Nervous System Pt 1 AUDIO
Overview of the Nervous System
Perception & Experience:
Controls perception and experience of the world.
Central to personality and consciousness.
Involves learning and memory formation.
Interaction Between Nervous and Endocrine Systems
Voluntary Nature:
Experience of the world is a voluntary event.
Homeostasis:
Nervous system and endocrine system work together to maintain homeostasis.
Regulates respiratory rate, blood pressure, body temperature, sleep/wake cycles, and blood pH.
Central and Peripheral Nervous Systems (CNS vs PNS)
Central Nervous System (CNS):
Comprises the brain and spinal cord.
Contains neurons that regulate body functions.
Involves negative feedback loops and positive feedback loops.
Peripheral Nervous System (PNS):
Includes cranial and spinal nerves.
Important for nurses to memorize the 12 pairs of cranial nerves and 31 pairs of spinal nerves.
Functions of the Nervous System
Divisions of Functions:
Sensory Function:
Conducted by the sensory division of the PNS.
Integrative Function:
Performed by the CNS (brain). Includes analysis and interpretation of stimuli.
Motor Function:
Responses are carried out by the motor division of the PNS.
Involves voluntary and involuntary responses.
Types of Sensory and Motor Divisions in PNS
Sensory Divisions:
Somatic Sensory Division:
Carries signals to skeletal muscles, conveys information from joints, skin, and organs involved in vision, hearing, taste, and smell.
Visceral Sensory Division:
Carries signals from internal organs (heart, lungs, stomach, intestines, kidneys).
Motor Divisions:
Somatic Motor Division:
Controls voluntary skeletal muscles.
Visceral Motor Division (Autonomic Nervous System):
Controls involuntary smooth and cardiac muscles.
Structural Components of Neurons
Neurons:
Excitable cells that send and receive signals via action potentials.
Comprise a cell body (soma), dendrites (receiving signals), and axons (sending signals).
Dendrites and Axons:
Dendrites: carry signals toward the cell body.
Axons: carry signals away from the cell body.
Action Potentials:
Responsible for sensory, integrative, and motor functions. Long-lived cells that are significant in physiological response.
Types of Neurons
Structural Classifications:
Multipolar:
Most common, multiple dendrites, one axon (over 99% of neurons).
Bipolar:
One axon and one dendrite, often sensory (e.g., in eyes).
Pseudounipolar:
One part directs sensory stimuli to cell body, the other toward the spinal cord (detects touch, pressure, pain).
Functional Classification of Neurons
Afferent Neurons (Sensory):
Carries signals toward the CNS, often pseudounipolar or bipolar.
Efferent Neurons (Motor):
Carries signals away from CNS to muscles and glands, typically multipolar.
Neuroglial Cells
Role of Neuroglia:
Supports, protects neurons, and maintains extracellular environment. Do not transmit signals but perform supportive functions.
Undergo mitosis, fill gaps when neurons die.
There are six types:
CNS Neuroglia:
Astrocytes: Star-shaped, anchor neurons and blood vessels, maintain extracellular environment, assist in forming the blood-brain barrier, repair damaged tissue.
Oligodendrocytes: Wrap around axons in CNS, form myelin sheath.
Microglia: Phagocytes that clean up dead neurons and reduce inflammation.
Ependymal Cells: Ciliated cells that circulate cerebrospinal fluid (CSF).
PNS Neuroglia:
Schwann Cells: Myelinate axons, participate in regeneration, repair damaged axons.
Satellite Cells: Support cell bodies in ganglia, regulate extracellular environment.
Myelin Sheath
Formation:
Comprised of oligodendrocytes (CNS) and Schwann cells (PNS).
Myelin consists of phospholipids, cholesterol, and proteins that insulate axons to prevent ion movement.
Function:
Increases speed of action potential transmission (15-150 times faster) in myelinated axons compared to unmyelinated.
Regeneration of Nervous Tissue
Limited Regeneration:
CNS has minimal regeneration capabilities; oligodendrocytes inhibit growth and astrocytes create scar tissue.
PNS can regenerate if the cell body is intact, following a series of steps involving the formation of growth processes and tubes of Schwann cells.
Electrophysiology of Neurons
Responsive Nature: Neurons respond to stimuli (chemical signals, mechanical deformation), causing electrical changes.
Types of Potentials:
Local Potentials: Short-distance potentials, can be depolarizing (less negative) or hyperpolarizing (more negative).
Action Potentials: Travel long distances along axons.
Membrane Potential:
Resting membrane potential is negative (-70 mV), defines polarized states.
Local potentials can change resting membrane potential to be less polarized or hyperpolarized.
Ion Channels and Gradients
Channel Types:
Ligand-Gated: Open in response to specific chemicals.
Voltage-Gated: Open/close based on membrane potential changes.
Mechanically Gated: Open/close in response to mechanical changes (pressure, stretching).
Sodium-Potassium Pump: Moves two potassium ions into the cytosol and three sodium ions out, maintaining concentration gradients critical for action potentials.
Action Potential Phases:
Depolarization: Influx of sodium ions (Na+).
Repolarization: Return to resting potential.
Hyperpolarization: Increase in negativity beyond resting potential, often due to potassium ions (K+) leaving.
Summary
Understanding the structure and function of the nervous system, the roles of different neuron types, the importance of myelin, and the principles of neuronal communication is essential for success in the field of biology and healthcare.
Upcoming lessons will cover action potentials in detail, including triggering mechanisms, conduction along axons, and implications for neurotransmission.
Overview of the Nervous System
The nervous system is fundamental to our perception and experience of the world, playing a central role in personality, consciousness, learning, and memory formation.
