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Exercises1. Briefly describe the structure of the following:(a) Brain (b) Eye (c) EarAnswer(a)
Brain: The human brain is well protected by the skull. Inside the skull, the brain is covered by cranial meninges consisting of an outer layer called dura mater, a very thin middle layer called arachnoid and an inner layer (which is in contact with the brain tissue) called pia mater. The brain can be divided into three major parts:
(i) Forebrain: The forebrain consists of cerebrum, thalamus and hypothalamus.
(ii) Midbrain: It is located between the thalamus/hypothalamus of the forebrain and pons of the hindbrain.
(iii) Hindbrain: The hindbrain comprises pons, cerebellum and medulla.
(b)
Eye: The adult human eye ball is nearly a spherical structure. The wall of the eye ball is composed of three layers. The external layer is composed of a dense connective tissue and is called the sclera. The anterior portion of this layer is called the cornea. The middle layer, choroid, contains many blood vessels and looks bluish in colour. The choroid layer is thin over the posterior two-thirds of the eye ball, but it becomes thick in the anterior part to form the ciliary body. The ciliary body itself continues forward to form a pigmented and opaque structure called the iris which is the visible coloured portion of the eye. The eye ball contains a transparent crystalline lens which is held in place by ligaments attached to the ciliary body. In front of the lens, the aperture surrounded by the iris is called the pupil whose diameter is regulated by the muscle fibres of iris.
The inner layer is the retina and it contains three layers of neural cells from inside to outside – ganglion cells, bipolar cells and photoreceptor cells. There are two types of photoreceptor cells, namely, rods and cones. The daylight (photopic) vision and colour vision are functions of cones and the twilight (scotopic) vision is the function of the rods. The innermost ganglionic cells give rise to optic nerve fibre that forms optic nerve in each eye and is connected with the brain.
(c)
Ear: It perform two sensory functions, hearing and maintenance of body balance. It can be divided into three major sections called the outer ear, the middle ear and the inner ear:
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Outer ear: It consists of the pinna and external auditory meatus (canal). The pinna collects the vibrations in the air which produce sound. The external auditory meatus leads inwards and extends up to the tympanic membrane (the ear drum). There are very fine hairs and wax-secreting glands in the skin of the pinna and the meatus. The tympanic membrane is composed of connective tissues covered with skin outside and with mucus membrane inside.
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Middle ear: It contains three ossicles called malleus, incus and stapes which are attached to one another in a chain-like fashion. The malleus is attached to the tympanic membrane and the stapes is attached to the oval window of the cochlea. The ear ossicles increase the efficiency of transmission of sound waves to the inner ear. An Eustachian tube connects the middle ear cavity with the pharynx. The Eustachian tube helps in equalising the pressures on either sides of the ear drum.
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Inner ear: It is also known as labyrinth. Labyrinth is divided into bony labyrinth and a membranous labyrinth. Bony labyrinth is filled with perilymph while membranous labyrinth is filled with endolymph. Membranous labyrinth is divided into two parts – Vestibular apparatus and Cochlea.
The vestibular apparatus is composed of three semi-circular canals and the otolith (macula is the sensory part of saccule and utricle). Each semi-circular canal lies in a different plane at right angles to each other. The membranous canals are suspended in the perilymph of the bony canals. The base of canals is swollen and is called ampulla, which contains a projecting ridge called crista ampullaris which has hair cells. The saccule and utricle contain a projecting ridge called macula. The crista and macula are the specific receptors of the vestibular apparatus responsible for maintenance of balance of the body and posture.
Cochlea is a long and coiled outgrowth of sacculus. It is the main hearing organ. Cochlea consists of three membranes. The organ of corti, a hearing organ, is located on the basilar membrane that has hair cells.
2. Compare the following:(a) Central neural system (CNS) and Peripheral neural system (PNS)(b) Resting potential and action potential(c) Choroid and retinaAnswer
(a) Central neural system (CNS) and Peripheral neural system (PNS)
| Central neural system (CNS) | Peripheral neural system (PNS) |
| It is the main coordinating centre of the body. | It is not the main coordinating centre of the body. |
| It lies inside the skull. | It does not lie inside the skull. |
| This includes brain and spinal cord. | This includes all the nerves of the body associated with the CNS (brain and spinal cord). |
(b) Resting potential and action potential
Resting potential | Action potential |
| It is the potential difference across the nerve fibre when there is no conduction of nerve impulse. | It is the potential difference across nerve fibre when there is conduction of nerve impulse. |
| The interior of the neuron is electronegative and the exterior is electropositive. | The interior of the neuron is electropositive and the exterior is electronegative. |
| An active sodium pump operates. | No sodium pump operates. |
(c) Choroid and retina
| Choroid | Retina |
| Choroid is the middle vascular layer of eye. | Retina is the innermost nervous coat of eye. |
| It is rich in blood cells | It is rich in neurons |
| Its function is to supply nutrients and oxygen to other parts of eye like retina | Its function is to form an image of an object over it. |
(a) When the resting potential of the membrane changes it becomes polarized. During resting condition, the axoplasm inside the axon contains high concentration of K
+ and negatively charged proteins and low concentration of Na
+. As a result, the potassium ions move faster from inside to outside as compared to sodium ions. Therefore, the membrane becomes positively charged outside and negatively charged inside. This is known as polarization of membrane or polarized nerve.
