Neural Control and Coordination

Functions of the organs/organ systems in our body must be coordinated to maintain
homeostasis

Coordination is the process through which two or more organs interact and complement the
functions of one another.
Ex. - the functions of muscles, lungs, heart, blood vessels, kidney and other organs are
coordinated while performing physical exercises.
• 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.
• Endocrine system provides chemical integration through hormones.

NEURAL SYSTEM

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

• Neural organisation is very simple in lower invertebrates.
  Ex. - in Hydra it is composed of a network of neurons.
• Neural system is better organised in insects, where a brain is present along with a number of
  ganglia and neural tissues.
• Vertebrates have a more developed neural system

HUMAN NEURAL SYSTEM

Divided into two parts

(1) The central neural system (CNS)

CNS includes the brain and the spinal cord and is the site of information processing and
control.

(2) The peripheral neural system (PNS)

PNS comprises of all the nerves of the body associated with the CNS (brain and spinal cord).

PNS are of two types

(a) Afferent fibres : nerve fibres transmit impulses from tissues/organs to the CNS.
(b) Efferent fibres : transmit regulatory impulses from the CNS to the concerned peripheral
tissues/organs.

PNS is divided into two divisions

(i) Somatic neural system : neural system relays impulses from the CNS to skeletal muscles.
(ii) Autonomic neural system : neural system transmits impulses from the CNS to the
involuntary organs and smooth muscles of the body.
Autonomic neural system is further classified into

☆ Sympathetic neural system
☆ Parasympathetic neural system.

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 NEURAL SYSTEM

Neuron is a microscopic structure composed of three parts
I. Cell body
ii. Dendrites
iii. Axon

i. Cell body : contains cytoplasm with typical cell organelles and
certain granular bodies called Nissl’s granules

These granules are of rough endoplasmic reticulum (RER) with
rosettes of free ribosomes, and are the site of protein synthesis.

ii. Dendrites :- Short fibres which branch repeatedly and project out
of the cell body also contain Nissl’s granules and are called
dendrites. Transmit impulses towards the cell body.

iii. 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.
• Axons transmit nerve impulses away from the cell body to a synapse or to a neuro-muscular
  junction.

Two types of axons :

1. Myelinated : nerve fibres are enveloped with Schwann cells, which form a myelin sheath
   around the axon.

• Myelin sheath is formed by schwann cells and oligodendrocyte . (NEET 2017)
• gaps between two adjacent myelin sheaths are called nodes of Ranvier.
• Myelinated nerve fibres are found in spinal and cranial nerves.
  

2.Nonmyelinated : nerve fibre is enclosed by a Schwann cell that does not form a myelin sheath
around the axon.

• Commonly found in autonomous and the somatic neural systems.
• Based on the number of axon and dendrites, the neurons are divided into three types

(a) Multipolar (with one axon and two or more dendrites; found in the cerebral cortex).
(b) Bipolar (with one axon and one dendrite, found in the retina of eye)
(c) Unipolar (cell body with one axon only; found usually in the embryonic stage

Generation and Conduction of Nerve Impulse

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 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 (K+) impermeable to sodium ions (Na+).
• Similarly, the membrane is impermeable to negatively charged proteins present in the
  axoplasm.
• Axoplasm inside the axon contains high concentration of K+ , negatively charged proteins and
  low concentration of Na+. (NEET 2013)

Fluid outside the axon contains a low concentration of K+ , and high concentration of Na+ and
form a concentration gradient. ionic gradients across the resting membrane are maintained by the active transport of ions by
the sodium-potassium pump which transports 3 Na+ outward for 2 K+ into the cell.

• So, the outer surface of the axonal membrane possess a positive chage and inner surface
  become negatively charged therefore Polarised.
• Resting Potential - Electrical potential difference across the resting plasma membrane is
  called as the resting potential.

Mechanisms of generation of nerve impulse and its conduction along an axon :

When a stimulus is applied at a site on the polarised membrane, the membrane at the site A
becomes freely permeable to Na+ to a rapid influx of Na+ (as shown in Fig 21.2)

• This leads 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.
• Polarity of the membrane at the site A is thus
  reversed and hence depolarised.
• Electrical potential difference across the plasma
  membrane at the site A is called the action potential,
  which is in fact termed as a nerve impulse.
• At sites immediately ahead, the axon (e.g., site B)
  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 site B to site A to complete the circuit of current flow.
• 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.
• Sequence is repeated along the length of the axon and consequently the impulse is conducted.
• Rise in the stimulus-induced permeability to Na+ is extremely short-lived.
• 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 further stimulation.

Transmission of Impulses

A nerve impulse is transmitted from one neuron to another through junctions called synapses.
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.

Two types of synapses

(i) Electrical synapses (ii) Chemical synapses

(I) Electrical synapses - membranes of pre- and post-synaptic neurons 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.
• Electrical synapse is always faster than that across a chemical synapse.
• Electrical synapses are rare in our system.

