\nThe neural system of all animals is composed neurons that receive and transmit different kinds of stimuli.<\/p>\n
Neural system in lower forms<\/p>\n
- \n
- The neural organisation is very simple in lower invertebrates.<\/li>\n
- For example, in Hydra it is composed of a network of neurons.<\/li>\n
- The neural system is better organised in insects, where a brain is present.<\/li>\n<\/ul>\n
The vertebrates have a more developed neural system.<\/p>\n
Human Neural System<\/span>
\nThe human neural system is divided into two parts:<\/p>\n\n\n
\n (i) Central neural system (CNS)
\n(ii) Peripheral neural system (PNS)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n- \n
- The CNS includes the brain and the spinal cord and is the site of information processing and control.<\/li>\n
- The PNS comprises of all the nerves of the body associated with the CNS.<\/li>\n<\/ul>\n
The nerve fibres of the PNS are of two types:<\/p>\n
\n\n
\n (a) afferent fibres
\n(b) efferent fibres<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\nFunction of afferent efferent of nerve fibres:
\nThe afferent nerve fibres transmit impulses from tissues\/organs to the CNS. The efferent fibres transmit regulatory impulses from the CNS to the peripheral tissues\/organs.
\nThe PNS is divided into two divisions<\/p>\n- \n
- Somatic neural system<\/li>\n
- Autonomic neural system.<\/li>\n<\/ol>\n
Function:
\nThe somatic neural system relays impulses from the CNS to skeletal muscles. The autonomic neural system transmits impulses from the CNS to the involuntary organs and smooth muscles of the body.
\nThe autonomic neural system is classified into<\/p>\n- \n
- sympathetic neural system<\/li>\n
- parasympathetic neural system.<\/li>\n<\/ol>\n
Neuron As Structural And Functional Unit Of Neural System<\/span>
\nA neuron is composed of three major parts,<\/p>\n\n\n
\n 1. cell body
\n2. dendrites and
\n3. axon<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n1. The cell body contains cytoplasm with cell organelles and certain granular bodies called Nissl\u2019s granules.<\/p>\n
2. Repeated branches project out of the cell body are called dendrites.<\/p>\n
3. The axon is a long fibre, the distal end of which is branched. Each branch terminates as a bulb-like structure called synaptic knob which possess synaptic vesicles containing chemicals called neurotransmitters. Based on the number of axon and dendrites, the neurons are divided into three types,<\/p>\n
- \n
- multipolar (with one axon and two or more dendrites; found in the cerebral cortex)<\/li>\n
- bipolar (with one axon and one dendrite, found in the retina of eye)<\/li>\n
- unipolar (cell body with one axon only found usually in the embryonic stage).<\/li>\n<\/ul>\n
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\n4. The myelinated nerve fibres are enveloped with Schwann cells, which form a myelin sheath around the axon.<\/p>\n5. The gaps between two adjacent myelin sheaths are called nodes of Ranvier.<\/p>\n
6. Myelinated nerve fibres are found in spinal and cranial nerves.<\/p>\n
8. Non-myelinated nerve fibre is enclosed by a Schwann cell that does not form a myelin sheath around the axon, and is commonly found in autonomous and somatic neural systems.<\/p>\n
Generation and Conduction of Nerve Impulse<\/span>
\nNeurons are excitable cells because their membranes are in a polarized state.
\nIn resting state of neuron:
\nThe axonal membrane is more permeable to potassium ions (K+<\/sup>) and impermeable to sodium ions (Na+<\/sup>) and negatively charged proteins present in the axoplasm. The fluid outside the axon contains a low concentration of K+<\/sup>, a high concentration of Na+<\/sup> and thus form a concentration gradient.<\/p>\nThese ionic gradients are maintained by the active transport of ions by the sodium-potassium pump which transports 3 Na+<\/sup> outwards for 2 K+<\/sup> into the cell.<\/p>\n
As a result, the outer surface of the axonal membrane possesses a positive charge while its inner surface becomes negatively charged and therefore is polarised. The electrical potential difference across the resting plasma membrane is called as the resting potential.<\/p>\n
When a stimulus is applied at a site on the polarised membrane:
\nThe membrane freely permeable to Na and influx of Na+<\/sup> followed by the reversal of the polarity at that site, i.e., the outer surface of the membrane becomes negatively charged and the inner side becomes positively charged.<\/p>\nThis is called depolarized state. The electrical potential difference across the plasma membrane is called the action potential At sites immediately ahead, the axon membrane has a positive charge on the outer surface and a negative charge on its inner surface. As a result, a current flows on the inner surface from site A to site B.<\/p>\n
On the outer surface current flows from site B to site A to complete the circuit of current flow. Hence, the polarity at the site is reversed, and an action potential is generated at site B. Thus, the impulse (action potential) generated at site A arrives at site B. The sequence is repeated along the length of the axon and consequently the impulse is conducted.
