Neural Control & Coordination

21.1 Neural System

21.2 Human Neural System

21.3 Neuron as Structural and Functional Unit of Neural System

21.4 Central Neural System (Forebrain, Midbrain, Hindbrain)

21.5 Reflex Action and Reflex Arc

21.6 Sensory Reception and Processing (Eye, The Ear)

Q 1 Briefly describe the structure of the following:

(a) Brain (b) Eye (c) Ear

Ans.

(a) Brain: 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: The forebrain consists of the cerebrum, thalamus and hypothalamus. Functions including temperature, reproductive functions, eating, sleeping and the display of emotion.

(ii) Midbrain: It is located between the thalamus/hypothalamus of the forebrain and pons of the hindbrain. The midbrain serves important functions in motor movement, particularly movements of the eye, and in auditory and visual processing.

(iii) Hindbrain: It is hindbrain comprises pons, cerebellum and medulla. Controls functions outside conscious control, such as breathing and blood flow

(b) Eye: The adult human eyeball is nearly a spherical structure. The wall of the eyeball is composed of three layers.

The external layer is composed of a dense connective tissue and is called the sclera.

The anterior portion of this layer is called the cornea. The middle layer, choroid, contains many blood vessels and looks bluish in colour.

The choroid layer is thin over the posterior two-thirds of the eyeball, but it becomes thick in the anterior part to form the ciliary body.

The ciliary body itself continues forward to form a pigmented and opaque structure called the iris which is the visible coloured portion of the eye.

The eyeball contains a transparent crystalline lens which is held in place by ligaments attached to the ciliary body. In front of the lens, the aperture surrounded by the iris is called the pupil whose diameter 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. The daylight (photopic) vision and colour vision are functions of cones and the twilight (scotopic) vision is the function of the rods. 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: It performs two sensory functions, hearing and maintenance of body balance. It can be divided into three major sections called the outer ear, the middle ear and the inner ear :

(i) Outer ear: It 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 eardrum). There are very fine hairs and wax-secreting glands in the skin of the pinna and the meatus. The tympanic membrane is composed of connective tissues covered with skin outside and with mucous membrane inside.

(ii) Middle ear: It 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 staps is attached to the oval window of the cochlea. The ear ossicles increase the efficiency of transmission of sound waves to the inner ear. A Eustachian tube connects the middle ear cavity with the pharynx. The Eustachian tube helps in equalizing the pressures on either side of the eardrum.

(iii) Inner ear: It is also known as the labyrinth. Labyrinth is divided into the bony labyrinth and a membranous labyrinth. Bony labyrinth is filled with perilymph while membranous labyrinth is filled with endolymph. The membranous labyrinth is divided into two parts-Vestibular apparatus and Cochlea.

The vestibular apparatus is composed of three semi-circular canals and the otolith (macula is the sensory part of saccule and utricle). The crista and macula are the specific receptors of the vestibular apparatus responsible for maintenance of the balance of the body and posture.

The 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.

Q 2 Compare the following:

(a) Central neural system (CNS) and Peripheral neural system (PNS)

(b) Resting potential and action potential

Ans.

Central neural system......................................................Peripheral neural system

1. It is the main coordinating centre of the body........1. It is not the main coordinating centre of the body.

2. It includes brain and spinal cord...............................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...................................1. It is the potential difference across nerve fibre when there is conduction of nerve impulse.

2.The membrane is more permeable to K+ ions than to Na+ ions......................................................2. The membrane is more permeable to Na+ ions than to K+ ions.

1. Choroid is the middle vascular layer of eye..............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.................2. It contains photoreceptor cells, rods and cones that are associated with twilight and colour vision respectively.

Q 3 Explain the following processes:

(a) Polarisation of the membrane of a nerve fibre

(b) Depolarisation of the membrane of a nerve fibre

(d) Transmission of a nerve impulse across a chemical synapse

Ans. 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 the polarization of membrane or polarized nerve.

