Study Notes on Neural Control and Coordination
Study Notes on Neural Control and Coordination
- Coordination is the process through which two or more organs interact and complement the functions of one another.
- 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.
- The neural system provides an organised network of point-to-point connections for a quick coordination. The endocrine system provides chemical integration through hormones.
- The neural system of all animals is composed of highly specialised cells called neurons which can detect,receive and transmit different kinds of stimuli.
- The neural organisation is very simple in lower invertebrates.
- For example, in Hydra it is composed of a network of neurons.
- The neural system is better organised in insects, where a brain is present along with a number of ganglia and neural tissues.
- The vertebrates have a more developed neural system.
- Nervous system is composed of nerual tissue.
- Nervous tissue is composed of neuron and neuroglia (Neuroglial cells)
HUMAN NEURAL SYSTEM
- Human has an elaborate and well-developed neural system
- The ability of man to control his own body is largely because of the specialised neural system
- The human neural system is divided into two parts :
(a) The central neural system (CNS)
(b) The peripheral neural system (PNS)
(a) Central Neural System (CNS) :
- The CNS lies along the central axis of the body. It includes the brain and the spinal cord.
- All the information of the external and internal environment is received by the various sensory receptors present in the body and is transmitted to the central neural system.
- Then CNS process the information and responds accordingly and controls the body.
(b) Peripheral Neural System (PNS)
- The PNS lies along the peripheries of the body and comprises of all the nerves of the body associated with the CNS (brain and spinal cord).
- A nerve is formed by the collection of many nerve fibres which is the components of the PNS.
Types of Nerve Fibres of the PNS
The nerve fibres of the PNS are of two types :
- Afferent fibres
- The afferent nerve fibres transmit impulses from tissues/organs to the CNS.
- The afferentnerve fibres are also called the sensory nerve fibres which carry the sensory information from various tissues/organs of the body towards the CNS so that the CNS can accordingly respond and control the body.
- Efferent fibres
- The efferent nerve fibres transmit the regulatory impulses from the CNS to the concerned peripheral tissues/organs.
- The efferent nerve fibres are also called the motor nerve fibre which carry the regulatory information from CNS to the concerned peripheral tissue/organ.
Divisions of PNS
The peripheral neural system is further divided into two divisions:
- Somatic neural system
- The somatic neural system relays impulses from the CNS to the skeletal (voluntary) muscles of the body.
- The somatic neural system consists of those efferent nerve fibres which transmit the regulatory impulses from the CNS to the concerned skeletal (voluntary) muscle of the body
- Autonomic neural system
- The autonomic neural system transmits impulses from the CNS to the involuntary organs (also called viscera),smooth muscles (involuntary muscles present in the wall of hollow internal organs like in the alimentary canal, reproductive tract, blood vessels and so on) and glands of the body.
Autonomic Neural System (ANS) :
Structurally and functionally, the ANS is further classified into two separate sub-systems called
(a) Sympathetic neural system
(b) Parasympathetic neural system
- Sympathetic and parasympathetic neural systems have antagonistic (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 neurotransmitter is adrenaline/ noradrenaline.
Visceral nervous system is the part of the peripheral nervous system that comprises the whole complex of nerves, fibres, ganglia, and plexuses by which impulses travel from the central nervous system to the viscera and from the viscera to the central nervous system.
NEURON AS STRUCTURAL AND FUNCTIONAL UNIT OF
- A neuron is a microscopic structure composed of three major parts, namely, cell body, dendrites and axon
- The cell body contains cytoplasm with typical cell organelles and certain granular bodies 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.
- 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.
- The axons transmit nerve impulses away from the cell body to a synapse or to a neuro-muscular junction.
- Based on the number of axon and dendrites, the neurons 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)
- unipolar (cell body with one axon only; found usually in the embryonic stage).
- There are two types of axons, namely, myelinated and nonmyelinated.
- The myelinated nerve fibres are 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.
