12.1 Coordination and Response
Coordination is the process of detecting stimuli from the external or internal environment, integrating the information, and initiating an appropriate response by effectors.
- Stimulus: A change in the internal or external environment that detects by receptors (e.g., light, sound, temperature, pressure).
- Receptor: Sensory structures or specialized cells that detect stimuli and convert them into nerve impulses.
- Integrating Centre: Central Nervous System (Brain and Spinal Cord) that processes incoming information and sends signals to effectors.
- Effector: Muscles or glands that carry out the body's response.
12.2 Nervous System
The human nervous system is divided into two main parts:
- Central Nervous System (CNS): Consists of the Brain (Cerebrum, Cerebellum, Hypothalamus, Pituitary Gland, Medulla Oblongata) and Spinal Cord.
- Cerebrum: Controls voluntary actions, reasoning, intelligence, memory, emotion, and sensory interpretation.
- Cerebellum: Maintains body posture, balance, and coordinates muscle contractions.
- Hypothalamus: Regulates body temperature, water balance, appetite, and links the nervous and endocrine systems via the pituitary gland.
- Medulla Oblongata: Controls involuntary actions such as heartbeat, peristalsis, vasoconstriction, and breathing rate.
- Peripheral Nervous System (PNS): Consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves connecting the CNS to the rest of the body.
Structure and Function of Neurones
- Sensory Neurone: Carries nerve impulses from receptors to the CNS. (Cell body located in the dorsal root ganglion).
- Relay Neurone (Interneurone): Transmits impulses between sensory and motor neurones within the CNS.
- Motor Neurone: Transmits impulses from the CNS to effectors (muscles or glands).
Reflex Arc and Reflex Action
A reflex action is an automatic, rapid, and involuntary response to a stimulus without conscious thought. The pathway travelled by nerve impulses during a reflex is the reflex arc:
Stimulus → Receptor → Sensory Neurone → Relay Neurone (Spinal Cord) → Motor Neurone → Effector → Response
12.3 Transmission of Impulse Across a Synapse
A synapse is a narrow microscopic gap between two neurones or between a neurone and an effector.
- When a nerve impulse reaches the axon terminal (presynaptic knob), it triggers synaptic vesicles to fuse with the presynaptic membrane.
- Neurotransmitters (e.g., acetylcholine, dopamine, noradrenaline) are released by exocytosis into the synaptic cleft.
- Neurotransmitters diffuse across the synaptic cleft and bind to specific receptor proteins on the postsynaptic membrane.
- This binding generates a new electrical impulse in the postsynaptic neurone. Enzymes subsequently break down neurotransmitters to prevent continuous stimulation.
12.4 Voluntary and Involuntary Actions
- Voluntary Actions: Conscious actions under the control of the cerebrum (e.g., writing, running, speaking).
Pathway: Stimulus → Receptor → Sensory Neurone → Cerebrum → Motor Neurone → Effector → Response.
- Involuntary Actions: Automatic actions managed by the medulla oblongata or hypothalamus without conscious effort (e.g., peristalsis, heartbeat, reflex actions like knee-jerk).
12.5 Health Issues Related to the Nervous System
- Multiple Sclerosis: Progressive destruction of the myelin sheath in the CNS, impairing nerve impulse conduction.
- Alzheimer's Disease: Degenerative brain disorder marked by loss of memory, cognitive decline, and loss of voluntary muscle control due to brain cell shrinkage.
- Parkinson's Disease: Tremors, muscle stiffness, and balance loss caused by the degeneration of dopamine-producing neurones in the brain.
- Drug & Alcohol Abuse: Substances like stimulants increase neurotransmitter activity, whereas depressants (e.g., alcohol) slow down central nervous system processing and reaction times.