Nervous System

(1)

Biology

Sylvia S. Mader Michael Windelspecht

Chapter 37 Neurons and Nervous

Systems

Lecture Outline

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Outline

• 37.1 Evolution of the Nervous System

• 37.2 Nervous Tissue

• 37.3 The Central Nervous System

• 37.4 The Peripheral Nervous System

(2)

37.1 Evolution of the Nervous

System

• Invertebrate Nervous System Organization

– Hydras

• Nerve net

composed of neurons in contact with one

another

• Also in contact with contractile cells in the body wall

– Planarians

• Ladderlike nervous system

• Cephalization

- a concentration of

ganglia

and

sensory receptors in the head

– Annelids, Arthropods and Molluscs

• Complex animals

• True nervous systems

3

Evolution of the Nervous System

nerve net

lateral nerve cords cerebral ganglia

brain eyespot

auricle

(3)

Evolution of the Nervous System

5

brain

tentacle brain

eye

thoracic ganglion

giant nerve fiber

cerebrum in forebrain

hindbrain

spinal cord

f. Cat e. Squid

d. Crab

Evolution of the Nervous

System

• Vertebrate Nervous-System Organization

– Central nervous system

• Develops from an embryonic dorsal neural tube

• Cephalization and bilateral symmetry result in

several paired sensory receptors

– Eyes, ears, olfactory structures

– Vertebrate brain is organized into three areas

• Hindbrain

• Midbrain

• Forebrain

(4)

Organization of the Vertebrate

Brain

7

hindbrain cerebellum thalamus

cerebrum

pituitary hypothalamus

olfactory bulb

optic lobe

spinal cord

medulla oblongata

midbrain forebrain

Evolution of the Nervous

System

• Division of Nervous System:

– The

Central nervous system (CNS)

• Includes the brain and spinal cord

– The

peripheral nervous system (PNS)

• Consists of all nerves and ganglia that lie outside the

CNS

• Two divisions

– Somatic nervous system

» Sensory and motor functions that control skeletal muscle

(5)

Organization of the Human Nervous System

9 radial nerve median nerve ulnar nerve a. sciatic nerve tibial nerve spinal cord cervical nerves brain cranial nerves common fibular nerve thoracic nerves lumbar nerves sacral nerves

Organization of the Human Nervous System

10

b.

Central Nervous

System brain and spinal cord

Peripheral Nervous System

autonomic motor fibers (to cardiac

and smooth muscle, glands) sympathetic division parasympathetic division visceral sensory fibers (internal organs) somatic sensory fibers (skin, special senses) somatic motor fibers (to skeletal

muscles)

(6)

37.2 Nervous Tissue

• Neurons

(nerve cells)

–

Cell body

contains nucleus and organelles

–

Dendrites

receive signals from sensory

receptors or other neurons

–

Axon

conducts nerve impulses to another

neuron or to other cells

• Covered by

myelin sheath

• Any long axon is also called a

nerve fiber

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Nervous Tissue

• Types of Neurons

• Motor (efferent) neurons

– Accept nerve impulses from the CNS

– Transmit them to muscles or glands

• Sensory (afferent) neurons

– Accept impulses from sensory receptors

– Transmit them to the CNS

• Interneurons

(7)

Neuron Anatomy

13

node of Ranvier

a. Motor neuron (multipolar)

cell body dendrite

axon

muscle direction

of conduction

myelin sheath

axon terminal

a: © M.B. Bunge/Biological Photo Service

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cell body

b. Sensory neuron (unipolar) axon

axon

skin myelin sheath

direction of conduction

sensory receptor

(8)

15

c: © Manfred Kage/Peter Arnold, Inc.

c. Interneuron (multipolar)

axon cell body

dendrite

Neuron Anatomy

Nervous Tissue

• Transmission of Nerve Impulses

–

Resting Potential

• The membrane potential (voltage) when the axon

is not conducting an impulse

– The inside of a neuron is more negative than

the outside, around -70 mV

(9)

Nervous Tissue

• An

action potential

is a rapid change in

polarity across a portion of an axonal

membrane

• An action potential is generated only after

a stimulus larger than the

threshold

• Gated channel proteins

– One channel protein suddenly allows sodium

to enter the cell

– Another channel protein allows potassium to

leave the cell

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Resting and Action Potential of the Axonal

Membrane

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K+

+ + + + + + + + + + – – – – – – – – – –

gated K+ channel gated Na+ channel Na+

a. Resting potential: more Na+outside the axon and more K+inside the axon causes polarization.

outside axon

reference electrode outside axon recording

electrode inside axon

axonal membrane

inside axon

(10)

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Resting and Action Potential of the Axonal

Membrane

direction of impulse

– – + + + + + + + +

+ + – – – – – – – –

– – + + + + + + + +

b. Action potential begins: depolarization occurs when Na+gates open and Na+moves to inside the axon.