Generation and conduction of Nerve impulse (Physiology of Nerve)
Different stage of nerve conduction
(1) Polarisation (Resting stage)
(2) Depolarisation (Excited stage) Nut infles
(3) Repolarisation (again resting stage)
Active and passive ion movements across the cell surface of an axon. The movements are responsible for the generation of a negative potential inside the axon. This is called the resting potential.
Active transport takes place through the sodium/potassium pump.
Ion channels (proteins) allow the passive movement of ions down their electrochemical gradient
- Neurons are excitable cells because their membranes are in a polarised state.
- Different types of ion channels are present on the neural membrane.
- These ion channels are selectively pemeable to different ions.
- When a neuron is not conducting any impulse, i.e.,resting, the axonal membrane is comparatively more permeable to potassium ions (K+) and nearly impermeable to sodium ions (Na+). Similarly, the membrane is impermeable to negatively charged proteins present in the axoplasm.
- Consequently, the axoplasm inside the axon contains high concentration of K+ and negatively charged proteins and low concentration of Na+
- In constrast, the fluid outside the axon contains a low concenration of K+,a high concentration of Na+ and thus from 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 Na+ outwards for 2 K+ 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 polarised.
- The electrical potential difference across the resting plasma membrane is called as the resting potential and this is about -70 mV (thenegative sign indicates that inside the cell is negative with respect to the outside). (Range -60 to -85 mV).
- Once the event of depolarization has occurred, a nerve impulse or spike is initiated.
- This is generated by a change in the sodium ion channels. These channels, and some of the potassium ion channels, are known as voltage gated channel, meaning they can be opened or closed with change in voltage.
- In resting state these channels are closed due to binding of Ca++
- An action potential is generated by a sudden opening of the sodium gates. Opening of gates increases the permeability of the axon membrane to sodium ions which enter by diffusion.
- This increases the number of positive ions inside the axon.
- A change of +10mV in potential difference from RMP (Resting Membrane Potenial) through influx is sufficiently significant to trigger a rapid influx of Na+ ions leading to generation of action potential.
- This change of +10 mV is called as threshold stimulus.
- At the point where membrane (Axolemma) is completely depolarised due to rapid influx of Na+ions, the negative potential is first cancelled out and becomes 0 (Depolarisation).
- This axolemma is called as excited membrane or depolarised membrane.
- Due to further entry of Na+, the membrane potential “over shoots” beyond the zero and becomes positive upto +30 to +45mV. This “over shoot” peak corresponds to maximum concentration of sodium inside the axon.
- This potential is called as action potential, In this state, the inner surface of axolemma becomes positively charged and outer surface becomes negatively charged.
- The rise in the stimulus-induced permeability to Na+ is extremely short lived. It is quickly followed by a rise in permeability to K.
- Within a fraction of a second, K 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 stimulation.
- The repolarization period returns the cell to its resting potential (-70 mV).
- The neuron is now prepared to receive another stimulus and conduct it in the same manner.
- Sodium pump starts working to maintain the normal resting membrane potential by expelling Na+ and intaking of K.
- The time taken for restoration of resting potential is called refractory period, because during this period the membrane is incapable of receiving another impulse
- The whole process of depolarisation and repolarisation is very fast. It takes only about 1 to 5 milli second (ms).
- Saltatory conduction occurs only in myelinated axon. Myelin sheath is electrically insulator that inhibits exchange of ionic transportation. so ionic exchanges occurs only at node of Ranveir.
- Saltatory conduction of nerve impulse :-In a myelinated nerve fibre the impulse jumps from one node of Ranvier to the other, this is called as Saltatory conduction of nerve impulse.
Transmission of Impulses
Synapse : A nerve impulse is transmitted from one neuron to another through junctions called synapses.
A synapse is formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron, which may or may not be separated by a gap called synaptic cleft.
Pre-synaptic neuron :That neuron which is
transmitting the impulse to the other neuron and is present before the synaptic cleft is called the pre-synaptic neuron.
