The maintenance of homeostasis within the body requires the coordinated functioning of various organs and organ systems. Coordination is the process by which multiple organs interact and complement each other's functions.

For example, during physical exercise, increased muscular activity demands more energy and oxygen. This triggers increased respiration, heart rate, and blood flow to meet the oxygen supply requirement. When exercise stops, these activities gradually return to normal levels.

In the human body, the neural system and the endocrine system work together to coordinate and integrate the activities of all organs, ensuring they function in a synchronized manner.

• The neural system provides rapid, point-to-point connections for quick coordination.
• The endocrine system provides slower, chemical integration through hormones.

Neural System

The neural system is composed of highly specialized cells called neurons. Neurons are capable of detecting, receiving, and transmitting different types of stimuli (signals).

The complexity of neural organization varies across the animal kingdom:

• Lower invertebrates (e.g., Hydra): Simple network of neurons.
• Insects: More organized system with a brain, ganglia, and neural tissues.
• Vertebrates: Possess a highly developed neural system.

Human Neural System

The human neural system is broadly divided into two main parts:

1. Central Neural System (CNS):
   • Includes the brain and spinal cord.
   • Acts as the site for information processing, control, and decision-making.
2. Peripheral Neural System (PNS):
   • Comprises all the nerves in the body that connect the CNS to the rest of the body.
   • Nerve fibers of the PNS are of two types:
     • Afferent fibres: Transmit sensory impulses *from* tissues/organs *to* the CNS.
     • Efferent fibres: Transmit motor/regulatory impulses *from* the CNS *to* the peripheral tissues/organs (effectors).

The PNS is further divided into two functional divisions:

• Somatic Neural System: Relays impulses from the CNS to skeletal muscles (involved in voluntary actions).
• Autonomic Neural System (ANS): Transmits impulses from the CNS to involuntary organs and smooth muscles (controls involuntary functions).
  • The ANS is further classified into the sympathetic neural system and the parasympathetic neural system, which often have opposing effects on organ functions.

The Visceral nervous system is considered part of the PNS. It comprises the complex of nerves, fibers, ganglia, and plexuses that transmit impulses between the CNS and the viscera (internal organs), coordinating involuntary organ functions.

Neuron As Structural And Functional Unit Of Neural System

A neuron (nerve cell) is the structural and functional unit of the neural system. It is a microscopic structure with three main parts (Figure 21.1):

1. Cell body (Soma): Contains the cytoplasm with typical cell organelles (nucleus, mitochondria, ER, Golgi, etc.). Also contains granular bodies called Nissl's granules (involved in protein synthesis).
2. Dendrites: Short, branched fibers projecting from the cell body. They also contain Nissl's granules. Dendrites receive impulses from other neurons and transmit them *towards* the cell body.
3. Axon: A single, long fiber extending from the cell body. Transmits impulses *away from* the cell body to other neurons or effector organs (muscles, glands). The distal end of the axon is branched, with each branch terminating in a bulb-like structure called a synaptic knob. Synaptic knobs contain synaptic vesicles filled with neurotransmitters.

Classification of neurons based on the number of axons and dendrites:

• Multipolar: One axon and two or more dendrites (e.g., cerebral cortex).
• Bipolar: One axon and one dendrite (e.g., retina of eye).
• Unipolar: Cell body with only one axon (e.g., usually found in embryonic stages).

Classification of axons based on myelin sheath:

• Myelinated axons: Enveloped by Schwann cells, which form an insulating myelin sheath around the axon. Gaps in the myelin sheath are called nodes of Ranvier. Found in spinal and cranial nerves. Impulse conduction is faster (saltatory conduction).
• Non-myelinated axons: Enclosed by Schwann cells but do not form a myelin sheath. Commonly found in the autonomous and somatic neural systems. Impulse conduction is slower.

---

Generation And Conduction Of Nerve Impulse

Neurons are excitable cells because their plasma membrane is maintained in a polarized state, meaning there is an electrical potential difference across the membrane.

Resting potential: In a resting neuron (not conducting an impulse), the axonal membrane is more permeable to K$^+$ ions and nearly impermeable to Na$^+$ ions and negatively charged proteins in the axoplasm (cytoplasm inside the axon). This creates ionic gradients:

• Inside the axon (axoplasm): High K$^+$ concentration, high concentration of negatively charged proteins, low Na$^+$ concentration.
• Outside the axon: High Na$^+$ concentration, low K$^+$ concentration.

These gradients are maintained by the sodium-potassium pump, which actively transports 3 Na$^+$ ions out for every 2 K$^+$ ions transported in. This results in the outer surface of the resting membrane being positively charged and the inner surface being negatively charged. The electrical potential difference across the membrane in this state is the resting potential.

