Neural Control and Coordination

Neural System

The neural system is a system of complex networks of specialised cells called neurons. It is responsible for coordinating the functions of different organs and systems in the body, allowing rapid communication and response to stimuli.

Functions of the Neural System:

• Receives sensory information from the environment and from within the body.
• Processes and interprets this information.
• Generates responses in the form of muscle contractions or glandular secretions.
• Coordinates rapid activities like movement and reflex actions.

Neural Organisation in Different Animals:

The complexity of the neural system varies among different animal groups:

• Lower invertebrates (e.g., Sponges): Lack a neural system.
• Coelenterates (e.g., Hydra): Have a diffuse network of neurons called a nerve net, spread throughout the body. There is no central brain.
• Platyhelminthes (e.g., Planaria): Have a more organised neural system with ganglia and nerve cords. Shows cephalisation (concentration of neural tissue at the anterior end).
• Annelids and Arthropods: Have a more developed neural system with a brain (ganglia in the head region) and a ventral nerve cord with segmentally arranged ganglia.
• Vertebrates: Possess a highly developed neural system with a central nervous system (brain and spinal cord) and a peripheral nervous system.

*(Image shows diagrams of a nerve net in Hydra and possibly a ladder-like nervous system in Planaria or a segmented nervous system in an annelid)*

Human Neural System

The human neural system is a highly complex and well-developed network responsible for all aspects of sensing, processing, and responding to stimuli, as well as higher functions like thought, memory, and consciousness.

Divisions of the Human Neural System:

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

1. Central Neural System (CNS):
   • Includes the Brain and the Spinal Cord.
   • It is the site of information processing and control.
2. Peripheral Neural System (PNS):
   • Includes all the nerves extending from the CNS to the rest of the body.
   • Nerves are bundles of nerve fibres (axons) covered by connective tissue.
   • The PNS is divided into two divisions:
     • Afferent Division: Carries sensory impulses from receptors to the CNS.
     • Efferent Division: Carries motor impulses from the CNS to effectors (muscles and glands).
   • The Efferent division is further divided into:
     • Somatic Neural System: Transmits impulses from the CNS to voluntary muscles (skeletal muscles).
     • Autonomic Neural System (ANS): Transmits impulses from the CNS to involuntary organs and smooth muscles. The ANS is further divided into Sympathetic and Parasympathetic nervous systems, which generally have opposite effects on target organs.

*(Image shows a flowchart illustrating the hierarchy of the human nervous system divisions)*

The CNS is protected by bony structures (skull and vertebral column) and cerebrospinal fluid (CSF). The PNS transmits information between the CNS and the body parts.

Neuron as Structural and Functional Unit of Neural System

The neuron (nerve cell) is the structural and functional unit of the neural system. It is a specialised cell capable of receiving, transmitting, and processing information in the form of electrical signals (nerve impulses).

Structure of a Neuron:

A neuron consists of three main parts:

1. Cell body (Soma or Cyton):
   • Contains the nucleus, cytoplasm, and characteristic granular bodies called Nissl's granules (made of ribosomes and RER, involved in protein synthesis).
   • Involved in metabolic maintenance of the neuron.
2. Dendrites:
   • Short, highly branched processes extending from the cell body.
   • Receive signals (impulses) from other neurons or sensory receptors and transmit them towards the cell body.
3. Axon:
   • A single, long, cylindrical process extending from the axon hillock (a cone-shaped region of the cell body).
   • Transmits nerve impulses away from the cell body to other neurons, muscles, or glands.
   • The axon may be covered by a myelin sheath (an insulating layer formed by Schwann cells in PNS or oligodendrocytes in CNS). Gaps in the myelin sheath are called Nodes of Ranvier. Myelinated axons conduct impulses faster (saltatory conduction).
   • The terminal end of the axon branches into bulb-like structures called synaptic knobs or axon terminals. Synaptic knobs contain vesicles filled with neurotransmitters.

Based on the number of axons and dendrites, neurons can be multipolar (most common, one axon, multiple dendrites), bipolar (one axon, one dendrite, e.g., retina), or unipolar (cell body with one axon-like process, e.g., in embryonic stage).