Post-synaptic neuron : That neuron which is present after the synaptic cleft and is receiving the nerve impulse from the other neuron (pre-synaptic neuron) is called the post-synaptic neuron.
Types of Synapses:
There are two types of synapses, namely,
- Electrical synapses
- Electrical synapses are specialised for the rapid signal transmission from one neuron to another.
- At electrical synapses, the membranes of pre- and post-synaptic neurons are in very close proximity.
- The electrical current or electrical impulse can flow directly from one neuron into the other across these synapses.
- The transmission of an impulse across electrical synapses is very similar to impulse conduction along a single axon because no special mechanism operates at the electrical synapses.
- At a chemical synapse, the membranes of the pre- and post-synaptic neurons are separated by a fluid-filled space called the synaptic cleft.
- Neurotransmitters are the chemicals which are involved in the transmission of nerve impulses across chemical synapses.
- The axon terminals contain vesicles filled with these neurotransmitters.
- When an impulse arrives at the axon terminal, it stimulates the movement of the synaptic vesicles towards the membrane where they fuse with the plasmamembrane 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
CENTRAL NEURAL SYSTEM
- The brain is the central information processing organ of our body, and acts as the ‘command and control system’.
- It controls the voluntary movements, 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.
- It is also the site for processing of vision, hearing, speech, memory, intelligence, emotions and thoughts.
Meninges /Meninx (Coverings of Brain)
- Brain is covered by three membranes of connective tissue termed as meninges or meninx
- Duramater: It is the outer layer of the cranial meninges. It is thick and tough layer made up of a tough connective tissue called fibrous tissue.
- Arachnoid : It is the middle meninx. It is a very thin layer made up of delicate connective tissue.
- Piamater : It is the inner meninx which is in contact with the brain tissue, It is also a thin layer and highly vascular.
- Cerebrum consist of two cerebral hemispheres. On the dorsal surfacea longitudinal groove present is present between two cerebral hemispheres caled as median fissure
- Both the cerebral hemispheres are partially connected with each other by a curved thick band (tract) of nerve fibres called corpus callosum.
- Each cerebral hemisphere is divided into 4 lobes – Anterior, middle, posterior and lateral.
- Anterior lobe is also called frontal lobe. Middle lobe is also called parietal lobe
- Central sulcus separates frontal lobe from parietal lobe. Lateral lobe or temporal lobe is separated from frontal lobe and parietal lobe by incomplete sulcus called lateral sulcus
- Many ridges and grooves are found on dorsal surface of cerebral hemisphere. Ridges are known as gyri while grooves are called sulci. These cover the 2/3 part of cerebrum.
- The layer of cells which covers the cerebral hemisphere is called cerebral cortex is referred to as the grey matter due to its greyish appearance.
- 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 lays and, hence, Is called the white matter.
It is small & posterior part of fore brain. It is covered by cerebrum. It consist of thalamus, hypothalamus, epithalamus.
- Thalamus : It forms upper lateral walls of Diencephalon. t forms 80% part of Diencephalon, It acts as a relay centre. The cerebrum wraps around a structure called thalamus, which is a major coordinating centre for sensory and motor signaling.
- Hypothalamus : It forms lower ventral part of Diencephalon. The hypothalamus contains a number of centres which control body temperature, urge for eating and drinking It also contain several group of neurosecretory cells, which secrete hormones called hypothalamic hormones.
The inner parts of cerebral hemispheres and a
- 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 (e.g., excitement, pleasure, rage and fear), and motivation.
3)Epithalamus -It forms the roof of diencephalon. Pineal body is found in epithalamus.
- Small & contracted part of brain
- The midbrain is located between the thalamus /hupothalamus of the forebrain and pons of the hindbrain
- A canal called the cerebral aqueduct passess through the midbrain.
