Chapter 8

The Nervous System: The The Nervous System: The Spinal Cord and Spinal Spinal Cord and Spinal

NervesNerves

Role of the Nervous SystemRole of the Nervous System

Nervous system coordinates all body systems• Detects and responds to stimuli• Brain and spinal cord act as switching centers• Nerves carry messages to and from centers

Structural Divisions

• Central nervous system (CNS)• Brain• Spinal cord

• Peripheral nervous system (PNS)• Cranial nerves• Spinal nerves

Anatomic divisions of the nervoussystem.

Functional Divisions

Somatic nervous system• Controlled voluntarily• Effectors are skeletal muscles• No further subdivisionsAutonomic (or visceral) nervous system (ANS)• Controlled involuntarily• Effectors are smooth muscle, cardiac muscle, and glands• Subdivided into

• Sympathetic nervous system• Parasympathetic nervous system

• Checkpoint 8-1: What are the two divisions of the nervous system based on structure?– Central nervous system – Autonomic nervous system

• Checkpoint 8-2: The nervous system can be divided functionally into two divisions based on type of control and effectors. What division is voluntary and controls skeletal muscle, and what division is involuntary and controls involuntary muscles and glands?– The somatic system is voluntary and controls skeletal

muscle– The autonomic system is involuntary and controls

involuntary muscles and glands

Neurons and Their FunctionsNeurons and Their Functions

Neurons• Functional cells of nervous system• Highly specialized• Unique structure

Structure of a Neuron

Cell body• Nucleus• Other organellesCell fibers• Dendrites• Axons

• Some are protected by myelin sheath

Diagram of a motor neuron.

ZOOMING IN • Is the neuron shown here a sensory or a motor neuron?

Motor neuron

A typical neuron as seen under the microscope.

Formation of a myelinsheath.

Checkpoint 8-3: The neuron, the functional unit of the nervous system, has long fibers extending from the cell body. What is the name of the fiber that carries impulses toward the cell body and what is the name of the fiber that carries impulses away from the cell body?

The dendrite carries impulses toward the cell body and the axon carries impulses away from the cell body. Try to remember it this way – DCBA

D – dendrite > CB – cell body > A - axon

Checkpoint 8-4: Myelin is a substance that covers and protects some axons. What color describes myelinated fibers, and what color describes unmyelinated tissue of the nervous system?

Myelinated fibers are white

Unmyelinated tissue is gray matter

Types of Neurons

• Sensory neurons (afferent neurons)• Conduct impulses to spinal cord, brain

• Motor neurons (efferent neurons)• Conduct impulses to muscles, glands

• Interneurons (central or association neurons)• Conduct information within CNS

Nerves and Tracts

• Nerve: fiber bundle within PNS• Tract: fiber bundle within CNS• Organized into fascicles• Connective tissue layers

• Endoneurium• Perineurium• Epineurium

Cross section of a nerve as seen under the microscope (x132).

Checkpoint 8-5: Nerves are bundles of neuron fibers in the PNS. These nerves may be carrying impulses either toward or away from the CNS. What name is given to nerves that convey impulses toward the CNS, and what name is given to nerves that transport away from the CNS?

Nerves that convey impulses toward the CNS are sensory or afferent.

Nerves that carry impulses away from the CNS are motor or efferent.

NeurogliaNeuroglia

Neuroglia (glial cells)• Protect nervous tissue• Support nervous tissue• Aid in cell repair• Remove pathogens and impurities• Regulation composition of fluids around and between cells

Examples of neuroglia.

• Checkpoint 8-6: The nonconducting cells of the nervous system serve in protection and support. What are these cells called?

– Neuroglia

The Nervous System at WorkThe Nervous System at Work

Electrical impulses are sent along neuron fibers and transmitted between cells at junctions

The Nerve Impulse

• Plasma membrane carries electrical charge (potential)• Plasma membrane is polarized (negative charge)• Membrane potential reverses, generates electrical

charge (action potential) • Resting state• Depolarization• Repolarization

• Myelin sheath speeds conduction

The action potential.

A nerve impulse.

• Checkpoint 8-7: An action potential occurs in two stages. In the first stage, the charge on the membrane reverses, and in the second stage, it returns to the resting state. What are the names of these two stages?– The first stage is depolarization and the second stage

is repolarization

• Checkpoint 8-8: What ions are involved in generating an action potential?– Sodium (Na+) and potassium (K-)

The Synapse

Junction point for transmitting nerve impulse• Axon (presynaptic cell) • Dendrite (postsynaptic cell)• Synaptic cleft• Neurotransmitter

• Epinephrine (adrenaline)• Norepinephrine (noradrenaline)• Acetylcholine

• Receptors

A synapse.