*(Image shows a detailed diagram of a neuron with labelled parts)*

Generation And Conduction Of Nerve Impulse

Nerve impulses are electrical signals that are generated and propagated along the axon of a neuron. This process involves changes in the electrical potential across the neuron's membrane.

1. Resting Membrane Potential:

• In a resting neuron (not transmitting an impulse), the membrane is relatively more permeable to $K^+$ ions than to $Na^+$ ions.
• A high concentration of $K^+$ ions is maintained inside the cell, and a high concentration of $Na^+$ ions is maintained outside the cell.
• This ionic gradient is maintained by the Sodium-Potassium pump ($Na^+/K^+$ pump), which actively transports 3 $Na^+$ ions out of the cell for every 2 $K^+$ ions pumped into the cell (using ATP).
• Also, negatively charged proteins are present inside the cell.
• This leads to a higher positive charge outside the membrane and a higher negative charge inside the membrane. The membrane is said to be polarised.
• The electrical potential difference across the membrane at rest is called the resting membrane potential (RMP), typically around $-70 \text{ mV}$ (inside negative relative to outside).

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2. Action Potential (Nerve Impulse):

• When a stimulus of sufficient strength (threshold stimulus) is applied to a neuron, it triggers a rapid change in the membrane potential, generating an action potential.
• This involves two main phases:
  • Depolarisation: Voltage-gated $Na^+$ channels in the membrane open, allowing a rapid influx of $Na^+$ ions into the cell. The inside of the membrane becomes positive relative to the outside. The potential reverses from $-70 \text{ mV}$ to about $+30 \text{ mV}$.
  • Repolarisation: Voltage-gated $Na^+$ channels close, and voltage-gated $K^+$ channels open. $K^+$ ions rapidly flow out of the cell, restoring the negative charge inside the membrane. The potential returns towards the resting potential.
  • Hyperpolarisation (brief): Sometimes, $K^+$ channels close slowly, causing a brief period where the membrane potential becomes more negative than the RMP (e.g., -75 mV). The $Na^+/K^+$ pump then restores the ionic balance over time.
• This rapid sequence of depolarisation and repolarisation is the action potential. It follows the 'all-or-none' principle (if the stimulus is threshold or above, a full action potential is generated; below threshold, none is generated).

*(Image shows a graph of membrane potential on the Y-axis versus time on the X-axis, illustrating the changes from RMP, depolarisation peak, repolarisation, and possible hyperpolarisation before returning to RMP)*

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3. Conduction of Nerve Impulse:

• Once an action potential is generated at one point on the axon membrane, it propagates along the axon as a wave of depolarisation.
• The influx of $Na^+$ at one point depolarises the adjacent region of the membrane, opening voltage-gated $Na^+$ channels there and generating a new action potential.
• This process continues along the axon, transmitting the impulse.
• In myelinated axons, the myelin sheath acts as an insulator. Action potentials only occur at the Nodes of Ranvier, where the membrane is exposed. The impulse 'jumps' from node to node (saltatory conduction), which is much faster than continuous conduction in unmyelinated axons.

*(Image shows illustrations comparing impulse conduction in unmyelinated (continuous) and myelinated (saltatory) axons)*

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Transmission Of Impulses

Information is transmitted from one neuron to another, or from a neuron to an effector cell (muscle or gland), at junctions called synapses.

Synapse:

• A synapse is the junction between the axon terminal of one neuron (presynaptic neuron) and the dendrite or cell body of another neuron (postsynaptic neuron), or an effector cell.
• There is a small gap between the presynaptic and postsynaptic membranes called the synaptic cleft.

Types of Synapses:

• Electrical Synapse: The membranes of presynaptic and postsynaptic neurons are in very close proximity. Electrical current can flow directly from one neuron to the other through gap junctions. This is faster but less common in mammals.
• Chemical Synapse: There is a synaptic cleft between the presynaptic and postsynaptic membranes. Information is transmitted by chemical messengers called neurotransmitters. This is slower than electrical synapses but allows for modulation of the signal. Most synapses in the human nervous system are chemical synapses.

