521.111022100 Help line 9357388588
Notes ;
NCERT QAs ;
Extra QAs ;
Comprehension QAs ;
http://rksvirtuals.com/Audios/1121.1.mp3
http://rksvirtuals.com/Audios/1121.2.mp3
http://rksvirtuals.com/Audios/1121.3.mp3
Click for notes
NCERT

The neural system provides an organised network of point-to-point connections for a quick coordination. The endocrine system provides chemical integration through hormones. In this chapter, you will learn about the neural system of human, mechanisms of neural coordination like transmission of nerve impulse, impulse conduction across a synapse and the physiology of reflex action.

Neural system
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.

Human neural system
The human neural system is divided into two parts :
(i) the central neural system (CNS)
(ii) the peripheral neural system (PNS)

The 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 (brain and spinal cord). The nerve fibres of the PNS are of two types:
(a) afferent fibres
(b) efferent fibres

The afferent nerve fibres transmit impulses from tissues/organs to the CNS and the efferent fibres transmit regulatory impulses from the CNS to the concerned peripheral tissues/organs.
The PNS is divided into two divisions called somatic neural system and autonomic neural system.

The somatic neural system relays impulses from the CNS to skeletal muscles while the autonomic neural system transmits impulses from the CNS to the involuntary organs and smooth muscles of the body.
The autonomic neural system is further classified into sympathetic neural system and 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
A neuron is a microscopic structure composed of three major parts, namely, cell body, dendrites and axon (Figure).
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) and 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.

⇉ 021121.O1 a. The neural system of all animals is composed of highly specialised cells called Neurons. b. Neuron can detect, receive and transmit different kinds of stimuli. c. In Hydra neural organisation is composed of a network of neurons. d. In insects a brain is present along witha number of ganglia and neural tissues. e. The human neural system is divided into two parts the central neural system (CNS) and the peripheral neural system (PNS). f. The CNS includes the brain and the spinal cord and is the site of information processing and control. g. The PNS comprises of all the nerves of the body associated with the CNS (brain and spinal cord). h. The nerve fibres of the PNS are of two types- afferent fibres and efferent fibres. i. The afferent nerve fibres transmit impulses from tissues/organs to the CNS j. The efferent fibres transmit regulatory impulses from the CNS to the concerned peripheral tissues/organs. k. The PNS is divided into two divisions called somatic neural system and autonomic neural system. l. The somatic neural system relays impulses from the CNS to skeletal muscles m. Autonomic neural system transmits impulses from the CNS to the involuntary organs and smooth muscles of the body. n. The autonomic neural system is further classified into sympathetic neural system and parasympathetic neural system. o. Visceral nervous system is the part of the peripheral nervous system that comprises the whole complex of nerves, fibres, ganglia, and plexuses. p. In visceral nervous system by impulses travel from the central nervous system to the viscera and from the viscera to the central nervous system. ⇔ ⇉
021121.O2. a. Neuron as structural and functional unit of Neural system. b. A neuron is composed of three major parts, namely, cell body, dendrites and axon. c. The cell body of neuron contains cytoplasm with typical cell organelles and certain granular bodies called Nissl’s granules. d. Short fibres which branch repeatedly and project out of the cell body also contain Nissl’s granules and are called dendrites. e. Short fibres present on dendrites transmit impulses towards the cell body. f. The axon is a long fibre, the distal end of which is branched. g. Each branch of axon terminates as a bulb-like structure called synaptic knob. h. synaptic knob of axon possess synaptic vesicles containing chemicals called neurotransmitters. i. The axons transmit nerve impulses away from the cell body to a synapse or to a neuro-muscular junction. j. Neurons are divided into three types, multipolar bipolar and unipolar k. multipolar Neuron has one axon and two or more dendrites and are found in the cerebral cortex. l. bipolar Neurons are with one axon and one dendrite and are found in the retina of eye. m. Unipolar axon has cell body with one axon only and found usually in the embryonic stage . n. There are two types of axons, namely, myelinated and nonmyelinated. o. The myelinated nerve fibres are enveloped with Schwann cells. p. Schwann cells form a myelin sheath around the axon. q. The gaps between two adjacent myelin sheaths are called nodes of Ranvier. r. Myelinated nerve fibres are found in spinal and cranial nerves. s. Unmyelinated nerve fibre is enclosed by a Schwann cell that does not form a myelin sheath around the axon. t. Unmyelinated nerve fibre is commonly found in autonomous and the somatic neural systems. ⇔