Interaction Between Nervous and Endocrine Systems
The experience of the world is a voluntary event, and closely intertwined with this is the nervous system's interaction with the endocrine system. Together, they are crucial for maintaining homeostasis, regulating vital functions such as respiratory rate, blood pressure, body temperature, sleep/wake cycles, and blood pH.
Central and Peripheral Nervous Systems (CNS vs PNS)
The nervous system is broadly divided into the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS consists of the brain and spinal cord, housing neurons responsible for regulating various body functions through complex negative and positive feedback loops. The PNS, on the other hand, encompasses the cranial and spinal nerves; specifically, there are 12 pairs of cranial nerves and 31 pairs of spinal nerves, which are important for healthcare professionals like nurses to memorize.
Functions of the Nervous System
The functions of the nervous system are categorized into three main divisions. The sensory function, carried out by the sensory division of the PNS, involves sensing stimuli. The integrative function, performed by the CNS, specifically the brain, involves the analysis and interpretation of these stimuli. Finally, the motor function, executed by the motor division of the PNS, generates responses, which can be both voluntary and involuntary.
Types of Sensory and Motor Divisions in PNS
Within the PNS, both sensory and motor divisions are further categorized. The sensory divisions include the Somatic Sensory Division, which transmits signals to skeletal muscles and relays information from joints, skin, and special senses like vision, hearing, taste, and smell. The Visceral Sensory Division carries signals from internal organs such as the heart, lungs, stomach, intestines, and kidneys. Correspondingly, the motor divisions comprise the Somatic Motor Division, responsible for controlling voluntary skeletal muscles, and the Visceral Motor Division, also known as the Autonomic Nervous System, which manages involuntary smooth and cardiac muscles.
Structural Components of Neurons
Neurons are excitable cells specialized in sending and receiving signals through action potentials, which are crucial for sensory, integrative, and motor functions. Each neuron typically consists of a cell body (soma), dendrites that receive incoming signals and carry them towards the cell body, and axons that transmit signals away from the cell body. Neurons are long-lived and play a significant role in physiological responses.
Types of Neurons
Neurons can be structurally classified into three main types. Multipolar neurons are the most common, featuring multiple dendrites and a single axon, constituting over 99% of all neurons. Bipolar neurons possess one axon and one dendrite, often found in sensory organs like the eyes. Pseudounipolar neurons have a single process that divides into two, with one part directing sensory stimuli to the cell body and the other toward the spinal cord, responsible for detecting touch, pressure, and pain.
Functional Classification of Neurons
Functionally, neurons are classified as either afferent (sensory) or efferent (motor). Afferent neurons carry signals towards the CNS and are often pseudounipolar or bipolar in structure. Conversely, efferent neurons transmit signals away from the CNS to muscles and glands and are typically multipolar.
Neuroglial Cells
Neuroglial cells play a crucial supportive and protective role within the nervous system, maintaining the extracellular environment without transmitting signals themselves. Unlike neurons, they can undergo mitosis and fill gaps left by dead neurons. There are six types of neuroglia, divided between the CNS and PNS. In the CNS, astrocytes are star-shaped cells that anchor neurons and blood vessels, maintain the extracellular environment, help form the blood-brain barrier, and repair damaged tissue. Oligodendrocytes wrap around axons to form the myelin sheath. Microglia act as phagocytes, cleaning up dead neurons and reducing inflammation, while ependymal cells are ciliated cells that circulate cerebrospinal fluid (CSF). In the PNS, Schwann cells myelinate axons and participate in regeneration and repair of damaged axons, and satellite cells support neuron cell bodies in ganglia and regulate their extracellular environment.
Myelin Sheath
The myelin sheath, formed by oligodendrocytes in the CNS and Schwann cells in the PNS, is composed of phospholipids, cholesterol, and proteins. Its primary function is to insulate axons, preventing ion movement across the membrane and significantly increasing the speed of action potential transmission. Myelinated axons can transmit action potentials 15 to 150 times faster than unmyelinated ones.
Regeneration of Nervous Tissue
The regeneration capabilities of nervous tissue are highly limited, especially in the CNS, where oligodendrocytes inhibit growth and astrocytes form scar tissue, preventing repair. In contrast, the PNS can regenerate, provided the neuron's cell body remains intact. This regeneration involves a sequential process, including the formation of growth processes and the guidance provided by tubes of Schwann cells.
Electrophysiology of Neurons
Neurons exhibit a responsive nature, reacting to various stimuli, such as chemical signals or mechanical deformation, by undergoing electrical changes. These changes manifest as different types of potentials: local potentials, which are short-distance electrical changes that can either depolarize (make the membrane less negative) or hyperpolarize (make it more negative), and action potentials, which can travel long distances along axons. The neuron's membrane potential, typically resting at negative (a polarized state), can be altered by local potentials. Key to these electrical events are ion channels, which can be ligand-gated (opening in response to specific chemicals), voltage-gated (opening/closing based on membrane potential changes), or mechanically gated (responding to pressure or stretching). The sodium-potassium pump is vital for maintaining the necessary concentration gradients for action potentials by moving two potassium ions into and three sodium ions out of the cytosol. The action potential itself proceeds through distinct phases: depolarization, characterized by an influx of sodium ions (); repolarization, where the membrane returns to its resting potential; and hyperpolarization, an increase in negativity beyond the resting potential, often due to the efflux of potassium ions ().
Summary
A comprehensive understanding of the nervous system's structure and function, including the various neuron types, the critical role of myelin, and the fundamental principles of neuronal communication, is indispensable for professionals in biology and healthcare. Future lessons will delve deeper into action potentials, exploring their triggering mechanisms, conduction along axons, and their broader implications for neurotransmission.