eye.

• Lacrimal apparatus

          It includes lacrimal glands, lacrimal ducts, lacrimal puncta, lacrimal canals, lacrimal sac and nasolacrimal duct.

Anatomy of eye

          Each eye is 2.5cm in diameter having the following three layers.

1. Fibrous tunic (outer layer) anterior one sixth is cornea and posterior five sixth is sclera.

2. Vascular tunic (middle layer)

3. Retina

Fibrous tunic

Cornea

         It is transparent avascular (no vessel) layer which is subdivided into three layers.

External is made up of non-keratinized stratified squamous epithelium.

Middle is of collagen fibers.

Internal layer is of simple squamous epithelium.

Sclera

         This portion of fibrous tunic is made up of dense connective tissue (collagen fibers and fibroblasts). It gives shape to the eye.

         There is a canal at junction of sclera and cornea called as canal of schlemm through which the aqueous humor drains to the venous sinus.

Vascular tunic (uvea)

          Choroid, ciliary body and iris are the three parts of the vascular tunic. Posterior two third is the choroid which is highly vascularized and helps in the nourishment of both sclera (outside) and retina (inside). Slightly posterior to the junction of sclera with cornea or at the level of ora serrata of the retina, choroid transforms in a slightly expanded part called ciliary body. Projections of ciliary body are called ciliary processes.  
          Ciliary processes secrete aqueous humor which helps in the nourishment of the lens and internal portion of cornea. They have thread like structures called as zonular fibers (suspensory ligaments) which attaches to the lens and helps in the accommodation of lens.

         Ciliary body has muscle called ciliary muscle which on contraction causes stretching of zonular fibers (suspensory ligaments) helping in accommodation phenomenon of the lens. Long extensions from ciliary body which surrounds the anterior part of the lens is called iris. Iris contains two types of muscles: circular and radial muscle.

         Parasympathetic innervation causes contraction of circular muscle and hence constricts the pupil. Pupil is the central hole in the iris. Sympathetic innervation causes contraction of the radial muscle and hence causes dilation of pupil.

Retina

         Retina is the innermost layer of the eye. It covers the posterior 2/3 circumference of eye. Retina is further divided into the following layers.

Pigmented layer

Neural layer

• Pigmented layer

          Pigmented layer is made up of melanin containing epithelium cells. This layer prevents scattering of light within the eye and prevent reflection of light. It is located in between the choroid of the vascular tunic and neural layer of the retina.
         Neural layer has three types of cells divided by two synaptic layers. The most external layer of cells is rods and cones. There are 120 million rods and 6 million cons in one eye. Rods are responsible for dim light. These cells are mostly active at night. Decrease number of rods will create difficulty night vision (black difficulty in night vision or black and white).
         Rods are concentrated towards the periphery. Cones are responsible for bright day light and color vision. Cones are mainly concentrated towards the center. Just opposite to the central axis of the eye in the retina is a yellow spot called macula lutea. In the center of macula lutea is a depression called fovea centralis. It contains only cones and no rods and is responsible for acuity (sharpness or resolution) of vision.
         Both rods and cones form the photoreceptors part of the neural layer o f the retina. From the photoreceptor cells (rods and cons) the visual impulses flow through the outer synaptic (plexiform) layer into the middle part of the neural layer and synapses with the bipolar cells. Bipolar cells in turn synapses with ganglion cells through the inner synaptic (plexiform) layer. Axons of ganglion cells then converge at the optic disc forming the optic nerve which then leaves the orbit of the eye. Optic disc is also called blind spot of the eye. If the light falls here no image will be formed.

Lens

          The cavity of the eye is divided into 2 parts: anterior and posterior cavity by the presence of lens. Lens is made up of crystalline cells which are arranged in the form of an onion (layers). Externally the lens is covered by a capsule. Lens like cornea is transparent and devoid of blood and lymphatic vessels. The anterior cavity of the eye is further divided into 2 chambers: anterior and posterior by the iris.
          Aqueous humor produced by the ciliary processes, first enters the posterior chamber and then flow into the anterior chamber through the pupil. From the anterior chamber aqueous humor enter the canal of schlemm and drain into the venous system. Posterior cavity is filled with vitrous body.
         Vitrous body is a gel like structures which is formed during embryological development. It helps to maintain the position of the retina against the choroid. Any damage or leakage of the vitrous body will cause displacement of the retina. Vitrous body along with the aqueous humor helps in the formation of intra ocular pressure (16mmhg).

