Neural Control and Coordination

Neural vs. Endocrine System

Introduction

• Coordination is the process by which two or more organs work together to maintain internal balance (homeostasis).
• For example, during physical exercise, muscles require more energy and oxygen.
• To meet this demand, breathing rate, heartbeat, and blood flow increase.
• Once exercise stops, these activities gradually return to normal.
• This shows coordinated functioning of muscles, lungs, heart, blood vessels, kidneys, and nerves.

In humans, coordination and integration of body activities are carried out jointly by two systems:

1. Neural system
2. Endocrine system

• The neural system provides rapid and precise coordination through electrical signals transmitted via nerves.
• The endocrine system provides slow but long-lasting coordination through chemical messengers called hormones.
• Together, both systems ensure smooth and synchronized functioning of organs.

Neural System

Basics

The neural system of animals is made up of specialised cells called neurons, which can detect, receive, and transmit stimuli.

• In lower animals like Hydra, the neural system is simple and consists of a nerve net.
• In insects, the system is better organised with a brain and ganglia.
• Vertebrates possess a highly developed neural system.

Human Neural System

Two Main Parts

1. Central Neural System (CNS)
   • Components:
     • Brain.
     • Spinal cord.
   • Function:
     • Processing and integration of information
     • Control and regulation of body activities
2. Peripheral Neural System (PNS)
   • Components: All nerves connected to CNS.
   • Types of nerve fibres:
     • Afferent (sensory) fibres – carry impulses from receptors/organs to the CNS
     • Efferent (motor) fibres – carry impulses from the CNS to effectors (muscles or glands)

Divisions of Peripheral Neural System

1. Somatic Neural System
   • Carries impulses from CNS to skeletal muscles
   • Controls voluntary movements
2. Autonomic Neural System
   • Controls involuntary activities of organs, smooth muscles, cardiac muscles, and glands
   • Self-regulated system

Subdivisions of ANS:

• Sympathetic Neural System (SNS) – thoracolumbar outflow
  • Active during stress
  • Increases heart rate and blood pressure
  • Dilates pupils, decreases salivation
• Parasympathetic Neural System (PSNS) – craniosacral outflow
  • Rest and relaxation responses
  • Acts antagonistically to SNS

Neuron – Structural and Functional Unit of Neural System

Parts of a Neuron

A neuron is a microscopic unit composed of three main parts:

1. Cell Body (Cyton)
   • Contains cytoplasm and cell organelles
   • Contains Nissl’s granules (rough endoplasmic reticulum with ribosomes)
   • Nissl’s granules are involved in protein synthesis
2. Dendrites
   • Short, branched processes
   • Contain Nissl’s granules
   • Transmit impulses towards the cell body
3. Axon
   • Long fibre that conducts impulses away from the cell body
   • Terminal end forms synaptic knobs
   • Synaptic knobs contain synaptic vesicles filled with neurotransmitters
   • Axon ends form synapse or neuromuscular junction

Types of Neurons

• Based on structure:
  • Unipolar neurons – single axon; usually found in embryonic stage
  • Bipolar neurons – one axon and one dendrite; found in retina of eye
  • Multipolar neurons – one axon and many dendrites; found in cerebral cortex
  • Pseudounipolar neurons – found in dorsal root ganglia
  • Non-polar neurons – found in coelenterates
• Based on function:
  • Afferent neurons – sensory; carry impulses from receptors to CNS
  • Efferent neurons – motor; carry impulses from CNS to effectors

Types of Axons

1. Myelinated Axons:
   • Covered by Schwann cells forming a myelin sheath
   • Gaps between sheaths are called nodes of Ranvier
   • Found in cranial and spinal nerves
   • Enable faster impulse conduction
2. Non-Myelinated Axons:
   • Enclosed by Schwann cells without myelin sheath
   • Common in autonomic and some somatic nerves
   • Slower impulse conduction

Generation and Conduction of Nerve Impulse

Neurons are excitable cells because their membranes exist in a polarised state. This property allows neurons to generate and transmit nerve impulses.

Polarised State of Neuron (Resting State)

In the resting condition, the neuron membrane is polarised due to unequal distribution of ions.