When an electrical stimulus is received by a nerve fibre, an action potential is generated. The membrane becomes permeable to sodium ions than to potassium ions. This results in a positive charge inside and negative charge outside the nerve fibre. Hence, the membrane is said to be depolarised. The potential generated at this phase is known as the action potential. As the action potential reaches its maximum value, the membrane potential gets reversed and this state is known as repolarization.

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 the axon, vesicles consisting of a 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.

Q4. Draw labelled diagrams of the following:

(a) Neuron (b) Brain (c) Eye (d) Ear

Q5 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

Ans.

(a) Neural coordination: The nervous system provides quick coordination of various parts of the body through electric impulses. These impulses are short lived but quick.

(b) Forebrain consists of the cerebrum, cerebral hemispheres, olfactory lobes and diencephalon (thalamus and hypothalamus). It helps in the interpretation of stimulus received by effector organs.

(c) Midbrain consists of tectum (visual and auditory stimuli) and tegmentum (contains nuclei for pain modulation, motor coordination and movement planning). It helps in the relay of impulse from effector organ to the forebrain.

(d) Pons, cerebellum and medulla together form the hindbrain. It helps to maintain the balance of body and body posture. It also has regulatory centres for controlling the involuntary actions.

(e) The retina is the innermost layer of the eyeball and contains photoreceptor rods and cones.

(f) Ear ossicles: Three small bones present between the tympanic membrane and oval window are collectively referred to as ossicles, they are namely malleus, incus and stapes. It increases the amplification of sound waves.

(g) Cochlea: Cochlea is a spiral hollow structure containing three fluid-filled canals. Organ of corti is located in middle cochlear canal and has hair cells (mechanoreceptors) on its basilar membrane. Thus, cochlea houses sensory system for hearing only and is not associated with balancing.

(h) Organ of corti: Organ of corti is located in middle cochlear canal and has hair cells (mechanoreceptors) on its basilar membrane. It generates the auditory impulse which is carried by auditory nerves.

(i) Synapse: Two neurons are never physically connected to each other and synapse is the region of close proximity between two neurons where information from one neuron is transmitted to the next one.

Q6 Give a brief account of:

(a) Mechanism of synaptic transmission

(b) Mechanism of vision

Ans.

(a) Synapse is a junction between two neurons. It is present between the axon terminal of one neuron and the dendrite of next neuron separated by a cleft.

There are two ways of synaptic transmission.

(1) Chemical transmission

(2) Electrical transmission

1. Chemical transmission: When a nerve impulse reaches the end plate of the axon, it releases a neurotransmitter (acetylcholine) across the synaptic cleft. This chemical is synthesized in the cell body of the neuron and is transported to the axon terminal. The acetylcholine diffuses across the cleft and binds to the receptors present on the membrane of next neuron. This causes depolarization of membrane and initiates an action potential.

2. Electrical transmission: In this type of transmission, an electric current is formed in the neuron. This electric current generates an action potential and leads to transmission of a nerve impulse across the nerve fibre. This represents a faster method of nerve conduction than the chemical method of transmission.

(b) Mechanism of vision

Retina is the innermost layer of the eye. It contains three layers of cells – inner ganglion cells, middle bipolar cells, and outermost photoreceptor cells. A photoreceptor cell is composed of a protein called as opsin and an aldehyde of vitamin A called as retinal. When light rays are focused on the retina through the cornea, it leads to the dissociation of retinal from opsin protein. This changes the structure of opsin. As the structure of opsin changes, the permeability of membrane changes, generating a potential difference in the cells. This generates an action potential in the ganglionic cells and is transmitted to the visual cortex of the brain via optic nerves. In the cortex region of the brain, the impulses are analysed and the image is formed on the retina.

