Neurobiology, Sensory Systems, and Endocrine Regulation
Glial Cells: Support and Function
Glial cells are the other type of cell found in nervous tissue, often considered support cells. Their functions are directed at helping neurons complete communication. They typically have a higher ratio to nerve cells.
Key Glial Functions
- Bind nerve cells and maintain the ionic milieu.
- Modulate the rate of nerve signal propagation.
- Modulate synaptic action by controlling the uptake of neurotransmitters.
- Provide a scaffold for neural development.
- Aid in recovery from neural injury.
Types of Glial Cells
- Astrocytes: Restricted to the brain and spinal cord (Central Nervous System – CNS), maintaining an appropriate chemical environment for neuronal signaling.
- Oligodendrocytes: Also restricted to the CNS, they lay down a laminated, lipid-rich wrapping called myelin around some, but not all, axons.
- Schwann Cells: These cells elaborate myelin in the Peripheral Nervous System (PNS).
Other glial cells act primarily as scavenger cells that remove cellular debris from sites of injury or normal cell turnover (e.g., Microglia).
Neuronal Electrophysiology
A neuronal action potential has three main stages: depolarization, repolarization, and hyperpolarization.
Action Potential Propagation
Action potentials propagate a signal along the length of an axon differently in myelinated versus unmyelinated axons.
Myelinated Axons (Saltatory Conduction)
In myelinated neurons, the stages of the action potential occur primarily at the Nodes of Ranvier (gaps in the myelin sheath). This pattern of node-to-node propagation, known as saltatory conduction, can increase the conduction velocity by more than an order of magnitude compared to unmyelinated axons.
Unmyelinated Axons (Continuous Conduction)
In unmyelinated axons, depolarization of the cell membrane must spread to the immediately adjacent region of the membrane, raising the potential passively until reaching the threshold voltage. The action potential propagates as a continuous wave of depolarization.
During the action potential sequence:
- Voltage-gated sodium channels activate, causing depolarization.
- Inactivation of sodium channels and activation of potassium channels cause repolarization.
- The potassium channels stay open longer, causing hyperpolarization (undershoot).
Major Brain Structures
Cerebrum
Interprets the five senses and regulates actions such as speech, memory, behavior, personality, movement, reasoning, and judgment. It is divided into left and right hemispheres, which are connected by nerve fiber bundles.Cerebellum
Maintains balance, posture, coordination, and motor skills. It is a small structure located in the back of the brain.Brainstem
Regulates automatic body functions like heart rate, breathing, sleep and wake cycles, and swallowing. It is the lower part of the brain that connects the brain to the spinal cord.Peripheral Nervous System (PNS)
The PNS extends from the brain and spinal cord to the rest of the body.
Neural Ganglia
Neural ganglia are groups of neuron cell bodies, which can be categorized as sensory ganglia.
Types of Ganglia
- Dorsal Root Ganglia: Contain cell bodies of sensory neurons whose axons are sensory endings.
- Cranial Nerve Ganglia: Their roots are located within the cranium.
- Sympathetic Chain Ganglia (Paravertebral): Constitute a row of ganglia alongside the vertebral column that receive central input. (Includes three ganglia in the cervical region).
- Prevertebral Ganglia: Three autonomic ganglia located outside of the sympathetic chain.
- Terminal Ganglia: Regulate parasympathetic aspects of homeostatic mechanisms.
The sympathetic and parasympathetic systems often project to the same organs to regulate overall function.
Divisions of the PNS
Somatic Nervous System
Consists of motor and sensory neurons that carry information to and from the CNS, primarily controlling voluntary movement and relaying sensory input.Autonomic Nervous System (ANS)
Controls internal organs and glands and is outside the realm of voluntary control. It is divided into two systems:- Sympathetic System: Prepares the body for stressful activities (“fight or flight”).
- Parasympathetic System: Returns the body to day-to-day operations (“rest and digest”).