Ion Distribution:

• Inside the axon (axoplasm):
  • High concentration of K⁺
  • Low concentration of Na⁺
  • Negatively charged proteins (impermeable to membrane)
• Outside the axon:
  • High concentration of Na⁺
  • Low concentration of K⁺

Membrane Permeability:

• Membrane is more permeable to K⁺
• Nearly impermeable to Na⁺
• Impermeable to negatively charged proteins

Sodium–Potassium Pump:

• Actively transports
  • 3 Na⁺ out of the axon
  • 2 K⁺ into the axon
• Maintains ionic gradients using ATP

Result:

• Outside of membrane becomes positively charged
• Inside becomes negatively charged

Resting Potential:

• Electrical potential difference across membrane
• Approximately –70 mV
• Neuron is said to be polarised

Generation of Nerve Impulse (Action Potential)

When a stimulus is applied at a point on the axon membrane (Point A):

• Depolarisation:
  • Membrane at Point A becomes freely permeable to Na⁺
  • Rapid influx of Na⁺ into the axon
  • Inner surface becomes positively charged
  • Outer surface becomes negatively charged
• Action Potential:
  • Polarity of membrane reverses
  • Electrical potential difference becomes +30 mV to +45 mV
  • This change is called action potential or nerve impulse

Conduction of Nerve Impulse Along Axon

Impulse Propagation:

• Adjacent region (Point B) is still polarised
• Current flows internally from A → B
• External current flows from B → A
• This reverses polarity at Point B

Sequential Conduction:

• Action potential moves from one region to the next
• Impulse travels along the length of the axon
• Conduction is unidirectional

Repolarisation (Restoration of Resting State):

• Na⁺ permeability decreases rapidly
• K⁺ permeability increases
• K⁺ diffuses out of the axon
• Resting potential is restored

Refractory Period:

• Short period of complete inexcitability
• Occurs between depolarisation and repolarisation
• Prevents backward conduction

Saltatory Conduction:

• Occurs in myelinated axons
• Impulse jumps from one node of Ranvier to the next
• Faster than conduction in non-myelinated axons

Transmission of Nerve Impulse (Synapse)

A nerve impulse passes from one neuron to another through synapses.

• Synapse:
  • Junction between pre-synaptic and post-synaptic neurons
  • May or may not have a synaptic cleft
• Types of Synapses:
  1. Electrical synapse
  2. Chemical synapse

1. Electrical Synapses

• Close Proximity: Pre- and post-synaptic membranes are very close
• Direct Flow: Electrical current flows directly between neurons.
• Fast Transmission: Quicker than chemical synapses.
  • Similar to impulse conduction along axon
• Rarity: Rare in human nervous system

2. Chemical Synapses

Structure:

• Pre-synaptic membrane
• Synaptic cleft (fluid-filled gap)
• Post-synaptic membrane

Neurotransmitters:

• Chemicals that transmit impulses
• May be excitatory or inhibitory
• Major neurotransmitters:
  • Acetylcholine
  • Noradrenaline
  • GABA (inhibitory)

Mechanism:

1. Action potential reaches axon terminal
2. Synaptic vesicles move to membrane
3. Neurotransmitters released into synaptic cleft
4. Neurotransmitters bind to specific receptors on post-synaptic membrane
5. Ion channels open, ions enter
6. New potential is generated in post-synaptic neuron

Nature of Response:

• Excitatory or inhibitory
• Transmission across chemical synapse is slower than electrical synapse

Central Neural System

• The central neural system consists of the brain and spinal cord.
• It acts as the command and control centre of the body.

The Brain: Command and Control Center

The brain is the central information processing organ of the human body.

