💀Anatomy and Physiology I Unit 12 – The Nervous System: Tissue & Function

Key Concepts and Terminology

• Action potential: Rapid, transient change in the membrane potential of a neuron or muscle cell, allowing for the transmission of electrical signals along the cell membrane
• Neurotransmitters: Chemical messengers released by neurons at synapses to communicate with target cells (other neurons, muscles, or glands)
  • Includes acetylcholine, dopamine, serotonin, and norepinephrine
• Synapse: Junction between two neurons or between a neuron and a target cell, where chemical or electrical communication occurs
• Myelin sheath: Insulating layer formed by Schwann cells (PNS) or oligodendrocytes (CNS) around axons, facilitating rapid signal transmission
  • Composed of lipids and proteins
• Resting membrane potential: Difference in electrical charge across a neuron's membrane when the cell is not actively transmitting a signal (typically -70 mV)
• Saltatory conduction: Rapid propagation of action potentials along myelinated axons, as the signal "jumps" from one node of Ranvier to the next
• Neuroglia: Non-neuronal cells in the nervous system that provide support, protection, and maintenance for neurons
  • Includes astrocytes, oligodendrocytes, microglia, and ependymal cells in the CNS; Schwann cells and satellite cells in the PNS

Nervous System Overview

• Nervous system divided into two main parts: central nervous system (CNS) consisting of the brain and spinal cord, and peripheral nervous system (PNS) comprising nerves outside the CNS
• PNS further divided into somatic nervous system (voluntary control of skeletal muscles) and autonomic nervous system (involuntary control of smooth muscles, cardiac muscle, and glands)
  • Autonomic nervous system has sympathetic (fight-or-flight response) and parasympathetic (rest-and-digest functions) divisions
• Nervous tissue composed of neurons (nerve cells) and neuroglia (supporting cells)
• Nervous system functions include sensory input, integration of information, motor output, and regulation of bodily functions
• Neurons communicate through electrical and chemical signals at specialized junctions called synapses
• Nervous system develops from the ectoderm layer during embryonic development
  • Neural tube forms the CNS, while neural crest cells give rise to parts of the PNS

Neurons: Structure and Types

• Neurons are specialized cells that transmit electrical and chemical signals in the nervous system
• Main parts of a neuron: cell body (soma), dendrites, and axon
  • Cell body contains the nucleus and organelles for protein synthesis and cellular metabolism
  • Dendrites are branched extensions that receive signals from other neurons
  • Axon is a long, thin extension that conducts electrical signals away from the cell body and releases neurotransmitters at synapses
• Three main types of neurons based on structure and function: sensory neurons, motor neurons, and interneurons
  • Sensory neurons (afferent neurons) convey information from sensory receptors to the CNS
  • Motor neurons (efferent neurons) transmit signals from the CNS to effector cells (muscles or glands)
  • Interneurons are found within the CNS and form connections between sensory and motor neurons, allowing for signal integration and processing
• Neurons can also be classified based on the number of processes extending from the cell body: unipolar, bipolar, and multipolar
• Axons are often covered by a myelin sheath, which is formed by Schwann cells in the PNS and oligodendrocytes in the CNS
  • Myelin sheath provides insulation and facilitates rapid signal transmission through saltatory conduction

Neuroglia: Supporting Cells

• Neuroglia are non-neuronal cells that provide support, protection, and maintenance for neurons in the nervous system
• Types of neuroglia in the CNS:
  • Astrocytes: Regulate neurotransmitter levels, maintain blood-brain barrier, and provide metabolic support for neurons
  • Oligodendrocytes: Form myelin sheaths around axons in the CNS, facilitating rapid signal transmission
  • Microglia: Immune cells of the CNS that protect against pathogens and remove cellular debris
  • Ependymal cells: Line the ventricles of the brain and central canal of the spinal cord, producing cerebrospinal fluid (CSF)
• Types of neuroglia in the PNS:
  • Schwann cells: Form myelin sheaths around axons in the PNS and aid in nerve regeneration
  • Satellite cells: Surround neuron cell bodies in ganglia, providing structural and metabolic support
• Neuroglia outnumber neurons in the nervous system and play crucial roles in maintaining homeostasis and proper neural function
• Neuroglia can respond to injury or damage in the nervous system by forming glial scars, which can inhibit nerve regeneration

