Nervous System and Special Senses: Structure, Function, and Physiology

11.1 Overview of the Nervous System

Major Functions and Divisions

The nervous system is responsible for coordinating and regulating bodily functions through rapid communication. It is divided into the central nervous system (CNS) and the peripheral nervous system (PNS).

• Central Nervous System (CNS): Consists of the brain and spinal cord; processes information and issues commands.

• Peripheral Nervous System (PNS): Composed of nerves and ganglia outside the CNS; transmits signals between the CNS and the rest of the body.

• Major Functions: Sensory input, integration, and motor output.

• Functional Divisions: Sensory (afferent) and motor (efferent) divisions; the motor division includes the somatic and autonomic nervous systems.

11.2.1 Neurons

Structure and Function of Neurons and Neuroglia

Neurons are the primary signaling cells of the nervous system, while neuroglia support and protect neurons.

• Neuron Structure: Consists of the cell body (soma), dendrites (receive signals), and axon (transmits signals).

• Types of Neurons: Sensory (afferent), motor (efferent), and interneurons (association neurons).

• Neuroglial Cells: Supportive cells in CNS (astrocytes, oligodendrocytes, microglia, ependymal cells) and PNS (Schwann cells, satellite cells).

• Function: Neurons transmit electrical impulses; neuroglia provide structural support, insulation, and metabolic support.

• Myelination: Schwann cells (PNS) and oligodendrocytes (CNS) form myelin sheaths, increasing conduction speed.

11.3 Electrophysiology of Neurons

Action Potentials and Electrical Properties

Neurons communicate via electrical signals called action potentials, generated by voltage-gated ion channels.

• Resting Membrane Potential: The difference in charge across the neuronal membrane at rest, typically around -70 mV.

• Action Potential: A rapid, temporary change in membrane potential due to the movement of Na+ and K+ ions.

• Phases: Depolarization (Na+ influx), repolarization (K+ efflux), and hyperpolarization.

• Absolute and Relative Refractory Periods: Times during which a neuron cannot or is less likely to fire another action potential.

• Conduction Velocity: Influenced by axon diameter and myelination.

Equation:

$V_m = V_{in} - V_{out}$

Where $V_m$ is the membrane potential.

11.4 Synapses

Chemical and Electrical Synapses

Synapses are specialized junctions where neurons communicate with other neurons or effector cells.

• Chemical Synapses: Use neurotransmitters to transmit signals across a synaptic cleft.

• Electrical Synapses: Allow direct passage of ions through gap junctions for rapid communication.

• Excitatory Postsynaptic Potential (EPSP): Depolarizes the postsynaptic membrane, increasing the likelihood of an action potential.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes the postsynaptic membrane, decreasing the likelihood of an action potential.

• Neurotransmitters: Chemical messengers such as acetylcholine, dopamine, and serotonin.

Example: At the neuromuscular junction, acetylcholine is released to stimulate muscle contraction.

14.1 Overview of the Autonomic Nervous System (ANS)

Structure and Function

The ANS regulates involuntary physiological processes, including heart rate, digestion, and respiratory rate. It consists of sympathetic and parasympathetic divisions.

• Sympathetic Division: Prepares the body for 'fight or flight' responses.

• Parasympathetic Division: Promotes 'rest and digest' activities.

• Target Cells: Smooth muscle, cardiac muscle, and glands.

• Somatic vs. Autonomic: Somatic controls voluntary muscles; autonomic controls involuntary functions.

14.2.3 Sympathetic Effects on Target Cells

Neurotransmitters and Receptors

The sympathetic division uses neurotransmitters such as norepinephrine and epinephrine, acting on adrenergic receptors.

• Adrenergic Receptors: Alpha and beta receptors mediate different physiological effects.

• Effects: Increased heart rate, bronchodilation, pupil dilation, decreased digestive activity.

14.3.3 Parasympathetic Effects on Target Cells

Neurotransmitters and Receptors

The parasympathetic division primarily uses acetylcholine, acting on muscarinic and nicotinic receptors.

• Effects: Decreased heart rate, increased digestive activity, pupil constriction.

14.4 PNS Maintenance of Homeostasis

Homeostatic Regulation

The parasympathetic nervous system maintains homeostasis by regulating organ function during restful states.

• Sympathetic Tone: Baseline level of sympathetic activity.

• Parasympathetic Tone: Baseline level of parasympathetic activity.

• Homeostasis: Balance between sympathetic and parasympathetic activity ensures stable internal conditions.

15.4.5 Anatomy & Physiology of the Eye

Structure and Function of the Eye

The eye is a complex organ responsible for vision, composed of several layers and specialized cells.

• Three Layers of the Eyeball: Fibrous (sclera, cornea), vascular (choroid, ciliary body, iris), and neural (retina).

• Retina: Contains photoreceptors (rods and cones) that detect light and initiate visual signals.

• Photoreceptors: Rods detect dim light; cones detect color and detail.

• Image Formation: Light is focused by the cornea and lens onto the retina, where photoreceptors convert it into electrical signals.

• Path of Light: Light passes through the cornea, aqueous humor, lens, vitreous humor, and strikes the retina.

• Visual Pathway: Action potentials travel from the retina via the optic nerve to various parts of the brain for processing.

Photoreceptor	Function	Location
Rods	Dim light, peripheral vision	Retina (periphery)
Cones	Color vision, sharp detail	Retina (fovea centralis)

Example: The fovea centralis contains a high density of cones, allowing for sharp central vision.