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<\/p>\nTransmission of Impulses<\/span>
\nA nerve impulse is transmitted from one neuron to another through junctions called synapses. The junction between pre-synaptic neuron and a post-synaptic neuron is called synaptic cleft. There are two types of synapses, namely<\/p>\n\n\n
\n 1. Electrical synapses and
\n2. Chemical synapses<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\nAt electrical synapses, erlectrical current flowdirectly from one neuron into the other across these synapses. It is very similar to impulse conduction along a single axon, impulse transmission across an electrical synapse is always faster than that across a chemical synapse.<\/p>\n
At a chemical synapse, the membranes ofthepre-and post-synaptic neurons are separated by a fluid-filled space called synaptic cleft Chemicals called neurotransmitters are involved in the transmission of impulses at these synapses.<\/p>\n
The released neurotransmitters bind to the 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.
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<\/p>\nCentral Neural System<\/span>
\nIt controls the voluntary movements such as balance of the body, functioning of vital involuntary organs (e.g., lungs, heart, kidneys, etc.), thermoregulation, hunger and thirst, circadian (24-hour) rhythms of our body, activities of several endocrine glands and human behaviour.<\/p>\nIt is also the site for processing of vision, hearing, speech, memory, intelligence, emotions and thoughts. 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 called pia mater.<\/p>\n
The brain is divided into three major parts:<\/p>\n
- \n
- forebrain<\/li>\n
- midbrain<\/li>\n
- hindbrain<\/li>\n<\/ul>\n
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<\/p>\nForebrain<\/span>
\nThe forebrain consists of cerebrum, thalamus and hypothalamus. Cerebrum forms the major part of the human brain. Cerebrum divided longitudinally into two halves the left and right cerebral hemispheres. The hemispheres are connected by nerve fibres called corpus callosum.<\/p>\nThe layer of cells which covers the cerebral hemisphere is called cerebral cortex. The cerebral cortex is referred to as the grey matter .The cerebral cortex contains motor areas, sensory areas and large regions These regions called as the association areas.They are responsible functions like memory and communication.<\/p>\n
The inner part of cerebral hemisphere gives white appearance to the layer and called as the white matter. Hypothalamus lies at the base of the thalamus contains a number of centres which control body temperature, urge for eating and drinking. Hypothalamus secrete hormones called hypothalamic hormones.<\/p>\n
he inner parts of cerebral hemispheres and a group of associated deep structures like amygdala, hippocampus, etc., form a complex structure called the limbic lobe or limbic system. Along with the hypothalamus,<\/p>\n
\n\n
\n it is involved in the regulation of sexual behaviour, expression of emotional reactions (eg: excitement, pleasure, rage and fear), and motivation<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n Midbrain<\/span>
\nThe 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.<\/p>\nThe dorsal portion of the midbrain consists mainly of four round swellings (lobes) called corpora quadrigemina. Midbrain and hindbrain form the brain stem..<\/p>\n
Hindbrain<\/span>
\nThe hindbrain comprises pons, cerebellum and medulla (also called the medulla oblongata). Pons consists of fibre tracts that interconnect different regions of the brain. Cerebellum provide additional space for many more neurons.<\/p>\nThe medulla of the brain is connected to the spinal cord. The medulla contains centres which control respiration, cardiovascular reflexes and gastric secretions.<\/p>\n
Reflex Action And Reflex Arc<\/span>
\nIt involves the sudden withdrawal of a body part which comes in contact with objects that are extremely hot, cold pointed or animals that are poisonous. The reflex pathway consists of one afferent neuron (receptor) and one efferent (effector or excitor) neuron arranged in a series.<\/p>\nThe afferent neuron receives signal from a sensory organ and transmits the impulse via a dorsal nerve root into the CNS (at the level of spinal cord). The efferent nueuron carries signals from CNS to the effector. The stimulus and response thus forms a reflex arc in the knee jerk reflex.
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<\/p>\nSensory Reception And Processing<\/span>
\nThe sensory organs detect all types of changes in the environment and sent to different parts\/centres of the brain. The sense organs are the eye (sensory organ for vision) and the ear (sensory organ for hearing).<\/p>\nEye<\/span>
\nOur paired eyes are located in sockets of the skull called orbits.<\/p>\nParts of an eye<\/span>
\nThe wall of the eye ball is composed of three layers. External layer is composed of a dense connective tissue and is called the sclera. Anterior portion of this layer is called the cornea. Middle layer, choroid, contains many blood vessels and looks bluish in colour.<\/p>\nThe choroid layer is thin over the posterior part of eye ball, but it becomes thick in the anterior part to form the ciliary body. The ciliary continues forward to form a pigmented and opaque structure called the iris which is the visible coloured portion ofthe eye.