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Mechanism of Transmission at a Chemical Synapse:

1. An action potential arrives at the axon terminal of the presynaptic neuron.
2. The depolarisation of the presynaptic membrane opens voltage-gated Calcium channels ($Ca^{2+}$).
3. $Ca^{2+}$ ions flow into the presynaptic terminal.
4. Influx of $Ca^{2+}$ causes the synaptic vesicles (containing neurotransmitters) to fuse with the presynaptic membrane and release their contents (neurotransmitters) into the synaptic cleft by exocytosis.
5. Neurotransmitters diffuse across the synaptic cleft and bind to specific receptors on the postsynaptic membrane.
6. Binding of neurotransmitters to receptors causes the opening of ion channels in the postsynaptic membrane, leading to changes in its membrane potential (postsynaptic potential).
   • If the postsynaptic potential is excitatory (e.g., causes depolarisation), it makes the postsynaptic neuron more likely to fire an action potential.
   • If the postsynaptic potential is inhibitory (e.g., causes hyperpolarisation), it makes the postsynaptic neuron less likely to fire an action potential.
7. The neurotransmitter is then quickly removed from the synaptic cleft (by enzymatic degradation or reuptake into the presynaptic terminal or glial cells) to terminate the signal.

*(Image shows a diagram of a chemical synapse illustrating the events involved in neurotransmitter release and binding)*

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Chemical synapses allow for integration of signals (excitatory and inhibitory) by a neuron, enabling complex information processing.

Central Neural System

The Central Neural System (CNS) is the control and processing centre of the human nervous system. It consists of the Brain and the Spinal Cord. The CNS receives information, processes it, and initiates responses.

The Brain:

The brain is the highest coordinating centre in the body. It is protected by the skull (cranium) and three layers of membranes called meninges (Dura mater, Arachnoid mater, Pia mater). Cerebrospinal fluid (CSF) is present between the arachnoid and pia mater, providing cushioning.

The brain is divided into three major parts:

1. Forebrain
2. Midbrain
3. Hindbrain

*(Image shows a sagittal view of the human brain highlighting the forebrain (cerebrum, thalamus, hypothalamus), midbrain, and hindbrain (pons, cerebellum, medulla))

Forebrain:

The forebrain is the anterior part of the brain, consisting of the Cerebrum, Thalamus, and Hypothalamus.

• Cerebrum:
  • The largest part of the brain, forming about 80% of the brain volume.
  • Divided into two cerebral hemispheres connected by a tract of nerve fibres called the corpus callosum.
  • The outer layer of the cerebrum is the cerebral cortex, composed of grey matter (neuron cell bodies). The cerebral cortex is highly folded, forming gyri (ridges) and sulci (grooves), which increase the surface area.
  • The cerebral cortex is the site of consciousness, voluntary movement, sensory perception, language, learning, memory, and intelligence. Different areas are specialised for different functions (e.g., sensory areas, motor areas, association areas).
  • The inner part of the cerebrum is composed of white matter (myelinated axons).
• Thalamus:
  • Located below the cerebrum.
  • Acts as a relay station for sensory and motor signals going to and from the cerebral cortex. Filters and processes sensory information before relaying it.
• Hypothalamus:
  • Located below the thalamus.
  • Controls many vital functions, including body temperature regulation (thermostat), thirst, hunger, emotions, sleep-wake cycle.
  • Links the nervous system to the endocrine system by controlling the pituitary gland.
• The inner parts of cerebral hemispheres and a group of associated deep structures like amygdala, hippocampus, etc., form the limbic system. The limbic system, along with the hypothalamus, is involved in regulating motivation, emotion (fear, pleasure, anger), sexual behaviour, and memory formation.

Midbrain:

• Located between the thalamus/hypothalamus of the forebrain and the pons of the hindbrain.
• A short, narrow region.
• Includes the corpora quadrigemina, four rounded swellings (lobes) on the dorsal portion. The upper pair is involved in visual reflexes, and the lower pair is involved in auditory reflexes.
• Ventral portion consists of nerve fibre tracts (cerebral peduncles).
• Connects the forebrain and hindbrain. Involved in processing visual and auditory information and controlling certain reflexes.

