Chapter 44 ~ Nervous System

Effector cells~ muscle or gland cells

Nerves~ bundles of neurons wrapped in connective tissue

Central nervous system (CNS)~ brain and spinal cord

Peripheral nervous system (PNS)~ sensory and motor neurons

Neuron~ structural and functional unit Cell body~ nucelus and organelles Dendrites~ impulses from tips to neuron Axons~ impulses toward tips Myelin sheath~ supporting, insulating layer Schwann cells~PNS support cells Oligodendrocytes- CNS support cells Synaptic terminals~ neurotransmitter releaser Synapse~ neuron junction

Sensory neuron: convey information to spinal cord

Interneurons: information integration

Motor neurons: convey signals to effector cell (muscle or gland)

Reflex: simple response; sensory to motor neurons

Ganglion (ganglia): cluster of nerve cell bodies in the PNS

Supporting cells/glia: nonconductiong cell that provides support, insulation, and protection

Membrane potential (voltage differences across the plasma membrane)

Intracellular/extracellular ionic concentration difference K+ diffuses out (Na+ in); large anions cannot

follow….selective permeability of the plasma membrane Net negative charge of about -70mV

Excitable cells~ cells that can change membrane potentials (neurons, muscle) Resting potential~ the unexcited state of excitable cells Gated ion channels (open/close response to stimuli): photoreceptors; vibrations

in air (sound receptors); chemical (neurotransmitters) & voltage (membrane potential changes)

Graded Potentials (depend on strength of stimulus): 1- Hyperpolarization (outflow of K+); increase in electrical gradient; cell becomes

more negative 2- Depolarization (inflow of Na+); reduction in electrical gradient; cell becomes

less negative

Threshold potential: if stimulus reaches a certain voltage (-50 to -55 mV)….

The action potential is triggered…. Voltage-gated ion channels (Na+; K+) 1-Resting state •both channels

closed 2-Threshold •a stimulus opens some

Na+ channels 3-Depolarization •action potential

generated •Na+ channels open; cell becomes positive (K+ channels closed)

4-Repolarization •Na+ channels close, K+ channels open; K+ leaves •cell becomes negative

5-Undershoot •both gates close, but K+ channel is slow; resting state restored

Refractory period~ insensitive to depolarization due to closing

of Na+ gates

“Travel” of the action potential is self-propagating Regeneration of “new” action potentials only after refractory

period Forward direction only Action potential speed: 1-Axon diameter (larger = faster; 100m/sec) 2-Nodes of Ranvier (concentration of ion channels); saltatory

conduction; 150m/sec

Video 1 Video 2 Video 3

Presynaptic cell: transmitting cell

Postsynaptic cell: receiving cell Synaptic cleft: separation gap Synaptic vesicles:

neurotransmitter releasers Ca+ influx: caused by action

potential; vesicles fuse with presynaptic membrane and release….

Neurotransmitter

Acetylcholine (most common) •skeletal muscle

Biogenic amines (derived from amino acids)•norepinephrine

•dopamine (Parkinson/Schizophrenia)•serotonin (SSRI)

Amino acids◦ GABA- IPSP

Neuropeptides (short chains of amino acids)•endorphin

Table 48.1

Drug Addiction and Neurotransmitters

1. Cocaine2. Nicotine

Nervous systems consist of circuits of neurons and supporting cells

All animals except sponges◦ Have some type of nervous system

What distinguishes the nervous systems of different animal groups◦ Is how the neurons are organized into circuits

The simplest animals with nervous systems, the cnidarians◦ Have neurons arranged in nerve nets

Figure 48.2a

Nerve net

(a) Hydra (cnidarian)

Sea stars have a nerve net in each arm◦ Connected by radial nerves to a central nerve ring

Figure 48.2b

Nervering

Radialnerve

(b) Sea star (echinoderm)

In relatively simple cephalized animals, such as flatworms◦ A central nervous system (CNS) is evident

Figure 48.2c

Eyespot

Brain

Nerve cord

Transversenerve

(c) Planarian (flatworm)

Annelids and arthropods◦ Have segmentally arranged clusters of neurons

called ganglia These ganglia connect to the CNS

◦ And make up a peripheral nervous system (PNS)

Brain

Ventral nervecord

Segmentalganglion

Brain

Ventralnerve cord

Segmentalganglia

Figure 48.2d, e (d) Leech (annelid) (e) Insect (arthropod)

Anteriornerve ring

Longitudinalnerve cords

Ganglia

Brain

Ganglia

Figure 48.2f, g (f) Chiton (mollusc) (g) Squid (mollusc)

Nervous systems in molluscs◦ Correlate with the animals’ lifestyles

Sessile molluscs have simple systems◦ While more complex molluscs have more

sophisticated systems

In vertebrates◦ The central nervous system consists of a brain and

dorsal spinal cord◦ The PNS connects to the CNS

Figure 48.2h

Brain

Spinalcord(dorsalnervecord)

Sensoryganglion

(h) Salamander (chordate)

The three stages of information processing◦ Are illustrated in the knee-jerk reflex

Figure 48.4

Sensory neurons from the quadriceps also communicatewith interneurons in the spinal cord.

