Chapter 7—The Nervous SystemPart 1

General Functions of the Nervous System

Sensory input – Gathering Information Monitor changes occurring inside and

outside the body Changes = stimuli

Integration of stimuli To process and interpret sensory input

and decide if action is needed Motor output—Responding to Stimuli

Sending instructions to activate muscles or glands

General Functions of the Nervous System

Central nervous system (CNS) Brain Spinal cord

Peripheral nervous system (PNS) Nerves that extend from the brain and spinal

cord Spinal nerves—carry impulses to & from the

spinal cord Cranial nerves—carry impulses to & from

brain Carry impulses from sensory receptors to CNS &

from CNS to appropriate muscles & glands

Structural Classification of the Nervous System

Figure 7.1

Functional Classification of the Peripheral Nervous System

Sensory (Afferent) Division Nerve fibers that carry information to the

central nervous systemSomatic – fibers from skin, muscles &

jointsVisceral – fibers from internal organs

Figure 7.1

Functional Classification of the Peripheral Nervous System

Motor (efferent) division Nerve fibers that carry impulses away

from the central nervous systemTo effector organs, muscles, glands

Figure 7.1

Functional Classification of the Peripheral Nervous System

Motor (efferent) division Two subdivisions

Somatic nervous system = voluntary (skeletal muscle)

Autonomic nervous system = involuntary (smooth & cardiac muscle, glands)2 parts: sympathetic & parasympathetic

Organization of the Nervous System

Figure 7.2

Nervous tissue made of 2 kinds of cells: Supporting cells = neuroglia (aka glial

cells)Cannot transmit nerve impulsesCan divideMost brain tumors are gliomasFunction – to support, insulate, and protect neurons

Neurons

Nervous Tissue—Structure & Function

Figure 7.3a

Nervous Tissue: Support Cells of CNS

Astrocytes (half of neural tissue) Abundant, star-shaped cells Brace neurons Form barrier

between capillaries and neurons

Control the chemical environment of the brain

Nervous Tissue: Support Cells Microglia

Spider-like phagocytes

Dispose of debris, including bacteria & dead brain cells

Figure 7.3b–c

Nervous Tissue: Support Cells

Ependymal Cells Line cavities of the brain and spinal

cord Circulate cerebrospinal fluid Cushions CNS tissue

Nervous Tissue: Support Cells

Oligodendrocytes Wrap around nerve fibers in the central

nervous system Produce myelin sheath that insulates

nerve fibers

Figure 7.3d

Figure 7.3e

Nervous Tissue: Support Cells of PNS

Satellite Cells Cushion & protect neuron cell bodies

Schwann Cells Form myelin sheath in the peripheral

nervous system

Nervous Tissue: Neurons

Neurons = Nerve Cells Cells specialized to transmit messages Major regions of neurons

Cell Body – nucleus and metabolic center of the cell

Processes – fibers that extend from the cell bodyNerve fibers covered in whitish, fatty

material = myelin Protects & insulates & increases

transmission rate of nerve impulse

Neuron Anatomy Cell Body

Nissl substanceSpecialized rough endoplasmic

reticulum Neurofibrils

Intermediate cytoskeleton Maintains cell shape

NucleusLarge nucleolus

Figure 7.4a

Neuron AnatomyExtensions outside the cell body (can be 3-4’ long)Dendrites – conduct impulses toward the cell body

Axons – conduct impulses away from the cell body

Neuron has 100’s of dendrites, but only 1 axon

Figure 7.4a

Figure 7.4a–b

Axons and Nerve Impulses

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

Nerve Fiber Coverings

Myelin sheath—whitish, fatty material covering axons

Schwann cells – produce myelin sheaths that form in jelly-roll like fashion around nerve Found in PNS only Neurilemma – cytoplasm on outside

Nodes of Ranvier – gaps in myelin sheath along the axon

Figure 7.5

Neuron

Neuron Cell Body Location Most neuron cell bodies are found in the

central nervous system Gray matter – cell bodies and

unmyelinated fibers Nuclei – clusters of cell bodies within

the gray matter of the central nervous system

White matter – myelinated fibers Ganglia – collections of cell bodies

outside the central nervous system

Functional Classification of Neurons Sensory (Afferent) Neurons

Cell bodies in ganglia of PNS Carry impulses from the sensory

receptors to CNSCutaneous sense organs (skin)Proprioceptors – detect stretch or tension (muscle)

Motor (Efferent) Neurons Cell bodies in CNS Carry impulses from the central

nervous system to viscera, muscles, glands

Functional Classification of Neurons

Free nerve endings (pain & temperature receptors)

Meissner’s Corpuscles (touch receptors

Lamellara Corpuscles (deep pressure receptors)

Golgi Tendon Organ (proprioceptors)

Muscle Spindle (proprioceptor)

Functional Classification of Neurons

Interneurons (Association Neurons)Cell bodies in CNSFound in neural pathways in the central nervous system

Connect sensory and motor neurons

Neuron Complex

Figure 7.6

Structural Classification of Neurons

Multipolar neurons – many extensions from the cell body (all motor & association neurons)

Figure 7.8a

Structural Classification of Neurons

Bipolar neurons – one axon and one dendrite Rare in adults In special sense organs (eye, nose)

Figure 7.8b

Structural Classification of Neurons Unipolar neurons – have a short, single

process leaving the cell body Axon conducts nerve impulses both toward

& away from cell body In PNS ganglia

Figure 7.8c

Functional Properties of Neurons Irritability – ability to respond to stimuli &

convert it into a nerve impulse Conductivity – ability to transmit an

impulse Resting Neuron (Baseline Anatomy):

The plasma membrane (of a neuron) at rest is polarized

Fewer positive ions are inside the cell than outside the cell (K+ inside, Na+ outside)

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 (aka “nerve impulse”) in the neuron

Figure 7.9a–c

Nerve Impulses

The Action Potential

If the action potential (nerve impulse) starts, it is propagated over the entire axon (all or none)

Impulses travel faster when fibers have a myelin sheath Saltatory conduction – impulse can’t

flow through myelin sheath, so it jumps across from node to node

Nerve Impulses

Repolarization Potassium ions rush out of the neuron

after sodium ions rush in, which repolarizes the membrane Restores electrical conditions at

membrane to polarized or resting state

The sodium-potassium pump restores the original configuration (Na+ outside, K+ inside) This action requires ATP

Nerve Impulses

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 next dendrite

Transmission of a Signal at Synapses

Transmission of a Signal at Synapses

Figure 7.10