🔬General Biology I Unit 35 – The Nervous System

Nervous System Basics

• Coordinates and controls body functions, enabling organisms to respond and adapt to their environment
• Consists of the brain, spinal cord, and a complex network of nerves that extend throughout the body
• Receives and processes sensory information from internal and external stimuli (touch, sight, hearing)
• Transmits signals to muscles, glands, and organs to initiate appropriate responses
• Divided into two main components: the central nervous system (CNS) and the peripheral nervous system (PNS)
  • CNS includes the brain and spinal cord, serving as the main control center
  • PNS consists of nerves that connect the CNS to the rest of the body
• Communicates using electrical and chemical signals called nerve impulses or action potentials
• Relies on specialized cells called neurons to transmit signals rapidly and efficiently

Neurons: Structure and Function

• Specialized cells that transmit electrical and chemical signals throughout the nervous system
• Consist of three main parts: cell body (soma), dendrites, and axon
  • Cell body contains the nucleus and other organelles essential for cellular functions
  • Dendrites are branched extensions that receive signals from other neurons
  • Axon is a long, thin fiber that conducts electrical impulses away from the cell body
• Classified into three main types based on their function: sensory neurons, motor neurons, and interneurons
  • Sensory neurons detect stimuli from the environment and transmit signals to the CNS
  • Motor neurons carry signals from the CNS to muscles and glands, initiating responses
  • Interneurons form connections between sensory and motor neurons, processing and integrating information
• Generate and transmit electrical signals called action potentials along their axons
• Insulated by a fatty substance called myelin, which is produced by specialized cells (Schwann cells in the PNS, oligodendrocytes in the CNS)
  • Myelin sheath enhances signal transmission speed and efficiency
• Communicate with other neurons or target cells through specialized junctions called synapses

Synapses and Neurotransmission

• Synapses are specialized junctions where neurons communicate with each other or with target cells (muscles, glands)
• Consist of a presynaptic neuron (signal-sending), a postsynaptic cell (signal-receiving), and a synaptic cleft (narrow gap between them)
• Neurotransmission is the process by which signals are transmitted across synapses using chemical messengers called neurotransmitters
  • Action potential arrives at the presynaptic terminal, causing voltage-gated calcium channels to open
  • Calcium influx triggers the release of neurotransmitters from synaptic vesicles into the synaptic cleft
  • Neurotransmitters bind to specific receptors on the postsynaptic cell membrane, initiating a response (excitatory or inhibitory)
• Neurotransmitters can have excitatory effects (increasing the likelihood of an action potential) or inhibitory effects (decreasing the likelihood)
  • Examples of excitatory neurotransmitters include glutamate and acetylcholine
  • Examples of inhibitory neurotransmitters include GABA and glycine
• Neurotransmitters are cleared from the synaptic cleft by reuptake into the presynaptic neuron or degradation by enzymes
• Synaptic plasticity refers to the ability of synapses to strengthen or weaken over time, which is crucial for learning and memory

Central Nervous System (CNS)

• Consists of the brain and spinal cord, serving as the main control center for the nervous system
• Brain is the most complex organ in the body, responsible for higher cognitive functions, sensory processing, and motor control
  • Divided into three main regions: forebrain (cerebrum, thalamus, hypothalamus), midbrain, and hindbrain (pons, medulla oblongata, cerebellum)
  • Cerebrum is the largest part of the brain, involved in conscious thought, learning, memory, and sensory processing
  • Cerebellum coordinates muscle movements, balance, and posture
  • Brainstem (midbrain, pons, medulla oblongata) regulates vital functions (breathing, heart rate, blood pressure)
• Spinal cord is a long, thin bundle of nervous tissue that extends from the brainstem and runs through the vertebral column
  • Serves as a conduit for signals between the brain and the rest of the body
  • Contains neural circuits that control reflexes and coordinate simple movements
• Protected by the skull (brain) and vertebral column (spinal cord), as well as layers of membranes called meninges
• Surrounded by cerebrospinal fluid (CSF), which provides cushioning, nourishment, and waste removal

Peripheral Nervous System (PNS)