Generation and Conduction of Nerve Impulse
Neurons are excitable cells because their membranes are in a polarised state. Do you know why the membrane of a neuron is polarised? 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⁺) 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 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 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. You might be curious to know about the mechanisms of generation of nerve impulse and its conduction along an axon. When a stimulus is applied at a site (Figure 21.2 e.g., point 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. The 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 (Figure 21.2) 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 Na+ is extremely shortlived. 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 further stimulation.
Transmission of Impulses
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. There are two types of synapses, namely, electrical synapses and chemical synapses. At electrical synapses, the 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. Impulse transmission across an electrical synapse is always faster than that across a chemical synapse. Electrical synapses are rare in our system.
At a chemical synapse, the membranes of the pre- and post-synaptic neurons are separated by a fluid-filled space called synaptic cleft (Figure 21.3). Do you know how the pre-synaptic neuron transmits an impulse (action potential) across the synaptic cleft to the post-synaptic neuron? Chemicals called neurotransmitters are involved in the transmission of impulses at these synapses. The axon terminals contain vesicles filled with these neurotransmitters. 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 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. The 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 and an inner layer (which is in contact with the brain tissue) called pia mater.

The brain can be divided into three major parts:
(i) forebrain,
(ii) midbrain, and
(iii) hindbrain (Figure).

Forebrain The forebrain consists of cerebrum, thalamus and hypothalamus (Figure). 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 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. 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 cerebral hemisphere.
They give an opaque white appearance to the layer and, hence, is called the white matter.

The cerebrum wraps around a structure called thalamus, which is a major coordinating centre for sensory and motor signaling.
Another very important part of the brain called hypothalamus lies 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 (e.g., excitement, pleasure, rage and fear), and motivation.

Midbrain

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

Hindbrain The hindbrain 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 medulla of the brain is connected to the spinal cord. The medulla contains centres which control respiration, cardiovascular reflexes and gastric secretions.
Brain stem forms the connections between the brain and spinal cord. Three major regions make up the brain stem; mid brain, pons and medulla oblongata.

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. The reflex pathway comprises at least one afferent neuron (receptor) and one efferent (effector or excitor) neuron appropriately arranged in a series (Figure 21.5). 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. You should carefully study Figure 21.5 to understand the mechanism of a knee jerk reflex.

sensory reception and processing Have you ever thought how do you feel the climatic changes in the environment? How do you see an object and its colour? How do you hear a sound? The 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. This is how you can sense changes in the environment.

Eye
Our paired eyes are located in sockets of the skull called orbits. A brief account of structure and functions of the human eye is given in the following sections.

Parts of an eye
The adult human eye ball is nearly a spherical structure. The wall of the eye ball is composed of three layers (Figure 21.6). 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 eye ball, but it becomes thick in the anterior part to form the ciliary body. 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. The eye ball 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. 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 cones and the twilight (scotopic) vision is the function of the rods. The rods contain a purplish-red protein called the rhodopsin or visual purple, which contains a derivative of Vitamin A. 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. When these cones are stimulated equally, a sensation of white light is produced.

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
The light rays in visible wavelength focussed on the retina through the cornea and lens generate potentials (impulses) in rods and cones.

As mentioned earlier, the photosensitive compounds (photopigments) 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 (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.
Top
Go back to Sense Organs

The Ear
The ears perform two sensory functions, hearing and maintenance of body balance. Anatomically, the ear can be divided into three major sections called the outer ear, the middle ear and the inner ear (Figure 21.7). The outer ear consists of the pinna and external auditory meatus (canal). 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 ear drum). 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 mucus membrane inside.
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. The 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. The Eustachian tube helps in equalising the pressures on either sides of the ear drum. The fluid-filled inner ear called labyrinth 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. The coiled portion of the labyrinth is called cochlea. 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 (Figure 21.8). 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 scala tympani terminates at the round window which opens to the middle ear.

