Modern Biology: Nervous System

• This course is titled Modern Biology (BT 101) and is worth 3006 credits.
• The prerequisite for this course is Nil.
• Division IV covers human physiology, including the nervous system, sensory systems, and introduction to ANN.
• Human physiology focuses on:
• Nervous system
• Biological axons and neurons
• Neuromuscular and synaptic junctions
• Sensory systems (hearing, taste, smell, and visual receptors)
• This course is taught by Dr. Souptick Chanda, BSBE, IIT Guwahati
• Mid semester exams are worth 40%
• Quizzes conducted before the mid semester exams are worth 10 marks
• Final semester exams are worth 60%
• Quizzes after the mid semester exams are worth 10 marks
• Re-examinations are not permitted
• Attendance guidelines are to be strictly followed

Nervous System

• Neurons are structural and functional units
• They communicate directly or indirectly through synapses
• Neurons are electrically excitable and transmit impulses via chemical and electrical signals
• The nervous system consists of:
• Nerve cells (neurons)
• Neuroglia
• Neurons transfer information around the body

Neuron Structure

• Dendrites receive signals from neighboring neurons, like radio antennas
• Axons transmit signals over a distance, like telephone wires
• Axon terminals transmit signals to other neuron dendrites, like a radio transmitter
• Myelin sheaths speed up signal transmission along the axon

Nerve Impulse and Resting Potential

• Electrical potential difference between the inside and outside of a cell is the membrane potential
• Resting membrane potential is when a nerve or muscle cell is at "rest"
• Resting membrane potential of neurons is about -50mv
• Resting membrane potential is impacted by:
• Na+
• K+
• Ions are drawn to opposing charges across the membrane
• Chemical diffusion force is related to concentration gradients
• Particles move in the direction from higher to lower concentration across neuron membranes

Electrochemical Forces

• Each ion is acted on by two forces to enable functions
• At equilibrium, chemical and electrical energy are balanced
• The membrane potential is the Nernst Equation at equilibrium
• Simplified Equation: Ex = (RT/zF) * ln([X]o/[X]i)
• T = room temperature
• z = charge
• F = Faraday constant

Neuron Function

• 100 billion neurons fire nearly 50 action potentials per second
• This activity controls our thoughts, movements, and senses
• Signals are sent in two steps:
• Action Potential: Along the cell (electrochemical cascade)
• Neurotransmitters: Between cells

Action Potentials

• Action potentials travel as an electrochemical cascade, allowing a net inward flow of positively charged ions into the axon
• Neurotransmitters are chemicals stored in membrane-bound vesicles at the axon terminal of neurons
• Neurotransmitters are released when Ca2+ enters the axon terminal
• Released neurotransmitters act by binding to receptors on the postsynaptic cell membrane
• Neurotransmitters can be:
• Excitatory: Firing a target neuron (e.g., Glutamate)
• Inhibitory: Making a target neuron less likely to fire (e.g., GABA)
• Action potentials occur when graded potentials reach threshold potential

Action Potential Characteristics

• Action potentials occur in the axons of neurons
• Action potentials do not decay and remain constant in duration
• Myelinated axons conduct action potentials faster
• Action potential speed in axons ranges from 1 to 100 meters/second
• Size of action potentials varies from neuron to neuron

Muscles

• Skeletal muscles consist of fascicles surrounded by connective tissue (perimysium)
• Fascicles are composed of numerous muscle fibers (muscle cells)
• Muscle cells consist of:
• Myofibrils, ensheathed by endomysium
• Myofilaments (thick and thin)
• Skeletal muscles attach to bone via tendons composed of connective tissue (called epimysium)
• The thick and thin protein filaments occur in subdivisions called sacromeres

Receptors

• Mechanoreceptors respond to mechanical compression or stretching of adjacent tissues
• Thermoreceptors respond to changes in temperature (cold/warmth)
• Nociceptors respond to pain
• Electromagnetic receptors respond to light
• Chemoreceptors respond to taste, smell, oxygen levels, osmolality, and CO2 concentrations

Vision

• Basic structure of the eye is responsible for the formation of images on the retina
• Common vision problems include:
• Farsightedness
• Nearsightedness
• Intraocular fluid and pressure are key components of eye health
• Glaucoma is related to issues with fluid and pressure
• Photochemistry of vision involves rods and cones
• Colour vision in reptiles and birds is significant

Eye Details

• The pigmented layer of the retina adjacent to the photoreceptors stores a large quantity of vitamin A
• Vitamin A contributes to the regeneration process in rods
• Rhodopsin generation decreases if the retina detaches from the pigmented layer
• Cone photopigments regenerate more quickly than rhodopsin and are less dependent on the pigmented layer
• Half of all rod pigments take 5 minutes to regenerate
• Half of all cone pigments take 90 seconds

