The Nervous System: The Spinal Cord and Spinal Nerves Chapter 8 The Nervous System: The Spinal Cord and Spinal Nerves

Role 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 nervous system.

Functional Divisions Somatic nervous system Controlled voluntarily Effectors are skeletal muscles No further subdivisions Autonomic (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 Functions Functional cells of nervous system Highly specialized Unique structure

Structure of a Neuron Cell body Nucleus Other organelles Cell 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 myelin sheath.

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 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.

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

Spinal cord and spinal nerves. ZOOMING IN • Is the spinal cord the same length as the spinal column? How does the number of cervical vertebrae compare with the number of cervical spinal 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 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 a somatic or an autonomic reflex arc? What type of neuron is located between the sensory and 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

The patellar (knee-jerk) reflex ZOOMING IN • How many total neurons are involved in this spinal reflex? What neurotransmitter is released at the synapse shown by number 5? 2 neurons are involved. Acetylcholine is the neurotransmitter

The 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 nerves carry impulses from the skin of the toes? From the anterior hand and fingers? Toes - Ist sacral, anterior hand and feet – 8th cervical

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

The Autonomic Nervous 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 Nervous System Sympathetic nervous system Thoracolumbar area Collateral ganglia Adrenergic Parasympathetic Craniosacral Terminal ganglia Cholinergic

Autonomic nervous system. ZOOMING IN • Which division of the autonomic nervous system has ganglia closer to the effector organ? Parasympathetic

Functions of the Autonomic Nervous System 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.