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 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.
The inner ear also contains a complex system called vestibular apparatus, 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.

Mechanism of Hearing
How does ear convert sound waves into neural impulses, which are sensed and processed by the brain enabling us to recognise a sound ? 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 recognised. Top

Exercises

1. Briefly describe the structure of the following: (a) Brain
(b) Eye
(c) Ear
2. Compare the following:
(a) Central neural system (CNS) and Peripheral neural system (PNS)
(b) Resting potential and action potential
(c) Choroid and retina

3. Explain the following processes:
(a) Polarisation of the membrane of a nerve fibre
(b) Depolarisation of the membrane of a nerve fibre
(c) Conduction of a nerve impulse along a nerve fibre
(d) Transmission of a nerve impulse across a chemical synapse
4. Draw labelled diagrams of the following:
(a) Neuron (b) Brain (c) Eye (d) Ear
5. Write short notes on the following:
(a) Neural coordination
(b) Forebrain
(c) Midbrain
(d) Hindbrain
(e) Retina
(f) Ear ossicles
(g) Cochlea
(h) Organ of Corti
(i) Synapse
6. Give a brief account of:
(a) Mechanism of synaptic transmission
(b) Mechanism of vision
(c) Mechanism of hearing
7. Answer briefly:
(a) How do you perceive the colour of an object?
(b) Which part of our body helps us in maintaining the body balance?
(c) How does the eye regulate the amount of light that falls on the retina.
8. Explain the following:
(a) Role of Na+ in the generation of action potential.
(b) Mechanism of generation of light-induced impulse in the retina.
(c) Mechanism through which a sound produces a nerve impulse in the inner ear.
9. Differentiate between:
(a) Myelinated and non-myelinated axons
(b) Dendrites and axons
(c) Rods and cones
(d) Thalamus and Hypothalamus
(e) Cerebrum and Cerebellum
10. Answer the following:
(a) Which part of the ear determines the pitch of a sound?
(b) Which part of the human brain is the most developed?
(c) Which part of our central neural system acts as a master clock?
11. The region of the vertebrate eye, where the optic nerve passes out of the retina, is called the
(a) fovea (b) iris (c) blind spot (d) optic chaisma
12. Distinguish between:
(a) afferent neurons and efferent neurons
(b) impulse conduction in a myelinated nerve fibre and unmyelinated nerve fibre
(c) aqueous humor and vitreous humor
(d) blind spot and yellow spot
(f) cranial nerves and spinal nerves.

