Chapter 9:

Nervous System

Functions Sensors

› Monitor external and internal environment Processing

› Receives information, integrates it, and decided what to do

Effectors› Carries messages to effectors and tells

them what to do

Types of Nervous Cells Neurons

› Main cell of nervous tissue› Relay and process messages

Neuroglial› Provide support to the neurons› Several types known› Ex: Schwann, microglial

Neuroglial Cells Microglial cells

› Scattered throughout CNS› Phagocytize bacteria or cellular debris

Oligodendrocytes› Along nerve fibers› Provide myelin sheath (made of a fatty

substance called myelin) around axon in CNS Schwann cells

› Same as oligodendrocytes but in PNS

Neuroglial Cells (cont) Astrocytes

› Provide connection between a neuron and a blood vessel

› Provide support, help regulate ion concentrations in tissue, make-up scar tissue after injury

Ependymal cells› Forms epithelial-like linings on the outsides

of specialized parts or lining cavities within the CNS

Neurons Remember: It’s a cell! Body of neuron

› Cell Body – contains cell organelles› Dendrites- carry messages to cell body› Axons – carry messages away from cell body

Cell Structures› Large nucleus with easily seen nucleolus› Chromatophilic substance – similar to rough ER

Scattered throughout cytoplasm, membranous› Neurofibrils- help support cell shape

Axons Can be myelinated or unmyelinated PNS

› Schwann cells form myelin sheath› Nodes of Ranvier- small breaks in myelin

sheath CNS

› Oligodendrocytes form myelin › Myelinated neurons form white matter › Unmyelinated neurons form gray matter

Structural Classification of Neurons

Multipolar› Many small branched dendrites› One axon › Found in CNS

Bipolar› Two processes off of cell membrane (one axon and

one dendrite)› Neurons in special sense organs

Unipolar› One process off of cell body (one axon)› Found throughout PNS

Functional Classification of Neurons

Sensory (afferent) neurons› Have sensitive dendrites that are stimulated by

changes in environment› Message is taken into CNS› Usually unipolar or sometimes bipolar

Interneurons› Transfer, direct, and process messages within CNS› Usually multipolar

Motor (efferent) neurons› Carries message out of CNS to effectors› Usually multipolar

A Neuron at Rest Inside the neuron

› High in K+

› High in negative ions Outside the neuron

› High in Na+

› High in positive ions Result

› K+ tends to diffuse out› Na+ tends to diffuse in› Negative ions cannot cross

A Neuron at Rest (cont) Na/K pump - helps to restore

concentration gradient across the cell membrane

Resting potential - difference is electrical charge across the membrane› Established by concentration gradients of

various ions› Inside of the membrane has a negative

charge of 70 mv› Membrane is said to be polarized

Potential Changes Stimuli cause changes to the resting

potential by making the inside of the membrane less negative

Once a stimulus happens:› If stimulus is not strong enough to reach

threshold potential = cell membrane will return to resting potential

› If stimulus is strong enough to reach threshold potential = start an action potential

Summation - when additive effect of stimuli causes action potential

Action Potential Starts at trigger zone of axon Threshold stimulus open sodium channels Sodium moves into axon

› Because of the concentration gradient› Because of the negative charge that attracts the

positive ions Depolarizes the membrane as negative

charge diminishes Potassium channels open and potassium

moves out of the axon, repolarizing the membrane

Animation #1 Animation #2

Action Potential Animation

Nerve Impulse Action potential at the trigger zone

stimulates the next part of the axon to do a action potential

Potentials spread along the axon like a wave

Unmyelinated axons › Wave continues uninterrupted; relatively slow

Myelinated axons› Wave goes through saltatory conduction (jump

from one node to the next); very fastAnimation

Neuron Responses All-or-nothing effect

› Neuron does not react until a threshold stimulus is applied, but once it is applied it reacts fully

Stimuli greater than threshold levels don’t change the size of the response but changes its frequency

Refractory period:› After a action potential› Brief period of time › The nerve cannot be stimulated again.

The Synapse The connection between two neurons Don’t touch, separated by synaptic

cleft One-way communication between axon

of presynaptic neuron and dendrite of postsynaptic neuron

Neurotransmitters are made in the synaptic knob of the axon, stored in synaptic vesicles, and cross the cleft when needed

Neurotransmitter Actions

Excitatory Action:› A neurotransmitter that puts a neuron

closer to an action potential (facilitation) or causes an action potential

Inhibitory Action:› A neurotransmitter that moves a neuron

further away from an action potential Response of neuron:

› Responds according to the sum of all the neurotransmitters received at one time

Neurotransmitters Acetylcholine Monoamines – modified amino acids Amino acids Neuropeptides- short chains of amino

acids Depression:

› Caused by the imbalances of neurotransmitters

Many drugs imitate neurotransmitters› Ex: Prozac, zoloft, alcohol, drugs, tobacco

Release of Neurotransmitters

When an action potential reaches the end of an axon, Ca+ channels in the neuron open

Causes Ca+ to rush in› Cause the synaptic vesicles to fuse with the cell

membrane› Release the neurotransmitters into the synaptic

cleft After binding, neurotransmitters will either:

› Be destroyed in the synaptic cleft OR› Taken back in to surrounding neurons (reuptake)

