NERVOUS (NEURAL) SYSTEM
- Controls and coordinates the body activities.
- Conducts and integrates the information.
Neuron is the structural and functional unit of neural system. It is composed of
- Cell body (cyton): Contains cytoplasm, cell organelles and Nissl’s granules (granular bodies).
- Dendron: Short fibres projecting from the cyton. Their sub branches (dendrites) transmit impulses towards the cyton.
- Axon: A long fibre which transmit impulses away from the cell body. The branching of axon is called axonite. Each axonite ends as a bulb-like structure called synaptic knob.
Types of Neurons
· Unipolar: One axon only. Found usually in embryo.
· Bipolar: one axon and one dendron. Found in the retina.
· Multipolar: One axon and 2 or more dendrons. Most common type. Found in the cerebral cortex.
Types of axon
· Myelinated axon: It is enveloped with Schwann cells that form a myelin sheath around the axon. Found in spinal and cranial nerves. The white coloured area, formed of myelinated nerve fibres is called white matter. The gaps between two adjacent myelin sheaths are called nodes of Ranvier.
· Non-myelinated axon: Schwann cells present but no myelin sheath. The gray coloured area without myelin sheath is called gray matter. Found in autonomous nerves.
HUMAN NERVOUS (NEURAL) SYSTEM
· Central neural system: Includes brain & spinal cord.
· Peripheral nervous system (PNS): Includes all nerves.
PERIPHERAL NEURAL SYSTEM (PNS)
It includes cranial nerves and spinal nerves. PNS has 2 divisions. They are
· Somatic neural system: Relays impulses from the CNS to skeletal muscles.
· Autonomic neural system: Transmits impulses from CNS to involuntary organs and smooth muscles. It includes sympathetic & parasympathetic nerves. Sympathetic system prepares body to cope with emergencies, stresses and dangers. It increases heartbeat, breathing rate, constricts arteries, and elevates BP. Parasympathetic system returns the body to a resting state after stressful situations and slows down heartbeat, dilates arteries, lowers BP etc.
Nerve fibres of PNS are 2 types:
· Afferent (sensory) fibres: Carry impulses from tissues or organs (such as sense organs) to CNS.
· Efferent (motor) fibres: Carry impulses from CNS to tissues or organs (such as muscles and glands).
GENERATION & CONDUCTION OF NERVE IMPULSES
Impulse transmission is electrochemical. It consists of 3 steps:
1. Maintenance of resting membrane potential
- Resting membrane potential is the potential difference existing in a resting neuron (unstimulated neuron).
- The resting membrane is electrically polarized i.e. outside is +vely charged and inside is –vely charged.
- The resting membrane has only a poor permeability for Na+ and has a higher permeability for K+. Similarly, the membrane is impermeable to negatively charged proteins present in the axoplasm. So there will be an ionic concentration gradient across the resting membrane.
- These ionic gradients are maintained by active transport of ions by sodium-potassium pump which transports 3 Na+ outwards for 2 K+ into the cell. So outer surface of axonal membrane possesses a +ve charge while its inner surface becomes –vely charged. This is called polarized state.
- The resting membrane potential is -70 mV.
2. Action potential
- The minimum strength of a stimulus required to stimulate a neuron is called Threshold stimulus.
- When a neuron is stimulated, Na+ pump stops temporarily in stimulated region. It leads to a heavy inflow of Na+, so that inner side of the membrane becomes +ve and outside –ve. This reversal of polarity is called depolarization.
- Immediately Na+ pump becomes active and resting membrane potential is restored (repolarisation).
- Depolarization and subsequent repolarization together constitute the action potential (i.e. a nerve impulse).
- The action potential is about +30 mV.
3. Propagation of action potential
- Action potential formed at a point becomes a stimulus for the next region, i.e. a current flows on the inner surface from stimulated site to adjacent site. As a result, depolarization and repolarisation occur there.
- This process is repeated and the action potential travels along the entire length of the neuron as nerve impulse.
- In myelinated fibre, ionic changes and depolarization occur only at the nodes of Ranvier.
- Between the nodes the myelin sheath acts as insulator. So the action potential jumps from one node to the other. This is called saltatory propagation. Conduction is very fast in myelinated fibre than non-myelinated fibre.
Synaptic transmission of impulses
Synapse is a functional junction between two neurons.
It is 2 types: Electrical & Chemical.
1. Electrical synapses
- In this, the membranes of pre- and post-synaptic neurons are in very close proximity. So impulse transmission is similar to the transmission along a single axon.