Vision Impairments

• Farsightedness (hyperopia/hypermetropia)
• Nearsightedness (myopia)
• Night blindness

The Eye

• The human eye has various structures
• Sclera
• Iris
• Pupil
• Cornea
• Lens
• The eye functions like a camera, with a lens system, variable aperture, and film (retina)
• The lens system is composed of four refractive interfaces
• Air to cornea
• Cornea to aqueous humor
• Aqueous humor to crystalline lens
• Lens to vitreous humor
• The refractory index corresponds to the following values:
• Air is 1
• Cornea is 1.38
• Aqueous humor is 1.34
• Crystalline lens is 1.41
• Vitreous humor is 1.34

Eye Physiology

• The eye is kept round and distended by maintaining pressure
• The eye has two chambers
• Aqueous
• Vitreous
• The Aqueous Humor is watery fluid filling the anterior and posterior chamber of the eyeball
• Volume: 0.31 ml
• Anterior chamber- 0.25ml
• Posterior chamber- 0.06ml
• Refractive Index: 1.336
• Density: 1.025-1.040 (greater than water)
• pH: 7.2 (acidic)
• Rate of formation: 2.3µl/min
• The Vitreous Humor is a clear, avascular gel which occupies the posterior compartment of the eye
• It has the following composition:
• Water (99%)
• Network of collagen fibrils
• Large molecules of hyaluronic acid
• Peripheral cells (hyalocytes)
• Inorganic salts, sugar and ascorbic acid

Ocular Tonometry

• Ocular tonometry measures intraocular pressure (IOP) to evaluate the risk of glaucoma
• A tonometer is a device measures intraocular pressure (IOP)

Glaucoma

• Glaucoma involves increased eye pressure that is caused by:
• Compression and death of optic nerve axons
• Blockage of axonal cytoplasm flow
• Lack of nutrition

Photoreceptors

• 6 million cones and 120 million rods
• Rods are responsible for night vision
• Cones provide colour vision in brighter light
• Light is converted into electrical signals in rods and cones
• This process is called the visual cycle (visual transduction)
• The cycle was discovered by George Wald, who received the Nobel Prize in 1967
• In darkness, retinal has a bent shape (cis-retinal) that is suited for opsin
• When light is absorbed, cis straightens to trans-retinal (isomerization)
• Trans-retinal detaches from opsin (bleaching)
• Trans-retinal converts back to cis-retinal with retinal isomerase
• Cis-retinal binds to opsin
• Half of all rod pigments take 5 minutes to regenerate
• Half of all cone pigments take 90 seconds

Hearing

• Is the ability to perceive sound
• The ear transforms sound vibrations into electrical signals
• This functions one thousand times faster than photoreceptors
• Hearing involves the outer, middle, and inner ear
• The ear also contains receptors for equilibrium
• The Outer ear consists of:
• auricle
• external auditory canal
• eardrum
• The auricle is made of elastic cartilage
• The external auditory canal carries to the eardrum
• The eardrum separates the outer and middle ear
• Ceruminous glands in the external auditory canal secrete earwax or cerumen
• The Middle ear relays sound from the outer ear to the inner ear
• It conveys sound vibrations to the oval window
• The Middle ear transfers external forces 22 times higher while using auditory ossicles as an amplifier
• Malleus
• Incus
• Stapes
• The Middle as an amplifier uses two means:
• Area difference: 55 to 3.2 sq mm
• Configuration of ossicles
• The Inner ear houses the receptors for hearing and equilibrium and is called the labyrinth
• Bony labyrinth (BL) and membranous labyrinth (ML)
• BL contains perilymph and ML contains endolymph
• The vestibule is the oval central portion of BL
• It houses sensory as well as motor neurons
• It synapses with receptors for equilibrium
• Located anterior to the vestibule, the cochlea is the bony, spiral canal
• Utricle and saccule are small sacs connecting the Vestibule to the main duct through ampulla

Hearing Process

• The auricle directs sound waves into the external auditory canal.
• Sound waves cause the eardrum to vibrate .
• The malleus vibrates along with the eardrum, transmitting it further to the incus and then to the stapes.
• The stapes moves back and forth and makes the oval window vibrate at an intensity ~22 times more than the eardrum.
• Fluid pressure waves are made by the cochlea.
• These vibrations cause the basilar membrane to vibrate.
• Electrical signals that are generated by hair cells in the basilar membrane are sensed as sound.
• Sound waves of various frequencies are sensed, depending on the location of the basilar membrane on the 'tuned' to a pitch.