Chapter 12 Nervous System AI Notes

Overview of the Nervous System

  • The nervous system acts as a communication and control network for approximately 100 trillion cells in the body.

Functions of the Nervous System (NS)

  1. Collects Information:

    • Utilizes receptors to gather data from external environment and internal bodily conditions.

  2. Processes Information:

    • Analyzes the collected data to ascertain the need for a response.

  3. Initiates Response:

    • Sends commands via effectors such as muscle tissues and glands to elicit action.

Organization of the Nervous System

Central Nervous System (CNS)

  • Function: Processes information.

  • Components:

    • a. Brain

    • b. Spinal Cord

Peripheral Nervous System (PNS)

  • Function: Transmits information to and from the CNS.

  • Components:

    • a. Nerves (bundles of neuron branches)

    • b. Ganglia (clusters of neuron cell bodies)

  • Divisions:

    • Motor Division: Delivers commands from CNS to effectors.

    • Sensory Division: Carries sensory information from receptors to CNS.

    • Visceral (Autonomic) Motor: Controls involuntary movements (e.g., cardiac/smooth muscles, glands).

    • Somatic Motor: Controls voluntary movements (e.g., skeletal muscles).

    • Visceral (Autonomic) Sensory: Not consciously perceived; originates from internal organs.

    • Somatic Sensory: Consciously perceived through senses such as sight and touch.

Functional Components of the Nervous System

Brain and Spinal Cord

  • Brain: Processes sensory input and controls motor output.

  • Spinal Cord: Conducts signals between the brain and body.

Neurons and Glial Cells

  1. Neurons:

    • Basic units of the nervous system.

    • Excitable cells that transmit electrical signals.

    • Exhibit extreme longevity but are typically amitotic (lost ability to divide).

  2. Glial Cells (Neuroglia):

    • Non-excitable cells that support and protect neurons:

      • Protect

      • Nourish

      • Support

      • Repair

      • Guide neurons.

Structure of Neuron

  • Dendrites:

    • Short branches that deliver messages to the cell body.

  • Cell Body:

    • Central nucleus responsible for the nerve cell's metabolism and message integration.

  • Axon:

    • A longer branch transmitting signals to other neurons, muscles, or glands.

    • Includes axon hillock (triangular region at the top) and axon terminals (branches at distal end).

Classification of Neurons

Structural Classification

  1. Multipolar Neurons: Many dendrites and one axon (common).

  2. Bipolar Neurons: One dendrite and one axon.

  3. (Pseudo)Unipolar Neurons: Single short branch that branches like a T.

Functional Classification

  1. Sensory Neurons (Afferent): Conduct input from receptors to the CNS (mostly unipolar, cell bodies outside CNS).

  2. Motor Neurons (Efferent): Conduct output from CNS to effectors (all multipolar, cell bodies within CNS).

  3. Interneurons: Make up 99% of neurons; process and store information within the CNS.

Glial Cells Functions

  • Astrocytes:

    1. Form blood-brain barrier.

    2. Regulate interstitial fluid composition.

    3. Provide structural support in the CNS.

    4. Assist with neuronal development.

    5. Replicate to replace dying neurons.

  • Ependymal Cells:

    1. Line brain ventricles and spinal canal, aiding CSF production and circulation.

  • Microglial Cells:

    1. Move through the CNS to protect by engulfing infectious agents and harmful substances.

  • Oligodendrocytes:

    1. Myelinate and insulate CNS axons.

Myelin Sheath Formation

  • Myelination:

    • In the PNS by Schwann cells (neurolemmocytes) and in the CNS by oligodendrocytes.

    • Myelinated axons allow faster action potential propagation.

Resting Membrane Potential (RMP)

  • RMP established by:

    • Na+/K+ pumps maintaining + outside and - inside the membrane.

    • Graded potentials arise from sodium-potassium pump activity and ion behavior through leak channels.

Action Potentials

  • Generated at the initial segment of a neuron and propagated along axon via voltage-gated channels.

  • Steps:

    1. Depolarization: Makes the inside positive (+30 mV) when Na+ channels open.

    2. Repolarization: Returns the membrane to RMP (-70 mV) via K+ channels opening.

    3. Hyperpolarization: Membrane becomes overly negative before returning to RMP.

Refractory Period

  • Brief period after initiation of the action potential:

    1. Absolute Refractory Period: No stimulus can generate a second AP.

    2. Relative Refractory Period: AP possible with greater stimulation due to neuronal hyperpolarization.

Factors Influencing Nerve Signal Velocity

  • Diameter of Axon: Larger diameter can transmit signals faster.

  • Myelination: Myelinated axons transmit action potentials faster than unmyelinated axons.

Neural Integration and Neuronal Pools**

  • Neuronal Pools: Groups of interneurons coordinating activities.

  • Types of Circuits:

    1. Converging Circuit: Multiple inputs converge on a single neuron.

    2. Diverging Circuit: Single input spreads information to multiple neurons.

Synaptic Transmission**

  • Arrival of Action Potential: Triggers Ca2+ channels to open, leading to neurotransmitter release from synaptic vesicles into the synaptic cleft.

  • Neurotransmitters result in excitation or inhibition of the postsynaptic neuron, muscle, or gland.

Neurotransmitter Elimination**

  1. Degradation: Chemical inactivation occurs in the synaptic cleft.

  2. Reuptake: Reabsorbed into the presynaptic neuron for reuse.