Bio 203 Exam 1 Review

Homeostasis, the maintenance of a balanced internal environment, is discussed, focusing on positive and negative feedback loops. Positive feedback is a self-amplifying cycle (e.g., childbirth, blood clotting). Negative feedback, more prevalent in the body, involves receptors, a control center (brain), and effectors to maintain a set point (e.g., blood glucose, body temperature, blood pressure).

The review begins by distinguishing anatomy (study of structure and form, 'what it is') from physiology (study of function, 'how it works'), emphasizing that form dictates function. It then covers the characteristics of living things: metabolic processes (catabolic and anabolic), generative processes (reproduction, growth, healing), responsiveness to internal and external environments (homeostasis), and structure and organization, particularly the cell doctrine.

The nervous system is divided into the central nervous system (CNS - brain and spinal cord) and the peripheral nervous system (PNS - everything else). Communication involves sensory (afferent, arriving) information from the PNS to the CNS, and motor (efferent, exiting) commands from the CNS. Further divisions include visceral sensory, somatic sensory, somatic motor (voluntary), and visceral motor (involuntary, also known as the autonomic nervous system).

The autonomic nervous system is explained with its two divisions: sympathetic ('fight or flight') and parasympathetic ('rest and digest'). The sympathetic division prepares the body for crisis, increasing heart rate and breathing, while the parasympathetic division calms the body and directs maintenance systems.

The structure of a multipolar neuron (soma, dendrites, axon, axon terminals) is detailed as the most common type, especially in the CNS. The role of supportive glial cells (neuroglia) is also highlighted, with two types in the PNS and four in the CNS, outnumbering neurons 10 to 1.

Regeneration of nerve fibers is possible only in the peripheral nervous system, provided certain criteria are met: the presence of Schwann cells and neurilemma, an intact soma, and a very close break. This process is often slow and imperfect.

Neuronal communication involves alternating between electrical (ion movement) and chemical (neurotransmitter) signals. The resting membrane potential (-70 mV) is maintained by the sodium-potassium pump. Ion channels, including ligand-gated and voltage-gated types, play a crucial role in electrical transmission. Calcium influx in the axon terminal triggers neurotransmitter release.

The process of generating an action potential is described: a strong enough local potential at the axon hillock opens voltage-gated channels, leading to depolarization (sodium influx), repolarization (potassium efflux), and hyperpolarization (brief overshoot of negative potential). The absolute refractory period (no new action potential) and relative refractory period (stronger stimulus needed) are also explained.

Three types of synapses are discussed: excitatory cholinergic (acetylcholine, leading to depolarization by sodium influx), inhibitory GABAergic (GABA, leading to hyperpolarization by chloride influx), and adrenergic (norepinephrine, using a secondary messenger system for amplified and flexible responses). The mechanism for neurotransmitter release and binding is consistent across these types.

Neurotransmitters are cleared from the synaptic cleft by diffusion, reuptake (endocytosis), or enzymatic degradation (e.g., acetylcholine esterase). This clearing mechanism ensures signal flexibility. The trade-off between speed (electrical synapses with gap junctions) and flexibility (chemical synapses for decision-making and integration) is also discussed.

The post-synaptic neuron integrates multiple excitatory and inhibitory signals. Two types of summation are explained: temporal summation (one neuron firing rapidly) and spatial summation (multiple neurons firing simultaneously). This integration determines whether an excitatory (EPSP) or inhibitory (IPSP) post-synaptic potential is generated, highlighting the flexibility and control in chemical synapses.