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• The human nervous system maintains homeostasis, ensures efficient bodily function, and protects from danger.
• It consists of the central and peripheral nervous systems.

Central Nervous System (CNS)

• Includes the brain and spinal cord.
• Functions as the primary control center for processing information.
• Transmits information throughout the body.

Peripheral Nervous System (PNS)

• Extends outside the CNS.
• Comprises cranial and spinal nerves.
• Divided into autonomic (involuntary) and somatic (voluntary) systems.
• The autonomic system further splits into sympathetic and parasympathetic systems.
• These regulate involuntary bodily functions.
• Consists of all nerves found outside the central nervous system.
• Includes 12 pairs of cranial nerves connected to the brain.
• Includes 31 pairs of spinal nerves connected to the spinal cord.

Key Components and Terms of the Nervous System

• Receptors detect stimuli and convert them into impulses.
• Effectors (glands or organs) respond to stimuli.
• Stimulus: a detectable change in the environment.
• Impulse: an electrical signal generated in response to a stimulus.
• Neurons: specialized nerve cells that transmit impulses.
• Synapses: gaps between neurons where impulses pass via neurotransmitters.
• Grey matter: consists of cell bodies and dendrites.
• White matter: comprises myelinated axons.
• The nervous system responds to external and internal stimuli.
• The nervous system coordinates body activities.
• The nervous system ensures survival by adapting to environmental changes.
• Works alongside the endocrine system, which uses hormones for slower, longer-lasting responses.
• Alzheimer's and multiple sclerosis are associated with nervous system dysfunctions.
• Dysfunctions affect memory, physical, and mental capabilities.

Brain

Cerebrum

• Largest part of the brain.
• Controls voluntary actions (walking, speaking, writing).
• Interprets sensations from sense organs (sight, sound, touch).
• Involved in higher thought processes (memory, reasoning, intelligence).

Cerebellum

• Coordinates all voluntary movements.
• Maintains balance and muscle tension.
• Ensures smooth, coordinated body movements.

Medulla Oblongata

• Transmits nerve impulses between the spinal cord and the brain.
• Controls involuntary actions (heartbeat, breathing).
• Critical for life support.

Corpus Callosum

• Connects the left and right hemispheres of the brain.
• Facilitates communication between both sides.
• Ensures both sides of the brain can work together efficiently.

Hypothalamus

• Acts as a control center for autonomic functions (hunger, thirst, sleep, body temperature, emotions).
• Plays a key role in maintaining the body's homeostasis.

Spinal Cord

• Main pathway for information connecting the brain and peripheral nervous system.
• Protected by the vertebral column and meninges.
• Transmits sensory information from the body to the brain.
• Transmits motor commands from the brain to the body.
• Coordinates reflexes independently from the brain.

Neurons

• Specialized cells that transmit impulses.

Sensory (Afferent) Neurons

• Detect stimuli and transmit impulses from sense organs to the spinal cord and brain.

Motor (Efferent) Neurons

• Transmit impulses from the brain and spinal cord to effectors (muscles and glands).
• Facilitate responses to stimuli.

Interneurons

• Connect sensory and motor neurons.
• Roles in the interpretation of sensory input and the generation of motor output.
• The brain handles higher function processing and sensory data interpretation.
• The brain coordinates voluntary muscle movement.
• Neurons facilitate the transmission of signals throughout the body.

PNS Divisions

Somatic Nervous System

• Controls voluntary muscles.
• Allows the body to react to changes in the external environment.
• Governs conscious movements.
• Governs sensory information processing from the external environment.

Autonomic Nervous System

• Controls involuntary muscles.
• Manages functions not under conscious control (heartbeat and digestion).

PNS Functions

• Transmits impulses from the receptors to the central nervous system via sensory neurons.
• Transmits impulses from the central nervous system to the body's effectors (muscles and glands) via motor neurons.
• Enables responses to internal and external stimuli.

Autonomic Nervous System (ANS)

• Controls involuntary actions in the body.
• Responds to changes in the internal environment to maintain homeostasis.

Sympathetic Nervous System

• Prepares the body for emergency situations ("fight or flight" response).
• Increases heart rate.
• Constricts blood vessels in the skin (vasoconstriction).
• Increases blood pressure.
• Widens bronchioles.
• Decreases peristalsis.
• Causes relaxation of the bladder wall.
• Stimulates sweat secretion.
• Dilates pupils.
• Stimulates the secretion of adrenaline.