Animation

Neuronal Pools Groups of highly interconnected

neurons that work together in the CNS Convergence

› Axons from different parts of the nervous system connect to the same neuron combining their affects

Divergence› A message from one neuron is sent to

many neurons at once; amplifies message

Convergence

Divergence

Nerves Nerves are made of bundled axons,

called nerve fibers Nerve fibers

› Sensory (afferent)- carry messages to CNS› Motor (efferent)- carry messages from CNS to

effectors Nerves

› Same definitions hold true› Most nerves contain both types of fibers and

are called mixed nerves

Structure of a nerve A nerve fiber (axon) is surrounded with

endoneurium Nerve fibers are bundled together and

surrounded with perineurium to form a fascicle

Fascicles are bundled together and surrounded with epineurium to form a nerve

Nerve Pathways

Path that the message takes through the body

Includes:› Sensor› Sensory neuron› Interneuron› Motor neuron› Effector

Reflex Arc Simplest nerve pathway is a reflex Reflexes without pain

› Involve only sensory and motor neurons› Ex: knee-jerk reflex

Reflexes with pain› Involve interneurons

in CNS› Ex: withdrawal reflex

The CNS Central nervous system Consists of brain and spinal cord Made of both gray and white matter Covered in meningies

The Meninges (Brain) Cranial Bone Dura mater

› First layer; tough, fibrous connective tissue › Forms inner periosteum of cranial bone › Folds into the cranium in some places to

form division walls in the brain Arachnoid mater

› Web-like membrane over CNS › Does not dip into crevices

The Meninges (Brain) Subarachnoid space

› Below arachnoid layer› Contains cerebrospinal fluid

Pia mater› Lower layer of meninges› Forms a tight covering over brain › Does dip into crevaces

Meningies (Spinal Cord) Same except:

› Vertebrae bones - protection› Epidural space- filled with loose connective

and adipose tissue› All other are the same

Cerebrospinal fluid Flows through ventricles (spaces in

brain) in the subarachnoid space, and through the central canal of the spinal cord

Fluid is made by the choroid plexus

Spinal Cord Stretches from brain to intervertebral

disk between first and second lumbar vertebrae

31 pairs of spinal nerves come of the cord

Gray matter core surrounded by white matter

Spinal Cord (cont) Responsible for communication

between brain and body and spinal reflexes

Ascending tracts› Nerves that send info to brain

Descending tracts› Nerves that send into to effectors

The Brain Made up off about 100 billion neurons Four main sections:

› 1) Cerebrum Nerves for processing sensory and motor

function Higher functions (like reasoning)

› 2) Diencephalon Processes sensory information

› 3) Brainstem Regulates certain body functions like breathing

› 4) Cerebellum Coordinates skeletal muscle movements

Cerebrum Divided into two hemispheres: right

and left Corpus callosum

› Connects the two sides Other structures

› Convolutions - ridges› Sulcus - shallow groove› Fissure - deep groove

Areas of the Cerebrum Frontal lobe Parietal lobe Temporal lobe Occipital lobe Insula

Each lobe has unique functions

Areas of the Cerebrum (cont)

Cerebral cortex› Thin layer of gray matter surrounding

cerebral hemisphere; contain most of the cell bodies in the cerebrum

Inner part of the cerebrum is mainly made of white matter

Functions of the Cerebrum Motor areas

› Primarily in frontal lobe› Send information out to effectors

Sensory areas› Interpret information from sensors› Area in parietal, temporal, and occipital

lobes Association areas

› Analyze information from sensors› Located in areas in all lobes mentioned

above

Diencephalon Located between the cerebral

hemispheres above the brainstem Contains:

› Thalamus› Hypothalamus› Pituitary gland› Pineal gland

Functions of the Diencephalon

Thalamus› Routes sensory impulses to the correct

region of the cerebrum Hypothalamus

› Monitors many internal conditions, link between nervous and endocrine system

Limbic system › Thalamus, hypothalamus, and basal nuclei› Controls experience and expression

(feelings)

Brainstem Connection between spinal cord and

the rest of the brain Contains:

› Midbrain› Pons› Medulla oblongata

Midbrain Located between diencephalon and

pons Contains some visual and auditory

reflexs Serves as the main connection for

motor neurons between spinal cord and upper part of brain

Pons Rounded bulge between midbrain and

medulla oblongata Relays impulses between medulla and

cerebrum or between cerebrum and cerebellum

Medulla oblongata Lowest part of brain, connect to spinal

cord All ascending and descending tracts

run through the oblongata Serves as a control center for many

vital function like heart rate, blood pressure, and respiratory center

Cerebellum Located in the lower back part of the

brain Structured liked cerebrum with inner

white matter core and gray matter covering

Controls posture and complex skeletal movements

The PNS Peripheral Nervous system Includes:

› 12 pairs of cranial nerves › 31 pairs of spinal nerves

Divided into:› 1) Somatic nervous system

Controls conscious activities; connects to skin and skeletal muscles

› 2) Autonomic nervous system Controls unconscious activities; connects to

internal organs or structures

Autonomic Nervous System

Two branches:› 1) Parasympathetic

Control under more normal conditions› 2) Sympathetic

Control under stress or emergency conditions (fight or flight)

Usually these have antagonistic effects› Work to counteract each other (one

increases, while other decreases) Either might be utilized to maintain

homeostasis