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<\/p>\n- \n
- The eye ball contains 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.<\/li>\n
- The inner layer is the retina contains three layers of cells – from inside to outside.<\/li>\n
- Ganglion cells, bipolar cells and photoreceptor cells.<\/li>\n
- There are two typ es of photoreceptor cells, namely, rods and cones<\/li>\n
- The daylight (photopic) vision and colour vision are functions of cones<\/li>\n
- The twilight (scotopic) vision is the function of the rods.<\/li>\n
- The rods contain a purplish-red protein called the rhodopsin which contains a derivative of Vitamin A. In the human eye, there are three types of cones that respond to red, green and blue lights.<\/li>\n<\/ol>\n
What is blind spot?
\nThe optic nerves and the retinal blood vessels enter above the posterior pole of the eye ball. Photoreceptor cells are not present in that region and hence it is called the blind spot.<\/p>\nWhat is macula lutea?
\nAt the posterior pole of the eye lateral to the blind spot, there is a yellowish pigmented spot called macula lutea with a central pit called the fovea where only the cones are densely packed. It is the point where the visual acuity (resolution) is the greatest.<\/p>\nThe space between the cornea and the lens is called the aqueous chamber and contains fluid called aqueous humor. The space between the lens and the retina is called the vitreous chamber and contains fluid called vitreous humor.<\/p>\n
Mechanism of Vision<\/span>
\nThe light rays falls on the retina through cornea and lens generate impulses in rods and cones. The photosensitive compounds (photopigments) in the human eyes is composed of opsin (a protein) and retinal (an aldehyde of vitamin A). Light induces the changes in the structure of the opsin.<\/p>\nThis causes membrane permeability changes. As a result, potential differences are generated in the photoreceptor cells. This produces a signal that generates action potentials in the ganglion cells through the bipolar cells.<\/p>\n
These action potentials are transmitted by the optic nerves to the visual cortex area of the brain, where the neural impulses are analysed and the image formed on the retina is recognised based on earlier memory and experience.<\/p>\n
The Ear<\/span>
\nThe ear is divided into three major sections called the outer ear, the middle ear and the inner ear. The outer ear consists of the pinna and external auditory meatus (canal).The pinna collects the vibrations of sound. The external auditory meatus leads inwards and extends up to the tympanic membrane (the ear drum).<\/p>\nThere are wax-secreting sebaceous glands in the skin of the pinna and the meatus. The middle ear contains three ossicles called malleus, incus and stapes which are attached to one another in a chain-like fashion.<\/p>\n
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.
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\nA Eustachian tube connects the middle ear cavity with the pharynx. The Eustachian tube helps in equalising the pressures on either sides of the eardrum. The fluid-filled inner ear called labyrinth consists of two parts, the bony and the membranous labyrinths.<\/p>\nThe bony labyrinth is connected with membranous labyrinth, which is surrounded by a fluid called perilymph. The membranous labyrinth is filled with a fluid called endolymph. The coiled portion of the labyrinth is called cochlea.<\/p>\n
The membranes constituting cochlea, the reissner\u2019s and basilar, divide the surounding perilymph filled bony labyrinth into an upper scala vestibuli and a lower scala tympani. The space within cochlea called scala media is filled with endolymph.<\/p>\n
At the base of the cochlea, the scala vestibuli ends at the oval window The organ of corti is a structure located on the basilar membrane which contains hair cells that act as auditory receptors. The hair cells are present in rows on the internal side of the organ of corti.<\/p>\n
The basal end of the hair cell is in close contact with the afferent nerve fibres. A large number of processes called stereo cilia are projected from the apical part of each hair cell. Above the rows of the hair cells is a thin elastic membrane called tectorial membrane. The inner ear also contains vestibular apparatus, located above the cochlea.<\/p>\n
The vestibular apparatus is composed of three semi-circular canals and the otolith organ consisting of the saccule and utricle. 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.<\/p>\n
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.
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<\/p>\nMechanism of Hearing<\/span>
\nHow does ear convert sound waves into neural impulses, which are sensed and processed by the brain enabling us to recognise a sound?
