Neural Control And Co-Ordination

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What is coordination

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Coordination is the process through which two or more organs interact and complement the functions of one another.

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The relation between neural system and endocrine system

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In our body, 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. Neural system provides an organised network of point to point connections for a quick coordination. The endocrine system provides chemical integration through hormones.

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What are neurones

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Neural system of all animals is composed of highly specialised cells called neurones which can detect, receive and transmit different kinds of stimuli.

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Parts of human neural system

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Human neural system is divided into two parts: the central neural system (CNS) and the peripheral level system (PNS). CNS includes the brain and the spinal cord and is the site of information processing and control. The PNS comprises of all the nerves of the body associated with the CNS.

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Nerve fibres of the PNS.

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They are of two types: afferent fibres and efferent fibres. Afferent nerve fibres transmit impulses from tissues or organs to the CNS and efferent fibres transmit regulatory impulses from the CNS to the concerned peripheral tissues or organs.

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Two divisions of PNS

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PNS is divided into two divisions called somatic neural system and autonomic neural system. Somatic neural system relays impulses from the CNS to skeletal muscles while the autonomic nervous system transmits impulses from the CNS to the involuntary organs and smooth muscles of the body. Autonomic nervous system is further classified into sympathetic nervous system and parasympathetic nervous system.

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Sympathetic vs parasympathetic neural systems

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Sympathetic and parasympathetic neural systems have opposite effects on the organs. In the parasympathetic neural system, the neurotransmitter between the axons of the neurons and target organ is acetylcholine whereas in the sympathetic neural system, the neural transmitter between the axons of the neurons and target organ is adrenaline or noradrenaline. If the sympathetic nerve ending excites a particular organ, the parasympathetic usually inhibits it. The sympathetic neural system accelerates the Heartbeat whereas, the parasympathetic neural system slows down the heartbeat. The sympathetic nervous system inhibits the secretion of saliva whereas parasympathetic nervous system stimulates its secretion.

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Parts of neuron

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Neuron is composed of three major parts, namely, body, dendrites and axon. Cell body contains cytoplasm with typical cell organelles and certain granular body is called Nissl's granules. Short fibres which branch repeatedly and project out of the cell body also contain Nissl's granules and are called dendrites. These fibres transmit impulses towards the cell body. 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.

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Classification of neurons

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Based on the number of axon and dendrites, the neurones are divided into three types, i.e., multipolar with (One axon and two or more dendrites; found in the cerebral cortex), bipolar (with one axon and one dendrite, found in the retina of eye) and unipolar (cell body with one axon only; found usually in the embryonic stage).

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Types of axons

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There are two types of axons, namely, myelinated and non myelinated. The myelinated nerve fibres our enveloped with Schwann cells, which form a myelin sheath around the axon. The gaps between two adjacent myelin sheaths are called Nodes of Ranvier. Myelinated nerve fibres are found in spinal and cranial nerves. Unmyelinated 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 the somatic neural systems.

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Why neurones are excitable cells?

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Neurons are excitable cells because their membranes are in a polarised state.

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How do a neurone gets polarized

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Different types of Ion channels are present on the neural membrane. These Ion channels are selectively permeable to different ions. When a neuron is not conducting any impulse, i.e., resting, the axonal membrane is comparatively more permeable to potassium ions and nearly impermeable to sodium ions. similarly, the membrane is impermeable to negatively charged proteins present in the axoplasm. Consequently, Axoplasm inside the axon contains high concentration of potassium ions and negatively charged proteins and low concentration of sodium ions. In contrast, the fluid outside the axon contains a low concentration of potassium ions, a high concentration of sodium ions and thus form a concentration gradient. These ionic gradients across the resting membrane are maintained by the active transport of Ions by the Sodium Potassium pump which transports 3 sodium ions outwards for two potassium ions into the cell. As a result, the outer surface of the axonal membrane possesses a positive charge while its inner surface becomes negatively charged and therefore is polarized. The electrical potential difference across the resting plasma membrane is called as the resting potential.

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Mechanism of generation of nerve impulse and its conduction along an axon.