(II) Chemical Synapse : membranes of the pre- and post-synaptic neurons are separated by a
fluid-filled space called synaptic cleft.

• Chemicals called neurotransmitters are involved in the transmission of impulses.
• Axon terminals contain vesicles filled with these neurotransmitters.
• 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. Released neurotransmitters bind to their specific receptors, present on the post-synaptic membrane. (NEET 2017)
• Binding opens ion channels allowing the entry of ions which can generate a new potential in
  the post-synaptic neuron.
• New potential developed may be either excitatory or inhibitory.

Central Nervous System

☆ Brain - The brain is the central information processing organ of our body, and acts as the
‘command and control system’.

• 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.
• Site for processing of vision, hearing, speech, memory, intelligence, emotions and thoughts. 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
• An inner layer (which is in contact with the brain tissue) called pia mater.

Brain can be divided into three major parts:

1. Forebrain 2. Midbrain 3. Hindbrain

1.Forebrain : consists of (i) Cerebrum (ii) Thalamus (iii) Hypothalamus
(i) 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.
• Hemispheres are connected by a tract of nerve fibres called corpus callosum. (NEET 2015,18)
• Layer of cells which covers the cerebral hemisphere is called cerebral cortex and is thrown into
  prominent folds.
• Cerebral cortex is referred to as the grey matter due to its greyish appearance.
• Neuron cell bodies are concentrated here giving the colour.
• Cerebral cortex contains motor areas, sensory areas and large regions that are neither clearly
  sensory nor motor in function.

These regions called as the association areas are responsible for complex functions like
intersensory associations, memory and communication.

• Fibres of the tracts are covered with the myelin sheath, which constitute the inner part of
  cerebral hemisphere.
• Give an opaque white appearance to the layer and, hence, is called the white matter.
• Cerebrum wraps around a structure called thalamus

(ii) Thalamus - Major coordinating centre for sensory and motor signalling

(iii) Hypothalamus - lies at the base of the thalamus.

• Hypothalamus contains a number of centres which control body temperature, urge for eating
  and drinking. (NEET 2009,2010,2014,2015,2018,2019)
• Contains several groups of neurosecretory cells, which secrete hormones called hypothalamic
  hormones.
  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.
• 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. (NEET 2018)

2. Midbrain - located between the thalamus/hypothalamus of the forebrain and pons of the
   hindbrain. A canal called the cerebral aqueduct passes through the midbrain. Dorsal portion of the midbrain consists mainly of four round swellings (lobes) called corpora
   quadrigeminal.

3.Hindbrain

Hindbrain comprises (i) Pons (ii) Cerebellum (iii) Medulla (medulla oblongata).

(i) Pons - consists of fibre tracts that interconnect different regions of the brain.
(ii) Cerebellum - very convoluted surface in order to provide the additional space for many more neurons.
(iii) Medulla of the brain is connected to the spinal cord. (NEET 2015,2018)

• contains centres which control respiration, cardiovascular reflexes and gastric secretions.
• Three major regions make up the brain stem; mid brain, pons and medulla oblongata.
• Brain stem forms the connections between the brain and spinal cord.

REFLEX ACTION AND REFLEX ARC

You must have experienced a sudden withdrawal of a body part which comes in contact with objects that are extremely hot, cold pointed or animals that are scary or poisonous. The

• Entire process of response to a peripheral nervous stimulation, that occurs involuntarily, i.e., without conscious effort or thought and requires the involvment of a part of the central nervous system is called a reflex action.

Reflex pathway comprises (arranged in a series)

• Afferent neuron (receptor) : receives signal from a sensory organ and transmits the impulse via a dorsal nerve root into the CNS (at the level of spinal cord).
• Efferent (effector or excitor) : carries signals from CNS to the effector.
• Stimulus and response thus forms a reflex arc .

SENSORY RECEPTION AND PROCESSING

Sensory organs detect all types of changes in the environment and send appropriate signals to the CNS, where all the inputs are processed and analysed. Signals are then sent to different parts/ centres of the brain.

Sense Organs.

Smell things by our nose    • Taste by tongue    • see objects by eyes         • Hear by ear

1. Nose

• Contains mucus-coated receptors which are specialised for receiving the sense of smell and called olfactory receptors.
• Made up of olfactory epithelium that consists of three kinds of cells.
• Neurons of the olfactory epithelium extend from the outside environment directly into a pair of broad bean-sized organs, called olfactory bulb, which are extensions of the brain’s limbic system.
• Nose and tongue detect dissolved chemicals.
• Chemical senses of gustation (taste) and olfactory (smell) are functionally similar and interrelated.

2. Tounge

Detects tastes through taste buds, containing gustatory receptors.

Brain integrates the differential input from the taste buds and a complex flavour is perceived.

3. Eye

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

Parts of an Eye

- Adult human eye ball is nearly a spherical structure.

Wall of the eye ball is composed of three layers

i. Sclera ii. Choroid        iii. Retina

i. External layer is composed of a dense connective tissue and is called the sclera.