Note: Homeostasis is the state of equilibrium where biological conditions are maintained at optimal levels.
Neurological Disorders
Alzheimer’s Disease
A neurodegenerative disease that damages areas of the brain responsible for cognition. It is characterized by the formation of beta-amyloid plaques and tau tangles within the brain.Epilepsy
A brain disorder where groups of neurons send incorrect signals, causing recurring seizures. Excessive electrical activity in the brain often leads to involuntary movements.Multiple Sclerosis (MS)
An autoimmune condition where the immune system attacks myelin cells, damaging the sheath and interrupting messages sent throughout the body, affecting sensory and motor functions.Parkinson’s Disease
An age-related brain condition causing deterioration, known for symptoms such as slowed movements, tremors, and balance problems.Cerebral Palsy (CP)
A group of permanent disorders limiting activity and causing disturbances of sensation, perception, cognition, communication, and behavior.Stroke
A medical emergency caused when something prevents the brain from getting proper blood flow (ischemic or hemorrhagic).Amyotrophic Lateral Sclerosis (ALS)
A neurodegenerative condition affecting how cells communicate with muscles, which can impact movement, speech, and breathing, leading to progressive muscle weakness.External Factors Affecting the Nervous System
Effect of Drugs on the Nervous System
Drugs interfere with the way neurons send, receive, and process neurotransmitters. Key brain areas affected by drug use include:
- The Basal Ganglia: Plays an important role in positive forms of motivation.
- The Extended Amygdala: Plays a role in stressful feelings like anxiety, irritability, and unease.
- The Prefrontal Cortex: Powers the ability to think, plan, and solve problems.
Interference with these areas can lead to addiction and drug-seeking behavior.
Effect of Toxins on Neuron Functioning (Neurotoxicity)
Neurotoxicity occurs when exposure to toxic substances changes the function of a part of the nervous system. Neurotoxicants damage, change, or kill cells in the PNS or CNS.
The severity of neurotoxicity is often greater when exposure happens during early life and fetal development.
Anatomy of the Eye
Cornea
The clear outer part located at the front of the eye.Iris
The colored part of the eye that surrounds the pupil and regulates the amount of light entering the eye.Lens
The clear part of the eye behind the iris that helps focus light and images onto the retina.Optic Nerve
The largest sensory nerve of the eye that carries impulses for sight from the retina to the brain.Pupil
The opening at the center of the iris through which light passes. The iris adjusts the size of the pupil to control the amount of light entering.Retina
Light-sensitive tissue at the back of the eye that converts light into electrical impulses sent to the brain via the optic nerve.Photoreceptors are light-detecting cells within the retina that convert incoming light into a form the brain can use for vision.
Major Parts of the Ear and Hearing
Outer Ear
- Pinna (Auricle)
- Ear Canal: Connects the outer ear to the middle ear.
Middle Ear
- Eardrum (Tympanic Membrane): Divides the external and middle ear.
- Ossicles: Three small bones (malleus, incus, stapes) that transmit sound waves to the inner ear.
Inner Ear
- Cochlea: Contains the hearing nerves.
- Semicircular Canals: Contain receptors for balance.
The Mechanism of Hearing
The Organ of Corti is the sensory epithelium within the cochlea where hair cells and nerve fibers interact to facilitate hearing. The structures of the cochlea vibrate in response to sound with a particular vibratory pattern, allowing the inner hair cells and their connections to the auditory nerve to send signals to the brainstem and brain regarding the sound’s vibration and frequency content.
The Vestibular System (Balance)
The vestibular system helps maintain a sense of balance. The Peripheral Vestibular Structures (PVS) are the five organs that sense posture, position, and movement. The Central Vestibular System (CVS) processes balance signals sent from the vestibular organs.