- Impulse transmission is faster than in chemical synapse.
- Electrical synapses are very rare in human system.
2. Chemical synapses
- In this, there is a fluid filled space (synaptic cleft) between the presynaptic neuron and postsynaptic neuron.
- The presynaptic regions have swellings called Synaptic knob (buttons). They contain synaptic vesicles filled with neurotransmitters like acetylcholine (Ach) or adrenaline.
- When the impulse reaches the presynaptic region, the synaptic vesicles break and release the neurotransmitters. It diffuses across the synaptic gap to combine with the receptors found on the post synaptic neuron.
- Based on neurotransmitter, chemical synapses are 2 types:
o Cholinergic synapse: Neurotransmitter is Acetylcholine.
o Adrenergic synapse: Neurotransmitter is Adrenalin.
It is the rapid, involuntary and unconscious actions of the body brought about by any part of the CNS through sudden stimulation from receptors. E.g.
¨ Sudden withdrawal of the hand when it touches a hot object.
¨ Touching lips of a nursing baby evokes a sucking reflex.
¨ Closing of the eyelids when a beam of light falls on them
¨ Knee jerk phenomenon.
¨ If a child sees or smells some food unknown to him he does not salivate. But if he sees or smells that food every time before tasting it, he begins to salivate (conditioned reflex).
Reflex arc (pathway of impulses in a reflex action) consists of
§ A receptor organ receiving the stimulus.
§ A sensory (afferent) neuron, which transmits the impulses from sense organ to CNS.
§ An intermediate neuron (connector neuron) that connects the sensory and motor neuron.
§ A motor (efferent) neuron that conducts the impulse from the CNS to effector organ.
§ An effector organ (muscle/ gland): responds to impulse.
SENSE ORGANS (EYE, EAR, NOSE, TONGUE & SKIN)
- Visual organ present in the orbit of the skull and held in place by 6 muscles.
- The lachrymal (tear) glands secrete tears which contain a bactericidal enzyme called Lysozyme.
- The excess tear is passed out by nasolachrymal duct that opens into the nasal chamber.
- Eyeball has 3 layers- sclera, choroid and retina.
- Outermost layer that protects the inner structure and helps to maintain rigidity of the eyeball.
- The transparent, anterior part of sclera is called cornea.
- Conjunctiva (a transparent covering) protects cornea.
- Highly vascular middle layer.
- It contains melanocytes which produces melanin.
- Just behind the cornea, choroid forms a circular opaque disc called Iris, which has a central opening (pupil).
- Around the base of iris, ciliary body is present which contains ciliary muscles.
- A biconvex lens is present just behind the iris, which is held in place by suspensory ligaments.
- Lens can change its focal length to view near and distant objects. This is called power of accommodation. It is brought about by altering convexity of lens by ciliary body.
- Innermost layer formed of 3 layers of cells-from inside to outside- ganglion cells, bipolar cells & photoreceptors.
- Photoreceptor cells are 2 types: cones and rods.
- Cone cells:
§ For photopic (day light) vision & colour vision.
§ Contain iodopsin (violet coloured photopigment).
§ There are 3 types of cones for discriminating the primary colours (red, green and blue).
§ Defect in cone cells results in colour blindness.
- Rod cells:
§ For scotopic vision (vision in dim light).
§ Contain rhodopsin (purple coloured photopigment synthesized from vitamin A).
§ Deficiency of vitamin A leads to lack of rhodopsin, which results in night blindness.
- The spot at the back of eye, where optic nerve originates is called blind spot. Rods and cones are absent here.
- Lateral to blind spot is a depressed area called Fovea centralis (yellow spot or macula lutea) that contains only cones and no rods. It is the area of keenest vision.
- The aqueous chamber lies between the cornea and lens. It is filled with aqueous humor, secreted by the ciliary body. It nourishes the cornea and lens. The vitreous chamber lies behind the lens. It is filled with vitreous humor.
- Both the fluids help in maintaining the form of eyeball and in focusing the light rays.
Mechanism of vision
- Light reflected from an object enters the eye through cornea and lens. These are focused on retina and generate potentials (impulses) in rods and cones.
- Eye contains photosensitive compounds (photopigments) formed of opsin (a protein) and retinal (an aldehyde of vitamin A). Light induces the dissociation of retinal from opsin. This changes membrane permeability.
- As a result, potential differences are generated in the photoreceptor cells. It generates action potentials in the ganglion cells through the bipolar cells.
