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When a neuron is not conducting any impulse; i.e resting, the axoplasm inside the axon contains high concentration of K⁺ and negatively charged proteins and low concentration of Na⁺ . In contrast, the fluid outside the axon contains a low concentration of K⁺ , a high concentration of Na⁺ 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 Na⁺ outwards for 2K⁺ 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 cerebrum wraps around a structure called thalamus, which is major coordinating centre for sensory and motor signaling. It functions as a relay station. Hypothalamus lies at the base of thalamus. This portion contains a number of centres which control body temperature, urge for eating and drinking. It also contains several groups of neuro secretory cells, which secrete hormones called hypothalamic hormones.

Cerebral cortex …

The two types of synapses present are:

• Electrical synapse
• Chemical synapse

Multipolar, bipolar and unipolar neurons.

Synaptic vesicles are found in synaptic knob. They contain chemicals called neurotransmitters which are involved in transmission of impulses.

Electrical synapses : Here the membrane of pre and post-syneptic neuron are in very close proximity. Electric current can flow directly from one neuron into other across these synapses, like impulse conduction along a single axon. 

Chemical synapses : Here the membrane of pre and post-syneptic neuron are separated by fluid filled space called synaptic cleft. Neurotransmitter are involved here. When an impulse arrives at the axon terminal, it stimulates the movement of the synaptic vesicles towards membrane and they fuse with the plasma membrane and release their neurotransmitter in the syneptic cleft. These chemicals bind to specific receptors, present on the post-syneptic membrane. Their binding opens ion channels and allow the entry of ion which generate new potential in post synaptic neuron.

The point where optic nerves leave the eye and the retinal blood vessels enter which is slightly above posterior pole of the eye ball is called blind spot. It lacks photoreceptor cells (rods and cones) and is devoid of vision. Hence called blind spot.

Perilymph of the inner ear floats.

Synaptic vesicles are found in a bulb like structure called synaptic knob. Synaptic vesicles contain chemicals called neurotransmitters. Neurotransmitters are involved in the transmission of impulses at the synapses. They bind to the receptors present on the post – synaptic membrane and generate new potential.

Malleus, Incus, and Stapes are the ear ossicles. The vibrations due to sound waves are transmitted through the ear ossicles to the oval window which are further transmitted to endolymph.

Organ of Corti

(i) Unipolar neurons:

• Cell body with one axon. 
• found in embryonic stage

(ii) Bipolar neurons:

• Cell body with one axon and one dendrite
• found in retina of the eye

(iii) Multipolar neurons:

• Cell body with one axon and two or more dendrites
• Found in cerebral cortex

The middle ear contains three ossicles called malleus, incus and stapes which are attached to one another in a chain like fashion. The malleus is attached to the tympanic membrane and the stapes is attached to the oval window of the cochlea. An Eustachian tube connects the middle ear cavity with the pharynx.

Functions:

• The ear ossicles increase the efficiency of transmission of sound waves to the inner ear.
• Eustrachian tube helps in equalising the pressures on either sides of the ear drum.

The outer ear consists of the pinna and external auditory meatus.

• Pinna collects the vibrations in the air which produce sound. 
• There are very fine hairs and wax secreting sebaceous glands in the skin of pinna and the meatus.

Eustachian tube

Cerebral aqueduct

(a)  Neural co-ordination:

Co-ordination 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 co-ordinations. Neural – co-ordination includes mechanisms like transmission of nerve impulse, impulse conduction across a synapse and reflex action.

(b) Forebrain:

The forebrain consists of cerebrum, thalamus and hypothalamus. Cerebrum is divided longitudinally into two halves, left and right cerebral hemispheres. The hemispheres are connected by a tract of nerve fibres called corpus callosum. The layer of cells which covers the cerebral hemisphere is called cerebral cortex and is thrown into prominent folds. It is referred to as grey matter due to greyish appearance. The cerebral cortex contains motor areas, sensory areas and association areas which 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. This layer is white in colour, hence called white matter. The cerebrum wraps around a structure called thalamus, which is a major co-ordinating centre for sensory and motor signalling. Hypothalamus lies at the base of the thalamus and contains several group 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.