Hindbrain:

The hindbrain consists of the Pons, Cerebellum, and Medulla oblongata.

• Pons:
  • Located below the midbrain, between the medulla and the cerebellum.
  • Consists of fibre tracts that interconnect different regions of the brain.
  • Contains the pneumotaxic and apneustic centres, involved in regulating respiration.
• Cerebellum:
  • Located below the cerebrum, at the back of the brain.
  • A large, highly convoluted structure.
  • Responsible for coordinating voluntary movements, maintaining posture, balance, and muscle tone. It integrates sensory information from muscles, joints, etc., with motor commands from the cerebrum.
• Medulla Oblongata:
  • Located at the base of the brain, connecting the brain to the spinal cord.
  • Contains centres that control vital involuntary functions, such as respiration (respiratory rhythm centre), heart rate, blood pressure, swallowing, vomiting, coughing, sneezing.

The midbrain, pons, and medulla oblongata together form the brainstem, which connects the cerebrum and cerebellum to the spinal cord and controls basic survival functions.

The Spinal Cord:

• A long, cylindrical structure extending from the medulla oblongata down through the vertebral column.
• Protected by the vertebral column and meninges.
• Acts as a pathway for nerve impulses between the brain and the rest of the body.
• Also serves as the centre for many reflex actions.
• In cross-section, it shows grey matter in a butterfly or H-shape (containing cell bodies and unmyelinated axons) and surrounding white matter (containing myelinated axon tracts).

*(Image shows a diagram of a spinal cord cross-section highlighting grey matter (dorsal/ventral horns) and white matter, and possibly a longitudinal view showing nerve roots)*

Reflex Action And Reflex Arc

A reflex action is a sudden, involuntary, and nearly instantaneous response to a stimulus. It is mediated by the spinal cord and/or brainstem and does not involve conscious thought from the higher brain centres.

Examples: Withdrawal of hand from a hot object, knee-jerk reflex, blinking of eyes.

Reflex Arc:

The pathway taken by the nerve impulse during a reflex action is called the reflex arc. It is the functional unit of a reflex action. A typical reflex arc involves the following components:

1. Receptor: Detects the stimulus (e.g., pain receptors in the skin).
2. Sensory neuron (Afferent neuron): Transmits the sensory impulse from the receptor to the CNS (spinal cord).
3. Integration centre: Located in the CNS (spinal cord). It processes the information. It may involve a single synapse between the sensory and motor neuron (monosynaptic reflex, e.g., knee-jerk) or multiple synapses involving an interneuron (polysynaptic reflex, e.g., withdrawal reflex).
4. Motor neuron (Efferent neuron): Transmits the motor impulse from the CNS to the effector organ.
5. Effector: The muscle or gland that responds to the motor impulse (e.g., muscle contracting to withdraw hand).

*(Image shows a diagram of a simple reflex arc, e.g., finger touching flame, sensory neuron to spinal cord, interneuron (optional), motor neuron to muscle, causing withdrawal)*

Significance of Reflex Actions:

• Provide rapid responses to potentially harmful stimuli, protecting the body from injury.
• Occur without conscious thought, freeing up the brain for higher-level processing.
• Involuntary, ensuring consistent responses.

Answer:

Sensory Reception And Processing

The nervous system receives information about the external and internal environment through sensory receptors, which are specialised structures or cells that detect stimuli. This information is then processed by the nervous system, leading to perception and response.

Sense Organs

Sensory receptors are often organised into complex structures called sense organs. Humans have several major sense organs:

• Eye: For vision (detects light).
• Ear: For hearing (detects sound waves) and balance (detects head position and movement).
• Nose: For olfaction (smell - detects chemical substances in the air).
• Tongue: For gustation (taste - detects chemical substances in food).
• Skin: Contains various receptors for touch, pressure, pain, temperature.

