Human Nervous System: Structure and Reflex Mechanisms

The Human Nervous System

Core Components

• CNS (Central Nervous System): Made up of the brain and spinal cord.
• PNS (Peripheral Nervous System): Made up of the rest of the nerves in the body. These connect the CNS with sensory organs, muscles, blood vessels, and glands.

Key Terminology

• Neurones: Connections of many long, thin nerve cells.
• Effectors: Carry out actions in response to messages from the CNS.
• Receptors: Sense changes in the environment (eyes, nose, mouth, skin, ears).
• Stimuli: Provide the information to the receptors.
• Sense Organ: Organs that contain the receptors.

Types of Neurones

• Sensory Neurone: Pass electrical messages from the receptor to the CNS.
• Motor Neurone: Pass messages from the CNS to effectors (muscles/glands).
• Relay Neurone: Pick up messages from one neurone and pass them onto neighbouring neurones.

Reflex Actions and Pathways

Reflex Arc: The pathway from receptors to CNS to the effector.

Pathway: Receptors → sensory neurones → relay neurones (spinal cord) → motor neurones → effectors.

Reflex Action: A quick, automatic response to a stimulus.

Synaptic Transmission

Synapse: A gap between adjacent neurones. Neurones never touch each other; a signal is sent from one neurone to the next by a chemical transmitter (Acetylcholine) across the synapse. These transmitters are then destroyed.

Neurone Structure

• Cell Body: Controls the metabolism of the nerve cell.
• Dendrites: Shorter fibres of cytoplasm that pick up messages from other neurones nearby.
• Nerve Fibres: Long, thin fibres of cytoplasm from the cell body.
• Axon: The longest fibre.
• Myelin: A layer of fat and protein (fatty sheath) that provides insulation, allowing electrical impulses to be carried faster.

Voluntary vs. Involuntary Actions

• Involuntary: Actions performed without conscious thought (e.g., reflex actions). These can be present from birth.
• Voluntary: Actions that we have conscious control over.

Sequence: Stimulus → Receptor → Sensory Neurone → Coordinator → Motor Neurone → Effector → Response.

Musculoskeletal System

• Antagonistic Muscles: When the bicep contracts, the triceps relax, causing the arm to flex (move upwards). Because muscles can only pull, the triceps must contract to pull the arm down (extension).
• Tendon: Joins muscle to bone; must not stretch to ensure efficient movement.
• Ligament: Joins bone to bone; must stretch to allow skeletal flexibility.