• Functions:
  • Controls voluntary movements and body balance
  • Regulates involuntary functions of vital organs (heart, lungs, kidneys)
  • Maintains body temperature (thermoregulation)
  • Regulates hunger, thirst and sleep–wake (circadian) rhythm
  • Controls activities of several endocrine glands
  • Responsible for behaviour, emotions, intelligence and motivation
  • Processes sensory inputs like vision, hearing, speech, memory and thoughts

Protection of the Brain

Skull:

• The brain is enclosed and protected by the skull

Cranial Meninges:

• The brain is covered by three protective membranes:
  1. Dura mater – tough outer layer
  2. Arachnoid – thin middle layer
  3. Pia mater – delicate inner layer, closely attached to brain tissue

Cerebrospinal Fluid (CSF):

• Present in brain ventricles and subarachnoid space
• Acts as a shock absorber
• Protects the brain from mechanical injury

Major Parts of the Brain

The brain is divided into three major regions:

1. Forebrain
2. Midbrain
3. Hindbrain

a. Forebrain (Prosencephalon)

The forebrain is the largest and most complex part of the brain.

• Components:
  • Cerebrum
  • Diencephalon (thalamus, hypothalamus, epithalamus)
  • Olfactory lobes

Cerebrum:

• Largest part of the human brain
• Divided into left and right cerebral hemispheres
• Hemispheres are connected by a thick band of nerve fibres called corpus callosum

Cerebral Cortex:

• Outermost layer of cerebrum
• Made of grey matter
• Highly folded into gyri (ridges) and sulci (grooves)
• Increases surface area

White Matter:

• Lies beneath the cortex
• Made of medullated nerve fibres

Lobes of Cerebral Hemisphere:

1. Frontal lobe
2. Parietal lobe
3. Temporal lobe
4. Occipital lobe

Functional Areas:

• Motor areas – control voluntary movements
• Sensory areas – receive sensory impulses
• Association areas – memory, learning, thinking, communication and intelligence

Thalamus

• Lies above the midbrain
• Composed mainly of grey matter
• Acts as a sensory relay station
• All sensory impulses (except smell) pass through thalamus before reaching cerebrum
• Coordinates sensory and motor signals

Hypothalamus

• Forms the floor of the diencephalon
• Lies above the pituitary gland
• Connected to pituitary by a stalk called infundibulum

Functions:

• Maintains homeostasis
• Regulates body temperature
• Controls hunger and thirst
• Regulates endocrine activity

Neurosecretory Cells:

• Secrete hypothalamic hormones
• Control pituitary gland

Limbic System

• Formed by inner parts of cerebral hemispheres and associated deep structures
• Includes amygdala and hippocampus

Functions:

• Regulation of emotions (fear, pleasure, rage)
• Sexual behaviour
• Motivation and memory
• Works closely with hypothalamus

Olfactory Lobes

• Paired, club-shaped structures
• Located at the anterior part of the forebrain
• Concerned with sense of smell
• Each lobe has:
  • Olfactory bulb (anterior)
  • Olfactory tract (posterior)

b. Midbrain (Mesencephalon)

• Location: Between forebrain (thalamus/hypothalamus) and hindbrain (pons).
• Cerebral Aqueduct: A narrow canal called cerebral aqueduct passes through it

Major Structures:

1. Corpora quadrigemina
2. Crura cerebri

Corpora Quadrigemina:

• Four rounded lobes
• Superior colliculi – visual reflexes
• Inferior colliculi – auditory reflexes

Crura Cerebri:

• Conduct nerve impulses between cerebrum, cerebellum, pons and medu

c. Hindbrain

• Components:
  1. Pons
  2. Cerebellum
  3. Medulla oblongata

Pons:

• Located below the midbrain
• Composed mainly of fibre tracts
• Connects different regions of the brain
• Contains pneumotaxic centre, which regulates breathing pattern

Cerebellum:

• Surface: Has a highly convoluted surface
  • Provides space for a large number of neurons
• Functions:
  • Coordination of voluntary movements
  • Maintenance of posture
  • Balance and equilibrium

Medulla Oblongata:

• Lowermost part of the brain
• Connection: Connects brain with spinal cord
• Control Centres:
  • Regulation of respiration
  • Control of cardiovascular reflexes
  • Control of gastric secretions

Brain Stem

• Connects brain to spinal cord
• Formed by:
  • Midbrain
  • Pons
  • Medulla oblongata
• Functions:
  • Maintains vital life support systems
  • Serves as entry and exit point for most cranial nerves