The interneurons inhibit motor neurons that supply the hamstring (flexor) muscle. This inhibition prevents the hamstring from contracting, which would resist the action of the quadriceps.

The sensory neurons communicate with motor neurons that supply the quadriceps. The motor neurons convey signals to the quadriceps, causing it to contract and jerking the lower leg forward.

4

5

6

The reflex is initiated by tapping

the tendon connected to the quadriceps

(extensor) muscle.

1

Sensors detecta sudden stretch in the quadriceps.

2 Sensory neuronsconvey the information to the spinal cord.

3

Quadricepsmuscle

Hamstringmuscle

Spinal cord(cross section)

Gray matter

White matter

Cell body of sensory neuronin dorsal root ganglion

Sensory neuron

Motor neuron

Interneuron

The vertebrate nervous system is regionally specialized

In all vertebrates, the nervous system◦ Shows a high degree of cephalization and distinct

CNS and PNS components

Figure 48.19

Central nervoussystem (CNS)

Peripheral nervoussystem (PNS)

Brain

Spinal cord

Cranialnerves

GangliaoutsideCNSSpinalnerves

Cranial nerves (brain origin) Spinal nerves (spine origin) Sensory division Motor division

•somatic system voluntary, conscious control •autonomic system

√parasympathetic

conservation of energy √sympathetic

increase energy consumption

In all vertebrates◦ The brain develops from three embryonic regions:

the forebrain, the midbrain, and the hindbrain

Figure 48.23a

Forebrain

Midbrain

Hindbrain

Midbrain Hindbrain

Forebrain

(a) Embryo at one month

Embryonic brain regions

By the fifth week of human embryonic development◦ Five brain regions have formed from the three

embryonic regions

Figure 48.23b

Telencephalon

Diencephalon

Mesencephalon

Metencephalon

Myelencephalon

(b) Embryo at five weeks

MesencephalonMetencephalon

Myelencephalon

Spinal cord

Diencephalon

Telencephalon

Embryonic brain regions

As a human brain develops further◦ The most profound change occurs in the forebrain,

which gives rise to the cerebrum

Figure 48.23c

Brain structures present in adult

Cerebrum (cerebral hemispheres; includes cerebralcortex, white matter, basal nuclei)

Diencephalon (thalamus, hypothalamus, epithalamus)

Midbrain (part of brainstem)

Pons (part of brainstem), cerebellum

Medulla oblongata (part of brainstem)

(c) Adult

Cerebral hemisphereDiencephalon:

Hypothalamus

ThalamusPineal gland(part of epithalamus)

Brainstem:

Midbrain

Pons

Medullaoblongata

Cerebellum

Central canal

Spinal cord

Pituitarygland

The brainstem consists of three parts◦ The medulla oblongata, the pons, and the midbrain

The medulla oblongata◦ Contains centers that control several visceral

functions The pons

◦ Also participates in visceral functions The midbrain

◦ Contains centers for the receipt and integration of several types of sensory information

A diffuse network of neurons called the reticular formation◦ Is present in the core of the brainstem

Figure 48.24

Eye

Reticular formation

Input from touch, pain, and temperature receptors

Input from ears

A part of the reticular formation, the reticular activating system (RAS)◦ Regulates sleep and arousal

The cerebellum◦ Is important for coordination and error checking

during motor, perceptual, and cognitive functions

The cerebellum ◦ Is also involved in learning and remembering motor

skills

The embryonic diencephalon develops into three adult brain regions◦ The epithalamus, thalamus, and hypothalamus

The epithalamus◦ Includes the pineal gland and the choroid plexus

The thalamus◦ Is the main input center for sensory information

going to the cerebrum and the main output center for motor information leaving the cerebrum

The hypothalamus regulates◦ Homeostasis◦ Basic survival behaviors such as feeding, fighting,

fleeing, and reproducing

The hypothalamus also regulates circadian rhythms◦ Such as the sleep/wake cycle

Animals usually have a biological clock◦ Which is a pair of suprachiasmatic nuclei (SCN)

found in the hypothalamus

The cerebrum ◦ Develops from the embryonic telencephalon

The cerebrum has right and left cerebral hemispheres◦ That each consist of cerebral cortex overlying white

matter and basal nuclei

Left cerebralhemisphere

Corpuscallosum

Neocortex

Right cerebralhemisphere

Basalnuclei

Figure 48.26

The basal nuclei◦ Are important centers for planning and learning

movement sequences In mammals

◦ The cerebral cortex has a convoluted surface called the neocortex

In humans, the largest and most complex part of the brain ◦ Is the cerebral cortex, where sensory information is

analyzed, motor commands are issued, and language is generated

A thick band of axons, the corpus callosum◦ Provides communication between the right and left

cerebral cortices

The cerebral cortex controls voluntary movement and cognitive functions

Each side of the cerebral cortex has four lobes◦ Frontal, parietal, temporal, and occipital

