Lesson 8: Organization of the Nervous System

Lecture Notes

Neurons are the fundamental units of the nervous system that conduct electrochemical impulses.
Three Major Parts:
- Cell Body: Contains the nucleus and integrates incoming signals.
- Dendrites: Short, branched structures that receive electrochemical signals and send them to the cell body.
- Axon: A long projection that transmits impulses away from the cell body.
Myelination:
- Schwann Cells form the myelin sheath in the PNS, while oligodendrocytes form it in the CNS.
- The myelin sheath insulates the axon and increases impulse transmission speed.
- Nodes of Ranvier are gaps in the myelin that help speed up nerve signal conduction.
- Myelinated axons = White matter, unmyelinated regions = Gray matter.

Central Nervous System (CNS): Composed of the brain and spinal cord.
Peripheral Nervous System (PNS): Composed of sensory and motor neurons outside the CNS.
- Sensory Neurons: Carry signals toward the CNS from sensory receptors.
- Motor Neurons: Carry signals away from the CNS to effectors (muscles & glands).
Motor Pathways:
- Somatic Nervous System: Controls voluntary movements (skeletal muscles).
- Autonomic Nervous System (ANS): Controls involuntary functions (heart rate, digestion).
  - Sympathetic Division: Active in "fight or flight" situations.
  - Parasympathetic Division: Active in "rest and digest" situations.

Largest and most developed region of the brain.
Divided into two hemispheres (left and right), connected by the corpus callosum.
Each hemisphere controls the opposite side of the body (contralateral control).
Four Lobes:
- Frontal Lobe: Higher-order thinking, decision-making, voluntary movement (motor cortex).
- Parietal Lobe: Spatial awareness, mathematical reasoning, sensory input processing.
- Temporal Lobe: Processes auditory information and language (Wernicke's & Broca’s areas).
- Occipital Lobe: Responsible for vision processing.
Cerebral Cortex:
- Outer layer of the cerebrum, densely packed with neurons.
- Highly convoluted surface increases surface area for neural activity.

Brain: Consists of the hindbrain, midbrain, and forebrain.
- Hindbrain: Includes the cerebellum, medulla oblongata, and pons (motor coordination and basic survival functions).
- Midbrain: Processes sensory information (especially vision).
- Forebrain: Includes the thalamus, hypothalamus, and cerebrum (higher-order processing, learning, and memory).
Spinal Cord:
- Gray Matter (Inner Zone): Contains cell bodies of motor neurons and interneurons.
- White Matter (Outer Zone): Contains myelinated axons transmitting signals.
- Relays signals between the brain and the body.
- Involved in reflex actions.

Composed of nerves and ganglia outside the CNS.
Sensory Pathway: Neurons carry information from sensory receptors to the CNS.
Motor Pathway: Neurons carry commands from the CNS to effectors (muscles or glands).

A reflex is a rapid, involuntary response to a stimulus.
Reflex Arc Components:
1. Stimulus activates a sensory receptor.
2. Sensory Neuron carries the impulse to the spinal cord.
3. Interneuron (if present) processes the signal.
4. Motor Neuron sends the response signal to an effector.
5. Effector (muscle or gland) carries out the response.
Example: Knee-Jerk Reflex
- A monosynaptic reflex (only involves one synapse).
- The sensory neuron directly synapses with the motor neuron in the spinal cord.
- Allows the body to react quickly without involving the brain.

Somatic Nervous System:
- Controls voluntary movements.
- Uses skeletal muscles as effectors.
- Example: Moving your arm.
Autonomic Nervous System:
- Controls involuntary functions.
- Uses smooth muscle, cardiac muscle, and glands as effectors.
- Example: Regulating heart rate or digestion.

Sympathetic ("Fight or Flight"):
- Increases heart rate, dilates pupils, slows digestion.
- Uses lots of energy for immediate action.
- Originates from the middle spinal cord.
Parasympathetic ("Rest and Digest"):
- Lowers heart rate, constricts pupils, increases digestion.
- Conserves energy for long-term survival.
- Originates from the brainstem and sacral spinal cord.

The nervous system regulates internal conditions to maintain stability (homeostasis).
Examples:
- Temperature Regulation:
  - Detects temperature changes and signals sweat glands or muscles to adjust.
- Blood Pressure & Heart Rate:
  - The autonomic nervous system adjusts heart rate and vessel constriction to maintain stable blood pressure.
- Oxygen & pH Levels:
  - The nervous system monitors blood oxygen and CO₂ levels and signals the respiratory system to adjust breathing.
- Digestive Regulation:
  - The parasympathetic nervous system promotes digestion when the body is at rest.
- Reflex Responses:
  - Reflexes protect the body from harm without requiring brain processing, allowing for immediate responses.

The nervous system is a highly complex network responsible for communication, movement, sensory processing, and maintaining homeostasis.
Key Takeaways:
- Neurons transmit signals via electrochemical impulses.
- The CNS (brain & spinal cord) integrates and processes information.
- The PNS (sensory & motor neurons) connects the CNS to the body.
- The somatic system controls voluntary movements, while the autonomic system regulates involuntary functions.
- The sympathetic and parasympathetic systems have opposing effects in regulating body functions.
- Reflexes provide rapid, protective responses to stimuli.
- The nervous system works with other systems to maintain homeostasis.

**Be sure to label neuron digram

Organization of Vertabrae Nervous System

CNS—> Brain and spinal cord
Peripheral NS—> sensory and motor pathways
- Somatic= voluntary (skeletal)
- Automatic= involuntary (smooth)

**Nerves work by electric and chemical pathway