Prep Guide: Nervous System

Key Concepts (Recall & Understanding)

Neuron: The basic functional unit of the nervous system, consisting of a cell body, dendrites, and an axon.
Myelin Sheath: An insulating layer around the axon that speeds up nerve impulse transmission (saltatory conduction).
Synapse: The junction between two neurons where a nerve impulse is transmitted via neurotransmitters.
Central Nervous System (CNS): Consists of the brain and spinal cord.
Brain:
- Cerebrum: Controls intelligence, memory, consciousness, and voluntary actions.
- Cerebellum: Coordinates voluntary movements and balance.
- Medulla Oblongata: Controls involuntary vital functions (heartbeat, breathing).
Peripheral Nervous System (PNS): Nerves branching from the CNS. Includes the Somatic (voluntary) and Autonomic (involuntary) systems.
Autonomic Nervous System (ANS): Divided into the Sympathetic ("fight or flight") and Parasympathetic ("rest and digest") systems.
Reflex Arc: The pathway of a reflex action: Receptor → Sensory Neuron → Interneuron (in CNS) → Motor Neuron → Effector.
Sense Organs:
- Eye: Organ of sight. Light is focused by the cornea and lens onto the retina, which contains photoreceptors (rods and cones).
- Ear: Organ of hearing and balance. Sound vibrations are transmitted to the cochlea. Balance is maintained by the vestibular system (semicircular canals).

Application Corner

A person suffers a stroke that damages their cerebellum. What kind of difficulties would they likely experience in their daily life?
- Answer: The cerebellum is crucial for coordinating movement and balance. The person would likely have difficulty with activities requiring fine motor control, such as writing or buttoning a shirt. They might have a clumsy, unsteady gait (ataxia) and difficulty maintaining their balance.
When you step from a brightly lit room into a dark room, you cannot see clearly at first, but your vision gradually improves. What is this process called, and which photoreceptor cells in the retina are primarily responsible for vision in low light?
- Answer: This process is called dark adaptation. The rod cells are responsible for vision in low light conditions (scotopic vision).

Analytical Thinking

Odd One Out: Cerebrum, Cerebellum, Medulla Oblongata, Cranial Nerve.
- Odd One: Cranial Nerve.
- Category: The rest are the three main parts of the brain.
Scenario: A person touches a hot stove. Their hand pulls away instantly, even before they consciously feel the pain. Explain the neural pathway that allows for this rapid response.
- Answer: This is a spinal reflex action. The heat is detected by receptors in the skin. A sensory neuron sends an impulse to the spinal cord. In the spinal cord, an interneuron immediately relays the signal to a motor neuron, which instructs the muscles in the arm to contract, pulling the hand away. The sensation of pain is processed by the brain, but the impulse reaches the brain slightly after the reflex action has already occurred, which is why the withdrawal is so fast.

Key Case Study

Multiple Sclerosis (MS): A Demyelinating Disease

Multiple Sclerosis is an autoimmune disease where the body's own immune system attacks and destroys the myelin sheath that insulates nerve fibers in the CNS. This damage, known as demyelination, disrupts the flow of nerve impulses. Instead of the fast, saltatory conduction, the nerve signals are slowed down, distorted, or stopped altogether. This leads to a wide range of neurological symptoms, including muscle weakness, difficulty with coordination and balance, vision problems, and fatigue. MS is a powerful illustration of the critical role the myelin sheath plays in ensuring rapid and efficient communication within the nervous system.

Assertion-Reason Practice

Assertion (A): The sympathetic nervous system prepares the body for a "fight or flight" response. Reason (R): It increases heart rate, dilates pupils, and redirects blood flow to the muscles.

(a) Both (A) and (R) are true and (R) is the correct explanation of (A). (b) Both (A) and (R) are true but (R) is not the correct explanation of (A). (c) (A) is true but (R) is false. (d) (A) is false but (R) is true.

Answer: (a) Both (A) and (R) are true and (R) is the correct explanation of (A). The physiological changes listed in the reason are the specific actions the sympathetic nervous system takes to prepare the body for an emergency, which is what is meant by the "fight or flight" response.

HOTS (Higher-Order Thinking Skills) Question

Reflex actions are rapid and involuntary. Voluntary actions are slower and under conscious control. From an evolutionary perspective, why is it advantageous for protective responses, like pulling your hand from a fire, to be governed by a reflex arc rather than being a voluntary action controlled by the brain?

Answer: It is advantageous for survival. A reflex arc involves a very short neural pathway, often just to the spinal cord and back, bypassing the brain for the initial response. This makes the reaction time extremely fast. If you had to rely on a voluntary action, the sensory information would have to travel all the way to the brain, be processed, a conscious decision made, and then a motor command sent back to the muscles. This entire process would take significantly longer. In a dangerous situation, like touching a fire, that extra fraction of a second can be the difference between a minor burn and a severe injury. Therefore, the speed of the reflex provides a crucial survival advantage by minimizing damage.

Specimen Prep - Nervous System