Notes 1. Briefly describe the structure of the following:
(a) Brain
(b) Eye
(c) Ear
1. (a) Structure of Brain The human brain is well protected by the skull. The brain can be divided into three major parts forebrain, midbrain and hindbrain.
(i) Forebrain The various parts of forebrain are cerebrum, thalamus and hypothalamus. Cerebrum is responsible for complex functions like intersensory associations, memory and communication. (ii) Midbrain The midbrain is located between the thalamus/hypothalamus of the forebrain and pons of the hindbrain. (iii) Hindbrain The hindbrain comprises pons, cerebellum and medulla (also called the medulla oblongata). The medulla contains centres, which control- respiration cardiovascular reflexes and gastric secretions. Forebrain: It is the main thinking part of the brain. It consists of cerebrum, thalamus, and hypothalamus. (a) Cerebrum: Cerebrum is the largest part of the brain and constitutes about four-fifth of its weight. Cerebrum is divided into two cerebral hemispheres by a deep longitudinal cerebral fissure. These hemispheres are joined by a tract of nerve fibre known as corpus callosum. The cerebral hemispheres are covered by a layer of cells known as cerebral cortex or grey matter. Cerebrum has sensory regions known as association areas that receive sensory impulses from various receptors as well as from motor regions that control the movement of various muscles. The innermost part of cerebrum gives an opaque white appearance to the layer and is known as the white matter. (b) Thalamus: Thalamus is the main centre of coordination for sensory and motor signalling. It is wrapped by cerebrum. (c) Hypothalamus: It lies at the base of thalamus and contains a number of centres that regulate body temperature and the urge for eating and drinking. Some regions of cerebrum, along with hypothalamus, are involved in the regulation of sexual behaviour and expression of emotional reactions such as excitement, pleasure, fear, etc. Midbrain: It is located between the thalamus region of the forebrain and pons region of hindbrain. The dorsal surface of midbrain consists of superior and inferior corpora bigemina and four rounded lobes called corpora quadrigemina. A canal known as cerebral aqueduct passes through the midbrain. Midbrain is concerned with the sense of sight and hearing. Hindbrain: It consists of three regions − pons, cerebellum, and medulla oblongata. (a) Pons is a band of nerve fibre that lies between medulla oblongata and midbrain. It connects the lateral parts of cerebellar hemisphere together. (b) Cerebellum is a large and well developed part of hindbrain. It is located below the posterior sides of cerebral hemispheres and above medulla oblongata. It is responsible for maintaining posture and equilibrium of the body. (c) Medulla oblongata is the posterior and simplest part of the brain. It is located beneath the cerebellum. Its lower end extends in the form of spinal cord and leaves the skull through foramen magnum. /
What is the purpose of ventricles in the brain?
Your brain floats in a bath of cerebrospinal fluid. This fluid also fills large open structures, called ventricles, which lie deep inside your brain. The fluid-filled ventricles help keep the brain buoyant and cushioned.
sulcus (plural sulci) (anatomy) A furrow or groove in an organ or a tissue, especially that marking the convolutions of the surface of the brain.
The optic chiasm, or optic chiasma, is the part of the brain where the optic nerves cross and is therefore of primary importance to the visual pathway. It is located at the base of the brain inferior to the hypothalamus, and approximately 10 mm superior to the pituitary gland within the suprasellar cistern.
The optic chiasm is located in the front part of the brain. It lies directly in front of the hypothalamus, the part of the brain that controls body temperature, hunger and mood. Long, threadlike nerve fibers, called axons, come together from the retinas to form the optic nerves of each eye. Once the optic nerves meet at the optic chiasm, axons from half of each retina cross over to the opposite side of the brain. The axons from the other half of the retina remain on the same side of the brain. After the partial crossover of nerve fibers at the optic chiasma, the resulting two bundles of fibers are called the optic tracts. Each optic tract contains nerve fibers from both eyes — parts of the retina that correspond to specific parts of the visual field. The optic tracts then relay this “binocular” information to the visual cortex of the brain.
The partial crossing over of optic nerve fibres at the optic chiasm allows the visual cortex to receive the same hemispheric visual field from both eyes. Superimposing and processing these monocular visual signals allow the visual cortex to generate binocular and stereoscopic vision.
What is the difference between optic nerve and optic chiasm? The optic nerves exit the orbit and pass through the optic canals (in the skull base) and into the intracranial space. A portion of each optic nerve crosses the midline above the pituitary gland to form the optic chiasm.
The mammillary bodies are a pair of small round bodies, located on the undersurface of the brain that, as part of the diencephalon, form part of the limbic system. They are located at the ends of the anterior arches of the fornix. The fornix acts as the major output tract of the hippocampus, arcing around the thalamus and connecting the medial temporal lobes to the hypothalamus. The fornix plays an important role in the formation and consolidation of declarative memories The mammillary bodies are part of the diencephalon, which is a collection of structures found between the brainstem and cerebrum. The diencephalon includes the hypothalamus, and the mammillary bodies are found on the inferior surface of the hypothalamus (the side of the hypothalamus that is closer to the brainstem). The mammillary bodies are a paired structure, meaning there are two mammillary bodies---one on either side of the midline of the brain. They get their name because they were thought by early anatomists to have a breast-like shape. The mammillary bodies themselves are sometimes each divided into two nuclei, the lateral and medial mammillary nuclei. The medial mammillary nucleus is the much larger of the two, and is often subdivided into several subregions. The primary function associated with the mammillary bodies is recollective memory. Why is the cerebellum called arbor vitae? The arbor vitae /ˌɑːrbɔːr ˈvaɪtiː/ (Latin for "tree of life") is the cerebellar white matter, so called for its branched, tree-like appearance. In some ways it more resembles a fern and is present in both cerebellar hemispheres. It brings sensory and motor information to and from the cerebellum. The arbor vitae /ˌɑːrbɔːr ˈvaɪtiː/ (Latin for "tree of life") is the cerebellar white matter, so called for its branched, tree-like appearance. In some ways it more resembles a fern and is present in both cerebellar hemispheres. It brings sensory and motor information to and from the cerebellum. Each gyrus is surrounded by sulci and together, the gyri and sulci help to increase the surface area of the cerebral cortex and form brain divisions. They form brain divisions by creating boundaries between the lobes, so these are easily identifiable, as well as serving to divide the brain into two hemispheres.
Is there a 5th ventricle in the brain?
The conus medullaris is the terminal end of the spinal cord, which typically occurs at the L1 vertebral level in the average adult. Conus medullaris syndrome (CMS) results when there is compressive damage to the spinal cord from T12-L2.

