Nervous Coordination, Neurons and the Reflex Arc: PECTAA Class 12 Biology, Punjab Board

When a doctor taps just below your knee, your leg kicks before you have decided anything. That kick travels through only two neurons. Chapter 15 of the book starts with how nervous coordination works, what a neuron looks like, and how this simplest pathway, the reflex arc, operates.

Chapter 15 of the PECTAA Biology 12 book is the Human Nervous System. After the urinary system (Chapter 14, which ended with kidney failure, dialysis and transplant), sections 15.1 and 15.2 cover nervous coordination, neurons and the reflex arc. This note is for FSc Part 2 (2nd year, Class 12) students in Lahore, Faisalabad, Sargodha, Rawalpindi, Multan, Gujranwala and the other Punjab boards. Gujrat district comes under the Gujranwala board. PECTAA writes the books. Your own board sets your paper.

What to know

Nervous coordination (section 15.1). The nervous system coordinates the body with the outside environment and among its own parts, using electrical signals. It is the most important system for homeostasis because it regulates other systems as well as keeping balance itself.

Basic elements (section 15.1.1).

1. Receptors: organs, tissues or cells (eyes, ears, skin) that detect stimuli, the changes in the external and internal environment. They act as transducers, turning one form of energy into another; rods and cones turn light into nerve impulses. Impulses travel in sensory neurons to the central nervous system.
2. Central nervous system (CNS): the brain and spinal cord, the control centre. Its inter-neurons (associative neurons) receive and process impulses, then send impulses to effectors through motor neurons.
3. Effectors: muscles, which respond by contracting, and glands, which respond by secreting.

Pathway: receptor → sensory neuron → CNS (inter-neurons) → motor neuron → effector.

Neuron (section 15.2). The neuron, or nerve cell, is the structural and functional unit of the nervous system.

Structure (section 15.2.1).

• Cell body: the enlarged region holding the nucleus and most organelles. Its cytoplasm has Nissl's granules, groups of ribosomes and rough endoplasmic reticulum.
• Dendrites: extensions that receive information, convert it into impulses and carry them towards the cell body. Motor and inter-neurons have highly branched dendrites because they receive many inputs at once.
• Axon: a long extension that carries impulses away from the cell body, ending in axon terminals that contact a muscle cell, gland cell or another neuron. The book notes that axons to the foot muscles are over a metre long.

Neuroglia. Glial cells support neurons: they supply nutrients, remove wastes and provide immunity. Schwann cells (PNS) and oligodendrocytes (CNS) wrap axons in myelin sheaths. Myelin insulates the axon and speeds conduction. Gaps in the sheath every 1 to 2 mm are the nodes of Ranvier. In the CNS, myelinated axons form white matter; cell bodies and non-myelinated dendrites form grey matter.

Types of neurons (section 15.2.2).

Reflex arc (section 15.2.3). The simplest neural pathway, controlling reflex actions: immediate, involuntary responses to stimuli.

The knee-jerk reflex.

1. A tap on the patellar tendon below the knee is the stimulus.
2. A receptor in the quadriceps muscle of the thigh generates an impulse in a sensory neuron.
3. In the spinal cord, the sensory neuron passes the impulse directly to a motor neuron, with no inter-neuron in between.
4. The motor neuron makes the quadriceps contract, so the lower leg kicks forward.
5. Meanwhile the sensory neuron also activates an inter-neuron that inhibits the motor neurons of the hamstrings, so they relax while the leg extends.

Key points

• Receptor → sensory neuron → CNS → motor neuron → effector.
• Dendrites bring impulses in; axons carry them out.
• Myelin speeds conduction; nodes of Ranvier interrupt it every 1 to 2 mm.
• The knee jerk uses one sensory and one motor neuron in its main path.

Worked examples

Example 1 (trace the pathway). You touch a hot pan and pull your hand away. Name the parts in order.

1. Receptor in the skin detects heat.
2. Sensory neuron carries the impulse to the spinal cord.
3. Inter-neuron processes it.
4. Motor neuron carries it to the arm muscle.
5. The muscle (effector) contracts and the hand moves away.

Example 2 (classify). A neuron has its cell body in a ganglion beside the spinal cord. Which type is it?

1. Cell bodies in ganglia outside the CNS belong to sensory neurons.
2. It is a sensory neuron.

Common mistakes in the exam

1. Reversing dendrites and axons. Dendrites carry towards the cell body.
2. Mixing Schwann cells and oligodendrocytes. Schwann: PNS; oligodendrocytes: CNS.
3. Adding an inter-neuron to the knee-jerk main pathway. It is sensory to motor directly.
4. Forgetting the hamstrings. Their relaxation is part of the full knee-jerk answer.
5. Calling glands receptors. Glands are effectors.

Exam technique

• Draw and label a neuron: cell body, Nissl's granules, dendrites, axon, myelin, nodes of Ranvier, axon terminals.
• For the reflex arc, number each step as above.
• Use a table to compare the three types of neuron.
• Check your board's notice for the sections in your paper.

Practice questions

1. What are Nissl's granules made of? Answer: Groups of ribosomes and rough endoplasmic reticulum.
2. Which cells form myelin in the CNS? Answer: Oligodendrocytes.
3. Name two effectors. Answer: Muscles and glands.
4. Why do receptors count as transducers? Answer: They turn one form of energy, such as light, into nerve impulses.
5. Which muscle contracts in the knee-jerk reflex? Answer: The quadriceps.

Ready to practise this? Continue in 2nd Year Biology.