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Anatomy of Ear

Ear has three main parts
External ear
Middle ear
Inner ear

• External ear:

          It is consist of auricles (pinna), external auditory meatus, and external auditory canal which ends at the the tympanic membrane (ear drum).

          Auricle is consist of an upper helix and anti helix. lower part is called  lobule. The upper part of lobule is called antitragus which lies opposite to the tragus. The opening to the external auditory meatus is called conchae. External auditory meatus is the bony opening into the external auditory canal. The external 1/3 of the external auditory canal is covered by the skin, while the internal 2/3 is covered by mucous membrane. This part of the external auditory canal  contains specialized ceruminous glands which produce cerumen (wax like substance). Cerumen helps to protect the middle ear from foreign particles .External auditory canal is to 2.5cm in length. it ends at the tympanic membrane. Tympanic membrane is 1cm in diameter.
          Point of attachment of the handle of the malleus produces a projection at the centre of the tmpanic membrane called as umbo. Part of the membrane which lies above the umbo is called as pars flaccida. The remaining lower part is called pars tensa. The margins of the tympanic membrane attaches to the groove inside the temporal bone called as annulus.

• Middle ear

          Also called as tympanic cavity. It is an air filled cavity which lies in the petrous part of the temporal bone. Externally it is separated from the external ear by the tympanic membrane (ear drum). Within the cavity of middle ear lie the three most smallest bones  of the body namely malleus (hammer), incus (anvil), stapes (stirrup). The handle of the malleus is attached to the internal surface of the tympanic membrane at its centre. the head of the malleus attaches to the incus. Incus in turn attaches to the head of the stapes.         

        Footplate (base) of the stapes attaches to the oval window. Two small muscles namely tensor tympani muscle is attached to the tympanic membrane, while the second muscle stapedius muscle is attached to the stapes. These muscle on contraction control the frequency of sound , by altering the vibrations of the the structures to which they are attached (tympanic membrane and stapes). Nerve damage cause hypercoustica. In the anterior wall of the middle ear is an opening which leads into the superior part of the nasopharnyx through a canal called Eusthacian tube (auditory tube).  This tube helps in the maintanance and regulation of atmospheric pressure inside and outside the middle ear.

• Internal ear

           It is consist of three parts.
Vestibule
Semicircular canals
Cochlea
          All these parts have a bony portion and a membranous portion. Bony part is lined with periosteum and contains a fluid called perilymph. Membranous part is lined with epithelium and contains fluid called endolymph.

          Vestibule is the mddle part of the internal ear. Oval window opens into the vestibule. Vestibule in turn contains two small cavities which are called the utricle and saccule (little sac). Utricle and saccule contains receptors for equilibrium. Utricle is connected posteriorly to three semicircular canals. These three canals are oriented at 90 to eachother. Inside the semicircular canal lie the semicircular ducts which has a confined swelling at each end called ampulla. Ampulla also contains receptors for equilibrium. The three semicircular canals are called anterior, posterior and lateral.

          Conchae is a snail shaped organ which has 2 1/2 (2S) turns. The oval window opens into the vestibule to cochlea through the scala vestibule.The bony part of the cochlea make turns around a central pillar called modiolus. The modiolus at its upper end diverge into Y shaped membranes called vestibular membrane and basilar membrane. On the one side of the modiolus and vestibular membrane lie the scala vestibuli. On the other side of modiolus and basilar membrane lies the scala tympani. Scala vestibuli and tympani communicates with each other only at the apex of cochlea which is helicotrema. Between the vestibular membrane and basilar membrane lies the membranous cochlea which is called scala media (cochlear duct). On the internal surface of basilar membrane lies coiled  and arranged  in coiled form cells called hair cells and supporting cells. Hair cells are further divided into inner and outer hair cells.  Inner hair cells are arranged in 1 layer, while outer hair cells are arranged in 3 layers. These cells are covered by a thin membrane called tectorial membrane.

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