CHAPTER 7

THE NERVOUS SYSTEM

Introduction

The Nervous System (NS) is the master controlling and communicating system of the body.

Functions of the NS:

It uses sensory receptors to monitor changes occurring inside and outside the body.

It processes and interprets the sensory input and makes decisions about what should be done.

It activates muscles or glands.

Organization of the Nervous System

Structural Classification – Includes ALL nervous system organsCentral NS (CNS) – consists of the brain and

spinal cordPeripheral NS (PNS) – consists of the nerves

that extend from the brain or spinal cord

Organization of the Nervous System

Functional ClassificationSensory (Afferent) Division – consist of nerve

fibers that carry impulses to the CNS from sensory receptors; helps keep the CNS constantly informed of events going on both inside and outside the body.

Motor (Efferent) Division – carries impulses from the CNS to the organs, muscles, and glands to activate them; has 2 smaller subdivisions:

Somatic NS – allows us to consciously control our skeletal muscles (voluntary)

Autonomic NS – regulates events that are involuntary

Nervous Tissue: Structure & Function

The NS is made up of 2 types of cells – supporting cells and neuronsSupporting Cells - Lumped together in the CNS as NEUROGLIA, which means “nerve glue.” Each different type of neuroglia is called GLIA. The CNS glia include: Astrocytes – star shaped cells that help make exchanges

between the neurons and the capillaries; MAKES UP NEARLY ½ OF THE NERUAL TISSUE

Microglia – spider shaped cells that dispose of debris, including bacteria and dead brain cells

Ependymal Cells – line the cavities of the brain and spinal cord; have cilia to circulate cerebrospinal fluid

Oligodendrocytes – provide insulation to the nerve fibers

Nervous Tissue: Structure & Function

Neurons – Anatomy: Also called NERVE CELLS – specialized to transmit

messages from one part of the body to another. All neurons have a cell body and one or more slender

processes extending from the cell body. Parts of the Neuron:

Cell Body – the metabolic center of the neuron; contains organelles Nucleus – center of the cell Mitochondrion – gives the cell its energy Nissl Substance – the rough ER that maintains the shape of the cell Dendrites – convey incoming messages TOWARD the cell body Axons – convey incoming messages AWAY from the cell body Axonal Terminals – where the axons end Schwann Cells – cells that wrap around the axon Nodes of Ranvier – the gaps or indentions between the Schwann

Cells

Nervous Tissue: Structure & Function

Neurons – Classification: Functional Classification – groups neurons according to

the direction the nerve impulse is traveling; 3groups: Sensory – carry impulses from sensory receptors to the CNS Motor – carry impulses from the CNS to the muscles and glands Association – they connect the motor and sensory neurons in a

pathway

Structural – groups neurons according to the number of processes extending from the cell body; 3 groups:

Multipolar – has several processes; includes all motor and association neurons

Bipolar – has 2 processes (axon and dendrite); found only in the eye and ear

Unipolar – have one process; includes sensory neurons in the PNS

Nervous Tissue: Structure & Function

PhysiologyNerve Impulse – an electrochemical event,

initiated by stimuli, that transmits to other neurons, muscle, or glands.

Reflexes – rapid, predictable, and involuntary responses to stimuli; 2 types:

Autonomic Reflex – secretion of saliva and changes in the size of the pupil

Somatic Reflex – pulling your hand away from a hot object

Reflex Arc

Central Nervous System

Functional Anatomy of the Brain:Size = about 2 good fistfuls of pinkish gray

matterWeight = a little over 3 pounds4 major parts of the brain:

Cerebrum – largest partCerebellumBrain stemDiencephalon

Parts of the Brain

CerebrumStructure:Consists of 2 large masses called CEREBRAL

HEMISPHERES – mirror images of each other4 lobes of the cerebral hemispheres:

Frontal Lobe = anterior portion of each cerebral hemisphere

Parietal Lobe = posterior to the frontal lobeTemporal Lobe = lies below the frontal and parietal

lobesOccipital Lobe = posterior portion of each cerebral

hemisphereCerebral Cortex – outermost portion of the

cerebrum

Structure of the Cerebrum

CerebrumFunctionsThe cerebrum is concerned with higher brain

functions.Three Functional Regions

Motor Areas – the motor area of the right cerebral hemisphere controls skeletal muscles on the left side of the body and vice versa

Frontal Lobe = PRIMARY MOTOR AREA – controls speech, movement of the eyes, and writing

Sensory Areas – involves several lobes Parietal Lobe = sensations from all parts of the skin Occipital Lobe = vision Temporal Lobe = hearing

CerebrumFunctions Three Functional Regions continued…

Association Areas – function in the analysis and interpretation of sensory experiences and are involved with memory, reasoning, verbalizing, judgement, and emotional feelings

Frontal Lobe = concentrating, planning, problem solving Parietal Lobe = understanding speech and choosing the words

needed to express thoughts and feelings Temporal Lobe = understanding speech and reading printed words,

memory of visual scenes and music Occipital Lobe = analyzing visual patterns and recognizing another

person or an object

Both cerebral hemispheres participate in basic functions. However, in most persons one side acts as a dominant hemisphere for certain functions.

