8.3 - 8.4 CNS and PNS

CNS Overview

• CNS stands for Central Nervous System.

• Key components: Brain and Spinal Cord.

The Human Nervous System

Nervous System Structures

• Central Nervous System (CNS)

  • Brain: Processes information, stores memories, generates thoughts and emotions.

  • Spinal Cord: Communication link between brain and Peripheral Nervous System (PNS).

• Peripheral Nervous System (PNS)

  • Motor Neurons: Conduct signals from CNS to muscles and glands to initiate responses and control reflex activities.

  • Sensory Neurons: Conducts sensory information to CNS from sensory organs.

Nervous System Divisions

• Somatic Nervous System: Controls voluntary movements.

• Autonomic Nervous System: Controls involuntary movements.

  • Sympathetic Division: "Fight or flight" response during stress.

  • Parasympathetic Division: "Rest and digest" response when relaxed.

Central Nervous System Components

• Grey Matter: Contains neurons with unmyelinated axons.

• White Matter: Contains neurons with myelinated axons.

The Spinal Cord

• A column of nerve tissue serving as a communication link between brain and PNS.

• Sensory Neurons: Carry messages from body receptors to the brain.

• Motor Neurons: Carry messages from the brain to the body’s effectors.

• Primary Reflex Centre: Site for processing reflex actions.

Brain Structure

Regions of the Brain

Three distinct regions:

• Forebrain

• Midbrain

• Hindbrain

Forebrain

• Cerebrum: Largest part of the brain; surface is the cerebral cortex covered in folds.

• Thalamus: Located below the cerebrum; acts as a relay station for sensory information.

• Hypothalamus: Below thalamus; critical for maintaining homeostasis; connects to pituitary gland.

Lobes of the Cerebrum

• Divided into four lobes:

  • Frontal Lobe: Involved in organization, movement, and personality.

  • Temporal Lobe: Involved in language, memory, and auditory processing.

  • Parietal Lobe: Processes sensory information and body awareness.

  • Occipital Lobe: Involved in vision and problem-solving.

• The brain is also divided into left and right hemispheres.

Functional Areas of the Brain

• Motor Area: Controls voluntary muscles.

• Sensory Area: Processes sensory information (temperature, pain).

• Wernicke's Area: Responsible for language comprehension.

• Broca's Area: Controls speech production.

• Cerebellum: Responsible for coordination and balance.

• Brain Stem: Controls consciousness and autonomic functions (breathing, heart rate).

Connectivity in the Brain

• Corpus Callosum: Bundle of nerve fibers connecting the left and right hemispheres, facilitating communication.

Midbrain

• Considered the least developed of the three brain regions; part of the brainstem along with hindbrain.

Hindbrain

• Cerebellum: Largest part of the hindbrain; involved in balance and coordination.

• Pons: Connects neurons between the cerebrum, cerebellum, and rest of the brain.

• Medulla Oblongata: Junction between spinal cord and brain; coordinates autonomic functions such as heart rate and breathing.

Brain Myths and Conditions

Myth

• "Humans use only 10% of their brain."

Interesting Brain Conditions

• Astasia-Abasia: Patients exhibit instability and are prone to falling.

• Broca's Aphasia: Patients cannot form coherent words despite understanding language.

• Foreign Accent Syndrome: Stroke or injury leads to unexpected foreign accent.

• Capgras Syndrome: Belief that a loved one is an imposter, linked to brain lesions.

• Phantom Pain Sensations: Perception of pain or sensations in a missing limb.

Peripheral Nervous System (PNS)

• Comprises the network of nerves outside the brain and spinal cord.

Somatic Nervous System

• Controls voluntary skeletal movements; connects to head, trunk, and limbs.

Autonomic Nervous System

• Controls involuntary actions (glandular secretions, cardiac muscle).

• Maintains homeostasis without conscious input.

• Divided into:

  • Sympathetic Nervous System: Activates in stress; responsible for "fight or flight" reaction.

  • Parasympathetic Nervous System: Activates at rest; responsible for "rest and digest" reaction.

Effects of Sympathetic Division

• Increases heart rate and blood pressure; digestion slows.

• Releases norepinephrine to enhance muscle activity.

Effects of Parasympathetic Division

• Slows down bodily functions; conserves energy.

• Releases acetylcholine to promote digestion and reduce heart rate.

Comparative Functions of the Two Divisions

• Eyes:

  • Sympathetic: Dilates pupils.

  • Parasympathetic: Constricts pupils.

• Salivary Glands:

  • Sympathetic: Inhibits salivation.

  • Parasympathetic: Stimulates salivation.

• Heart:

  • Sympathetic: Accelerates heartbeat.

  • Parasympathetic: Slows heartbeat.