Eye

The eyes are the organs of vision. They are complex structures that convert light energy into neural signals, which are then interpreted by the brain as images.

Parts Of An Eye

The human eye is roughly spherical in shape. The wall of the eyeball is composed of three layers:

1. Sclera: The outermost layer. A dense connective tissue layer. The anterior portion is transparent and called the cornea. The cornea is the main refractive surface of the eye.
2. Choroid: The middle layer. It is richly supplied with blood vessels and is bluish in colour. The anterior portion thickens to form the ciliary body. The ciliary body continues forward to form the pigmented and opaque structure called the iris, which is the visible coloured part of the eye. The iris regulates the size of the pupil (opening in the centre of the iris) to control the amount of light entering the eye.
3. Retina: The innermost layer. It is the light-sensitive layer and contains the photoreceptor cells (rods and cones). The retina contains three layers of cells (from inside to outside): ganglion cells, bipolar cells, and photoreceptor cells.

Other Structures:

• Lens: A transparent, biconvex, crystalline structure located behind the iris and pupil. It is held in place by ligaments attached to the ciliary body. The lens focuses light onto the retina.
• Suspensory ligaments: Connect the ciliary body to the lens, helping to change the shape of the lens for focusing (accommodation).
• Pupil: The opening in the centre of the iris that allows light to enter the eye.
• Aqueous humour: Fluid filling the space between the cornea and the lens (anterior chamber). Provides nourishment to the cornea and lens and maintains eye shape.
• Vitreous humour: Gel-like substance filling the space between the lens and the retina (posterior chamber). Maintains the shape of the eyeball and supports the retina.

Photoreceptor Cells in the Retina:

• Rods: Contain a pigment called rhodopsin (visual purple), a derivative of Vitamin A. They are sensitive to dim light and are responsible for scotopic vision (twilight vision) and detecting black and white. They do not detect colour.
• Cones: Contain pigments sensitive to bright light and colour vision. There are three types of cones, sensitive to red, green, and blue light. Photopigments in cones are called iodopsin. Cones are responsible for photopic vision (daylight vision) and colour vision.

The fovea (or fovea centralis) is a central pit in the macula lutea (yellowish spot) of the retina. It has a high density of cones and is the area of sharpest vision.

The blind spot is the region where the optic nerve leaves the eye. It contains no photoreceptor cells, so vision is not possible in this area.

*(Image shows a cross-section diagram of the eye highlighting the cornea, iris, pupil, lens, retina, choroid, sclera, optic nerve, fovea, blind spot, aqueous humour, vitreous humour)*

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Mechanism Of Vision

The process of vision involves the conversion of light energy into a neural signal and its transmission and interpretation in the brain.

1. Light enters the eye: Light rays pass through the cornea, aqueous humour, pupil, lens, and vitreous humour to reach the retina.
2. Focusing on the retina: The cornea and lens refract (bend) the light rays to focus them onto the retina, forming an inverted real image. The lens adjusts its shape (accommodation) to focus objects at different distances.
3. Activation of photoreceptors: Light energy causes photochemical changes in the photopigments (rhodopsin in rods, iodopsin in cones) in the photoreceptor cells. This causes the dissociation of the pigment into its protein component (opsin) and retinal (an aldehyde of Vitamin A).
4. Generation of electrical signal: This dissociation of the pigment leads to changes in the membrane permeability of the photoreceptor cells, generating a potential difference (receptor potential).
5. Synaptic transmission: The receptor potential in photoreceptors influences the release of neurotransmitters, which affect the bipolar cells. Bipolar cells transmit signals to ganglion cells.
6. Generation of action potential: Ganglion cells generate action potentials (nerve impulses).
7. Transmission to the brain: The axons of ganglion cells bundle together to form the optic nerve, which transmits the nerve impulses to the visual cortex in the occipital lobe of the brain.
8. Processing and perception: The visual cortex processes the electrical signals, interprets them, and allows us to perceive the visual image (upright and in correct orientation).

*(Image shows a simplified pathway of light through the eye and a diagram of the retinal layers showing interaction between photoreceptors, bipolar cells, and ganglion cells)*

The Ear

The ears are sense organs that perform two main functions: hearing (detecting sound) and balance (maintaining body equilibrium).