Chemical Senses: Smell and Taste
Major Structures Involved in Smell (Olfaction)
Nasal Cavity
Its role is to humidify and warm the air and remove airborne particles and debris before air reaches other airways. It communicates with the external environment.Olfactory Epithelium
A sheet of neurons and supporting cells that lines part of the nasal cavities. It secretes mucus that traps and neutralizes harmful particles.Olfactory Receptor Cells
A bipolar cell that gives rise to an unmyelinated axon at its basal surface and a process that expands into a thick layer of mucus.Olfactory Bulb
An array of neuropil that lies beneath the surface of the bulb that transduces and relays odorant information from the receptor neurons.Odor molecules bind to a receptor, initiating an electrical signal that travels from the sensory neuron to the olfactory bulb. A collection of neurons located behind the bulb works to identify the smell.
Major Structures Involved in Taste (Gustation)
Tongue
A muscular organ in the mouth that aids chewing, speaking, and breathing. It is made of strong tissue and a moist pink lining, containing papillae and taste buds.Taste Buds
Cells located all over the tongue that enable taste perception. They are made up of taste receptor cells, basal cells, and supporting cells.Taste Receptor Cells
Connect to nerves that transmit taste signals to the brain, which registers the chemical encountered.Gustatory Nerves
Three cranial nerves are associated with taste transmission:- The Facial Nerve (CN VII): Provides fibers to the anterior two-thirds of the tongue.
- The Glossopharyngeal Nerve (CN IX): Provides fibers to the posterior third of the tongue.
- The Vagus Nerve (CN X): Provides fibers to the epiglottis region.
Sensory System Disorders
Vision Disorders
Myopia (Nearsightedness)
Impaired distance vision occurring when problems with the shape of the cornea or lens cause light to focus in front of the retina, resulting in blurriness.Hyperopia (Farsightedness)
Caused by a relatively short eyeball or a flatter cornea, causing the focal point to be misaligned.Presbyopia
Impaired near vision (difficulty seeing things up close) that happens when the lens hardens and loses its ability to focus light on the retina.Nyctalopia (Night Blindness)
Symptoms that make it hard to see in dark places or adjust to light and dark, often caused by the retinas working improperly.Astigmatism
A type of refractive error where the cornea or lens is irregularly curved, causing blurry vision.Conjunctivitis (Pink Eye)
Inflammation that occurs when the membrane covering the white of the eye is irritated, often from an infection.Color Blindness
A deficiency that happens because specialized nerve cells (cones) in the retinas are missing or do not work properly.Hearing Disorders
Otitis Externa (Swimmer’s Ear)
An outer ear infection, often caused when water trapped in the ear canal creates an environment for bacteria and fungi to thrive.Otitis Media
A middle ear infection occurring when bacteria infect the space behind the eardrum, often traveling through the Eustachian tube.Types of Deafness/Hearing Loss
- Conductive Hearing Loss: Keeps sounds from passing through the ear canal or the middle ear.
- Sensorineural Hearing Loss: Happens when something damages the inner ear (cochlea or auditory nerve).
- Mixed Hearing Loss: Occurs when there are issues in both the middle/outer ear and the inner ear.
Smell and Taste Disorders
Anosmia
The inability to detect odors, resulting from interference with signals sent from the scent cells to the brain.Dysosmia
A distorted sense of smell where the olfactory system sends incorrect signals.Dysgeusia
A taste disorder causing people to perceive all foods as tasting metallic, sweet, sour, or bitter, due to changes in the nerve functions responsible for taste perception.The Endocrine System
Three Classes of Hormones
Peptide Hormones
Consist of multiple amino acids (e.g., some are produced by the heart to decrease blood pressure).Steroid Hormones
Derived from lipid cholesterol (e.g., hormones from the reproductive organs) and have a much more complex structure.Amine Hormones
Synthesized from amino acids tryptophan or tyrosine (e.g., melatonin, which helps regulate circadian rhythm).Hormone Action Mechanisms
- Nuclear Hormones: Regulate the transcription of target genes by binding to regulatory sequences.
- Cytoplasmic Hormones: Bind ligands, such as steroid hormones, within the cytoplasm.