💀Anatomy and Physiology I Unit 12 – The Nervous System: Tissue & Function
The nervous system is a complex network of cells and tissues that control our body's functions. It's divided into the central nervous system (brain and spinal cord) and peripheral nervous system (nerves outside the CNS). Neurons, the main cells of the nervous system, communicate through electrical and chemical signals.
Nervous tissue is organized into gray and white matter, with supporting cells called neuroglia. The system processes sensory input, integrates information, and generates motor output. Understanding its structure and function is crucial for diagnosing and treating neurological disorders, from neurodegenerative diseases to peripheral neuropathies.
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
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