open Na+ channel

Resting and Action Potential of the Axonal

Membrane

open K+ direction of impulse

(11)

21

Resting and Action Potential of the Axonal

Membrane

d. An action potential can be visualized if voltage changes are graphed over time.

resting potential threshold K+ moves

to outside axon Na+ moves

to inside axon

action potential

Vo

lta

g

e

(m

V)

0

0 1 2 3 4 5 6

Time (milliseconds) +60

–60 –40 –20 +20 +40

Nervous Tissue

• Propagation of Action Potentials

– In nonmyelinated axons, the action potential travels

down an axon one small section at a time

– In myelinated fibers, an action potential at one node

causes an action potential at the next node

• Saltatory(jumping) Conduction

– Conduction of a nerve impulse is an all-or-nothing

event

• Intensity of signal is determined by how many impulses are generated within a given time span

(12)

Nervous Tissue

• Transmission Across a Synapse

– A

synapse

is a region where neurons nearly touch

– Small gap between neurons is the

synaptic cleft

– Transmission across a synapse is carried out by

neurotransmitters

• Sudden rise in calcium in the axon terminal of one neuron • Calcium stimulates synaptic vesicles to merge with the

presynaptic membrane

• Neurotransmitter molecules are released into the synaptic cleft

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Synapse Structure and Function

path of action potential

synaptic cleft

neurotransmitter Ca2+

1. After an action potential arrives at an axon terminal, Ca2+enters, and synaptic vesicles fuse with the presynaptic membrane. cell body of

postsynaptic neuron axon terminal synaptic vesicles enclose neuro-transmitter 2. Neuro-transmitter molecules are released and bind to receptors on the postsynaptic membrane. presynaptic membrane postsynaptic membrane 3. When an excitatory neuro-transmitter binds to a receptor, Na+diffuses

neuro-transmitter

(13)

Nervous Tissue

• Synaptic Integration

– A single neuron is on the receiving end of

• Many excitatory signals, and

• Many inhibitory signals

–

Integration

• The summing of excitatory and inhibitory signals

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Synaptic Integration

26

(a): Courtesy Dr. E.R. Lewis, University of California Berkeley

a.

b.

threshold resting potential axon terminals cell body of the neuron

excitatory signal integration inhibitory signal

Time (milliseconds) +20

–20 –40 –70 –80 0

(14)

37.3 The Central Nervous

System

• The Central Nervous System

– Consists of the brain and spinal cord

– Three specific functions:

• Receives sensory input

• Performs integration

• Generates motor output

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The Central Nervous System

• The Central Nervous System

– Consists of the brain and spinal cord

– Spinal cord and brain are wrapped in three

protective membranes called

meninges

• Spaces between meninges are filled with

cerebrospinal fluid

(15)

The Central Nervous System

• The

Spinal Cord

– Two main functions

• Center for many reflex actions

• Means of communication between the brain and

spinal nerves

– Composed of

grey matter

and

white matter

• Cell bodies and short unmyelinated fibers give the

gray mater its color

• Myelinated long fibers of interneurons running in

tracts

give white mater its color

29

The Central Nervous System

• The Brain

– The

cerebrum

is the largest portion of the

brain in humans

• Communicates with, and coordinates the activities

of, the other parts of the brain

• Longitudinal fissure divides the cerebrum into left

and right

cerebral hemispheres

(16)

The Human Brain

31 skull meninges third ventricle pituitary gland pineal gland fourth ventricle spinal cord Diencephalon Cerebellum hypothalamus midbrain pons Brain stem

b. Cerebral hemispheres a. Parts of brain

Cerebrum (telencephalon) corpus callosum medulla oblongata

opening to lateral ventricle

thalamus (surrounds the third ventricle)

The Lobes of a Cerebral

Hemisphere

Frontal lobe Occipital lobe Parietal lobe premotor area trunk arm hand face tongue leg primary motor area

motor speech (Broca’s) area prefrontal area

central sulcus

primary somatosensory area somatosensory association area primary taste area

general interpretation area

primary visual area visual association area lateral sulcus Temporal lobe auditory association area primary auditory area sensory speech (Wernicke’s) area

(17)

The Central Nervous System

• Cerebral Cortex

• A thin but highly convoluted outer layer of

gray matter

• Covers the cerebral hemispheres

• Contains motor areas and sensory areas as

well as association areas

–

Primary motor area

is in the frontal lobe, just

ventral to the central sulcus

–

Primary somatosensory area

is in the parietal lobe,

just dorsal to the central sulcus

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The Central Nervous System

• Basal nuclei

– Integrate motor commands

– Ensures that the proper muscle groups are

either activated or inhibited

(18)

