Chapter 7 Human Physiology

The nervous system is divided into:

1. Central Nervous System (CNS): Comprises the brain and spinal cord.

2. Peripheral Nervous System (PNS): Includes cranial nerves from the brain and 31 pairs of spinal nerves.

Neurons

• Neurons are the functional units of the nervous system, conducting electrical impulses and releasing chemical regulators, which enable sensory perception, learning, memory, and muscle and gland control.

Neuroglial Cells

• Comprise about 80% of the nervous system's cells. They support, protect, and maintain neurons. The CNS has four types of neuroglial cells:

  1. Oligodendrocytes: Produce myelin in the CNS.

  2. Microglia: Provide immune support in the brain.

  3. Astrocytes: Maintain the blood-brain barrier and nutrient support.

  4. Ependymal cells: Produce cerebrospinal fluid (CSF).

• In the PNS, two types of neuroglial cells are found:

  • Schwann cells: Produce myelin in the PNS.

  • Satellite cells: Support neuronal health.

Structure of Neurons

• Neurons typically consist of four parts:

  1. Dendrites: Receive information.

  2. Cell Body: Processes information.

  3. Axon: Conducts action potentials.

  4. Axon Terminal: Relays information via neurotransmitters.

• Each neuron can connect with thousands of others, highlighting the brain's complex networking.

Nuclei and Ganglia

• Nuclei: Clusters of similar neurons in the CNS.

• Ganglia: Clusters of similar neurons in the PNS.

Classification of Neurons

• Neurons are classified by their function:

  1. Sensory (Afferent): Carry information into the CNS.

  2. Motor (Efferent): Transmit information away from the CNS.

  3. Interneurons: Connect sensory and motor neurons within the CNS.

• A nerve consists of a bundle of axons outside the CNS and can contain motor, sensory, or mixed fibers.

Myelin Sheath and Nodes of Ranvier

• The myelin sheath, formed by Schwann cells, insulates axons, increasing action potential speed. Exposed gaps between myelin segments, known as Nodes of Ranvier, facilitate quicker impulse transmission via saltatory conduction.

Electrical Activity in Axons

• Action potential generation occurs when ions move through gated channels, leading to changes in membrane potential:

  • Depolarization: Cell interior becomes less negative.

  • Repolarization: Restores resting potential after an action potential.

  • Hyperpolarization: Cell interior becomes more negative.

Neurotransmission

• At chemical synapses, neurotransmitters released from axon terminals influence postsynaptic cells, facilitating communication between neurons or to muscles and glands. Key neurotransmitters include:

  • Acetylcholine (ACh): Can be excitatory or inhibitory.

  • Monoamines (e.g., dopamine, norepinephrine): Modulate various brain functions, such as mood and alertness.

Synaptic Integration and Plasticity

• Through synaptic integration, multiple signals converge (spatial summation) to influence neuron activation. Synaptic plasticity enables neural pathways to strengthen with repeated use, forming memories and adapting learning processes. Long-Term Potentiation (LTP) is a key mechanism underpinning memory formation, characterized by lasting improvements in communication between neurons after repeated stimulation.