Parts of the Human Ear:

The ear is divided into three main parts:

1. External Ear:
   • Pinna (Auricle): The visible outer flap of cartilage covered by skin. Collects sound waves.
   • External auditory meatus (Ear canal): A tube leading inwards from the pinna to the tympanic membrane. Lined with skin containing hair and wax-secreting glands (ceruminous glands) to trap dust and foreign bodies.
   • Tympanic membrane (Eardrum): A thin membrane separating the external ear from the middle ear. Vibrates when hit by sound waves.
2. Middle Ear:
   • An air-filled cavity containing three small bones (ossicles): Malleus, Incus, and Stapes. These ossicles are attached to each other and transmit sound vibrations from the tympanic membrane to the inner ear. Malleus is attached to the tympanic membrane, Stapes is attached to the oval window of the cochlea.
   • The Eustachian tube connects the middle ear cavity to the pharynx. It helps in equalising the pressure on either side of the tympanic membrane.
3. Inner Ear:
   • Fluid-filled and consists of two main parts:
     • Bony Labyrinth: A series of channels within the bone, enclosing the membranous labyrinth.
     • Membranous Labyrinth: A series of fluid-filled sacs and tubes (filled with endolymph). Located within the bony labyrinth (space filled with perilymph).
   • The inner ear contains structures for both hearing and balance:
     • Cochlea: A coiled, spiral-shaped structure (looks like a snail shell). The main organ of hearing. Contains the Organ of Corti (with hair cells, the auditory receptors).
     • Vestibular apparatus: Involved in balance and equilibrium. Consists of three semicircular canals and the otolith organ (utricle and saccule).

*(Image shows a diagram of the human ear highlighting the pinna, ear canal, tympanic membrane, ossicles (malleus, incus, stapes), Eustachian tube, cochlea, semicircular canals, auditory nerve)*

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Mechanism Of Hearing

Hearing is the process of converting sound waves into neural signals and interpreting them in the brain.

1. Sound waves collection: The pinna collects sound waves and directs them into the external auditory meatus.
2. Tympanic membrane vibration: Sound waves cause the tympanic membrane to vibrate.
3. Ossicles transmission: The vibrations are transmitted through the middle ear ossicles (malleus $\rightarrow$ incus $\rightarrow$ stapes). The ossicles amplify the vibrations.
4. Vibration transfer to inner ear: The stapes transmits the vibrations to the oval window (opening into the cochlea).
5. Generation of fluid waves: Vibrations at the oval window create waves in the perilymph (fluid) of the cochlea.
6. Basilar membrane vibration: These waves cause vibrations in the basilar membrane, located within the cochlea. Different frequencies of sound cause vibrations at different locations along the basilar membrane.
7. Hair cell bending: The vibrations of the basilar membrane cause the hair cells (sensory receptors) of the Organ of Corti (resting on the basilar membrane) to bend against the tectorial membrane (membrane above the hair cells).
8. Generation of nerve impulses: The bending of hair cells generates electrical signals (nerve impulses) in the associated neurons.
9. Transmission to the brain: The nerve impulses are transmitted via the auditory nerve (part of the vestibulocochlear nerve) to the auditory cortex in the temporal lobe of the brain.
10. Processing and perception: The brain interprets the impulses as sound.

Mechanism of Balance:

• The vestibular apparatus (semicircular canals, utricle, saccule) is responsible for maintaining balance and posture.
• Semicircular canals detect rotational movements of the head. They contain fluid (endolymph) and hair cells. Movement of the head causes the endolymph to move, bending the hair cells and generating nerve impulses.
• Utricle and saccule (otolith organ) detect linear acceleration and changes in head position relative to gravity. They contain a jelly-like membrane with embedded calcium carbonate crystals (otoliths) and hair cells. Movement of the otoliths bends the hair cells, generating nerve impulses.
• These impulses are transmitted to the brain (cerebellum and brainstem), which helps in maintaining balance and coordinating movements.