Nervous Tissue Organization

• Nervous tissue is organized into two main components: gray matter and white matter
  • Gray matter consists primarily of neuron cell bodies, dendrites, and unmyelinated axons
  • White matter is composed mainly of myelinated axons, giving it a lighter appearance due to the lipid content of myelin
• In the CNS, gray matter is found in the cerebral cortex, cerebellar cortex, and nuclei deep within the brain and spinal cord
  • White matter is located beneath the gray matter in the cerebrum and cerebellum and surrounds the gray matter in the spinal cord
• PNS nerves are composed of bundles of axons called fascicles, surrounded by connective tissue layers (endoneurium, perineurium, and epineurium)
• Ganglia are clusters of neuron cell bodies in the PNS, found along the path of nerves or near organs
  • Sensory ganglia contain cell bodies of sensory neurons (dorsal root ganglia, trigeminal ganglia)
  • Autonomic ganglia contain cell bodies of motor neurons in the autonomic nervous system (sympathetic chain ganglia, parasympathetic ganglia)
• Nervous tissue is highly specialized and requires a constant supply of oxygen and glucose for proper function
  • Blood-brain barrier and blood-nerve barrier regulate the exchange of substances between the bloodstream and nervous tissue

Synapses and Neurotransmission

• Synapses are specialized junctions between neurons or between a neuron and a target cell (muscle or gland) where communication occurs
• Two main types of synapses: chemical synapses and electrical synapses
  • Chemical synapses are more common and involve the release of neurotransmitters from the presynaptic neuron to bind to receptors on the postsynaptic cell
  • Electrical synapses are gap junctions that allow direct electrical coupling between cells, facilitating rapid signal transmission
• At a chemical synapse, the presynaptic neuron releases neurotransmitters from synaptic vesicles into the synaptic cleft
  • Neurotransmitters diffuse across the cleft and bind to specific receptors on the postsynaptic cell membrane
• Binding of neurotransmitters to receptors can result in excitatory or inhibitory postsynaptic potentials (EPSPs or IPSPs), depending on the neurotransmitter and receptor type
  • EPSPs depolarize the postsynaptic cell, increasing the likelihood of an action potential
  • IPSPs hyperpolarize the postsynaptic cell, decreasing the likelihood of an action potential
• Neurotransmitters are cleared from the synaptic cleft by enzymatic degradation, reuptake into the presynaptic neuron, or uptake by glial cells
• Synaptic plasticity refers to the ability of synapses to strengthen or weaken over time in response to activity, which is the basis for learning and memory
  • Long-term potentiation (LTP) and long-term depression (LTD) are examples of synaptic plasticity mechanisms

Nervous System Function and Integration

• The nervous system's primary functions are to receive sensory input, process and integrate information, and generate appropriate motor output or physiological responses
• Sensory receptors detect various stimuli (touch, temperature, pain, light, sound, chemicals) and convert them into electrical signals (receptor potentials)
  • Sensory information is relayed to the CNS via afferent neurons for processing and integration
• CNS integrates sensory information, compares it with stored memories, and makes decisions based on the current context and goals
  • Involves complex neural circuits and networks in the brain and spinal cord
• Motor output is generated by the CNS and transmitted via efferent neurons to effector cells (muscles or glands) to produce a response
  • Somatic nervous system controls voluntary movements of skeletal muscles
  • Autonomic nervous system regulates involuntary functions of smooth muscles, cardiac muscle, and glands
• Nervous system maintains homeostasis by continuously monitoring internal and external environments and adjusting physiological processes accordingly
  • Examples include regulation of heart rate, blood pressure, respiration, digestion, and body temperature
• Higher-order functions of the nervous system include cognition, emotion, memory, and learning
  • Involves complex interactions between various regions of the brain, such as the cerebral cortex, limbic system, and basal ganglia

Clinical Applications and Disorders

• Nervous system disorders can result from damage to neurons, neuroglia, or nerves, leading to impaired function
• Neurodegenerative diseases involve progressive loss of neurons, leading to cognitive and motor deficits
  • Examples include Alzheimer's disease (loss of neurons in the cerebral cortex and hippocampus), Parkinson's disease (loss of dopaminergic neurons in the substantia nigra), and Huntington's disease (loss of neurons in the basal ganglia and cerebral cortex)
• Demyelinating disorders, such as multiple sclerosis, involve damage to the myelin sheath, disrupting signal transmission and causing neurological symptoms
• Peripheral neuropathies are caused by damage to nerves in the PNS, leading to sensory and motor deficits
  • Can be caused by diabetes, vitamin deficiencies, autoimmune disorders, or toxins
• Neuromuscular disorders affect the communication between motor neurons and muscles, resulting in muscle weakness or paralysis
  • Examples include myasthenia gravis (autoimmune attack on acetylcholine receptors) and amyotrophic lateral sclerosis (ALS, progressive degeneration of motor neurons)
• Neuroimaging techniques, such as MRI, CT, and PET scans, allow for non-invasive visualization of the nervous system and aid in the diagnosis of disorders
• Pharmacological treatments for nervous system disorders often target neurotransmitter systems to alleviate symptoms or slow disease progression
  • Examples include acetylcholinesterase inhibitors for Alzheimer's disease, levodopa for Parkinson's disease, and immunomodulatory drugs for multiple sclerosis
• Advances in neuroscience research continue to provide insights into the functioning of the nervous system and the development of new therapies for neurological disorders