• Consists of nerves that connect the CNS to the rest of the body, transmitting signals to and from the brain and spinal cord
• Divided into two main components: the somatic nervous system and the autonomic nervous system
  • Somatic nervous system controls voluntary movements and receives sensory information from the external environment
  • Autonomic nervous system regulates involuntary functions (heart rate, digestion, respiration) and is further divided into the sympathetic and parasympathetic divisions
    • Sympathetic division activates the "fight or flight" response, increasing heart rate, blood pressure, and glucose release
    • Parasympathetic division promotes "rest and digest" functions, slowing heart rate, increasing digestion, and conserving energy
• Consists of 12 pairs of cranial nerves (originating from the brain) and 31 pairs of spinal nerves (originating from the spinal cord)
  • Cranial nerves primarily serve the head and neck region, controlling sensory and motor functions (vision, hearing, facial movements)
  • Spinal nerves innervate the trunk and limbs, transmitting sensory and motor signals
• Includes sensory receptors that detect various stimuli (touch, temperature, pain) and transmit signals to the CNS
• Plays a crucial role in maintaining homeostasis by coordinating organ systems and responding to changes in the internal and external environment

Sensory and Motor Pathways

• Sensory pathways transmit information from sensory receptors to the CNS for processing and interpretation
  • Sensory receptors detect stimuli (touch, temperature, light, sound) and convert them into electrical signals
  • Sensory neurons carry these signals to the spinal cord or brain for further processing
  • Sensory information is relayed through the thalamus to the appropriate cortical areas for conscious perception
• Motor pathways carry signals from the CNS to muscles and glands, initiating responses and controlling movement
  • Upper motor neurons originate in the motor cortex or brainstem and synapse with lower motor neurons in the spinal cord or cranial nerve nuclei
  • Lower motor neurons directly innervate muscles, causing them to contract or relax
  • Cerebellum and basal ganglia modulate motor commands, ensuring smooth, coordinated movements
• Reflex arcs are simple neural circuits that allow rapid, automatic responses to stimuli without conscious control
  • Involve sensory neurons, interneurons, and motor neurons in the spinal cord
  • Examples include the knee-jerk reflex and the withdrawal reflex (pulling away from a hot surface)
• Sensory and motor pathways work together to enable organisms to interact with their environment, respond to stimuli, and maintain homeostasis

Nervous System Disorders

• Neurodegenerative diseases involve the progressive loss of structure or function of neurons, often leading to cognitive and motor impairments
  • Examples include Alzheimer's disease (memory loss, cognitive decline), Parkinson's disease (tremors, rigidity, slowness of movement), and Huntington's disease (involuntary movements, cognitive decline)
• Neurodevelopmental disorders arise from abnormalities in brain development, often resulting in cognitive, social, or behavioral challenges
  • Examples include autism spectrum disorder (difficulties with social interaction and communication), attention deficit hyperactivity disorder (ADHD), and intellectual disability
• Neurological infections can be caused by viruses, bacteria, or other pathogens, leading to inflammation and damage to the nervous system
  • Examples include meningitis (inflammation of the meninges), encephalitis (inflammation of the brain), and polio (viral infection that can cause paralysis)
• Traumatic brain injury (TBI) results from a sudden, external force to the head, causing damage to brain tissue and disrupting normal function
  • Can lead to a range of symptoms, including loss of consciousness, memory problems, mood changes, and motor impairments
• Psychiatric disorders involve disturbances in thoughts, emotions, and behaviors that cause significant distress or impairment in daily functioning
  • Examples include depression, anxiety disorders, bipolar disorder, and schizophrenia
• Many nervous system disorders have complex causes involving genetic, environmental, and lifestyle factors
• Ongoing research aims to better understand the underlying mechanisms of these disorders and develop effective treatments and therapies

Key Takeaways and Clinical Applications

• The nervous system is a complex network of cells and tissues that coordinates and controls body functions, enabling organisms to respond and adapt to their environment
• Neurons are the fundamental units of the nervous system, transmitting electrical and chemical signals through specialized structures like dendrites, axons, and synapses
• The central nervous system (brain and spinal cord) serves as the main control center, processing information and generating appropriate responses
• The peripheral nervous system connects the CNS to the rest of the body, transmitting sensory and motor signals and maintaining homeostasis
• Sensory pathways convey information from the environment to the CNS, while motor pathways carry signals from the CNS to muscles and glands, initiating responses
• Nervous system disorders can arise from a variety of causes, including neurodegenerative diseases, neurodevelopmental abnormalities, infections, and traumatic injuries
• Understanding the structure and function of the nervous system is crucial for developing targeted therapies and interventions for neurological and psychiatric disorders
• Advances in neuroscience, such as brain imaging techniques and optogenetics, are providing new insights into the workings of the nervous system and potential treatment strategies
• Interdisciplinary collaborations between neuroscientists, clinicians, and engineers are driving innovations in neurorehabilitation, brain-computer interfaces, and personalized medicine approaches for nervous system disorders