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When a stimulus is applied at a site on the polarized membrane, the membrane at the site A becomes freely permeable to sodium ions. This leads to a rapid influx of sodium ions 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. The polarity of the membrane at the site A is thus reversed and hence depolarized. The electrical potential difference across the plasma membrane at the site A is called the action potential, which is in fact termed as the nerve impulse. 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. On the outer surface, current flows from the 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. The rise in the stimulus induced permeability to sodium ions is extremely short lived. It is quickly followed by a rise in permeability to potassium ions. Within a fraction of a second, potassium ions diffuses outside the membrane and restores the resting potential of the membrane at the site of excitation and the fibre becomes once more responsive to further stimulation.

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How do a nerve impulse is transmitted

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It is transmitted from the one neuron to another through junctions called synapses.

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What is synapse

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A synapse is formed by the membranes of a presynaptic neurone and up postsynaptic neurone, which may or may not be separated by a gap called synaptic cleft.

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Types of synapses

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There are two types of synapses, namely, electrical synapses and chemical synapses. At electrical synapses, the membranes of pre and post synaptic neurones are in very close proximity. Electrical current can flow directly from one neuron into the other across these synapses. Transmission of an impulse across electrical synapses 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. Electrical synapses are there in our system. At a chemical synapse, the membranes of the pre and post synaptic neurones are separated by a fluid filled space called synaptic cleft.

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Which Chemicals are involved in the transmission of impulses

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Chemicals called neurotransmitters are involved in the transmission of impulses at these synapses. In the exact lasm present inside the synaptic knob, numerous, tiny, rounded structures are present called as synaptic vesicles. Each synaptic vesicles contains neurotransmitter.

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Importance of myelin sheath

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Myelin sheath acts as a biological electrical insulation. It creates a region of high electrical resistance on the axon. The wrapping of the lipid rich myelin sheath acts like a biological insulator for the flow of electrical impulse through an axon. Unmyelinated nerve fibres are also enclosed by the Schwann cells. But these do not produce a myelin sheath but form neurilemma (sheath of Schwann. Neurilemma do not have ability to produce electrical insulation and thus electrical insulation is absent in unmyelinated sheaths. Neurilemma is not found in neurones of CNS.

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Mechanism of transmission of impulse

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When an impulse (action potential) 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 postsynaptic membrane. This binding opens Ion channels allowing the entry of Ions which can generate a new potential in the postsynaptic neurone. The new potential developed maybe either excitatory or inhibitory.

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Steps in the mechanism of transmission of nerve impulse through chemical synapse

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Examples of excitatory neurotransmitters

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Acetylcholine, epinephrine, norepinephrine, glutamate etc

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Examples of inhibitory neurotransmitters

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Dopamine, serotonin, glycine, GABA etc

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Scienaptic delay

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There is a delay in transmission of the nerve impulses at each synapse. The interval called the synaptic delay results from the time taken in releasing the neurotransmitter and in stimulating the next neuron by it.

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Scienaptic fatigue

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On repeated transmission of nerve impulse, there occurs a temporary suspension of impulse transmission at the synapse. This is called the synaptic fatigue. It results from an exhaustion of the neurotransmitter in the synaptic vesicles of the axon terminal.

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Brain

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Brain is the central information processing organ and acts as the command and control system. It controls the voluntary movements, balance of the body, functioning of vital involuntary organs, thermoregulation, hunger and thirst circadian (24 hour) rhythms of our body, activities of several endocrine glands and human behaviour. It is also the site for processing of vision, hearing, speech, memory, intelligence, emotions and thoughts.

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Location of brain

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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, 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: (1) forebrain (2) midbrain and (3) hindbrain.

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Embryological history of brain

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Embryological components from which brain arises are :-

1) Prosencephalon- forebrain

2) Mesencephalon- midbrain

3) Rhombencephalon- hindbrain

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Space between skull and the brain

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A very narrow space that exist below dura mater or between the dura mater and the arachnoid membrane is called the subdural space. A narrow that exists between arachnoid membrane and Pia mater is called subarachnoid space. The subarachnoid space contains the cerebrospinal fluid(CSF).