The choroid plexus is part of the blood-brain barrier that protects the central nervous system from harmful chemicals, and is the primary source of the various components of cerebrospinal fluid. The choroid plexus or plica choroidea borders the membrane of the pia mater and the ventricles of the brain. It is an area of specialized cells that surrounds a direct blood source (capillary). The choroid plexus also plays neuroendocrine, neuro-immune, and excretory roles.

Anterior choroid plexus, is present on the roof of the diencephalon. A human brain is grossly divided into forebrain, midbrain and hindbrain. The forebrain is further divisible into the cerebrum, olfactory lobes and diencephalon. The choroid plexus (ChP) is a secretory tissue found in each of the brain ventricles, the main function of which is to produce cerebrospinal fluid (CSF)
The choroid plexus is located in the posterior medullary velum which partially forms the roof of the fourth ventricle. The choroid plexus is supplied by the branches of the posterior inferior cerebellar arteries. The choroid plexus resides in the innermost layer of the meninges (pia mater) which is in close contact with the cerebral cortex and spinal cord. It is a highly organized tissue that lines all the ventricles of the brain except the frontal/occipital horn of the lateral ventricles and the cerebral aqueduct. Where is most of the choroid plexus? Choroid plexus is found in each lateral ventricle and the third and fourth ventricle. It is involved in the production of cerebrospinal fluid. Choroid plexus is composed of cuboidal epithelial cells resting on a basal lamina which are adjacent to highly fenestrated blood vessels separated by the stroma. Posterior Choroid plexus is in the fourth ventricle, in the bottom half of the cerebellum. (B) Eye: Eyes are spherical structures that consist of three layers. (a) The outer layer is composed of sclera and cornea. (i) Sclera is an opaque tissue that is usually known as white of the eye. It is composed of a dense connective tissue. (ii) Cornea is a transparent anterior portion of eye that lacks blood vessels and is nourished by lymph from the nearby area. It is slightly bulged forward and helps in focusing light rays with the help of lens. (b) The middle layer of eye is vascular in nature and contains choroid, ciliary body, and iris. (i) Choroid lies next to the sclera and contains numerous blood vessels that provide nutrients and oxygen to the retina and other tissues. (ii) Ciliary body: The choroid layer is thin over posterior region and gets thickened in the anterior portion to form ciliary body. It contains blood vessels, ciliary muscles, and ciliary processes. (iii) Iris: At the junction of sclera and cornea, the ciliary body continues forward to form thin coloured partition called iris. It is the visible coloured portion of eye. The eye contains a transparent, biconvex, and elastic structure just behind the iris. It is known as lens. The lens is held in position by suspensory ligaments attached to the ciliary body. The lens divides the eye ball into two chambers – an anterior aqueous and posterior vitreous chamber. (c) The innermost nervous coat of eye contains retina. Retina is the innermost layer. It contains three layers of cells – inner ganglion cells, middle bipolar cells, and outermost photoreceptor cells. The receptor cells present in the retina are of two types – rod cells and cone cells. (a) Rod cells –The rods contain the rhodopsin pigment (visual purple) that is highly sensitive to dim light. It is responsible for twilight vision. (b) Cone cells –The cones contain the iodopsin pigment (visual violet) and are highly sensitive to high intensity light. They are responsible for daylight and colour visions. The innermost ganglionic cells give rise to optic nerve fibre that forms optic nerve in each eye and is connected with the brain. (C) Ear: Ear is the sense organ for hearing and equilibrium. It consists of three portions – external ear, middle ear, and internal ear. 1. External ear: It consists of pinna, external auditory meatus, and a tympanic membrane. (a) Pinna is a sensitive structure that collects and directs the vibrations into the ear to produce sound. (b) External auditory meatus is a tubular passage supported by cartilage in external ear. (c) Tympanic membrane is a thin membrane that lies close to the auditory canal. It separates the middle ear from external ear. 2. Middle ear: It is an air-filled tympanic cavity that is connected with pharynx through eustachian tube. Eustachian tube helps to equalize air pressure in both sides of tympanic membrane. The middle ear contains a flexible chain of three middle bones called ear ossicles. The three ear ossicles are malleus, incus, and stapes that are attached to each other. 3. Internal ear: It is also called labyrinth. It has two parts bony labyrinth and a membranous labyrinth. Bony labyrinth is filled with perilymph while membranous labyrinth is filled with endolymph. Membranous labyrinth is divided into 2 parts. (a) Vestibular apparatus (b) Cochlea: (a) Vestibular apparatus Vestibular apparatus is a central sac-like part that is divided into utriculus and sacculus. A special group of sensory cells called macula are present in sacculus and utriculus. Vestibular apparatus also contains three semi-circular canals. The lower end of each semi-circular canal contains a projecting ridge called crista ampularis. Each ampulla has a group of sensory cells called crista. Crista and macula are responsible for maintaining the balance of body and posture. (b) Cochlea: Cochlea is a long and coiled outgrowth of sacculus. It is the main hearing organ. Cochlea consists of three membranes. The organ of corti, a hearing organ, is located on the basilar membrane that has hair cells. 2. Compare the following: (a) Central neural system (CNS) and Peripheral neural system (PNS) (b) Resting potential and action potential (c) Choroid and retina 2. (a) Central neural system (CNS) and Peripheral neural system (PNS) Central neural system Peripheral neural system 1. It is the main coordinating centre of the body. 2. It includes brain and spinal cord. 1. It is not the main coordinating centre of the body. 2. It includes cranial and spinal nerves that connect central nervous system to different parts of the body. (b) Resting potential and action potential Resting potential action potential 1. It is the potential difference across the nerve fibre when there is no conduction of nerve impulse. 2. The membrane is more permeable to K+ ions than to Na+ ions. 1. It is the potential difference across nerve fibre when there is conduction of nerve impulse. 2. The membrane is more permeable to Na+ ions than to K+ ions. (c) Choroid and retina Choroid Retina 1. Retina is the innermost nervous coat of eye. 2. It contains numerous blood vessels that provide nutrients and oxygen to retina and other tissues. 1. Choroid is the middle vascular layer of eye. 2. It contains photoreceptor cells, rods and cones that are associated with twilight and colour vision respectively. 3. Explain the following processes: (a) Polarisation of the membrane of a nerve fibre (b) Depolarisation of the membrane of a nerve fibre (c) Conduction of a nerve impulse along a nerve fibre (d) Transmission of a nerve impulse across a chemical synapse 3. (a) Polarisation of the membrane of a nerve fibre During resting condition, the concentration of K+ ions is more inside the axoplasm while the concentration of Na+ ions is more outside the axoplasm. As a result, the potassium ions move faster from inside to outside as compared to sodium ions. Therefore, the membrane becomes positively charged outside and negatively charged inside. This is known as polarization of membrane or polarized nerve. (b) Depolarisation of the membrane of a nerve fibre When an electrical stimulus is given to a nerve fibre, an action potential is generated. The membrane becomes permeable to sodium ions than to potassium ions. This results into positive charge inside and negative charge outside the nerve fibre. Hence, the membrane is said to be depolarized. (c) Conduction of a nerve impulse along a nerve fibre There are two types of nerve fibres – myelinated and non-myelinated. In myelinated nerve fibre, the action potential is conducted from node to node in jumping manner. This is because the myelinated nerve fibre is coated with myelin sheath. The myelin sheath is impermeable to ions. As a result, the ionic exchange and depolarisation of nerve fibre is not possible along the whole length of nerve