(b) When an electrical stimulus is given to a nerve fibre, an action potential is generated. The membrane becomes permeable to sodium ions than to potassium ions. This results into positive charge inside and negative charge outside the nerve fibre. Hence, the membrane is said to be depolarized.
(c) There are two types of nerve fibres – myelinated and non-myelinated. In myelinated nerve fibre, the impulse is conducted from node to node in jumping manner as myelinated nerve fibre is are enveloped with Schwann cells, which form a myelin sheath around the axon. The myelin sheath is impermeable to ions. As a result, the ionic exchange and depolarisation of nerve fibre is not possible along the whole length of nerve fibre. It takes place only at some point, known as nodes of Ranvier. In non-myelinated nerve fibre, the ionic exchange and depolarization of nerve fibre takes place along the whole length of the nerve fibre because of this ionic exchange, the depolarized area becomes repolarised and the next polarized area becomes depolarized.
(d) At a chemical synapse, the membranes of the pre- and post-synaptic neurons are separated by a fluid-filled space called synaptic cleft. When an impulse arrives at the axon terminal, it stimulates the movement of the synaptic vesicles towards the membrane where they fuse with the plasma membrane and release their neurotransmitters in the synaptic cleft. The released neurotransmitters bind to their specific receptors, present on the post-synaptic membrane. This binding opens ion channels allowing the entry of ions which can generate a new potential in the post-synaptic neuron. The new potential developed may be either excitatory or inhibitory.
4. Draw labelled diagrams of the following:(a) Neuron (b) Brain (c) Eye (d) EarAnswer(a) Neuron
(b) Brain
(c) Eye
(d) Ear
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5. Write short notes on the following:
(a) Neural coordination (b) Forebrain (c) Midbrain (d) Hindbrain (e) Retina (f) Ear ossicles
(g) Cochlea (h) Organ of Corti (i) Synapse
Answer
(a) The process through which two or more organs interact and complement the functions of one another through the neural system, it is called neural coordination. All the physiological processes in the body are closed linked and dependent upon each other. The neural system and the endocrine system jointly coordinate and integrate all the activities of the organs so that they function in a synchronised fashion. The neural system provides an organised network of point-to-point connections for a quick coordination. The endocrine system provides chemical integration through hormones.
(b) The forebrain consists of cerebrum, thalamus and hypothalamus.
→ Cerebrum forms the major part of the human brain. A deep cleft divides the cerebrum longitudinally into two halves, which are termed as the left and right cerebral hemispheres. The hemispheres are connected by a tract of nerve fibres called corpus callosum. The layer of cells which covers the cerebral hemisphere is called cerebral cortex and is thrown into prominent folds. The cerebral cortex is referred to as the grey matter due to its greyish appearance. The cerebral cortex contains motor areas, sensory areas and large regions that are neither clearly sensory nor motor in function. These regions called as the association areas are responsible for complex functions like intersensory associations, memory and communication.Fibres of the tracts are covered with the myelin sheath, which constitute the inner part of cerebral hemisphere. They give an opaque white appearance to the layer and, hence, is called the white matter.
→ Thalamus: It is a region present at the centre of the forebrain and wrapped by cerebrum. It is coordination center for sensory and motor signalling.
→ Hypothalamus: It lies at the base of the thalamus which contains a number of centres which control body temperature, urge for eating and drinking. It also contains the nerve centres for temperature regulation, hunger, thirst, heart beat and respiration regulation and emotions such as anger, love, cool, etc. It has connection with pituitary gland hence also controls growth and sexual behaviour.
(c) The midbrain is located between the thalamus/hypothalamus of the forebrain and pons of the hindbrain. A canal called the cerebral aqueduct passess through the midbrain. The dorsal portion of the midbrain consists mainly of four round swellings (lobes) called corpora quadrigemina. Midbrain and hindbrain form the brain stem.
(d) The hindbrain comprises pons, cerebellum and medulla.
→ Pons consists of fibre tracts that interconnect different regions of the brain.
→ Cerebellum has very convoluted surface in order to provide the additional space for many more neurons.
→ The medulla of the brain is connected to the spinal cord. The medulla contains centres which control respiration, cardiovascular reflexes and gastric secretions.
(e) Retina is the innermost layer which contains three layers of neural cells – from inside to outside – ganglion cells, bipolar cells and photoreceptor cells. There are two types of photoreceptor cells, namely, rods and cones. The daylight vision and colour vision are functions of cones and twilight vision is the function of the rods. The light enters through cornea, the lens and the images of objects are formed on the retina.