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What is basal ganglia in brain

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Basal ganglia is a collection of subcortical nuclei in the forebrain at the base of cortex. The largest nucleus in it is the Corpus striatum. It regulates planning and execution of stereotyped movements.

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Disorders of brain

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Destruction of dopamine secreting par compacta part of basal nucleus called substantia nigra leads to Paralysis agitans or Parkinson's disease. Huntington's chorea is due to degeneration of GABA secreting neurons of Corpus striatum and acetylcholine secreting neurons of other parts.

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Forebrain

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It 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 hemisphere are connected by a tract of nerve fibres called Corpus calossum. 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 neuron cell bodies are concentrated here giving the colour. 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 the cerebral hemisphere. They give an opaque white appearance to the layer and, hence, is called the white matter. The cerebrum wraps around the structure called thalamus, which is a major coordinating centre for sensory and motor signalling. Hypothalamus lie at the base of the thalamus. The hypothalamus contains a number of centres which control body temperature, urge for eating and drinking. It also contains several groups of neurosecretory cells, which secrete hormones called hypothalamic hormones. The 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, It is involved in the regulation of sexual behaviour, expression of emotional reactions and motivation.

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Midbrain

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Midbrain is located between the thalamus or hypothalamus of the forebrain and pons of the hindbrain. A canal called the cerebral aqueduct passes through the midbrain. The dorsal position of the midbrain consists mainly of 4 round swellings called corpora quadrigemina. Midbrain and hindbrain from the brainstem.

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Hindbrain

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It comprises pons, cerebellum and medulla (also called the medulla oblongata). 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 middle of the brain is connected to the spinal cord. The medulla contains centres which control respiration, cardiovascular reflexes and gastric secretions.

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Some facts about the components of brain

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Amygdala controls the moods, especially anger and rage. Hippocampus makes the lower portion of limbic fork. Thalamus is also called relay centre of the cerebral cortex.

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Ventricles in the brain

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Ventricles are the cavities present in the brain. There are four cavities present within the brain that are called cerebral ventricles. The cerebral aqueduct is a canal that passes through the midbrain and connects the third ventricle with fourth ventricle of the brain.

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Pneumotaxic centre of pons

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Pons many acts as the neuronal link between the cerebral cortex and cerebellum. A center present in the pons called pneumotaxic centre can moderate the functions of the respiratory rhythm centre located in the medulla oblongata.

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Facts about cerebellum

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Cerebellum is the second largest part of brain. It is also made up of two cerebral hemisphere and a vermis and has its grey matter on the outer side and white matter on the inner side. Cerebellum has its grey matter comprising of three layers of cells and fibres. The middle layer contains flask shaped purkinje cells. These are the most complex of all neurones. The white and grey matter form Arbor vitae. Central portion of the cerebellum has worm like appearance as it is narrowed and forward and called vermis. 3 paired bundles of myelinated nerve fibres called cerebellar peduncles form communication pathways between the cerebellum and other parts of the CNS. Cerebellum does not initiate the movements of the body but modulates or reorganises the motor commands. It is vital to the control of rapid muscular activities. All the activities are involuntary but may involve learning in early stages. Cerebrospinal fluid is the shock absorber for CNS.

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What is reflex action

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The entire process of response to a peripheral nerve stimulation, that occurs involuntarily i.e., without conscious effort or thought and requires the involvement of a part of the central nervous system is called a reflex action. The reflex pathway comprises at least one afferent neurones (receptor) and one efferent neurones (effector or exciter) appropriately arranged in a series. The afferent neurones receives signal from a sensory organ and transmits the impulses via a dorsal nerve root into the CNS (at the level of spinal cord). The different neuron then carry signal from CNS to the effector. The stimulus and response thus forms a reflex arc.

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Location of Eyes

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Our paired eyes are located in sockets of the skull called orbits.

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Parts of an eye

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The wall of the eyeball 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 eyeball, but it becomes thick in the anterior part to form the ciliary body. The ciliary body itself continuous forward to form a pigmented and opaque structure called the Iris which is the visible coloured portion of the eye. The eyeball 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. The diameter of the pupil 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.