In Anterior part 2 longitudinal fibres (myelinated)
In Posterior part 4 opitc lobes or four round swellings (corpora quadrigemina)
Function of Mid – Brain-
- Four optic lobes or colliculus present superior optic lobes are the main centres of pupillary light reflexes.
3 Parts –
(3) Medulla Oblongata (M.O.)
(1)Pons or Pons varolii :
- It is small spherical projection, which is situated below the midbrain upper side of the M.O.
- It consists of many transverse and longitudinal nerve fibres.
- Transverse nerve fibres connect with cerebelium.
- Longitudinal fibres connect cerebrum to M.O.
- Pons consists of fibre tracts that interconnect different regions of the brain
Function of Pons :-It regulates the breathing reaction through pneumotaxic centre.
(2) Cerebellum :
- Made up of 3 lobes (2 lateral lobes & 1 vermis divide in 7 to 9 segments
- Both lateral lobes become enlarged & spherical in shape
Function of Cerebellum
- Received impulses from different voluntary muscles and joints and then controlling of the their movements and their regulation and coordination
Medulla Oblongata (M.O.):
- Posterior part of brain is tubular and cylindrical in shape.
- The medulla of the brain is connected to the spinal cord.
Functon of Medulla Oblongata
It is also concerned with Reflex- Sneezing reflex , Salivation reflex, Coughing reflex, Swallowing reflex,Vomiting reflex, yawning reflex
- Reflex actions are spontaneous, automatic, involuntary actions which are completed very fast/quickly as compared to normal actions, and mechanical responses produced by stimulating specific receptors.
- The reflex pathway comprises at least one afferent neuron (receptor) and one efferent (effector or excitor) neuron appropriately arranged in a series
- The 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 then carries signals from CNS to the effector.
- The stimulus and response thus forms a reflex arc as shown below in the knee jerk reflex.
SENSORY RECEPTORS (EYE AND EAR)
Sensory organs detect all types of changes in the environment and send appropriate signals to the CNS,
These are photosensitive organs.
- Each eye is an empty ball like or nearly spherical structure, it is called eye ball.
- Each eye ball is situated in the sockets or notch of bone in the skull. It is called “Eye orbit“.
- Only 1/5th part of whole eye is seen from out side in between the eye lashes.Remaining 4/5th part is in the eye orbit.
The wall of remaining eye ball has three layers.
- Fibrous tunic
It is the outermost covering of eye ball. It is composed af dense connective tissue.The layer is divided into two parts
- It is the outer visible part of fibrous tunic, covered by Nonkeratinized Stratified Squamous Epithelium.
- Sclerotie layer/Sclera
- It is made up of white, hard, opaque thick fibrous connective tissue. It is the inner porton of eye ball
- It is non-vascularised.
- This layer is of white colour, so it is also called “White of eye
- This part of eye is mesodermal in origin.
- It is called conjunctiva.
- It is made up of epidermis of skin.
2) Vascular tunic
Inner layer of eyelids remain stretched over anterior part of sclera (limbus) in the form of translucent membrane.
It is the middle layer of eyeball.
This layer has three parts :-
- Choroid layer is found below the sclerotic layer.
- It contains abundant pigment cells, blood vessels and bluish in colour.
- It darkens the cavity of eyeball to prevent internal reflection of light.
- It is the lower swollen portion below limbus
- It has ciliary processes which project into eyeball.
- 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.
- 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
(e) Radial dilatory muscles
- These are outer unstriated muscles, these are expanded in the iris breadth wise.
- Iris becomes constricted if these muscles contract and diameter of pupil is increased at that time. It happens in dim light
It is the inner most layer of eye ball
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.
Receptor cells also known as photoreceptors/visual cells.
- These are long, thin, cylindrical structures/cells. These are numerous in number.
- Rods differentiates between light and dark.
- These are more sensitive than cones.
- There are present a purple coloured pigment in rods called Rhodopsin/Visual purple,
- These are thick and small cells which differentiate among different colours in full light.
- Lodopsin /Visual violet is present in cones.