Reflex Action and Reflex Arc

Reflex Action:

• Reflex actions are rapid, automatic and involuntary responses to specific stimuli.
• They occur without conscious involvement of the brain and help protect the body from harmful situations.
• Example:
  • Touching a hot object and immediately withdrawing the hand.
• Key Features:
  • Very fast response
  • Involves spinal cord (mostly)
  • Protective in nature

Reflex Arc:

• A reflex arc is the pathway followed by nerve impulses during a reflex action.
• Components of a Reflex Arc:

1. Receptor
   • Detects the stimulus (e.g., pain, heat)
2. Afferent (Sensory) Neuron
   • Carries impulse from receptor to CNS
   • Enters spinal cord through the dorsal root
3. Integration Centre
   • Usually spinal cord
   • Interneurons may be involved
4. Efferent (Motor) Neuron
   • Carries impulse from CNS to effector
   • Leaves spinal cord through ventral root
5. Effector
   • Muscle or gland that produces response

Example: Knee Jerk Reflex

• Tapping the patellar tendon stretches thigh muscles
• Sensory neuron carries impulse to spinal cord
• Motor neuron sends impulse back to muscle
• Leg extends suddenly

This reflex involves a simple reflex arc and helps maintain posture and balance.

Sensory Reception and Processing

Sensory Reception

Sensory organs detect changes (stimuli) in the internal and external environment.

Sensory Processing

• Sensory organs send signals to CNS
• CNS processes and analyses the information
• Appropriate response is generated

This mechanism allows organisms to perceive and respond to their surroundings.

Sense Organs

• Major Sense Organs and Their Functions:
  • Eyes – vision
  • Ears – hearing and balance
  • Nose – smell
  • Tongue – taste

Taste (gustation) and smell (olfaction) are chemical senses and are closely related.

The Eye – Organ of Vision

Location and Shape

• Eyes are located in bony sockets called orbits
• Each eyeball is nearly spherical

Layers of the Eyeball:

The eyeball has three concentric layers:

1. External Fibrous Layer

• Sclera
  • White, opaque
  • Made of dense connective tissue
  • Maintains shape of eyeball
• Cornea
  • Transparent anterior part of sclera
  • Avascular but richly supplied with nerves
  • Allows light to enter the eye
• Conjunctiva
  • Thin membrane covering cornea
  • Secretes mucus and oils
  • Protects and lubricates the eye

2. Middle Vascular Layer

• Choroid
  • Lies beneath sclera
  • Rich in blood vessels
  • Bluish in colour
  • Nourishes the retina
• Ciliary Body
  • Thickened anterior part of choroid
  • Holds lens in position
  • Helps in accommodation
• Iris
  • Pigmented visible part of the eye
  • Surrounds the pupil
  • Regulates amount of light entering the eye
• Pupil
  • Opening in the iris
  • Diameter controlled by iris muscles

3. Inner Nervous Layer

Retina

• Photosensitive inner layer
• Contains four layers of cells:
  • Pigment cells
  • Photoreceptor cells
  • Bipolar cells
  • Ganglion cells

Photoreceptor Cells:

1. Rods
   • Function in dim light (scotopic vision)
   • Contain rhodopsin
   • Responsible for black and white vision
2. Cones
   • Function in bright light (photopic vision)
   • Responsible for colour vision
   • Three types respond to red, green and blue light

Special Regions of Retina

• Blind Spot (Optic Disc)
  • Point where optic nerve leaves the eye
  • No rods or cones present

• Macula Lutea
  • Yellowish pigmented spot on retina

• Fovea Centralis
  • Central pit in macula lutea
  • Only cones present
  • Point of highest visual acuity

Chambers of the Eye

• Aqueous Chamber
  • Between cornea and lens
  • Filled with aqueous humour
  • Maintains shape and nourishes cornea and lens
• Vitreous Chamber
  • Between lens and retina
  • Filled with vitreous humour (gel-like)
  • Maintains shape of eyeball

Lens

• Transparent, biconvex
• Held by suspensory ligaments
• Focuses light on retina

Important Terms

• Rhodopsin:
  • Light-sensitive pigment in rods
  • Derived from Vitamin A
• Photopigments:
  • Light-sensitive proteins in rods and cones
• Optic Nerve:
  • Carries visual impulses from retina to brain

Mechanism of Vision:

• How Vision Works:
  • Light rays coming from an object enter the eye through the cornea.
  • They pass sequentially through aqueous humour, pupil, lens and vitreous humour and finally fall on the retina.
  • The lens focuses the light precisely on the retina.

