The Peculiarities of Human Body - Assignment Example

Describe and explain the structure and function of the eye and ear. Human eye is considered as the most intricate organ of the body. Eye contains eyeball which is supplied with nerve fibres and blood vessels, it has muscles for movement. The structure of the human eye is spherical with a bulge in front. Parts of eye are- Sclera- the tough white portion of the eye (opaque), in the front (anterior) it is transparent and called as conjunctiva (present on the edge of cornea). It covers moist surfaces on the anterior and posterior side of the eyeball. Cornea- is the transparent dome like structure present on the anterior side of the eye. It is transparent and allows light to enter the eye, focuses light on lens. It acts as a protective layer of the eye. From cornea light travels through pupil. Pupil- the aperture or the opening which dilates in dim light and constricts in bright light. It acts like an aperture of camera Iris- it is the pigmented part of the eye. It gives colour to the eye. It controls the size of aperture of the pupil through papillary sphincter muscle and dilator muscle. Lens- present behind the iris, supported by the zonula (fine fibrils). It changes shape to focus light on retina with the help of ciliary muscles. It becomes thick when it focuses on nearby objects while it becomes thinner when it focuses on distant objects. Choroid- it is vascular coat of the eye and provides nutrition to retina at its inner side. The ciliary body is present on the anterior side and contains smooth ciliary muscles, contraction of these muscles adjusts the diameter of lens and enables the eye to focus on the objects. Aqueous humour (AH) (transparent) is secreted by the ciliary epithelium it maintains ocular pressure. Ciliary muscles provide attachment to the iris Iridocorneal angle contains meshwork of cells, it contains Schlemm’s canal which carry out AH from anterior chamber into venous system, the process is called as aqueous drainage. Anterior chamber is between cornea (anterior) and lens and iris (posterior), from the posterior part of the lens up to retina, translucent fluid is present and this is vitreous humour (VH). Eye is present in the bony part- orbit and at its posterior apes is the optic canal containing optic nerve which takes the signals of vision to the brain. Blood vessels and cranial nerves enter the eye through opening called orbital fissures. Lachrymal glands are present on the anterior side in the super-lateral facet, a fossa is present on the anterior medial wall for lachrymal sac, secretions of lachrymal glands prevent the eyes from drying. Retina- is the most important layer on which inverted image is formed by the lens of the eye. It contains rods (which have rhodopsin and are responsible for night vision), and cones (which are responsible for coloured vision). Image falling on the black spot cannot be seen but image is sharpest at the yellow spot which is present on the optic nerve and sends the message to the brain. Optic nerve- originates from the eye and is surrounded by dura mater, arachnoid mater and the pia mater which continues up to the brain. Eye rotation. Primary muscles of the eye that are responsible of movement in all direction are agonist, synergist are the same muscles that move the eye in the same direction as agonist, while antagonist move the eye in the opposite direction (Segre & Bagi, n.d.). Eye is the sense organ for sight, light enters through cornea, enters pupil and adjusted by lens to form image on retina. Structure and Function of Ear The human ear is divided into three parts- the outer ear, the middle ear and the inner ear. Ear is the sense organ for hearing. The outer ear - Pinna- the auricle, visible on the head. It acts like a funnel, collects and directs sound waves into the external auditory canal. Tympanic membrane- commonly known as ear drum. It extends across the end of auditory canal. It transfers vibrations to the middle ear, thus, tympanic membrane separates external ear from middle ear. The Middle ear - Middle ear- contains three bones the ear ossicles. These are hammer shaped “malleus”, anvil shaped “incus” and stirrup shaped “stapes”. They carry the sound vibrations to the inner ear. Oval window connects ear ossicles to the cochlea Round window- separates middle ear, function is to maintain proper air pressure in the cochlea. Inner ear - Cochlea- the fluid filled chamber, contains cochlear fluid which transfers the sound vibrations, it has snail-like spiral structure, which is tubular. It contains receptors for sound. The cochlear duct contains organ of Corti, they comprise tiny auditory receptors. It changes sound vibrations into electromagnetic impulses which are sent to the brain through auditory nerve. Vestibule- associated with balance of the body. Eustachian tube- a tube between pharynx and middle ear, it remains closed and open during yawning and swallowing. It is important as it is responsible for equalizing pressure on both side of the ear drum. Ear collects sound waves (through pinna), converts them into pressure waves leading to the vibration of tympanic membrane, which vibrates to transfer sound waves to the middle ear, the three ossicles vibrate and direct the vibration towards oval window, a pressure wave is generated in cochlea, which travels through the cochlear fluid and round window is pushed the vibrations are sensed by