Parasympathetic Nervous System

• Restores the body to a normal state after an emergency ("rest and digest" response).
• Decreases heart rate.
• Dilates blood vessels in the skin (vasodilation).
• Decreases blood pressure.
• Narrows bronchioles.
• Increases peristalsis.
• Causes contraction of the bladder wall.
• Constricts pupils.
• There is no effect on sweat secretion.
• There is no stimulation of adrenaline secretion from this branch.
• The systems work antagonistically to each other.
• They ensure that the body can respond appropriately to different situations.
• They ensure that the body maintains internal balance.
• Adrenaline plays a crucial role in preparing the body for 'fight or flight' situations.

Neuron Structure

Nucleus

• Controls the metabolism and activities of the neuron.

Cell Body

• Contains the nucleus and cytoplasm.
• Maintains the neuron's health and functionality.

Cytoplasm

• Fills the cell body.
• Supports various cellular processes.

Myelin Sheath

• Covers the axon.
• Speeds up the transmission of electrical impulses.
• Provides insulation.

Axon

• A long, thin fiber.
• Transmits impulses away from the cell body.

Dendrites

• Branch-like structures.
• Receive signals from other neurons.
• Transmit these impulses towards the cell body.
• Connection between neurons occurs at a synapse.
• Synapse is where the terminal branches of one neuron's axon are in close proximity to another neuron's dendrites or cell body.
• Synapses enable the transmission of signals.
• Nerve impulse transmission is electro-chemical.
• Nerve impulse transmission involves the generation and propagation of electrical signals along the neuron and the release of neurotransmitters at synapses to communicate with other neurons.

Reflex Action

• A rapid, automatic response to a stimulus that involves the spinal cord.
• Does not involve the brain.
• Essential function for protecting the body from harm.
• Examples: blinking the eye, coughing, sneezing, dilation and constriction of the pupil of the eye, and quickly withdrawing the hand when it touches a hot surface.
• A reflex arc is the path along which an impulse is transmitted to bring about a response to a stimulus during a reflex action.

Reflex Arc Components

Receptor

• Receives a stimulus.
• Converts it into an impulse.
• Heat sensor of the finger feels the burn.

Sensory Neuron

• Carries the impulse from the receptor to the spinal cord.

Interneuron

• Located in the spinal cord.
• Carries the impulse from the sensory neuron to the motor neuron.

Motor Neuron

• Transports the impulse from the spinal cord to the effectors.
• Effectors are the muscles or glands responding to the impulse.

Effector

• Produces the response.
• Muscles in the finger contracting to pull the finger away from the flame.
• The functioning of a reflex arc involves the transmission of impulses from the receptor to the effector in a specific sequence: Receptor → Sensory Neuron → Interneuron → Motor Neuron → Effector.
• This sequence ensures a quick and automatic reaction to potentially harmful stimuli.
• It occurs without involving the brain's conscious decision-making processes.
• Provides an immediate response to avoid or mitigate injury.
• Significance of a reflex action lies in its ability to provide an immediate response to avoid danger or harm without the need for conscious thought.
• Synapses allow for the direction of the nerve impulse through the correct pathway.
• Synapses ensure the speed and efficiency of the response.

Disorders of the CNS

Alzheimer's Disease

• A neurodegenerative condition.
• Characterized by progressive brain cell death over time.
• Leads to symptoms such as memory loss, confusion.
• Irreversible.
• Symptoms typically emerge after the age of 60, though cases have been observed in individuals as young as 40.
• There is no cure.
• Symptoms can be managed.

Multiple Sclerosis (MS)

• Typically affects young adults between the ages of 20 and 40.
• An autoimmune disease.
• The body's immune system attacks the myelin sheath covering neurons.
• Impairs their proper function.
• There is no cure.
• Symptoms can be managed with medication.
• Symptoms include loss of speech and vision, difficulty walking, pain, fatigue, and memory loss.

Receptors Role

• Detect changes in the external and internal environments (stimuli).
• Convert these stimuli into electrical signals (nerve impulses).
• Photoreceptors in the eye detect light.
• Mechanoreceptors in the skin detect touch or pressure.
• Neurons transmit impulses from the receptors to the central nervous system (brain and spinal cord).
• Neurons transmit impulses from the central nervous system to the effectors.
• Neurons are specialized to carry information through electrical and chemical signals.
• They consist of dendrites, a cell body, and an axon.
• Effectors: glands or muscles that respond to the impulses sent by the nervous system.
• Muscle cells contract, leading to movement.
• Glands secrete hormones or other substances, influencing body functions.
• The body responds to various stimuli, including light, sound, touch, temperature, pressure, and chemicals (taste and smell).
• Receptors in the eye detect light and color.
• Hair cells in the ear respond to sound vibrations.
• The autonomic nervous system controls involuntary actions.
• The somatic nervous system controls voluntary actions.

Human Eye

Sclera

• Tough, white inelastic layer covering the posterior eye.
• Protects inner structures.
• Maintains eye shape.

Cornea

• Transparent, convex surface at the front of the eye.
• Allows light to pass through.
• Causes refraction to create an image on the retina.

