Chapter Nine The Nervous System

Introduction to the Nervous System Section One Introduction to the Nervous System

Functions of the Nervous System Sensory input- information gathered by sensory receptors about internal and external changes Integration- interpretation of sensory input Motor output- activation of effector (muscles and glands) produces a response

Functions of the Nervous System

Divisions of the Nervous System Central Nervous System (CNS) Brain and spinal cord Integration and command center Peripheral nervous system (PNS) Nerves outside brain & spinal cord Paired spinal and cranial nerves carry messages to and from the CNS

Peripheral Nervous System Functional Classification Sensory (afferent) division: Nerve fibers that carry information to the central nervous system 1 Somatic Afferent Fibers voluntary convey impulses from skin, skeletal muscles, and joints 2 Visceral Afferent Fibers Involuntary convey impulses from visceral organs

Motor (Efferent) Division Transmits impulses AWAY from the CNS and takes it to the effector organs 2 Types: Somatic and Autonomic Nervous Systems Somatic Nervous System Voluntary Conscious control of skeletal muscles (Good pic on next slide) Autonomic Nervous System Involuntary Visceral Nerve Fibers (for smooth muscle, cardiac, & glands) 2 types (continued……)

Functional Division of the Autonomic NS Sympathetic Fight or Flight Parasympathetic Rest and digest Feed and breed

Section Two Nervous System Cells

Histology of Nerve Cells

Neurons (Nerve Cells) excitable cells that transmit electrical signals 3 parts and accessory structures: Cell Body Dendrites Axons

Cell Body Nucleus & metabolic center of the nerve cell Large Nucleolus Associated structures: Axon Hillock: Cone shaped area where the axon arises Neurofibrils: Intermediate cytoskeleton that maintains cell shape Clusters of cell bodies in the: CNS: Nuclei PNS: Ganglia

Processes of the Cell Body Dendrites: conduct impulses toward the cell body Axons: conduct impulses away from the cell body One long axon (“nerve fiber”) per cell body. Axons end in axonal terminals Axonal terminals contain vesicles with neurotransmitters Axonal terminals are separated from the next neuron by a gap Synaptic cleft – gap between adjacent neurons Synapse – junction between nerves

The Nerve Cell and Associated Structures

Axon  Away from cell body Dendrite  toward cell body

Axon in comparison to the rest of the nerve bundle

Nerve Fiber (long axons) Coverings Schwann cells – produce myelin sheaths in jelly-roll like fashion Nodes of Ranvier – gaps in myelin sheath along the axon

You need to know this structure! Notice the difference?!

Neuron Cell Body Locations: 1. Most are found in the central nervous system Gray matter – cell bodies and unmyelinated fibers White matter – dense collection of myelinated fibers Nuclei – clusters of cell bodies within the white matter of the central nervous system 2. Ganglia – collections of cell bodies outside the central nervous system

Functional Classification of Neurons Sensory (afferent)- transmit impulses from sensory receptors toward the CNS Interneurons (association neurons)- Connect sensory and motor neurons in the neural pathways of CNS Motor (efferent)- carries impulses from the CNS to effectors

Neuron Classification

Structural Classification of Neurons Multipolar neurons – many extensions from the cell body Most common in vertebrates Bipolar neurons – one axon and one dendrite Eyes, nose, ears Unipolar neurons – have a short single process leaving the cell body Most common in invertebrates Figure 7.8a

CNS Vs PNS Neuroglia (Support Cells) This is not in your notes. It is for a reference. There are 4 CNS support cells Astrocytes Microglia Ependymal Oligodendrocytes There are 2 PNS support cells Satellite Cells Schwann Cells

Nervous Tissue: Support Cells (Neuroglia) (Glial Cells) Astrocytes MOST abundant, star-shaped cells Support & Brace neurons Form barrier between capillaries and neurons Control the chemical environment of the brain Microglia Spider-like phagocytes (swallows invaders) Dispose of debris Figure 7.3a

Nervous Tissue: Support Cells Ependymal cells Line cavities of the brain and spinal cord Circulate cerebrospinal fluid Oligodendrocytes Produce insulating myelin sheath around nerve fibers (axons) in the central nervous system Nodes of Ranvier-myelin sheath gaps between Schwann cells, sites where axon collaterals can emerge Figure 7.3b–c

Nervous Tissue: Support Cells Satellite cells Surround and protect neuron cell bodies in PNS Schwann cells Form myelin sheaths & surrounds the peripheral nerve fibers Helps regeneration of damaged peripheral nerve fibers Figure 7.3e

Chapter Nine Section 3: Delivering the Message End of Day 1, Start of Day 2 Chapter Nine Section 3: Delivering the Message

Functional Properties of Neurons Irritability – ability to respond to stimuli Conductivity – ability to transmit an impulse The plasma membrane at rest is polarized Fewer positive ions are inside the cell than outside the cell

Starting a Nerve Impulse Depolarization – a stimulus depolarizes the neuron’s membrane A depolarized membrane allows sodium (Na+) to flow inside the membrane The exchange of ions initiates an action potential in the neuron Figure 7.9a–c

The Action Potential If the action potential (nerve impulse) starts, it is propagated over the entire axon Potassium ions rush out of the neuron after sodium ions rush in, which repolarizes the membrane The sodium-potassium pump restores the original configuration This action requires ATP

Nerve Impulse Propagation The impulse continues to move toward the cell body Impulses travel faster when fibers have a myelin sheath Figure 7.9d–f

Continuation of the Nerve Impulse between Neurons Impulses are able to cross the synapse to another nerve Neurotransmitter is released from a nerve’s axon terminal The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter An action potential is started in the dendrite

How Neurons Communicate at Synapses Figure 7.10

Section 4: Developmental aspects of neurons Cell death About 2/3 of neurons die before birth Death results in cells that fail to make functional synaptic contacts Many cells also die due to apoptosis (programmed cell death) during development (we have webbed fingers and toes in the womb!!!)

Multiple Sclerosis (MS) An autoimmune disease that mainly affects young adults Myelin sheaths in the CNS become nonfunctional scleroses Shunting and short-circuiting of nerve impulses occurs, impulse condition slows and eventually ceases Symptoms: visual disturbances, weakness, loss of muscular control, speech disturbances, and urinary incontinence.