Overview of the Nervous System

Overview of the Nervous System

• The nervous system and endocrine system work together to control body actions
• The nervous system transmits rapid electrical signals for quick responses
• The endocrine system uses slower chemical signals for prolonged responses
• The nervous system is crucial for immediate reactions to stimuli
• The endocrine system regulates longer-term processes
• Both systems are essential for maintaining homeostasis and responding to environmental changes

Organization of the Nervous System

• The nervous system is divided into two main parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS)
• The CNS consists of the brain and spinal cord, primarily made up of interneurons that coordinate information
• The PNS includes all other nerves, categorized into sensory (afferent) and motor (efferent) neurons
• Sensory neurons carry information from receptors to the CNS
• Motor neurons transmit signals from the CNS to effectors
• The PNS connects the CNS to the rest of the body, facilitating communication and response

Components of Neurons

• Neurons are specialized cells that transmit nerve impulses and depend on glial cells for support
• Glial cells nourish neurons, remove waste, and defend against infections
• Key neuron parts include the cell body, axon, dendrites, myelin sheath, axon hillock, axon terminals, and Nodes of Ranvier
• The myelin sheath, produced by Schwann cells, insulates the axon and increases the speed of signal transmission
• Dendrites receive signals from other neurons or sensory receptors
• Axons conduct impulses away from the cell body

Classes of Neurons

• Sensory (afferent) neurons transmit information from sensory receptors to the CNS
• Motor (efferent) neurons convey impulses from the CNS to effector cells (muscles and glands)
• Interneurons connect sensory and motor neurons within the CNS, playing a role in reflexes and neural circuits

Neural Circuits and Reflex Arcs

• Neural circuits are pathways allowing communication between different parts of the nervous system
• Simplest neural circuits involve direct pathways to enable quick reflex actions
• Essential components include receptors, sensory neurons, interneurons, motor neurons, and effectors
• This organization allows for rapid responses to stimuli, crucial for survival
• Reflex arcs exemplify how the nervous system responds to stimuli without conscious thought

Electrochemical Impulse and Membrane Potential

• Neurons transmit signals through electrochemical currents, primarily involving ion movement across the membrane
• Depolarization is crucial for impulse conduction; the inside of the neuron becomes less negative relative to the outside
• All animal cells have a voltage difference across their membranes (membrane potential) due to charge separation
• The resting membrane potential is approximately -70 mV, indicating the inside is negatively charged compared to the outside

Action Potential and Its Phases

• Action potential is the electrical impulse that travels along the axon, characterized by rapid changes in membrane potential
• The process involves five stages: stimulus/initiation, depolarization, repolarization, hyperpolarization, and recovery of resting membrane potential
• Stimulus initiates depolarization to threshold (-50 to -55 mV)
• Depolarization: Na+ channels open, causing a rapid increase in membrane potential (+40 mV)
• Repolarization: K+ channels open, K+ flows out, decreasing membrane potential back to resting levels
• Hyperpolarization: Slow closing of K+ channels causes a brief undershoot of resting potential
• Recovery: The membrane stabilizes back to resting potential, ready for another action potential

Propagation of Impulse

• Action potentials propagate along the axon as a wave of depolarization
• Neighboring membrane areas depolarize due to local electrical currents, creating a chain reaction

Saltatory Conduction

• Myelinated axons have Schwann cells, insulating the axon with Nodes of Ranvier, allowing rapid depolarization
• The action potential "jumps" from node to node, significantly increasing transmission speed (up to 120 m/s)

Synaptic Transmission

• Electrical synapses have direct contact between cells for rapid transmission
• Chemical synapses use neurotransmitters across a synaptic cleft to transmit signals
• Neurotransmitters are stored in vesicles and released upon action potential arrival, binding to receptors on the postsynaptic membrane
• This causes ion channels to open and influences the postsynaptic cell's potential

Neurotransmitter Examples and Functions

• Neurotransmitters have excitatory or inhibitory effects on the postsynaptic cell, depending on the receptor type
• Key neurotransmitters include dopamine, serotonin, endorphins, and norepinephrine, each having specific functions and associated conditions