Checkpoint 8-9: Chemicals are needed to carry information across the synaptic cleft at a synapse. As a group, what are all these chemicals called?

Neurotransmitters

The Spinal CordThe Spinal Cord

• Links PNS and brain• Helps coordinate impulses within CNS• Contained in and protected by vertebrae

Spinal cord and spinalnerves.

ZOOMING IN• Is thespinal cord the samelength as the spinalcolumn? How does thenumber of cervicalvertebrae compare withthe number of cervicalspinal nerves?

No, it isn’t. There are 7 cervical vertebrae but 8 cervical spinal nerves

Structure of the Spinal Cord

• Unmyelinated tissue (gray matter)• Dorsal horn• Ventral horn• Gray commissure• Central canal

• Myelinated axons (white matter)• Posterior median sulcus• Anterior median fissure• Ascending and descending tracts

The spinal cord.

• Checkpoint 8-10: The spinal cord contains both gray and white matter. How is this tissue arranged in the spinal cord?– The gray matter is in the central portion and the white

matter on the outside

• Checkpoint 8-11: What is the purpose of the tracts in the white matter of the spinal cord?– Sensory (afferent) impulses to the brain are

transmitted in the ascending tracts and motor (efferent) impulses are transmitted in the descending tracts to the peripheral nervous system.

The Reflex Arc

• Receptor detects stimulus• Sensory neuron transmits impulses to CNS• CNS coordinates impulses and organizes response• Motor neuron carries impulses away from CNS• Effector carries out response

Typical reflex arc.

ZOOMING IN • Is this asomatic or an autonomic reflexarc? What type of neuron islocated between the sensoryand motor neuron in the CNS?

This is a somatic reflex arc. Interneurons are located between the sensory and motor neuron.

• Checkpoint 8-12: What name is given to a pathway through the nervous system from a stimulus to an effector?– Reflex arc

Reflex Activities

• Simple reflex• Rapid• Uncomplicated• Automatic

• Spinal reflex• Coordinated in spinal cord• Stretch reflex is example

ZOOMING IN• How many total neurons are involved in this spinal reflex? Whatneurotransmitter is released at the synapse shown by number 5?

The patellar (knee-jerk) reflex

2 neurons are involved. Acetylcholine is the neurotransmitter

The Spinal NervesThe Spinal Nerves

• 31 pairs• Each nerve attached to spinal cord by two roots

• Dorsal root• Dorsal root ganglion

• Ventral root• Mixed nerves

Branches of the Spinal Nerves

• Plexus (network)• Cervical plexus

• Phrenic nerve• Brachial plexus

• Radial nerve• Lumbosacral plexus

• Sciatic nerve• Dermatomes

Dermatomes.

ZOOMING IN• Which spinal nervescarry impulses from the skin of thetoes? From the anterior hand andfingers?

Toes - Ist sacral, anterior hand and feet – 8th cervical

Checkpoint 8-13: How many pairs of spinal nerves are there?

31 pairs

The Autonomic NervousThe Autonomic NervousSystem (ANS)System (ANS)

Regulates the action of glands, smooth muscles of hollow organs and vessels, and heart muscle

• Preganglionic neuron connects spinal cord to ganglion• Postganglionic neuron connects ganglion to effector

Checkpoint 8-14: How many neurons are there in each motor pathway of the ANS?

All autonomic pathways contain 2 motor neurons connecting the spinal cord with the effector organ.

Divisions of the Autonomic NervousSystem

• Sympathetic nervous system• Thoracolumbar area• Collateral ganglia• Adrenergic

• Parasympathetic• Craniosacral• Terminal ganglia• Cholinergic

Autonomic nervous system.

ZOOMING IN• Which divisionof the autonomic nervous systemhas ganglia closer to the effectororgan?

Parasympathetic

Functions of the Autonomic NervousSystem

• Sympathetic nervous system• Fight-or-flight response

• Parasympathetic nervous system• Returns body to normal

• Systems generally have opposite effects on organ

Checkpoint 8-15: Which division of the ANS stimulates a stress response, and which division reverses the stress response?

The sympathetic division stimulates a stress response and the autonomic division reverses it.