\nThe external ear receives sound waves and directs them to the eardrum. The eardrum vibrates in response to the sound waves and these vibrations are transmitted through the ear ossicles (malleus, incus and stapes) to the oval window.<\/p>\nThe vibrations are passed through the oval window on to the fluid of the cochlea, where they generate waves in the lymphs. The waves in the lymphs induce a ripple in the basilar membrane. These movements of the basilar membrane bend the hair ceils, pressing them against the tectorial membrane.<\/p>\n
As a result, nerve impulses are generated in the associated afferent neurons. These impulses are transmitted by the afferent fibres via auditory nerves to the auditory cortex of the brain, where the impulses are analysed and the sound is recognized.<\/p>\n
The vibrations are passed through the oval window on to the fluid of the cochlea, where they generate waves in the lymphs. The waves in the lymphs induce a ripple in the basilar membrane. These movements of the basilar membrane bend the hair cells, pressing them against the tectorial membrane.<\/p>\n
As a result, nerve impulses are generated in the associated afferent neurons. These impulses are transmitted by the afferent fibres via auditory nerves to the auditory cortex of the brain, where the impulses are analysed and the sound is recognized.<\/p>\n
NCERT SUPPLEMENTARY SYLLABUS<\/span>
\nSense organs:
\nThe environmental changes (both internal and external) called stimuli detected by the special sensory cells, are conveyed to the brain in the form of nerve impulses. The response for each stimulus from brain is sent to the various body parts for its well being. There are five senses: touch, vision, hearing, smell and taste.<\/p>\nWhile touch is a complex general sense, the other four are special senses. The general sensory receptors are simple receptors that are present in the skin, mucous membranes, connective tissues and muscles.<\/p>\n
These sense the information such as tactile sensation (a mix of touch, pressure, stretch and vibration), heat, cold, pain and muscle sense.<\/p>\n
Special sensory receptors are present in the head especially sensory organs like eyes and ears and tissues of the taste buds and olfactory epithelium. These sensory organs and tissues of eye and ear are photoreceptors and the auditory receptors respectively.<\/p>\n
The chemical senses: the taste and smell:
\nThe receptors fortaste and smell are called as chemoreceptors found as film of liquid coating in the membranes of the receptor cells. The taste receptors are specialized cells that detect chemicals present in the mouth while smell receptors are modified sensory neurons in the nasal passage which detect the volatile chemicals.<\/p>\nThese two types of receptors complement each other and often respond to the same stimulus. The smell receptors are 3,400 times more sensitive than the taste receptors.<\/p>\n
Sense of smell (olfaction):
\nNose contains the receptors of smell, in the mucous coated thin, yellowish patch (about 5 cm2) of modified pseudostratified epithelium called olfactory epithelium. It is located at the roof of the nasal cavity on either sides of the nasal septum.
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\nThe olfactory epithelium contains three types of cells:<\/p>\n- \n
- millions of olfactory receptor cells<\/li>\n
- columnar supportive cells<\/li>\n
- short basal cells.<\/li>\n<\/ul>\n
Olfactory receptors bear a cluster of about 20 modified cilia which function as receptor sites. These cilia extend from the olfactory epithelium into the thin coat of nasal mucous secreted by the supportive cells and olfactory glands. This mucous dissolves the airborne odour molecules.<\/p>\n
Once dissolved, the chemicals bind to the specific receptors on the cilia stimulating the receptor cells. This causes depolarization then action potential in the receptor cell.<\/p>\n
The axons of the olfactory receptors unite to form the olfactory nerve which transmits the information directly to olfactory bulb, a relay station in the brain. The nasal cavity contains pain receptors that respond to irritants such as ammonia, vinegar or hot chilly pepper.<\/p>\n
Impulses from these pain receptors reach the brain. The brain combines these sensations with those of smell to identify the odours Humans can detect about 10,000 different odours but the olfactory capability of fish and mammals such as a dog is high.<\/p>\n
Sense of taste (gustation):
\nThe receptor cells fortaste are located in taste buds. Humans have about 10,000 taste buds are located in pockets around the papillae on the surface and sides of the tongue, but some on the surface of the pharynx and the larynx.<\/p>\nEach taste bud contains about 40 specialized receptor cells or gustatory cells, that helps to replace the worn out cells of the taste buds. The receptor cells for taste are not neurons, but they are microvilli .The microvilli protrude into the surrounding fluids through a narrow opening called the taste pore.<\/p>\n
Dissolved chemicals contacting the microvilli bind to specific receptor proteins on the microvilli, thereby depolarizing the cell,it releases neurotransmitter which leads to the generation of an action potential in the associated sensory neuron.<\/p>\n
Each dendrite receives signals from several receptor cells within the taste bud pass to the brain stem. From here the nerve impulse is relayed to the taste centre in the cerebral cortex of the brain that perceives the taste sensation.
\nIn humans there are four basic taste senses:<\/p>\n