Action potential (Nerve impulse): When a stimulus is applied at a point (e.g., point A) on the membrane, the membrane becomes permeable to Na$^+$ ions at that site. This causes rapid influx of Na$^+$ ions, leading to a reversal of polarity: the outer surface becomes negative, and the inner surface becomes positive (depolarization). The electrical potential difference across the membrane at this point is called the action potential (Figure 21.2).

Conduction of impulse: The action potential generated at point A causes a current flow between the depolarized region (A) and the adjacent polarized region (B). On the inner surface, current flows from A (+) to B (-). On the outer surface, current flows from B (+) to A (-). This current flow depolarizes the membrane at point B, generating an action potential there. This process repeats along the axon, propagating the nerve impulse as a wave of depolarization. The rapid influx of Na$^+$ is quickly followed by increased permeability to K$^+$. K$^+$ diffuses outwards, restoring the resting potential (repolarization) and making the fiber ready for further stimulation.

---

Transmission Of Impulses

Nerve impulses are transmitted from one neuron to another neuron, or to an effector cell, at junctions called synapses (Figure 21.3).

A synapse is formed by the membrane of the pre-synaptic neuron and the membrane of the post-synaptic neuron. A gap between them is called the synaptic cleft.

Two types of synapses:

• Electrical synapses: Membranes of pre- and post-synaptic neurons are very close, allowing direct flow of electrical current. Transmission is very fast but less common in humans.
• Chemical synapses: Pre- and post-synaptic membranes are separated by a synaptic cleft filled with fluid. Impulse transmission is mediated by neurotransmitters.

Transmission at a chemical synapse:

1. An impulse (action potential) arrives at the axon terminal of the pre-synaptic neuron.
2. This stimulates synaptic vesicles containing neurotransmitters to move towards the pre-synaptic membrane.
3. Vesicles fuse with the pre-synaptic membrane and release neurotransmitters into the synaptic cleft.
4. Neurotransmitters diffuse across the synaptic cleft and bind to specific receptors on the post-synaptic membrane.
5. Binding of neurotransmitters opens ion channels in the post-synaptic membrane.
6. Movement of ions across the post-synaptic membrane generates a new electrical potential in the post-synaptic neuron. This potential can be either excitatory (promotes impulse generation) or inhibitory (suppresses impulse generation).

Central Neural System

The Central Neural System (CNS) consists of the brain and spinal cord. The brain is the primary information processing and control center of the body.

Functions of the brain:

• Controls voluntary movements and body balance.
• Regulates vital involuntary organs (lungs, heart, kidneys).
• Thermoregulation, hunger, and thirst.
• Circadian rhythms (sleep-wake cycle).
• Activities of endocrine glands.
• Human behavior, memory, intelligence, emotions, thoughts, speech, vision, hearing, etc.

Protection: The human brain is protected by the skull (cranium). Inside the skull, it is covered by three membranes called cranial meninges (from outer to inner): dura mater, arachnoid, and pia mater (in contact with brain tissue).

Divisions of the brain (Figure 21.4):

1. Forebrain:

---

Forebrain

Consists of cerebrum, thalamus, and hypothalamus.

• Cerebrum: Largest part of the brain, divided into left and right cerebral hemispheres by a deep cleft. Hemispheres are connected by a tract of nerve fibers called the corpus callosum. The outer layer is the cerebral cortex, folded into prominent gyri and sulci ("grey matter" due to neuron cell bodies). Cortex contains motor areas, sensory areas, and large association areas (for complex functions like memory, communication, intersensory associations). The inner part of the hemisphere contains nerve fibers covered with myelin sheath ("white matter").
• Thalamus: Located below the cerebrum. A major coordinating center for sensory and motor signals passing to and from the cerebral cortex.
• Hypothalamus: Lies at the base of the thalamus. Contains centers that control body temperature, urge for eating and drinking. Also contains neurosecretory cells producing hypothalamic hormones.
• Limbic system (limbic lobe): Formed by inner parts of cerebral hemispheres and associated deep structures (amygdala, hippocampus, etc.). Involved with hypothalamus in regulating sexual behavior, emotions (excitement, pleasure, rage, fear), and motivation.
2. Midbrain:

---

Midbrain

Located between the forebrain (thalamus/hypothalamus) and hindbrain (pons). Contains the cerebral aqueduct canal. The dorsal part has four round swellings called corpora quadrigemina (involved in visual and auditory reflexes).

3. Hindbrain:

---

Hindbrain

Comprises pons, cerebellum, and medulla oblongata.