Role of Photoreceptors

• The retina contains two types of photoreceptor cells:
  1. Rods – sensitive to dim light, responsible for twilight (scotopic) vision
  2. Cones – function in bright light, responsible for colour (photopic) vision

Photopigments and Signal Generation

• Photopigments present in rods and cones are composed of:
  1. Opsin – a protein
  2. Retinal – an aldehyde of Vitamin A

• When light falls on photoreceptors:
  • Rhodopsin dissociates into opsin and retinal
  • Structural changes occur in opsin
  • Membrane permeability of photoreceptor cells changes
  • This creates potential differences across the membrane

Transmission to Brain

• Action potentials travel through optic nerves
• They reach the visual cortex of the brain
• The brain analyses these signals using memory and experience
• The image formed on the retina is recognised

Additional Points

• Binocular vision is present in mammals and birds
• Pupil becomes smaller in bright light and larger in dim light
• Tear is a watery fluid containing lysozyme with antibacterial properties

The Ear

Functions of Ear:

• Hearing
• Maintenance of body balance and posture

Parts of the Ear:

1. Outer ear
2. Middle ear
3. Inner ear

1. Outer ear

• Components
  1. Pinna
  2. External auditory meatus

• Functions
  • Pinna collects sound vibrations from air
  • External auditory meatus directs sound waves to the ear drum

• Special Features
  • Skin of pinna and meatus contains fine hairs
  • Ceruminous (wax-secreting) glands produce ear wax
  • Ear wax moistens ear drum and protects it

• Tympanic Membrane (Ear Drum)
  • Forms boundary between outer and middle ear
  • Made of connective tissue
  • Covered by skin externally and mucous membrane internally
  • Vibrates in response to sound waves

2. Middle Ear

• Components
  1. Ear ossicles: malleus, incus, stapes
  2. Eustachian tube

• Ear Ossicles
  • Malleus is attached to tympanic membrane
  • Incus lies between malleus and stapes
  • Stapes is attached to the oval window of cochlea
  • Ossicles are arranged in a chain-like manner
  • They amplify and efficiently transmit sound vibrations

• Special Facts
  • Stapes is the smallest bone in the human body
  • Stapedius muscle attached to stapes is the smallest muscle

• Eustachian Tube
  • Connects middle ear to pharynx
  • Equalises air pressure on both sides of ear drum

3. Inner Ear (Labyrinth)

The inner ear is fluid filled and called labyrinth. It has two parts:

1. Bony Labyrinth
   • Series of channels
   • Filled with perilymph
2. Membranous Labyrinth
   • Suspended inside bony labyrinth
   • Filled with endolymph

Cochlea – Organ of Hearing

Structure

• Coiled part of inner ear
• Divided by Reissner’s membrane and basilar membrane into:
  • Scala vestibuli (upper, ends at oval window)
  • Scala tympani (lower, ends at round window)
• Scala media lies between them and is filled with endolymph

Organ of Corti

• Located on basilar membrane
• Contains rows of hair cells (auditory receptors)
• Basal end of hair cells connected to afferent nerve fibres
• Apical ends have stereo cilia
• Stereo cilia are covered by tectorial membrane

Mechanism of Hearing:

1. Sound Wave Journey:
   • Sound waves enter through the external ear and hit the ear drum.
   • The ear drum vibrates in response to these sound waves.
2. Transmission of Vibrations:
   • Vibrations pass through ossicles (malleus → incus → stapes)
   • The stapes passes these vibrations to the oval window.
3. Fluid Waves in the Cochlea:
   • Vibrations from the oval window create waves in the fluids (lymphs) inside the cochlea.
   • These waves cause ripples in the basilar membrane within the cochlea.
4. Hair Cell Activation:
   • Hair cells bend and press against tectorial membrane
   • Nerve impulses are generated in afferent neurons
5. Transmission to the Brain:
   • Nerve impulses travel via auditory nerves to the auditory cortex in the brain.
   • Brain analyses impulses and recognises sound