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Photoreceptor cells

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There are two types of photoreceptor cells, namely, rods and cones. These cells contain the light sensitive proteins called the photopigments. The daylight (photopic) vision and colour vision are functions of cons and the twilight (scotopic) vision is the function of rods. The rods contain a purple red protein called the rhodopsin or visual purple, which contains a derivative of vitamin A. Vitamin A deficiency leads to night blindness. In the human eye, there are three types of cones which possess their own characteristic photopigments that respond to red, green and blue lights. The sensations of different colours are produced by various combination of these cones and their photopigments. When these cones are stimulated equally, sensation of white light is produced.

Photopigments of cones are called cone pigments. Cones are of three different types, short wavelength (sensitive to blue), medium wavelength (sensitive to green) and long wavelength (sensitive to red) cones. The visual pigments for colour vision are erythropsin (sensitive to red), chloropsin (sensitive to green) and cyanopsin (sensitive to blue). In moonlight, we can see colours because only the rods are functional. The relationship of photoreceptor cells to bipolar cells to ganglion cells is 1:1:1 within the fovea only. Absorption of light is actually not by opsin but by 11-cis retinal which is converted into 11-trans retinal isomer. This conformational change results in dissociation of retinal and opsin which is referred as bleaching.

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Blindspot

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The optic nerves leave the eye and the retinal blood vessels enter it at a point medial to and slightly above the posterior pole of the eyeball. Photoreceptor cells are not present in that region and hence it is called the blind spot.

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What is fovea

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At 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. The fovea is a thinned-out portion of the retina where only the cons are densely packed. It is the point where the visual acuity(resolution) is the greatest.

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Aqueous chamber and vitreous chamber

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The space between the cornea and the lens is called the Aqueous chamber and contains a thin watery fluid called aqueous humour. The space between the lens and the retina is called the vitreous chamber and is filled with a transparent gel called vitreous humour.

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Mechanism of vision

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Light rays in visible wavelength focused on the retina through the cornea and lens generate potentials in rods and cones. As mentioned earlier, the photosensitive compounds in the human eyes is composed of opsin (a protein) and retinal (an aldehyde of vitamin A). Light induces dissociation of the retinal from opsin resulting in changes in the structure of the opsin. This 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. 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. Visual cortex area is the part of occipital lobes of cerebrum.

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Disorders of eye

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Functions of ear

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The ears perform 2 sensory functions, hearing and maintenance of body balance.

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Parts of ears

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It can be divided into three major sections called the outer ear, the middle ear and the inner ear. Outer ear consists of the pinna and external auditory meatus (canal). The middle ear contains three ossicles called malleus, incus and stapes which are attached to one another in a chain like fashion. The fluid filled inner ear called labyrinth consists of two parts, the bony and the membranous labyrinths.

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Inner ear

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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 eardrum). 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 mucous membrane inside.

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Ear ossicles

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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. And Eustachian tube connects the middle ear cavity with the pharynx. The Eustachian tube helps in equalising the pressures on either sides of the eardrum.

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Labyrinth

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The Bony Labyrinth is a series of channels. Inside these channels lies the 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. The membrane constituting cochlea, the reissner's and basilar, divide the surrounding perilymph filled Bony Labyrinth into an upper scala vestibuli and lower scala tympani. The space within cochlea called scala media is filled with endolymph. At the base of the cochlea, the scala vestibuli ends at the oval window, while the scale tympani terminates at the round window which opens to the middle ear.

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Organ of corti

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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. The Basal end of the hair cells is in close contact with the afferent nerve fibres. A large number of processes called stereocilia 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.

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Ears maintaining balance of body

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Crista ampullaris is the structure present in the semicircular canals acts as the sensory receptor for sensing the change in the position of head or body. Cristae in the three semicircular canals maintain the dynamic balance of the body. The macula present in both saccule and utricle acts as the specific sensory receptor for sensing the changes in the position of head in relation to the force of gravity. The ottilith organ (saccule and utricle) maintains the static balance of the body.

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Mechanism of hearing

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External ear receive 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 to the oval window. 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. As a result, nerve impulses are generated in the associated afferent neurones. 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 recognised.