• Pons: Contains fiber tracts connecting different regions of the brain, including tracts between cerebrum, cerebellum, and medulla.
• Cerebellum: Located below the cerebrum and behind the pons. Has a highly convoluted surface, increasing surface area for neurons. Primarily involved in coordinating voluntary movements, maintaining posture and balance.
• Medulla oblongata (Medulla): Connected to the spinal cord. Contains vital centers controlling respiration, cardiovascular reflexes (heart rate, blood pressure), and gastric secretions.

Brain stem: Formed by the midbrain, pons, and medulla oblongata. It connects the brain to the spinal cord.

Reflex Action And Reflex Arc

A reflex action is an involuntary, rapid, and automatic response to a peripheral nervous stimulation, occurring without conscious thought and involving part of the CNS (spinal cord or brainstem).

Example: Sudden withdrawal of a hand from a hot object, the knee-jerk reflex.

Reflex arc: The pathway followed by nerve impulses during a reflex action. It is the neural pathway from the stimulus to the response. A simple reflex arc involves at least one afferent neuron and one efferent neuron, connected in the CNS (Figure 21.5).

Pathway of a simple reflex arc (e.g., knee-jerk reflex):

1. Receptor: Sensory receptor in a sensory organ (e.g., stretch receptor in a muscle). Detects the stimulus (e.g., tapping the patellar tendon).
2. Afferent neuron (Sensory neuron): Transmits the impulse from the receptor to the CNS (spinal cord) via a dorsal nerve root.
3. Processing in CNS: The afferent neuron synapses with an efferent neuron (or via an interneuron) in the spinal cord.
4. Efferent neuron (Motor neuron): Transmits the impulse from the CNS back to the effector organ.
5. Effector: Muscle or gland that carries out the response (e.g., muscle contracts).

Sensory Reception And Processing

Our ability to perceive changes in the environment (like temperature, light, sound, smell, taste) relies on sensory organs. These organs detect stimuli and send signals to the CNS for processing and analysis. The brain then sends signals back to other parts for appropriate responses.

---

Sense Organs

Specialized organs for different senses:

• Nose: For smell (olfaction). Contains mucus-coated olfactory receptors in the olfactory epithelium (three cell types). Neurons from this epithelium extend to the olfactory bulb (extension of the limbic system in the brain).
• Tongue: For taste (gustation). Detects tastes through taste buds containing gustatory receptors. The brain integrates input from taste buds to perceive complex flavors.
• Ear: For hearing and body balance.
• Eye: For vision.

Chemical senses (gustation and olfaction) detect dissolved chemicals and are functionally similar and interrelated.

The following sections describe the eye (vision) and ear (hearing and balance).

---

Eye

Our paired eyes are located in the orbits (sockets) of the skull. The eyeball is nearly spherical.

---

Parts Of An Eye

The wall of the eyeball has three layers (Figure 21.6):

1. External layer (Fibrous layer):
   • Sclera: Posterior dense connective tissue, the white part of the eye.
   • Cornea: Anterior transparent portion, allowing light to enter.
2. Middle layer (Vascular layer):
   • Choroid: Contains many blood vessels, looks bluish, thin posteriorly.
   • Ciliary body: Thickened anterior part of the choroid, attached to the lens via ligaments, involved in focusing.
   • Iris: Pigmented, opaque anterior extension of the ciliary body, the visible colored part of the eye. Regulates pupil size.
3. Inner layer (Neural layer):
   • Retina: Innermost layer containing three layers of neural cells (from inside to outside): Ganglion cells, Bipolar cells, and Photoreceptor cells.
   • Photoreceptor cells: Contain light-sensitive photopigments. Two types:
     • Rods: Function in dim light (twilight or scotopic vision). Contain rhodopsin (visual purple), a protein with a derivative of Vitamin A.
     • Cones: Function in bright light (daylight or photopic vision) and are responsible for color vision. Humans have three types of cones responding to red, green, and blue light. Combinations of stimulation create sensations of different colors; equal stimulation gives white light sensation.

Other structures:

• Lens: Transparent, crystalline structure held by ligaments from the ciliary body, focuses light onto the retina.
• Pupil: Aperture in front of the lens, surrounded by the iris. Its diameter is regulated by iris muscles to control the amount of light entering.
• Optic nerve: Leaves the eye carrying visual impulses to the brain.
• Retinal blood vessels: Enter the eye at the point where the optic nerve leaves.
• Blind spot: Region where optic nerve leaves and blood vessels enter; lacks photoreceptor cells, so no vision is possible here. Located medial to and slightly above the posterior pole.
• Macula lutea: Yellowish pigmented spot at the posterior pole, lateral to the blind spot.
• Fovea: Central pit in the macula lutea. Thinned-out retinal area with a dense concentration of only cones. Provides the highest visual acuity (sharpest vision).