- These are transmitted by optic nerves to visual cortex of brain. Impulses are analyzed and the image is recognized based on the earlier memory and experience.
- Human being looks at objects with both eyes. So 2 retinal images are formed but only one image is seen. This is called binocular vision.
2. EAR (STATO-ACOUSTIC ORGAN)
Organ for hearing and balancing. It has 3 divisions- External ear, middle ear and inner ear.
a. External ear
- Consists of pinna (ear lobe) & auditory meatus (ear canal).
- The ear canal contains numerous ceruminous glands (modified sweat glands) which secrete wax (cerumen).
- Hairs are present at the opening of ear canal.
- Wax and hairs prevent entry of foreign objects like insects.
- Ear canal ends in tympanic membrane (ear drum).
b. Middle ear
- Consists of tympanic cavity and ear ossicles.
- Tympanic cavity is an air filled space that separates the external and inner ear portions.
- Tympanum is a semi-transparent membrane covered by a thin layer of skin on its outer surface and by mucous membrane on the inside.
- An auditory tube (Eustachian canal) connects middle ear to the pharynx. It maintains an equal pressure on either side of the eardrum and thus protects the tympanic cavity.
- Ear ossicles include 3 small bones namely Malleus, Incus and stapes. Malleus is attached to tympanum.
- Stapes is the smallest bone of the body. It is attached to membrane of oval window (fenestra ovalis) of inner ear.
c. Inner ear
- It consists of bony labyrinth and membranous labyrinth.
- Bony labyrinth is a cavity filled with perilymph.
- The membranous labyrinth consists of 3 semicircular canals, vestibule (utriculus + sacculus) and cochlea.
- Of 3 semicircular canals, 2 are vertical and one is horizontal. The canals are attached to utriculus. One end of each canal has a bulging called ampulla. Inside it is a jelly-like lump called crista ampullaris. The long cilia from many cells of crista are grouped together in a bundle (cupula) that is covered with a gelatinous coating.
- The inner lining of the ampulla and vestibule are formed of sensory hair cells.
- Small masses of calcareous particles called otoliths (otoconia or ear stones) are located within the vestibule. The hairs of the sensory cells are in contact with otoliths.
- Cristae (receptors on the ampulla) and macula (receptors on the utriculus & sacculus) are the organs of equilibrium and posture of body.
- Cochlea (organ of hearing) is a coiled structure having 3 canals namely, upper scala vestibula, middle scala media and lower scala tympani.
- A membrane called Reissner’s membrane separates the scala vestibula and scala media. The scala media and scala tympani are separated by basilar membrane.
- The outer canals (S. vestibula and S. tympani) are filled with perilymph and scala media is filled with endolymph.
- Terminally, Scala vestibula and S. tympani communicate with each other by a small opening called helicotrema.
- Resting on the basilar membrane and projecting into scala media is complex receptor organ called Organ of Corti. It consists of row of sensory hair cells. The hairs of these cells project upwards and lie in close contact with tectorial membrane, which projects above them.
Mechanism of hearing
Pinna collects sound waves → waves pass through ear canal → reach the tympanic membrane → tympanic membrane vibrates → vibrations are transmitted to ear ossicles & oval window → perilymph in the vestibular canal vibrates → vibrations reach the scala tympani and force the basilar membrane to vibrate → the hair endings of the sensory hair cells press against the tectorial membrane → sensory hair cells are excited → the impulses produced are carried by auditory nerve to the auditory centre of the brain.
The sound pitch is determined by the frequency of sound. Human ear register sounds of frequencies b/w 20- 20,000 Hz.
CHEMORECEPTORS (NOSE & TONGUE)
3. NOSE (Olfactoreceptors): Organ of smell
- The olfactory epithelium consists of Olfactoreceptors (confined to roof of the nasal cavity) and supporting cells.
- The odorants dissolve in the mucous (secreted by supporting cells) and attach to receptor cells. The receptor cells generate olfactory impulses which are transmitted into brain by olfactory nerves.
4. TONGUE (Gustatoreceptors): Organ of taste
- Taste buds (Gustatoreceptors + supporting cells) are seen around the bases of taste papillae.
- The food mixed with saliva enters the taste pores of the buds and stimulate the sensory cells. These impulses are carried to the brain, which gives the sense of the taste.
- 4 primary tastes are sweet, salt, sour and bitter.
5. SKIN (Cutaneous receptors)
- Largest sense organ.It contains receptors for heat, cold, touch, pain & pressure.