The pinna of the external region collects the sound waves and directs it towards ear drum or external auditory canal. These waves strike the tympanic membrane and vibrations are created. Then, these vibrations are transmitted to the oval window, fenestra ovalis, through three ear ossicles, named as malleus, incus, and stapes. These ear ossicles act as a lever and transmit the sound waves to internal ear. These vibrations from fenestra ovalis are transmitted into the cochlear fluid. This generates sound waves in the lymph. The formation of waves generates a ripple in the basilar membrane. This movement bends the sensory hair cells present on the organ of Corti against tectorial membrane. As a result of this, sound waves are converted into nerve impulses. These impulses are then carried to the auditory cortex of brain via auditory nerves. In cerebral cortex of the brain, the impulses are analysed and the sound is recognized.

Q7 . 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?

Ans. a) Photoreceptors are cells that are sensitive to light. They are of two types – rods and cones. These are present in the retina. Cones help in distinguishing colours. There are three types of cone cells – those responding to green light, those responding to blue light, and those responding to red light. These cells are stimulated by different lights, from different sources. The combinations of the signals generated help us see the different colours.

b) The inner ear is part of an ear is responsible for maintaining the body balance. The vestibular apparatus and the semicircular canal is responsible for maintaining the balance of the body.

c) A pupil is like an aperture in an eye. It dilates in low light and constricts in intense light when the light falls on the retina.

Q8 Explain the following:

(a) Role of Na+

in the generation of action potential.

(b) Mechanism of generation of light-induced impulse in the retina.

inner ear.

Ans.

Sodium ions play an important role in the generation of action potential. When a nerve fiber is stimulated, the membrane potential decreases. The membrane becomes more permeable to Na+ ions than to K+ ions. As a result, Na+ diffuses from the outside to the inside of the membrane. This causes the inside of the membrane to become positively charged, while the outer membrane gains a negative charge. This reversal of polarity across the membrane is known as depolarization. The rapid inflow of Na+ ions causes the membrane potential to increase, thereby generating an action potential.

b) In the eye, retina have photopigments like retinal and opsin. Light dissociates the retinal from opsin which changes the structure of opsin and generates an action potential.

c) When sound falls over the eardrum, it is then transmitted to the inner ear by ear ossicles. The vibrations are passed through the oval window onto the fluid of the cochlea, where they generate waves in the lymph. These waves induce the hair cell. As a result nerve impulses are generated in the associated afferent neurons and transmitted to the auditory cortex of the brain via auditory nerves.

Q9 Differentiate between:

(a) Myelinated and non-myelinated axons

(b) Dendrites and axons

(d) Thalamus and Hypothalamus

(e) Cerebrum and Cerebellum

Ans.

(a) A myelinated neuron is a neuron whose axon is covered by the myelin sheath (myelin means white). The conduction of nerve impulse is faster in this neuron than non-myelinated neuron due to the presence of myelin sheath over the axon. Myelin sheath avoids the loss of impulse during conduction.

Whereas non-myelinated neuron is the neuron whose axon is not covered by the myelin sheath. The conduction of nerve impulse in this neuron is slow than myelinated neuron due to the absence of myelin sheath. There are more chances of loss of impulse during conduction.

(b) Axon is a single long, thick neurite structure in the neuron. It contains neurofibrils. It is the efferent component of the impulse. Whereas dendrites are multiple short and thick neurites in the neuron. It is the branched structure which contains both neurofibrils and Nissl's granules. It is the afferent components of the neurons.

Structure of Typical Neuron

(c) Rods are the photoreceptor cells of the retina that are sensitive to dim light. They have the visual purple pigment called as rhodopsin.

Whereas cones are the photoreceptor cells of the retina that are sensitive to bright light. They have the visual violet pigment called as iodopsin.

(d) Thalamus and hypothalamus are both names of structures in the brain. While the hypothalamus is cone-shaped, the thalamus consists of two connected lobes, one located in each hemisphere. The hypothalamus regulates the body's vital metabolic processes, affecting temperature, blood pressure, hunger, thirst and sleep. It controls the endocrine system by affecting the pituitary gland's production of hormones. The thalamus takes information from a number of different areas of the brain and relays it to the cerebral cortex, the outer layer of gray matter where higher level brain functions take place.