Fluid-filled spaces:

• Aqueous chamber: Space between cornea and lens, filled with watery aqueous humor.
• Vitreous chamber: Space between lens and retina, filled with transparent gel-like vitreous humor.

---

Mechanism Of Vision

Vision begins when light rays focused by the cornea and lens fall on the retina, stimulating photoreceptor cells (rods and cones). The light-sensitive photopigments in these cells are composed of opsin (protein) and retinal (aldehyde of Vitamin A).

Steps:

1. Light strikes the retina.
2. Light causes dissociation of retinal from opsin.
3. This changes the structure of opsin.
4. Altered opsin structure changes membrane permeability of the photoreceptor cell.
5. This generates a potential difference in the photoreceptor cell.
6. The signal is transmitted to bipolar cells, then to ganglion cells.
7. Ganglion cells generate action potentials (nerve impulses).
8. Action potentials are transmitted via the optic nerves to the visual cortex of the brain.
9. In the visual cortex, impulses are analyzed and interpreted based on memory and experience, allowing recognition of the image formed on the retina.

---

The Ear

The ear is a sensory organ with two functions: hearing and maintenance of body balance. It is divided into three main sections (Figure 21.7):

1. Outer ear:
   • Pinna: Collects sound waves from the air.
   • External auditory meatus (canal): Tube leading inwards from the pinna to the ear drum. Contains fine hairs and wax-secreting glands.
   • Tympanic membrane (Ear drum): Separates the outer and middle ear. Composed of connective tissue covered with skin outside and mucus membrane inside. Vibrates in response to sound waves.
2. Middle ear: An air-filled cavity. Contains three tiny bones (ossicles) in a chain:
   • Malleus: Attached to the tympanic membrane.
   • Incus: Connected to the malleus and stapes.
   • Stapes: Attached to the oval window of the cochlea.

   Ear ossicles amplify and transmit sound vibrations from the ear drum to the inner ear.

   Eustachian tube: Connects the middle ear cavity to the pharynx, helping to equalize pressure on either side of the ear drum.

3. Inner ear (Labyrinth): Fluid-filled. Consists of a bony labyrinth (series of channels) and a membranous labyrinth (suspended inside, filled with endolymph, surrounded by perilymph).
   • Cochlea: Coiled portion of the labyrinth (Figure 21.8), involved in hearing. Divided by membranes (Reissner's and basilar) into three fluid-filled canals: scala vestibuli (upper, filled with perilymph, ends at oval window), scala tympani (lower, filled with perilymph, ends at round window), and scala media (middle, filled with endolymph).
   • Organ of Corti: Structure on the basilar membrane in the scala media. Contains hair cells (auditory receptors) arranged in rows. Stereocilia (projections) on hair cells project towards the tectorial membrane (elastic membrane above hair cells). Basal ends of hair cells are in contact with afferent nerve fibers.
   • Vestibular apparatus: Located above the cochlea, involved in balance. Complex system composed of three semi-circular canals and otolith organs (saccule and utricle). Semi-circular canals are arranged at right angles to each other and detect rotational movements of the head. Otolith organs (saccule and utricle) detect linear acceleration and static equilibrium (head position relative to gravity) via sensory spots called macula (containing hair cells embedded in a gelatinous layer with calcium carbonate crystals called otoliths). The base of each semi-circular canal has a swollen ampulla with a projecting ridge called crista ampullaris (containing hair cells) that detects angular movements.

---

Mechanism Of Hearing

The ear converts sound waves into neural impulses interpreted by the brain:

1. Sound waves are collected by the pinna and directed down the external auditory meatus to the tympanic membrane.
2. The tympanic membrane vibrates in response to sound waves.
3. Vibrations are transmitted and amplified by the ear ossicles (malleus $\rightarrow$ incus $\rightarrow$ stapes) to the oval window.
4. Vibrations at the oval window transfer pressure waves to the fluid (perilymph) of the cochlea (in the scala vestibuli).
5. Waves in the perilymph cause vibrations in Reissner's membrane and the basilar membrane.
6. Movement of the basilar membrane causes the hair cells (stereocilia) of the organ of Corti to bend, pressing against the tectorial membrane.
7. Bending of stereocilia generates mechanical stimulation, which is converted into electrical signals (nerve impulses) in the afferent neurons in contact with the hair cells.
8. Nerve impulses are transmitted via the auditory nerves to the auditory cortex of the brain.
9. The brain analyzes the impulses, interpreting them as sound.

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer: