Nervous System
Nervous System Divisions:
Nervous system is divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS).
CNS consists of the brain and spinal cord, which are responsible for processing information and coordinating responses.
PNS connects the CNS to the rest of the body and includes sensory and motor neurons.
PNS can be divided into the somatic nervous system, which controls voluntary movements, and the autonomic nervous system, which regulates involuntary functions such as heart rate and digestion.
Autonomic nervous system is further categorized into the sympathetic (fight-or-flight) and parasympathetic divisions (rest-and-digest), which work together to maintain homeostasis.
Careers in Neuroscience:
Principal Investigator (PI): Person in charge of a scientific research grant or clinical trial. The PI is considered the lead researcher, and their role is to manage and maintain the integrity of the study being conducted. Sometimes there is more than one PI.
Lab Manager: Person who generally oversees the lab, scheduling, ordering supplies, maintenance of equipment, and other tasks to keep the lab operating smoothly.
Lab Technicians: People responsible for carrying out the hands-on work in the lab. They may collect specimens or samples, perform tests, calibrate equipment, collect data, or other tasks based on the work or research being conducted.
Postdoctoral Students: PhD graduates continuing their education, research, and training beyond their doctoral studies.
CNS: Parts of the Brain
Outer Parts:
Cerebrum: Largest brain part, managing thought, action, and emotion; splits into left and right hemispheres.
Cerebellum: Located below the cerebrum, it coordinates balance and fine motor skills.
Frontal lobe: Front cerebrum part, handles reasoning, planning, and voluntary movement control.
Motor cortex: Frontal lobe area crucial for planning and executing voluntary movements.
Parietal lobe: Behind the frontal lobe, processes sensory details like touch and spatial awareness.
Sensory cortex: Part of the parietal lobe processing body sensory input for touch and temperature perception.
Temporal lobe: Below the lateral fissure, involved in auditory processing, language, and memory.
Occipital lobe: Back cerebrum portion responsible for visual information processing.
Pons: Structure connecting parts of the nervous system, regulates breathing and ferrying signals between cerebellum and cerebrum.
Brain stem: Connects cerebrum and spinal cord, controlling vital functions like heart rate and respiration.
Medulla oblongata: Part of the brain stem managing autonomic functions like heart rate and reflexes.
Gyri/gyrus: Raised brain ridges increasing cerebral cortex surface area for cognitive functions.
Sulci/sulcus: Grooves between gyri also enhancing surface area and organizing brain functions.
The bends and folds increase the surface area of the brain, allowing many more neurons to be packed into the overall structure, which increase the brains ability to process information.
Inner Parts:
Corpus callosum: A thick band of nerve fibers connecting the left and right hemispheres of the brain, facilitating interhemispheric communication and coordination of sensory and motor functions.
Thalamus: A relay station for sensory and motor signals; plays a crucial role in processing and transmitting information to appropriate areas of the cerebral cortex.
Hypothalamus: Responsible for regulating various autonomic functions, including temperature control, hunger, thirst, circadian rhythms, and managing the endocrine system through its influence on the pituitary gland.
Hippocampus: A critical structure involved in the formation of new memories and associated with learning and emotions, playing a key role in converting short-term memory to long-term memory.
Pituitary gland: Often referred to as the "master gland," it regulates vital hormonal functions and controls various endocrine glands throughout the body, influencing growth, metabolism, and reproductive processes.
Amygdala: Involved in processing emotions, particularly fear and pleasure, and plays a significant role in the formation of emotional memories.
7 Major Nerves:
Brachial plexus: A network of nerves that originates from the spinal cord in the neck region, responsible for innervating the upper limb and providing motor and sensory functions to the shoulder, arm, and hand.
Radial nerve: A major nerve that branches from the brachial plexus, responsible for innervating the muscles of the posterior arm and forearm, allowing for extension of the elbow, wrist, and fingers.
Ulnar nerve: A major nerve that also branches from the brachial plexus, primarily responsible for innervating the muscles of the anterior forearm and intrinsic muscles of the hand, facilitating movements such as flexion and fine motor skills.
Spinal nerves: The spinal nerves emerge from the spinal cord and are responsible for transmitting sensory and motor information between the central nervous system and the rest of the body, playing a crucial role in reflex actions and voluntary movements.
Median nerve: Responsible for the sensation and movement in the hand and forearm, particularly the thumb, index, and middle fingers.
Sciatic nerve: A large nerve that originates from the lumbosacral plexus, it innervates the posterior thigh, lower leg, and foot, playing a vital role in leg movement and sensation.
Cranial nerves: These nerves emerge directly from the brain and are primarily responsible for sensory and motor functions in the head and neck, including vision, smell, taste, and facial movements.
The 12 Cranial nerves:
Cranial Nerve I: Also called the olfactory nerve, it relays information about the sense of smell.
Cranial Nerve II: Also called the optic nerve, it relays information about the sense of sight.
Cranial Nerve III: Also called the oculomotor nerve, it relays information so the eye can move.
Cranial Nerve IV: Also called the trochlear nerve, it enables eye movement. Specifically, it controls the ability to look down and move your eyes toward your nose.
Cranial Nerve V: Also called the trigeminal nerve, it is in charge of sensation to the face and controls the muscles of mastication (chewing).
Cranial Nerve VI: Also called the abducens nerve, it enables eye movement. Specifically, it controls the ability to move your eyes away from your nose.
Cranial Nerve VII: Also called the facial nerve, it is in charge of expression. In Unit 1, you learned about Bell’s palsy. This temporary facial palsy is often caused by injury or disease to the facial nerve.
Cranial Nerve VIII: Also called the vestibulocochlear nerve, it relays hearing information and is in charge of balance.
Cranial Nerve IX: Also called the glossopharyngeal nerve, it is responsible for oral sensation, taste, and salivation.
Cranial Nerve X: Also called the vagus nerve, it is the longest nerve in the body and controls heart rate and blood pressure.
Cranial Nerve XI: Also called the accessory nerve, it controls shoulder elevation and head-turning.
Cranial Nerve XII: Also called the hypoglossal nerve, it controls tongue movement.
Why do biomedical scientists do dissections: Dissections allow biomedical scientists to gain a deeper understanding of the anatomical structures and functions of the nervous system, enabling them to observe the intricate relationships between nerves, muscles, and organs.
Basic Neuron Design:
Structural Classification of Neurons:
Unipolar Neurons
Purpose: Transmit sensory information, especially touch, pain, and temperature.
Location: Found primarily in sensory neurons of the Peripheral Nervous System (PNS), such as dorsal root ganglia.
Abundance: Common in invertebrates, but rare in vertebrates (only found in sensory neurons).
Appearance: Have one process extending from the cell body that splits into two branches, one acting like a dendrite and the other like an axon.
Multipolar Neurons
Purpose: Responsible for motor control and integrating information from other neurons.
Location: Found in the Central Nervous System (CNS), including the brain and spinal cord, and in motor neurons that control muscles.
Abundance: Most common type in vertebrates but less common in invertebrates.
Appearance: Have many dendrites and one axon, allowing for complex information processing.
Pseudounipolar Neurons
Purpose: Carry sensory signals from the PNS to the CNS, particularly for pain, touch, and proprioception (body position awareness).
Location: Found in sensory ganglia (clusters of nerve cell bodies), such as dorsal root ganglia in the spinal cord.
Abundance: Common in vertebrates as sensory neurons, rare in invertebrates.
Appearance: Start as bipolar neurons during development, but later, their dendrite and axon fuse into a single elongated process that carries signals.
Bipolar Neurons
Purpose: Specialize in processing sensory input, particularly for vision, smell, and hearing.
Location: Found in sensory organs like the retina (eyes), olfactory epithelium (nose), and inner ear (cochlea & vestibular system).
Abundance: Rare in both vertebrates and invertebrates, found only in specialized sensory pathways.
Appearance: Have one dendrite and one axon extending from opposite sides of the cell body, giving them a symmetrical look.
Functional Classification of Neurons:
Motor Neurons (Efferent Neurons)
Purpose: Send commands from the CNS to muscles and glands, causing movement or secretion.
Location: Found in the spinal cord and brainstem, with axons extending to muscles and glands throughout the body.
Abundance: Common in vertebrates and invertebrates, as all animals need movement control.
Appearance: Typically multipolar, with a large cell body, many dendrites, and a long axon to reach distant muscles.
Sensory Neurons (Afferent Neurons)
Purpose: Detect external (touch, temperature, pain) and internal (blood pressure, oxygen levels) stimuli and send signals to the CNS.
Location: Found in the PNS, especially in sensory organs, skin, and internal tissues, with cell bodies in sensory ganglia near the spinal cord.
Abundance: Common in both vertebrates and invertebrates, as all organisms need sensory input.
Appearance: Usually unipolar or pseudounipolar, with one long extension to carry signals quickly.
Interneurons (Association Neurons)
Purpose: Connect sensory and motor neurons, process information, and facilitate reflexes and decision-making.
Location: Found entirely in the CNS (brain and spinal cord), acting as signal relays between neurons.
Abundance: Most numerous neuron type in vertebrates, but less common in invertebrates due to simpler nervous systems.
Appearance: Typically multipolar, with many short dendrites and a short axon, optimized for quick signal transmission within the CNS.
Why there are different types of neurons: Diversity of neuron types enables the nervous system to adapt to various stimuli, ensuring that organisms can respond appropriately to their environment.
Glial cells (neuroglia): Cells in the nervous system that provide protection and maintain homeostasis for neurons; have no axons or dendrites and cannot generate action potentials or nerve impulses.
Function: Glial cells support neurons by providing structural support, insulating them, and facilitating communication between neurons; they also play a role in the repair and regeneration of nervous tissue.
Different types of glial cells:
Astrocytes: Star-shaped glial cells that provide structural support, regulate blood flow, and maintain the blood-brain barrier.
Oligodendrocytes: Cells that produce myelin in the central nervous system, which insulates axons and enhances signal transmission.
Satellite cells: Glial cells located in the peripheral nervous system that support neuron function and maintain the microenvironment around sensory neurons.
Microglia: Immune cells of the central nervous system that act as the first line of defense against pathogens and help remove debris from damaged neurons.
Schwann cells: Glial cells that form myelin sheaths around axons in the peripheral nervous system, aiding in faster nerve impulse conduction.
Microglia: Immune cells of the central nervous system that respond to injury and disease by removing debris and pathogens.
Why there are different glial cells: Each type of glial cell has specialized functions that support neuronal health, facilitate communication, and maintain homeostasis within the nervous system.
Improper functioning of glial cells can lead to significant issues, including brain tumors from mutations in glial cells and neurodegenerative diseases like multiple sclerosis, which involves damage to myelin sheaths produced by Schwann cells.
Major Parts of a Neuron:
Axon: A long projection that transmits electrical impulses away from the cell body to other neurons or muscles.
Axon terminals: The end points of an axon where neurotransmitters are released to communicate with other cells.
Cell body (soma): Contains the nucleus and organelles, responsible for the overall metabolic functions of the neuron.
Cell membrane: Flexible barrier around the neuron that controls what enters and exits the cell; helps maintain the neuron’s resting state and aids in the transmission of electrical signals (action potentials).
Dendrites: Branch-like structures that receive signals from other neurons and transmit them to the cell body.
Myelin sheath: A protective layer that surrounds the axon of some neurons, facilitating faster signal transmission through electrical insulation.
Neurotransmitter: Chemical messengers that transmit signals across the synapse from one neuron to another, playing a crucial role in modulating various physiological functions and behaviors.
Nodes of Ranvier: Small gaps in the myelin sheath where ion exchange occurs, allowing for the rapid conduction of nerve impulses through saltatory conduction.
Schwann cell: A type of glial cell that wraps around axons in the peripheral nervous system, providing insulation and facilitating the regeneration of damaged nerve fibers.
Synapse: The junction between two neurons where neurotransmitters are released, allowing for communication between the nerve cells and the continuation of nerve impulses.
Action potential: A rapid rise and fall in voltage across a cellular membrane, which propagates along the axon and is essential for the transmission of nerve signals.
Action Potential:
An action potential is an electrical signal that travels down a neuron to send information.
At rest, the inside of a neuron is negative, and the outside is positive because of the way sodium (Na⁺) and potassium (K⁺) ions are distributed. When a neuron receives a strong enough signal, sodium channels open, and Na⁺ enters the neuron, making the inside positive. This is called depolarization.
Then, potassium channels open, and K⁺ leaves the neuron, bringing the charge back to normal. This is called repolarization. After that, the neuron returns to its resting state and is ready to send another signal.
This process happens very fast and allows neurons to communicate with each other and with muscles.
Resting potential: The electrical potential of a neuron when it is not actively transmitting a signal, typically around -70 mV, which is maintained by the sodium-potassium pump.
Depolarization: The membrane potential becomes less negative (more positive) when sodium ions flow into the neuron, which starts an action potential.
Hyperpolarization: When the inside of the neuron becomes more negative than its normal resting state. It often happens when potassium ions leave the neuron or chloride ions enter it, making it harder for the neuron to send a signal.
Repolarization: After depolarization, this phase occurs when the membrane potential becomes more negative again. This happens mainly because potassium ions leave the neuron, helping to return it to its resting state and getting it ready for the next action potential.
Polarization: Process of establishing a difference in electrical charge across the neuronal membrane, which is crucial for the generation and propagation of action potentials.
Refractory period: The time following an action potential during which a neuron is unable to fire another action potential, ensuring that signals only travel in one direction and allowing the neuron to recover.
What do sodium, potassium, and chloride channels do during action potential?
During action potential, sodium channels open rapidly, allowing Na+ ions to flow into the neuron, causing depolarization.
Potassium channels open, permitting K+ ions to exit the neuron, which helps repolarize and restore the resting potential.
Chloride channels also play a role in stabilizing the membrane potential by allowing Cl- ions to enter, contributing to the overall excitability and inhibition of the neuron.
Each of these ions contributes to the dynamic balance of excitatory and inhibitory signals that regulate neuronal firing.
The precise timing of these ion channel openings and closings is crucial for the propagation of action potentials along the axon, ensuring rapid communication between neurons.