The Nervous System Overview

The Nervous System

• The nervous system is a complex communication system.
• It contains over 100 billion nerve cells, primarily in the brain.
• It is organized into two main divisions: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

Organization of the Nervous System

• Central Nervous System (CNS): Comprised of the brain and spinal cord, responsible for coordinating incoming and outgoing information.

• Peripheral Nervous System (PNS): Consist of nerves that connect the CNS to all parts of the body. This system is further divided into two categories: the somatic nervous system, and the autonomic nervous system.

PNS: The Somatic Nervous System

• Controls voluntary movements of skeletal muscles.
• Carries information from the environment (e.g., heat, light, touch) back to the brain.
• This system allows for conscious control of actions.

PNS: The Autonomic Nervous System

• Controls involuntary bodily functions like the internal organs.
• This includes things like digestion, stomach churning, and movement of food through intestines.
• It is further divided into the sympathetic and parasympathetic divisions.

PNS: Autonomic: Sympathetic vs. Parasympathetic

• These divisions are always active, opposing one another to control functions like heart rate.
• Sympathetic: Dominates in stressful, dangerous, exciting, or strenuous situations. Its actions include increasing heartbeat, blood pressure, and pupil dilation.
• Parasympathetic: Dominates in calm, relaxed situations. Its actions include slowing heartbeat, promoting digestion, and pupil constriction.

The Nerve Cell

• The neuron is the basic functional unit of the nervous system.
• There are three main types:
  • Sensory neurons (afferent): Transmit information from the environment to the CNS. Located in clusters called ganglia outside the spinal cord.
  • Interneurons (association neurons): Connect neurons within the brain and spinal cord.
  • Motor neurons (efferent): Transmit information from the CNS to muscles, organs, and glands to cause a response to the perceived change in environment.

Neurons: Structure

• All neurons contain:
  • Dendrites: Receive information from other nerve cells.
  • Cell body: Contains cell organelles. Responsible for the cell's metabolic function and forms the extension of the cytoplasm called the axon
  • Axon: Directs the nerve impulse away from the cell body. Many axons have a myelin sheath.
    • Myelin sheath: Insulating material of fatty proteins created by glial cells (like Schwann cells). Increases the speed of nerve impulse transmission.
    • Nodes of Ranvier: Gaps in the myelin sheath that speed up signal transmission by allowing the impulse to "jump" between the nodes.

Electrochemical Impulse

• Resting potential: (-70 mV) Charge difference across the neuron membrane when not transmitting a signal, due to ion concentration differences.
  • Sodium-Potassium Pump: Maintains the resting potential by pumping three sodium ions out for every two potassium ions pumped in.
• Action potential: (+40mV) Charge difference across the neuron membrane during signal transmission. The stimulus reaches a threshold, causing a rapid change in ion permeability, resulting in a signal being sent.
• Depolarization: The membrane becomes more permeable to sodium ions, and positive ions rush into the cell, making the inside more positive.
• Repolarization: Potassium ions leave the cell, restoring the original negative charge inside the neuron.
• Refractory period: The period after an action potential where the neuron cannot immediately fire again. Crucial in preventing signal overlap and ensuring a consistent speed of transmission.

Synaptic Transmission

• Synapse: The small space between the ends of two neurons.
• Neurotransmitters: Chemical messengers released by the axon terminal of one neuron that cross the synapse to signal the next neuron.
• Action on the postsynaptic neuron: The binding of the neurotransmitters to receptors on the next neuron cause a response that either leads to excitation or inhibition of the next neuron's function
• When the signal moves from one neuron to the next, the process of diffusion across the synapse slows the transmission. Multiple synapses lead to slower responses.
• Acetylcholine: Example neurotransmitter.

Botulism

• Botulism is a form of bacterial toxin that interferes with neurotransmitters' (like acetylcholine) function.
• This toxin paralyzes the nerves, prevents muscles from contracting.
• Interference is often permanent to the axon or neuromuscular junction (NMJ)

Reflex Arc

• Reflex arc: A very fast, automatic response to a stimulus, occurring without conscious thought or involvement of the brain to conserve time.
• Typical components of a reflex arc are:
  • Receptor: Detects the stimulus
  • Sensory neuron: Carries information from the receptor to the CNS
  • Interneuron (in spinal cord): Integrates the sensory information and stimulates responses.
  • Motor neuron: Carries information from the CNS to the effector.
  • Effector: Responds to the efferent signal

Threshold Level

• Threshold Level: The minimum stimulus intensity required to trigger an action potential.
• All- or- nothing response: A stimulus that is below the threshold will not trigger an action potential. A stimulus that is at or above it always creates a response of the same magnitude, regardless of its intensity,

Explore the intricate structure and function of the nervous system, including the Central and Peripheral Nervous Systems. This quiz delves into the roles of the somatic and autonomic nervous systems, highlighting how they control voluntary and involuntary actions.