biol 3410 11/7 lec

Overview of the Nervous System

  • The nervous system is responsible for the flow of information through sensory input and motor output.

Key Functions

  • Sensory Functions:

    • Involves the reception of sensory information, which is monitored by receptors detecting changes in conditions.

    • This information is sent to the spinal cord or brain as sensory input.

  • Motor Functions:

    • Involves signals leaving the central nervous system (CNS) to produce responses at effectors.

    • This is referred to as motor output.

  • Integration:

    • The central nervous system processes incoming sensory information and determines an appropriate response.

    • This is essentially the decision-making process of the CNS.

Division of the Nervous System

  • The nervous system is classified into two main parts:

    • Central Nervous System (CNS):

      • Comprises the brain and spinal cord.

    • Peripheral Nervous System (PNS):

      • Includes all other neural elements outside the CNS such as nerves and ganglia.

Sensory and Motor Pathways

  • Sensory Nervous System (Afferent):

    • Responsible for transmitting sensory information to the CNS from the peripheral regions.

    • Includes sensory neurons related to somatic, autonomic, and enteric functions.

  • Motor Nervous System (Efferent):

    • Involves signals that exit the CNS to initiate responses.

    • Divided into the somatic nervous system (skeletal muscle effectors) and the autonomic nervous system (smooth muscle, cardiac muscle, and glands).

Somatic and Autonomic Nervous Systems

Somatic Nervous System

  • Involves voluntary control over skeletal muscles.

  • The only effector is skeletal muscle.

  • Sensory input comes from proprioceptors in the muscles.

Autonomic Nervous System

  • Controls involuntary functions involving smooth and cardiac muscles, and glands.

  • Divided into:

    • Sympathetic Division: Prepares the body for fight or flight responses.

    • Parasympathetic Division: Engages in rest and digest activities.

Enteric Nervous System

  • A specialized part of the ANS associated with the digestion system.

  • Operates somewhat independently but communicates with the CNS about digestive functions.

Neurons and Glial Cells

Neurons

  • The primary electrically active cells communicating signals in the nervous system.

  • Composed of:

    • Cell body

    • Dendrites (input)

    • Axon (output)

Glial Cells

  • Previously thought to play a minor role; now recognized as crucial for supporting neurons. Types include:

    • Astrocytes: Maintain the blood-brain barrier, regulate ion concentrations, and modulate neuron activity.

    • Ependymal Cells: Line ventricles and create cerebrospinal fluid (CSF).

    • Oligodendrocytes: Form myelin in the CNS.

    • Microglia: Act as immune cells in the CNS, removing pathogens and debris.

    • Schwann Cells: Form myelin in the peripheral nervous system.

    • Satellite Cells: Provide support around neuron cell bodies in ganglia.

Myelin and Nerve Fibers

  • Myelination: Critical for efficient signal transmission.

  • White matter consists of myelinated axons, while gray matter contains unmyelinated cell bodies and dendrites.

  • Myelin acts as an insulator, promoting faster signal conduction.

Electrical Signals and Ion Channels

Resting Membrane Potential

  • The resting state of a cell has a slight negative charge, primarily due to the distribution of sodium and potassium ions across the membrane.

Action Potential

  • An all-or-none electrical signal that travels along the neuron's axon:

    • Initiated once the threshold potential is reached due to depolarization caused by sodium influx.

    • Followed by repolarization as potassium exits the cell.

Graded Potentials

  • Local changes in membrane potential that can vary in amplitude depending on stimulus strength.

  • Known as local potentials; can be depolarizing (making the inside of the cell more positive) or hyperpolarizing (making it more negative).

Summary of Action Potential Phases

  • Depolarization: Sodium channels open, allowing sodium to enter, pushing the potential up to +30mV.

  • Repolarization: Sodium channels close while potassium channels open, potassium exits, returning the potential to resting state.

  • After Hyperpolarization: Temporary increase in negativity before returning to resting state.

Conclusion

  • The nervous system is complex and involves intricate feedback loops, pathways, and cellular interactions necessary for body functions. Understanding the roles of neurons, glial cells, and the divisions of the nervous system is crucial for grasping how the nervous system operates.