Choroid

• Dark-colored layer containing blood vessels and pigments.
• Absorbs light to prevent reflection.
• Supplies nutrients and oxygen to the retina.

Ciliary Body

• Contains ciliary muscles.
• Alters tension on suspensory ligaments to change the lens's shape for accommodation.

Iris

• The colored part of the eye.
• Contains muscles to control the pupil size.
• Controls the amount of light entering the eye.

Lens

• An elastic, biconvex structure.
• Changes shape to focus light on near and distant objects.

Aqueous Humour

• A watery fluid between the cornea and lens.
• Maintains the cornea's shape.
• Assists in light refraction.

Vitreous Humour

• A jelly-like substance behind the lens.
• Maintains the eyeball's shape.
• Plays a minor role in light refraction.

Retina

• Contains light-sensitive cells (rods for dim light, cones for bright light and color vision).
• Transforms light into electrical impulses.

Optic Nerve

• Transmits impulses from the retina to the brain for image interpretation.

Yellow Spot (Macula)

• The area of clearest vision.
• Highest concentration of cones.

Blind Spot

• The point where the optic nerve leaves the eye.
• No photoreceptor cells present.
• No vision is possible in this area.

Binocular Vision

• Use of two eyes with overlapping fields of view.
• Forms one image.
• Provides depth perception and a wider field of vision.
• Enables the ability to judge distance, depth, and the size of objects.

Accommodation

• The process through which the eye changes the lens's shape to focus on near and distant objects.
• When viewing objects closer than 6 meters, the ciliary muscles contract, the lens becomes more convex, and light refraction increases.
• When viewing objects further than 6 meters, the ciliary muscles relax, the lens becomes less convex, and light refraction reduces.

Pupillary Mechanism

• The iris adjusts the pupil size to control the amount of light entering the eye.
• In bright light, radial muscles relax, and circular muscles contract, making the pupil smaller and reducing light entry.
• In dim light, radial muscles contract, and circular muscles relax, enlarging the pupil and allowing more light to enter.

Visual Defects

Short-sightedness (Myopia)

• The inability to see distant objects clearly.
• Corrected by wearing concave lenses.

Long-sightedness (Hyperopia)

• Difficulty in seeing close objects clearly.
• Corrected by wearing convex lenses.

Astigmatism

• Blurred vision due to uneven curvature of the cornea or lens.
• Corrected by glasses with specially shaped lenses.

Cataracts

• Clouding of the lens.
• Leads to blurred vision.
• Corrected by surgical removal of the cloudy lens and replacement with a synthetic lens.

Human Ear

Outer Ear

• Includes the pinna and the auditory (ear) canal.
• Pinna directs sound waves into the auditory canal.
• Auditory canal transmits sound waves to the tympanic membrane (eardrum).
• Contains little hairs and wax to prevent foreign bodies and to keep the tympanic membrane from drying out.

Middle Ear

• Air-filled cavity containing the ossicles: the hammer (malleus), anvil (incus), and stirrup (stapes).
• Separated from the outer ear by the tympanic membrane.
• Separated from the inner ear by the oval and round windows.
• Ossicles transmit and amplify vibrations from the tympanic membrane to the inner ear.
• Eustachian tube connects the middle ear to the throat, equalizing pressure on both sides of the tympanic membrane.

Inner Ear

• Located within the skull's bones.
• Includes the semi-circular canals, vestibule (sacculus and utriculus), and cochlea.
• Structures are part of the bony labyrinth filled with perilymph.
• Membranous labyrinth contains endolymph.

Hearing Function

• Sound waves pass into the auditory canal.
• Waves cause the tympanic membrane to vibrate.
• Vibrations are passed to the ossicles.
• Ossicles amplify vibrations and transmit them to the oval window.
• Movement of the oval window generates pressure waves in the cochlea's endolymph.
• The organ of Corti (containing sensory hair cells) is stimulated.
• These impulses are transmitted via the auditory nerve to the cerebrum for interpretation.

Maintaining Balance

• Cristae in the semi-circular canals detect changes in the body's speed and direction.
• Maculae in the sacculus and utriculus detect changes in the head's position relative to gravity.
• When stimulated, these receptors send impulses via the auditory nerve to the cerebellum.
• Cerebullum coordinates muscle activity to maintain balance.

Hearing Defects and treatments

Middle Ear Infection

• Caused by pathogens entering through the Eustachian tube.
• Leads to fluid buildup and inflammation.
• Treatment includes medication and the insertion of grommets (draining tubes) into the tympanic membrane.

Deafness

• Caused by injury to the ear, nerves, or brain.
• Hardening of ear tissues like the ossicles.
• Treatment options include hearing aids and cochlear implants.