• Anterior portion of this layer is called the cornea. (NEET 2019)
• Cornea is convex, transparent layer which is non- Vascular.
• Cornea is clear part of eye's protective covering.
• Cornea consist of dense matrix of collagen and is the most sensitive portion of the eye.

ii. Middle layer, choroid, contains many blood vessels and looks bluish in colour.

• Choroid layer is thin over the posterior two-thirds of the eye ball, but it becomes thick in the anterior part to form the ciliary body.
• Ciliary body itself continues forward to form a pigmented and opaque structure called the iris which is the visible coloured portion of the eye.
• Eye ball contains a transparent crystalline lens which is held in place by ligaments attached to the ciliary body. (NEET 2018)
• Front of the lens, the aperture surrounded by the iris is called the pupil.
• Diameter of the pupil is regulated by the muscle fibres of iris.

iii.The inner layer is the retina and it contains three layers of neural cells

• (A) ganglion cells (inside)

• (B) bipolar cells (middle)

• (C) photoreceptor cells (outside)

               ☆ Two types of photoreceptor cells    • Rods           • Cones.

- Rods and Cones contain the light-sensitive proteins called the photopigments.

• Cones - daylight (photopic) vision and colour vision are functions of cones.

• Three types of cones which possess their own characteristic photopigments that respond to red, green and blue lights.
• Sensations of different colours are produced by various combinations of these cones and their photopigments.
• When these cones are stimulated equally, a sensation of white light is produced.

• Rods - twilight (scotopic) vision is the function of the rods.

- Rods contain a purplish-red protein called the rhodopsin or visual purple, which contains a derivative of Vitamin A. (formed from carotene)

• 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.
• 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.

fovea is a thinned-out portion of the retina where only the cones are densely packed where the visual acuity (resolution) is the greatest. (NEET 2015)

• The space between the cornea and the lens is called the aqueous chamber and contains a thin watery fluid called aqueous humor.
• 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 in visible wavelength focussed on the retina through the cornea and lens generate potentials (impulses) in rods and cones.
• Photosensitive compounds (photopigments) in the human eyes is composed of opsin (a protein) and retinal (an aldehyde of vitamin A). (NEET 2014,2016,2017)
• Light induces dissociation of the retinal from opsin resulting in changes in the structure of the opsin and causes membrane permeability changes.
• Potential differences are generated in the photoreceptor cells.
• Produces a signal that generates action potentials in the ganglion cells through the bipolar cells.
• Action potentials (impulses) 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.

4. Ear

• Ears perform two sensory functions, hearing and maintenance of body balance.
• Ear can be divided into three major sections

(1) Outer ear          (2) Middle ear               (3) Inner ear

(1) Outer ear consists of the pinna and external auditory meatus (canal).

• Pinna collects the vibrations in the air which produce sound.
• External auditory meatus leads inwards and extends up to the tympanic membrane (the ear drum).
• Very fine hairs and wax-secreting glands in the skin of the pinna and the meatus.
• Tympanic membrane is composed of connective tissues covered with skin outside and with mucus membrane inside.

(2) Middle Ear -

• Contains three ossicles called malleus, incus and stapes which are attached to one another in a chain-like fashion.
• Malleus is attached to the tympanic membrane.
• Stapes is attached to the oval window of the cochlea.
• Ear ossicles increase the efficiency of transmission of sound waves to the inner ear.
• • An Eustachian tube connects the middle ear cavity with the pharynx.
• Eustachian tube helps in equalising the pressures on either sides of the ear drum.

(3) Inner Ear

 Fluid-filled inner ear called labyrinth consists of two parts,

(I) Bony labyrinth

(II) Membranous labyrinth

• Bony labyrinth is a series of channels and Inside these channels lies the membranous labyrinth, which is surrounded by a fluid called perilymph.
• Membranous labyrinth is filled with a fluid called endolymph and coiled portion of the labyrinth is called cochlea.
• 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 endolymph.
• At Base of the cochlea, the scala vestibuli ends at the oval window.
• Scala tympani terminates at the round window which opens to the middle ear.

Organ of corti is a structure located on the basilar membrane which contains hair cells that act as auditory receptors.

Hair cells are present in rows on the internal side of the organ of corti.

• Basal end of the hair cell is in close contact with the afferent nerve fibres.
• 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.
• Inner ear also contains a complex system called vestibular apparatus, located above the cochlea.
• 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.
• membranous canals are suspended in the perilymph of the bony canals.
• Base of canals is swollen and is called ampulla, which contains a projecting ridge called crista ampullaris which has hair cells.
• Saccule and utricle contain a projecting ridge called macula.

• Crista and macula are the specific receptors of the vestibular apparatus responsible for maintenance of balance of the body and posture. (NEET 2012,2015)

▪ Mechanism of Hearing

The external ear receives sound waves and directs them to the ear drum.

• 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.
• Vibrations are passed through the oval window on to the fluid of the cochlea, where they generate waves in the lymphs.
• 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.
• Nerve impulses are generated in the associated afferent neurons and 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.