In 90% of the population, the left hemisphere is dominant for speech, writing, and reading.

Cerebellum

The cerebellum is a large mass of tissue located below the occipital lobes of the cerebrum and posterior to the pons and medulla oblongata of the brainstem.Functions: It communicates with other parts of the CNS It transmits sensory information concerning the position of

the limbs and joints It stimulates skeletal muscles to cause the desired body

movement It helps maintain posture

Damage to the cerebellum can result in tremors, inaccurate movements of voluntary muscles, the loss of muscle tone, and the loss of equilibrium

Brainstem

Brain Stem – a bundle of nervous tissue that connects the cerebrum to the spinal cord3 parts:Midbrain

A short section of the brain stem located between the diencephalons and pons

Serves as a reflex centerResponsible for moving the eyes to view

something as the head is turned It contains the auditory reflex centers that operate

when a person needs to move his/her head in order to hear sounds more distinctly

Brainstem

3 Parts Continued:Pons

Appears as a rounded bulge on the underside of the brain stem

Relays impulses to and from the medulla oblongata to the cerebrum

Medulla OblongataAn enlarged continuation of the spinal cord

extending from the pons to the skullThe nerve fibers that connect the brain and spinal

cord MUST pass through the medulla olongata

Brain Stem

Diencephalon

Located between the cerebral hemispheres and above the midbrainContains the thalamus – central relay station for sensory impulses ascending from other parts of the NS to the cerebral cortexContains the hypothalamus – plays a key role in maintaining homeostasis by regulating a variety of activities such as blood pressure, body temperature, body weight, sleep, and hunger

Protection of the CNSNervous tissue is very soft and delicate

Meninges - 3 connective tissue membranes covering and protecting the CNS structures Dura Mater – outermost layer; very tough and leathery Arachnoid Mater – the middle layer; weblike Pia Mater – innermost layer; clings tightly to the surface

of the brain and spinal cord

Cerebrospinal Fluid Provides a watery cushion around the brain and spinal

cord Any significant change in CSF may be a sign of

meningitis

Brain DysfunctionsTraumatic Brain InjuriesConcussion – occurs when brain injury is

slight; The victim may be dizzy, “see stars,” or lose consciousness briefly; no permanent brain damage

Contusion – the result of marked tissue destruction; can result in coma lasting from hours to a lifetime

Cerebral Edema – swelling of the brain due to inflammatory response to injury; can result in death

Spinal Cord

Structure – a cylindrical shaped structure which is a continuation of the brain stem31 pairs of spinal nerves arise from the cord

and exit from the vertebral column to serve the body area close by

Function – provides a 2-way conduction pathway to and from the brain

Peripheral Nervous SystemConsist of nerves found outside the CNSStructure of a Nerve Nerve – a bundle of neuron fibers found outside the

CNS. Nerves are classified according to the direction in

which they transmit impulses: Mixed Nerves – nerves carrying both sensory and motor

fibers Sensory Nerves – nerves carrying impulses toward the

CNS Motor Nerves – nerves carrying impulses away from the

CNS

Cranial Nerves - 12 pairs that arise from the brain and serve the head and neckSpinal Nerves – 31 pairs that arise from the spinal cord and serve the limbs

Peripheral Nervous SystemAutonomic Nervous System Structure:

The motor subdivision of the PNS that controls body activities automatically

Also known as the INVOLUNTARY NERVOUS SYSTEM Autonomic Functioning – 2 divisions:

Sympathetic Division Referred to as the “fight-or-flight” system Its activity is evident when we are excited or find ourselves in

emergency or threatening situations Signs of Activity = pounding heart, deepbreathing, sweaty skin It allows the body to cope rapidly with situations that threaten

homeostasis Parasympathetic Division

Most active when the body is at rest and not threatened in any way. Concerned with normal digestion and elimination of wastes and

conserving body energy Example: Relaxing after a meal

Developmental Aspects of the Nervous System

Because the NS is formed during the first month of embryonic development, any maternal infection early in pregnancy can have harmful effects on the fetal NS.One of the last areas of the CNS to mature is the hypothalamus, which regulates body temperature. This is why premature babies usually have to be monitored closely and put under a heating element.The brain reaches its maximum weight in the young adultNeurons die throughout life and are not replaced – brain mass declines with age