Frontal lobe

Temporal lobe Occipital lobe

Parietal lobe

Frontalassociationarea

Speech

Smell

Hearing

Auditoryassociationarea

Vision

Visualassociationarea

Somatosensoryassociationarea

Reading

Speech

TasteSom

atos

enso

ry c

orte

x

Mot

or c

orte

x

Figure 48.27

Each of the lobes◦ Contains primary sensory areas and association

areas

Specific types of sensory input◦ Enter the primary sensory areas

Adjacent association areas◦ Process particular features in the sensory input and

integrate information from different sensory areas

In the somatosensory cortex and motor cortex◦ Neurons are distributed according to the part of the

body that generates sensory input or receives motor input

Figure 48.28

Tongue

JawLips

Face

Eye

Brow

Neck

Thumb

Fingers

HandW

ristForearmE

lbowShoulder

Trunk

Hip

Knee

Primarymotor cortex Abdominal

organs

Pharynx

Tongue

TeethGumsJaw

Lips

Face

Nose

Eye

Fingers

HandForearm

Elbow

Upper arm

Trunk H

ip

Leg

Thumb

Neck

Head

Genitalia

Primarysomatosensory cortex

Toes

Parietal lobeFrontal lobe

During brain development, in a process called lateralization◦ Competing functions segregate and displace each

other in the cortex of the left and right cerebral hemispheres

The left hemisphere◦ Becomes more adept at language, math, logical

operations, and the processing of serial sequences The right hemisphere

◦ Is stronger at pattern recognition, nonverbal thinking, and emotional processing

Studies of brain activity◦ Have mapped specific areas of the brain responsible

for language and speech

Figure 48.29

Hearingwords

Seeingwords

Speakingwords

Generatingwords

Max

Min

Portions of the frontal lobe, Broca’s area and Wernicke’s area◦ Are essential for the generation and understanding

of language

The limbic system ◦ Is a ring of structures around the brainstem

Figure 48.30

HypothalamusThalamus

Prefrontal cortex

Olfactorybulb

Amygdala Hippocampus

This limbic system includes three parts of the cerebral cortex◦ The amygdala, hippocampus, and olfactory bulb

These structures interact with the neocortex to mediate primary emotions ◦ And attach emotional “feelings” to survival-related

functions

Structures of the limbic system form in early development◦ And provide a foundation for emotional memory,

associating emotions with particular events or experiences

The frontal lobes◦ Are a site of short-term memory◦ Interact with the hippocampus and amygdala to

consolidate long-term memory

Many sensory and motor association areas of the cerebral cortex◦ Are involved in storing and retrieving words and

images

In the vertebrate brain, a form of learning called long-term potentiation (LTP)◦ Involves an increase in the strength of synaptic

transmission

Figure 48.32

PRESYNAPTIC NEURON

NO

Glutamate

NMDAreceptor

Signal transduction pathways

NO

Ca2+

AMPA receptor

POSTSYNAPTIC NEURON

Ca2+ initiates the phos-phorylation of AMPA receptors,making them more responsive.Ca2+ also causes more AMPAreceptors to appear in thepostsynaptic membrane.

5

Ca2+ stimulates thepostsynaptic neuron toproduce nitric oxide (NO).

6

The presynapticneuron releases glutamate.

1

Glutamate binds to AMPAreceptors, opening the AMPA-receptor channel and depolarizingthe postsynaptic membrane.

2

Glutamate also binds to NMDAreceptors. If the postsynapticmembrane is simultaneouslydepolarized, the NMDA-receptorchannel opens.

3

Ca2+ diffuses into thepostsynaptic neuron.

4

NO diffuses into thepresynaptic neuron, causing it to release more glutamate.

7

P

Modern brain-imaging techniques ◦ Suggest that consciousness may be an emergent

property of the brain that is based on activity in many areas of the cortex

CNS injuries and diseases are the focus of much research

Unlike the PNS, the mammalian CNS◦ Cannot repair itself when damaged or assaulted by

disease Current research on nerve cell development

and stem cells◦ May one day make it possible for physicians to

repair or replace damaged neurons

Signal molecules direct an axon’s growth ◦ By binding to receptors on the plasma membrane of

the growth cone

Forebrain (Prosencephalon)•cerebrum~memory,

learning, emotion •cerebral cortex~sensory and motor nerve cell bodies •corpus callosum~connects left and right hemispheres

•thalamus; hypothalamus Midbrain ( Mesencephalon)

•inferior (auditory) and superior (visual) colliculi

Hindbrain(Rhombencephalon) •cerebellum~coordination of movement •medulla oblongata/ pons~autonomic, homeostatic functions