Nervous System Organization: Chapter 15

• Lecture relates to nervous system organization
• Chapter 15 is relevant
• Lecture Date: 2/24/25

Types of Neurons

• Interneurons are confined to the CNS
• Sensory neurons convey information to the CNS
• Motor neurons convey information out of the CNS to control muscles or other effectors
• Effector cells carry out functions such as motion or secretion under the direction of the nervous system

Central Nervous System (CNS)

• The CNS consists of the brain and spinal cord
• Large numbers of neurons and support cells interact within the CNS
• The CNS works to achieve integrative functions
• The CNS is composed of gray and white matter
• Gray matter consists of intermingled neuronal cell bodies, processes, and synaptic contacts
• White matter consists entirely of tracts of myelinated axons
• The brain is organized into forebrain, midbrain, and hindbrain
• The cerebral cortex becomes increasingly prominent in higher vertebrates

Brain Anatomy

• Neurons in different anatomical regions of the brain have different functional roles
• Functional regions exist, this does not mean any one area of the brain is involved in only a single function
• The brain has maps that maintain information about the body’s anatomical organization
• Somatotopic maps mean the sensory surface of the body is mapped onto the cerebral cortex

Mammalian Brain Divisions

• Forebrain: Telencephalon includes the Cerebral cortex, higher sensory, motor, and integrative functions
• Forebrain: Telencephalon includes the Hippocampus, for learning and memory
• Forebrain: Telencephalon includes the Basal ganglia, for motor control
• Forebrain: Telencephalon includes the Limbic system, for emotions
• Forebrain: Diencephalon includes the Thalamus, a major sensory relay
• Forebrain: Diencephalon includes the Hypothalamus, for homeostatic and endocrine regulation and circadian clock
• Midbrain: Mesencephalon area includes the Superior colliculus, for visual integration
• Midbrain: Mesencephalon include the Inferior colliculus, for auditory integration
• Hindbrain: Metencephalon area include the Cerebellum, responsible for motor coordination
• Hindbrain: Metencephalon area include the Pontine motor nuclei, for descending motor control
• Hindbrain: Myelencephalon area include the Medulla oblongata, for autonomic and respiratory control

Peripheral Nervous System (PNS)

• The PNS consists of all processes and cell bodies of sensory and motor neurons outside the CNS
• A nerve consists of the axons of multiple neurons bundled together
• Cranial nerves are connected to the brain
• Spinal nerves are connected to the spinal cord
• The PNS divides into the somatic and autonomic nervous systems

Somatic Nervous System

• The somatic nervous system is part of the PNS
• The somatic nervous system controls skeletal muscles and voluntary control
• Somatic motor neurons directly synapse on muscle fibers
• Somatic motor neurons release acetylcholine (ACh)
• Somatic motor neurons excites muscles through nicotinic ACh receptors

Autonomic Nervous System

• The autonomic nervous system (ANS) is part of the PNS
• The ANS controls autonomic effectors like cardiac and smooth muscles and glands such as those in the gut, blood vessels, and eye
• The ANS has three divisions: sympathetic, parasympathetic, and enteric
• The sympathetic division increases activity, i.e. "fight or flight" response
• The parasympathetic division activates "rest and digest" processes
• The enteric division controls gut motility, absorption, and secretion
• The enteric division likely has more neurons than the spinal cord

Peripheral Synapses

• Sympathetic and parasympathetic nervous systems have a peripheral synapse
• The peripheral synapse is an "extra" synapse between the CNS and the effector
• Peripheral synapses are located within clusters of neuronal cell bodies called autonomic ganglia
• Preganglionic neurons are before the ganglia and release ACh (are cholinergic)
• Postganglionic neurons occur after these ganglia
• Postganglionic neurons release ACh in the parasympathetic system
• Postganglionic neurons release norepinephrine in he sympathetic system (are adrenergic)

CNS Nerve Exits

• Axons of parasympathetic neurons exit the CNS in the cranial and sacral region
• Axons of sympathetic preganglionic neurons exit the CNS in nerves of the thoracic and lumbar regions

Nervous System Diversity

• Central nervous systems can be described as either ganglionic or columnar
• Ganglionic nervous systems are found in annelids, mollusks, and arthropods
• Ganglionic CNS consists of a chain of segmental ganglia
• Chained ganglia are linked by bundles of axons called connectives
• Ganglionic nervous systems are ventral and solid
• Columnar nervous systems are found in vertebrates and consist of a continuous column of nervous tissue
• Columnar tissue is dorsal
• Columnar tissue is hollow and filled with cerebrospinal fluid
• Columnar has one pair of spinal nerves per vertebra
• Axons of sensory neurons enter the spinal cord in the dorsal roots of the spinal nerves
• Ventral roots of spinal nerves contain axons of somatic motor neurons and autonomic neurons
• Dorsal and ventral roots fuse together
• Centralization constitutes when neurons are collected into central integrating areas rather than being randomly dispersed
• Cephalization means the concentration of nervous structures and functions at one end of the body in the head
• Birds and mammals have evolved more complex forebrain structures