fibre. It takes place only at some point, known as nodes of Ranvier, whereas in non-myelinated nerve fibre, the ionic exchange and depolarization of nerve fibre takes place along the whole length of the nerve fibre. Because of this ionic exchange, the depolarized area becomes repolarised and the next polarized area becomes depolarized. (d) Transmission of a nerve impulse across a chemical synapse Synapse is a small gap that occurs between the last portion of the axon of one neuron and the dendrite of next neuron. When an impulse reaches at the end plate of axon, vesicles consisting of chemical substance or neurotransmitter, such as acetylcholine, fuse with the plasma membrane. This chemical moves across the cleft and attaches to chemo-receptors present on the membrane of the dendrite of next neuron. This binding of chemical with chemo-receptors leads to the depolarization of membrane and generates a nerve impulse across nerve fibre. The chemical, acetylcholine, is inactivated by enzyme acetylcholinestrase. The enzyme is present in the post synaptic membrane of the dendrite. It hydrolyses acetylcholine and this allows the membrane to repolarise. 4. Draw labelled diagrams of the following: (a) Neuron (b) Brain (c) Eye (d) Ear 4.
5. Write short notes on the following: a. Neural coordination a. b. Forebrain b. c. Midbrain c. d. Hindbrain d. e. Retina e. f. Ear ossicles f. g. Cochlea g. h. Organ of Corti h. i. Synapse i.
6. Give a brief account of: a. Mechanism of synaptic transmission a. b. Mechanism of vision b. c. Mechanism of hearing c. Sound waves enter the external auditory meatus and set the tympanum to vibrate. The vibrations are passed to membrane overlying fenestra vestibuli with an amplification of 20 - 22 times through ear ossicles. It sets the perilymph (scala vestibuli to scala tympani) and then endolymph in motion. Tectorial and basilar membranes move in opposite directions. This brings about contact between hair of sensory cells and tectorial membrane. Vibrations of different types sensitise different sensory cells (human capacity is 16 - 20000 cycles/see) -low frequency near tip of cochlea and high frequency towards the oval window. Impairment of hearing is mainly caused by damage to sensory hair due to over activity and noise pollution. The sensory phonoreceptor cells produce impulses that are transferred through their basal ends to some 30000 neurons and nerve fibres, which for cochlear branch of auditory nerve for carrying electrical to temporal lobe of cerebrum, hardly 2 cm away. The vibrations of perilymph are passed back into tympanic cavity through fenestra rotunda. Equilibrium. It is controlled by vestibular system. Besides equilibrium or balance the system makes us aware of orientation, acceleration and rotation. The maculae present in utriculus and sacculus have an otolithic membrane overlaying sensory cells. With the change in position of head and during linear movements, otoliths present in the gelatinous membrane come in contact with hair of sensory cells at particular points generating impulses that are carried through vestibular branch of auditory nerve for interpretation in cerebellum. The cerebellum sends reflex signals for restoring static balance. Receptors for dynamic balance are present in the cristae of ampullae present at ends of semicircular canals. Movements in any direction arc sensed by initiation of sensory impulses in one or the other crista when cupula comes in contact with hair of sensory cells. The sensory impulses are carried to brain through vestibular branch of auditory nerve for interpretation and correction. Dizziness after spinning or long travel is due to continued disturbance in endolymph or excessive sensitisation. However, balance is not a single sense phenomenon. Actually input comes from four sources. (i) Ears (ii) Stretch and strain guages in muscles, tendons, joints in head, neck, body, limbs (iii) Touch and pressure sensors in skin. (iv) Eyes for seeing vertical and horizontals. Cerebellum analyses data from all sources and with the help of cortex sends information to muscles for proper adjustment.

SVP1→

1. What is EPSP?
2. What is an excitatory or inhibitory potential
3. What does it mean if a neuron is excitatory?

↔

→

↑

→

→

→

↑

↑

→

↑

→

↑

→

↑

→

↑

→

↑