organ of Corti which sends the message through auditory nerve to brain (Structure and Function of Human Ear, n.d.). Explain the changes that occur within a nerve cell and across its membrane during the process of nerve impulse initiation. Nerve impulse is transmitted along a neuron by means of electrical changes. Polarisation- In an un-stimulated or polarised neuron more negative ions i.e. potassium ions (K+) are present on the inner side, and more sodium ions (Na+) are present on outer side. Some amount of Na+ and K+ ions are always leaking by means of leakage channels, but their substantial concentration across the membrane is maintained through Na+/K+ pumps present on the neuronal membrane. This is called as resting potential (-70 mv). Gated channels- these channels come in action in response to changes across the membrane, stimulus or in response to neurotransmitters. Graded potential- it occurs when stimulus results in opening of Na+ and K+ gated channels. When stimulus opens Na+ channel, positive Na+ enters and membrane depolarises (it becomes positive). When K+ channels are stimulated, K+ moves outside the membrane and membrane becomes more negative, i.e. it hyperpolarizes. Graded potential is local event and takes place in dendrites and cell bodies and is generated by any stimulus, does not travel far. Action potential- it can travel long distances. If the graded potential is strong Na+ channels open, in the reply of Na+ the membrane becomes depolarised on outside, strong stimulus may further open the Na+ gates allowing more Na+ to move in and the membrane become depolarised (from -70 to +30 mv) causing more Na+ channels to open this enables the impulse to travel along the nerve fibre, bringing nerve conduction. Repolarisation- in reply of Na+ inflow, K+ channels also open, permitting K+ to move from inside to outside of the cell, which results in repolarisation and thus membrane polarisation is restored. This is different than resting potential as in repolarisation Na+ are inside and K+ are outside the cell, the stage is followed by opening of K+ gates while Na+ gates close. Hyperpolarisation of membrane occurs because, as the K+ channels close down more K+ moves out of the cell than required for establishing polarisation thereby membrane becomes hyperpolarised. Refractory period- membrane is polarised with Na+ and K+ on the wrong side, no response to stimulus occurs (Transmission of Nerve Impulses, n.d.). Explain how the synapse is involved in transmitting the nerve impulse from one neuron to another. Neurons align themselves to form a electrical network and therefore they are interconnected. Due to action potential, nerve impulse reaches the terminal ends of axons, but neurons do not have direct connectivity (cell to cell), instead there is a fluid-filled gap called synapse, the signals cannot jump out to reach the next cell, therefore neurotransmitters are released from the presynaptic (sending neuron) to reach the postsynaptic membrane, as soon as the neurotransmitters bind to postsynaptic membrane, action potential is generated and nerve conduction continues. Types of synapses- Electrical synapse- they help in direct transfer of ionic current. They are found in gap junctions. Chemical synapse- electric impulse is transmitted when one neuron releases s chemical substance called neurotransmitter into synaptic cleft, this is detected by the second neuron and gets stimulated and various physiological changes takes place in the second neuron, which are essential for the conduction of impulse. However, these changes are induced as a result of depolarisation of the membrane of first neuron. Excitatory synapses- are responsible of bringing (EPSP) excitatory electrical changes in postsynaptic potential. Inhibitory synapses- are responsible of bringing (IPSP) inhibitory postsynaptic potential. Neurotransmitters They are simple molecules and are categorised as- 1. Choline- e.g. acetylcholine 2. Biogenic amines- catecholamine (such as dopamine and norepinephrine), histamine and serotonin. 3. Amino acids- aspartate and glutamate are recognised as excitatory neurotransmitters, on the other hand, taurine, Glycine and GABA (gamma-aminobutyric acid) are inhibitory neurotransmitters. 4. Neuropeptides- they are produced by long sequence of amino acids. 50 neuropeptides are present in brain 5. Dopamine- it is an important neurotransmitter, responsible for controlling arousal levels as well as motor control in many parts of the brain. Reduction in the level of dopamine causes Parkinson’s disease 6. Serotonin- its concentration is increased by antidepressants therefore it is called as feel-good neurotransmitter. 7. Acetylcholine- controls various functions of brain encompassing learning, attention and memory. Lower concentration leads to Alzheimer’s disease 8. Noradrenaline- it is responsible for elevated mood and mental arousal. (Cardoso, n.d.) References Cardoso SH. N.d. Communication Between Nerve Cells. [online]. Available at: . [Accessed 7 May 2015] Figure: Structure of eye- Garrity, J. N.d. Structure and Function of the Eyes. [online]. Available at: . [Accessed 7 May 2015]. Segre L, Bagi S. n.d. Human Eye Anatomy: Parts of the Eye. [online] Available at: . [Accessed 7 May 2015]. Structure and Function of Human Ear, n.d. . [online] Available at: . [Accessed 7 May 2015]. Transmission of Nerve Impulses, n.d. [online]. Available at: . [Accessed 7 May 2015]. Read More