Hearing Range – Humans can hear frequencies from 20 Hz to 20,000 Hz

Vestibular Apparatus – Balance

• Location
  • Present above cochlea in inner ear

• Components
  • Three semicircular canals
  • Otolith organs – utricle and saccule

• Semicircular Canals
  • Arranged at right angles to each other
  • Each canal has a swollen base called ampulla
  • Ampulla contains crista ampullaris with hair cells

• Otolith Organs
  • Utricle and saccule contain macula
  • Macula and crista act as balance receptors

• Function
  • Maintain balance
  • Maintain posture
  • Detect head movement and orientation

Vestibular apparatus is a type of proprioceptor.

Chapter Summary:

• The neural system coordinates and integrates the activities of all organs and also helps in maintaining metabolic and homeostatic balance of the body.
• Neurons are the structural and functional units of the neural system.
• They are excitable cells due to differences in ion concentration across their membranes.
• The electrical potential difference across the resting neural membrane is called the resting potential.
• When stimulated, nerve impulses travel along the axon as a wave of depolarization followed by repolarization.

• A synapse is formed between a pre-synaptic neuron and a post-synaptic neuron.
• The synaptic cleft may or may not be present.
• Transmission of impulses across synapses occurs either electrically or chemically.
• In chemical synapses, neurotransmitters act as chemical messengers to transmit impulses.

• The human neural system is divided into two main parts: the Central Neural System (CNS) and the Peripheral Neural System (PNS).
• The CNS consists of the brain and spinal cord and functions as the main site for information processing and control.

• The brain is divided into three major regions: forebrain, midbrain and hindbrain.
• The forebrain includes the cerebrum, thalamus and hypothalamus.
• The cerebrum is divided into two cerebral hemispheres connected by the corpus callosum and is responsible for sensory perception, voluntary actions, memory, intelligence and emotions.
• The hypothalamus regulates body temperature, hunger, thirst and several homeostatic functions.
• The limbic system, which is part of the forebrain, is involved in olfaction, autonomic responses, sexual behaviour, emotional reactions and motivation.

• The midbrain serves as an integrating centre for visual, auditory and tactile reflexes.
• The hindbrain includes the pons, cerebellum and medulla oblongata.
• The cerebellum coordinates voluntary movements, posture and balance and receives input from the semicircular canals of the ear.
• The pons connects different regions of the brain through fibre tracts.
• The medulla oblongata contains centres that regulate respiration, cardiovascular reflexes and gastric secretions.

• Reflex action is a quick, involuntary response to a stimulus and is mediated through a reflex arc without involving conscious effort.
• Sensory organs detect changes in the external and internal environment and transmit this information to the CNS for processing and appropriate response.

• The eye is the organ of vision and its wall is composed of three layers: the external fibrous layer consisting of the cornea and sclera, the middle vascular layer called the choroid, and the innermost nervous layer called the retina.
• The retina contains rod and cone photoreceptor cells.
• Rods are responsible for twilight or scotopic vision, while cones function in daylight and colour vision.
• Light entering through the cornea and lens forms an image on the retina, which is converted into nerve impulses and transmitted to the visual cortex of the brain.

• The ear is the organ of hearing and balance and is divided into three parts: outer ear, middle ear and inner ear.
• The middle ear contains three auditory ossicles namely malleus, incus and stapes, which amplify sound vibrations.
• The inner ear is a fluid-filled structure called the labyrinth.
• Its coiled part, the cochlea, contains the organ of Corti on the basilar membrane.
• Hair cells of the organ of Corti act as auditory receptors and convert sound vibrations into nerve impulses that are transmitted to the auditory cortex of the brain.

• The vestibular apparatus of the inner ear helps in maintaining balance and posture.
• It responds to gravity and movements of the head, enabling orientation and equilibrium.