The Nervous System and Senses: Structure, Function, and Disorders

The Nervous System and Senses

Introduction

The nervous system is a complex network responsible for regulating and coordinating body activities, maintaining homeostasis, and enabling perception and response to stimuli. This guide covers the structure and function of the nervous system, mechanisms of nerve impulse transmission, and the anatomy and physiology of the senses, with a focus on vision and hearing.

Systems Regulating Change in Human Organisms (Homeostasis)

• Homeostasis: The process by which a constant internal environment is maintained despite changes in the external environment (e.g., regulation of blood sugar).

• Nervous System: Provides quick and short-term responses.

• Endocrine System: Provides slow and long-term responses.

Nervous System Overview

• Functions: Involves sensation, transmission, integration, and response.

• Two Main Subsystems:

  • Central Nervous System (CNS): Brain and spinal cord.

  • Peripheral Nervous System (PNS): Includes involuntary (autonomic) and voluntary (somatic) systems.

Organizational Ladder of Nervous Control

The nervous system is organized hierarchically, with the CNS at the top, integrating and processing information, and the PNS relaying signals to and from the CNS.

Neurons: Structure and Function

• Neurons: Nerve cells specialized to conduct electrochemical impulses.

• Types of Neurons:

  • Sensory Neurons: Carry impulses from receptors to the CNS (afferent).

  • Motor Neurons: Carry impulses from the CNS to effectors (muscles/glands) (efferent).

  • Association/Interneurons: Connect sensory and motor neurons within the CNS.

Basic Components of a Neuron

• Dendrites: Receive stimuli from the environment or other neurons.

• Cell Body (Soma): Contains the nucleus and most organelles; integrates incoming signals.

• Axon: Long fiber that transmits impulses away from the cell body toward effectors.

Myelination

• Myelinated Fibers: Axons covered by myelin sheath (Schwann cells in PNS, oligodendrocytes in CNS); impulses travel faster due to saltatory conduction.

• Unmyelinated Fibers: Lack myelin sheath; impulses travel slower.

Myelinated Neurons	Unmyelinated Neurons
Have myelin sheath (Schwann cells/oligodendrocytes)	No myelin sheath
Impulse travels faster	Impulse travels slower
Found in PNS and CNS	Found only in CNS

Nerve Impulse: Fiber Transmission

1. Neuron at Rest (Resting Potential)

• Neuron is polarized; inside is negative relative to outside.

• Resting potential typically about $-70\text{mV}$ or $-0.07\text{V}$.

• Maintained by sodium-potassium pump: $\text{Na}^+$ out, $\text{K}^+$ in.

2. Action Potential

• Begins with a stimulus causing depolarization (membrane potential becomes less negative).

• If threshold is reached, voltage-gated $\text{Na}^+$ channels open, $\text{Na}^+$ rushes in, causing rapid depolarization.

• Followed by repolarization as $\text{K}^+$ channels open and $\text{K}^+$ exits the cell.

3. Refractory Period

• Brief period after an action potential when the neuron cannot fire again until resting potential is restored.

Threshold

• The minimum potential difference needed to trigger an action potential (e.g., 50mV).

• All-or-none response: If threshold is reached, the impulse is generated and propagated.

Impulse Characteristics

• Action potentials are self-propagating and cannot be stopped once initiated.

• Impulse speed varies with myelination and axon diameter.

Synapse

• Synaptic Cleft: Gap between axon terminal of one neuron and dendrite of another.

• Neurotransmitters: Chemicals (e.g., acetylcholine, norepinephrine) that transmit signals across the synapse.

• Types of Synaptic Effects:

  • Excitatory: Increase likelihood of action potential in postsynaptic neuron.

  • Inhibitory: Decrease likelihood of action potential.

Substances Affecting Synaptic Function

• Caffeine, nicotine, and amphetamines stimulate synaptic transmission.

• Alcohol and depressants inhibit synaptic transmission.

Central Nervous System (CNS)

• Includes the brain and spinal cord.

• Brain is protected by bone, meninges, and cerebrospinal fluid.

• Major regions: cerebrum, cerebellum, brainstem (medulla, pons, midbrain), thalamus, hypothalamus.

Cerebrum

• Largest part of the brain; responsible for higher functions (thought, memory, voluntary movement).

• Divided into left and right hemispheres, connected by the corpus callosum.

• Four lobes: frontal, parietal, temporal, occipital.

Cerebellum

• Coordinates movement, balance, and muscle tone.

Brainstem

• Controls vital functions (breathing, heart rate, digestion).

• Includes medulla oblongata, pons, and midbrain.

Thalamus and Hypothalamus

• Thalamus: Relay station for sensory information.

• Hypothalamus: Regulates homeostasis, links nervous and endocrine systems.

Spinal Cord

• Transmits nerve impulses between the brain and body.

• Contains gray matter (cell bodies) and white matter (myelinated axons).

Peripheral Nervous System (PNS)

• Somatic Nervous System: Voluntary control of skeletal muscles.

• Autonomic Nervous System: Involuntary control of smooth muscle, cardiac muscle, and glands.

• Autonomic system divided into:

  • Sympathetic: Prepares body for stress ("fight or flight").

  • Parasympathetic: Conserves energy, promotes "rest and digest" functions.

Parasympathetic	Sympathetic
Decreases heart rate	Increases heart rate
Stimulates digestion	Inhibits digestion
Constricts pupils	Dilates pupils
Vasodilation of skeletal muscle	Vasoconstriction of skeletal muscle

Sensory Systems: Eye and Ear

Eye: Structure and Function

• Eyelids and Eyelashes: Protect the eye.

• Extrinsic Muscles: Move the eye in various directions.

• Sclera: White outer layer, maintains shape.

• Choroid Layer: Contains pigment to absorb light and prevent reflection.

• Retina: Contains photoreceptor cells (rods and cones).

• Lens: Focuses light onto the retina.

• Pupil: Opening in the iris that regulates light entry.

• Fovea Centralis: Area of sharpest vision, contains only cones.

Photoreceptors

• Rods: Sensitive to low light, enable night vision.

• Cones: Detect color and detail, function best in bright light.

Chemistry of Vision: Rhodopsin

• Rhodopsin (in rods) requires vitamin A for function.

• Light converts rhodopsin to retinal and opsin, triggering nerve impulses.

Common Eye Disorders

Disorder	Problem	Effect
Near sighted	Eye too long	Image focused before retina
Far sighted	Eye too short	Image focused after retina
Astigmatism	Irregular curvature of lens/cornea	Blurred vision
Cataract	Lens or cornea clouds	Blocks light, impairs vision

Ear: Structure and Function

• Outer Ear: Pinna, ear canal, tympanic membrane (eardrum).

• Middle Ear: Ossicles (malleus, incus, stapes), oval window, Eustachian tube.

• Inner Ear: Cochlea (organ of Corti), vestibule, semicircular canals.

Hearing Mechanism

• Sound waves vibrate the eardrum, transmitted via ossicles to the cochlea.

• Hair cells in the cochlea convert vibrations into nerve impulses.

Balance Mechanism

• Semicircular canals and vestibule contain fluid and otoliths, detect head position and movement (static equilibrium).

Common Ear Disorders

• Tinnitus: Ringing in the ears.

• Nerve Deafness: Damage to hair cells or auditory nerve.

• Conduction Deafness: Damage to eardrum, ossicles, or cochlea.

Summary

• The nervous system integrates sensory input and coordinates responses to maintain homeostasis.

• Specialized structures in the eye and ear enable vision and hearing, with specific disorders affecting their function.

Additional info: Some explanations and tables have been expanded for clarity and completeness based on standard biology textbooks.