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.