The Central Nervous System

• Other Parts of the Brain

• Diencephalon

• A region encircling the third ventricle

• Includes three structures

–

Hypothalamus

• Forms the floor of the third ventricle

• Integrating center that maintains

homeostasis

• Controls the pituitary gland

35

The Central Nervous System

• Other Parts of the Brain

• Diencephalon (continued)

• Thalamus

• Consists of two masses of gray matter located in the sides and roof of the third ventricle

• Receives all sensory input except smell

• Integrates sensory information and sends it to the cerebrum

• Pineal gland

(19)

The Central Nervous System

• Other Parts of the Brain

• Cerebellum

• Separated from the brain stem by the fourth ventricle • Largest portion of the brainstem

• Receives sensory input from the eyes, ears, joints, and muscles

• Sends motor impulses out the brain stem to the skeletal muscles

37

The Central Nervous System

• Other Parts of the Brain

• Brain Stem

• Contains the

midbrain

,

pons

, and the

medulla

oblongata

• Midbrain

• Acts as a relay station for tracts passing between the cerebrum and the spinal cord or cerebellum

• Pons

• Contains axons that form a bridge between the cerebellum and the rest of the central nervous system

• Medulla Oblongata

• Contains reflex centers for vomiting, coughing, sneezing, hiccupping, and swallowing

(20)

The Central Nervous System

• The Reticular Activating System (RAS)

– A complex network of:

• Nuclei (masses of gray matter)

• Nerve fibers that extend the length of the brain

stem

– The reticular formation is a major component

of the RAS

– The RAS arouses the cerebrum via the

thalamus and causes a person to be alert

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The Reticular Activating System

thalamus

reticular formation

(21)

The Central Nervous System

• Limbic System

– Complex network of tracts and “nuclei”

– Incorporates

• Medial portions of the cerebral lobes, • The basal nuclei, and

• The diencephalon

– Integrates higher mental functions and primitive emotions

– Important structures in the limbic system are

• The hippocampus and the amygdala

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The Limbic System

42

corpus callosum

olfactory bulb olfactory tract hypothalamus

hippocampus amygdala thalamus

(22)

The Central Nervous System

• Learning and Memory

–

Memory

• The ability to hold a thought in mind or recall events from the past

– Learning

• Takes place when we retain and use past memories

– Short term memory is associated with the prefrontal area

of the frontal lobe

– Long term memory is associated with the hippocampus

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37.4 The Peripheral Nervous

System

• Somatic system

– Includes cranial nerves and spinal nerves

• Gather information from sensors and conduct decisions to effectors • Controls the skeletal muscles

– Voluntary

• Autonomic system

– Controls the smooth muscles, cardiac muscles, and glands – Innervates all internal organs

– Usually involuntary – Divided into two divisions

• Sympathetic division

(23)

Cranial and Spinal Nerves

45

dorsal root

ventral root vertebra spinal cord

white matter central canal gray matter frontal lobe

olfactory bulb olfactory tract optic chiasma

temporal lobe optic nerve

cerebellum medulla

gray matter white matter

vertebra

b.

c. a.

dorsal root ganglion spinal nerve

The Peripheral Nervous System

• Reflex Arc

• Sensory receptors generate a nerve impulse that moves along

sensory axons through a dorsal root ganglion toward the spinal cord

• Sensory neurons pass signals on to many interneurons in the gray matter of the spinal cord

• Nerve pulses travel along motor axons to an effector, which brings about a response to the stimulus

(24)

A Reflex Arc Showing the

Path of a Spinal Reflex

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white matter

ventral root axon of motor neuron

axon of sensory neuron pin

dorsal root ganglion

interneuron dendrites

dendrites Dorsal gray matter central canal

ventral horn sensory

receptor (in skin)

cell body of sensory neuron

cell body of motor neuron effector

(muscle)

Ventral

dorsal horn

The Peripheral Nervous System

• Sympathetic division

– Especially important during fight or flight

responses

– Accelerates heartbeat and dilates bronchi

• Parasympathetic division

– Promotes all internal responses associated

with a relaxed state

(25)

Autonomic System Structure and Function

49

inhibits tears stimulates tears constricts pupils ganglion dilates pupils Sympathetic Division inhibits salivation stimulates salivation cranial nerves Parasympathetic Division slows heart speeds heart dilates air passages cervical nerves thoracic nerves lumbar nerves constricts bronchioles stimulates liver to

release glucose stimulates adrenal secretion vagus nerve inhibits activity of kidneys, stomach, and pancreas increases intestinal activity decreases intestinal activity ganglion inhibits urination stimulates urination sacral nerves stimulates gallbladder to release bile

increases activity of stomach and pancreas sympathetic ganglia causes orgasmic contractions causes erection

of genitals Acetylcholine is neurotransmitter. Norepinephrine is neurotransmitter.