(e) The cerebrum is the part of the forebrain that controls voluntary functions. It is the place where intelligence, will power, memory, etc., reside. It is the largest part of the brain, forming four fifths of its weight. Whereas cerebellum is the part of the hindbrain that controls voluntary functions and controls the equilibrium. It is the second largest part of the brain, forming one eight of its mass.

Q10 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?

Ans.

(a) Cochlea determines the pitch of a sound.

(b) Forebrain is largest and the most developed part of the human brain.

Q11. The region of the vertebrate eye, where the optic nerve passes out of the retina, is

called the

(a) fovea

(b) iris

(d) optic chaisma

Ans. Blind spot is the part where the optic nerve passes out of the retina. Photoreceptors are absent in this region. Blind spot is a small portion of the visual field of each eye that corresponds to the position of the optic disc (also known as the optic nerve head) within the retina. There are no photoreceptors (i.e., rods or cones) in the optic disc, and, therefore, there is no image detection in this area. So, the correct answer is option C.

Q12 Distinguish between:

(a) afferent neurons and efferent neurons

(b) impulse conduction in a myelinated nerve fibre and unmyelinated nerve fibre

(d) blind spot and yellow spot

(f) cranial nerves and spinal nerves.

Ans. (a) Afferent neuron conducts nerve impulses toward the brain or the spinal cord and are also called as sensory neurons. While efferent neuron conducts nerve impulses from the brain or spinal cord to the effector organs such as muscles or glands and are also called as motor neurons.

(b) In a myelinated nerve fibre, the action potential is conducted from one node to another and the conduction of impulses is faster. whereas in an unmyelinated nerve fibre, the action potential is not conducted from node to node. It is carried along the whole length of the nerve fibre and the conduction of impulses is slower.

(c) Aqueous humor is a clear liquid found between the cornea and the lens of eye, whereas vitreous humor is a clear gelatinous mass found in the rear part of the eyeball between the lens and retina. Aqueous humor is continuously generated and continuously drains from the front of the eye throughout the lifetime, whereas vitreous humor is produced only during the embryonic stage and stays for the entire lifetime. Vitreous humor does not replenish while aqueous humor does. The volume of vitreous humor is higher than that of aqueous humor in a single eye.

(d) Blind spot is a spot on the retina present at the point of origin of the optic nerve and are insensitive to light as both rods and cones are absent.

Whereas yellow spot is a small area on the retina present at the posterior pole of the eye, lateral to the blind spot and is sensitive to bright light as cones are present.

(e)There are 12 pairs of cranial nerves arises from the brain. While spinal nerves arise from the spinal cord and there are 31 pairs of spinal nerves.

Basic terms

A ganglion is a collection of neuronal bodies found in the voluntary and autonomic branches of the peripheral nervous system (PNS). Ganglia can be thought of as synaptic relay stations between neurons. The information enters the ganglia, excites the neuron in the ganglia and then exits.

Sympathetic Nervous System (SNS): The SNS works to prepare your body for fight or flight situations by increasing heart rate, breathing rate, blood pressure, and blood glucose. These are all bodily systems that need to be increased when an individual is under stress or perceives a threat in order for the body to have enough energy to fight or flee from danger.
Parasympathetic Nervous System (PNS): The PNS opposes the SNS by promoting the opposite bodily functions. This is when the body needs to relax, digest food in order to produce energy, and lower heart rate in cases of emergencies where an individual does not need high amounts of energy for fight or flight situations.

The autonomic nervous system is a component of the peripheral nervous system that regulates involuntary physiologic processes including heart rate, blood pressure, respiration, digestion, and sexual arousal. It contains three anatomically distinct divisions: sympathetic, parasympathetic, and enteric.