(c) Midbrain:

The midbrain is located between the thalamus / hypothalamus of the forebrain and pons of the hind brain. A canal called the cerebral aqueduct passes through the mid brain. The dorsal portion of the midbrain consists of four round swellings (lobes) called Corpora quadrigemina. Midbrain and hindbrain form the brain stem

The functions of the hind brain are

• It regulates posture and postural activities.
• It plays an important part in muscular co ordination and maintenance of balance
• Cerebellar hemisphere controls muscle tone and posture on its side
• It regulates smooth and precise goal oriented movements.

Corpus callosum.

(a) Afferent, Efferent

(b) Nodes of Ranvier

(c) Association areas

(d) Cochlea

(e) Organ of Corti

The nerve fibres that transmit regulatory impulses from the CNS to the concerned peripheral tissues / organs are called efferent nerve fibres.

Rhodopsin the photo pigment present in the rod cells.

Corpus callosum

The connective tissue membranes around brain are called meninges. They protect the delicate nerve structure, carry the blood vessels to it and by the secretion of a fluid (C S F) minimize any blow or concussion.

There are three layers of Meninges;

• Durometer
• The arachnoid 
• Piamater

Rods and cones are photoreceptor cells that contain light sensitive proteins called photo pigments. The daylight vision and colour vision are functions of cones. There are three types of cones which respond to red, green and blue lights. The sensations of different colours are produced by various combinations of these cones and their photo pigments. When these cones are stimulated equally, a sensation of white light is produced.

Three semicircular canals and otolith organ.

The saccule and utricle contain a projecting ridge called macula.

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

The crista and macula are the specific receptors of the vestibular apparatus responsible for maintenance of balance of the body.

Crista and macula.

Pons, cerebellum and medulla.

It contains centres which control respiration, cardiovascular reflexes and gastric secretions.

Corpus callosum: A curved thick bundle of nerve fibres that joins two cerebral hemisphere. 

Depolarisation: A condition when polarity of the plasma membrane of nerve fibre is reversed. 

Endolymph: The fluid filled within membranous labyrinth. 

Eustachian tube: A tube which connects ear cavity with the pharynx. 

Fovea: An area of highest vision on the retina which contain only cones.

Meninges: Three sheets of covering of connective tissue wrapping the brain. 

Grey Matter: This shows many convolutions which increase the amount of vital nerve tissue. Medulla 

Oblongata: Posterior most part of the brain which is continuous with spinal cord and control respiration, heart rate, swallowing, vomiting. 

Pons: Thick bundles of fibres on the ventral side of brain below cerebellum. 

Foramen magnum: A big aperture in the skull posteriorly through which spinal cord emerges out. 

Spinal cord: A tubular structure connected with medulla oblongata of brain and situated in the neural canal of the vertebral column, covered by meninges.

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 nervous systems.

(A) To collect sound waves 

(B) Eustachina tube 

(C) Colour vision 

(D) Iris

(i) A : Cerebrum B : Corpus callosum 

(ii) C : Balancing of body and maintain posture 

D : Vomiting, coughing, breathing, salivation. 

(iii) Piameter, arachnoid and duramater.

A sudden change in the electrical charges in the plasma membrane of a nerve fibre.

A nerve impulse is transmitted from one neuron to another through junctions called synapses. At a chemical synapse, the 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 at these synapses. When an impulse 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 post-synaptic membrane. This binding opens ion channels allowing the entry of ions which can generate a new potential in the postsynaptic neuron. The new potential developed may be either excitatory or inhibitory.

When a stimulus is applied at a site, (eg: site A) on the polarised membrane, the membrane at the site A becomes freely permeable to Na⁺ . This leads to a rapid influx of Na⁺ 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 depolarised. Thus action potential is generated across the plasma membrane.

The path through which the electro-chemical impulses are generated in response to a stimulus is called reflex arc. It is the simplest functional unit of the nervous system by which an impulse produces a reflex action.The quick, spontaneous and involuntary action induced by the nervous system in response to a stimulus is called reflex action.

Neuron is composed of cell body, dendrites and axon.