- Only rods are found in the retina of owl, because it is nocturnal animal, unlike hen, cock and squirrel which have only cones in Its retina
- 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 combinations of these cones and their photopigments
- 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 eye ball
- Photoreceptor cells are not present in that region and hence it is called the blind spot.
- 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 cones are densely packed.
- It is the point where the visual acuity (resolution) is the greatest.
- The space between the cornea and the lens is called the aqueous chamber and contains a thin watery fluid called aqueous humor.
- The space between the lens and the retina is called the vitreous chamber and is filled with a transparent gel called vitreous humor.
Mechanism of Vision
- Light rays emitted by any object enter the eye, a small, real and inverted image of object is formed at retina.
- Sensory cells of retina are sensitized, and optic nerve carries this impulse to brain.At this time animal is able to see the object.
- Cornea, aqueous humor and biconvex lens completely refract the light rays coming from object,
- As a result of this an inverted image is formed at retina. Just like diaphragm of a camera iris of eye, decreases or increases the diameter of pupil according to light.
- Iris expands to decrease the pupil in high intensity of light so a small amount of light touches the retina.
- When light is dim, iris itself constricts to increase the diameter of pupil.
- The ability to change the focal length of lens by changing the curvature of lens, is called accommodation power.
- Only mammals and birds have this accommodation power in their eyes
All the vertebrates have one pair of ears back to the eyes,
There are two main functions of ears:-
- To receive sound waves (hearing)
- To maintain body balance Main function of ear is to maintain the balance of body
Structurally ear may be divided into three parts :-
(a) External ear
(b) Middle ear
(c) Internal ear
(a) External ear
- It is the outer part of ear. It is well developed in mammals only, External ear may be divided again into 2 pans
(i) ear pinna,
(ii) ear canal
- These may be small or large, fan like structure, important feature of mammals.
- The skin of ear pinna is hairy.
- The pinna collects the vibrations in the air which produce sound.
Ear canal or External auditory meatus
- It is a 24 mm long canal which is expanded from base of pinna to inner side.
- At the end of ear canal a stretched, thin, obliquely placed membrane in present, it is called ear drum or tympanic membrane. This separates the ear canal to middle ear.
- Ear drum is a part of middle ear.
- The tympanic membrane is composed of connective tissues covered with skin outside and with mucus membrane inside
- In the wall of external auditory meatus or ear canal there are found modified sweat glands (ceruminous glands) and sebaceous glands.
- These secrete cerumen or ear wax, which moisten the ear drum and protects it.
- Middle ear is also called tympantc cavity. It is filled with air. There are present 3 ear ossicles arranged in a chain with movable joints connected together in tympanic cavity
- This has 3 ear ossicles are :-
- Malleus :- It is situated towards external ear. It is the largest of three and of hammer shaped. Malleus. It is attached to the tympanic membrane.
- Incus :- The ossicle is anvil shaped. Its outer broad part is connected by malleus and inner thin part is connected by stapes.
- Stapes :- It is the smallest bone of body It looks like stirrup of horse. The malleus is attached to the tympanic membrane and the stapes is attached to the oval window of the cochlea Internal ear
Other internal parts of Ear
- 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.
- Labyrinth is the fluid-filled inner ear consists of two parts, the bony and the membranous labyrinths.
- 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.
- Cochlea is the coiled portion of the labyrinth.The membranes constituting cochlea, the reissner’s 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
- At the base of the cochlea, the scala vestibuli ends at the oval window, while the scala tympani terminates at the round window which opens to the middle ear
- .Organ of Corti:-it 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 intemal side of the organ of corti.
- 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.
- Vestibular apparatus- The inner ear also contains a complex system , located above the 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.
WORKING OF EAR
- The external ear receives sound waves and directs them to the ear drum.
- The ear drum vibrates in response to the sound waves and these vibrations are transmitted through the ear ossicles (malleus, incus and stapes) 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 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.