• Chapter 48 ~ Nervous System

               

     

Nervous systems• Effector cells:

muscle or gland cells that react to stimuli

• Nerves: bundles of neurons wrapped in connective tissue

• Central nervous system (CNS): brain and spinal cord

• Peripheral nervous system (PNS): sensory and motor neurons

Overview of the Nervous System 

Structural Unit of Nervous System

• Neuron: structural and functional unit• Cell body: nucleus and organelles• Dendrites: receive impulses from neurons• Axons: carry impulse to another cell• Myelin sheath: supporting, insulating layer

-Oligodendrocytes (CNS)

-Schwann cells (PNS)• Synaptic terminals: neurotransmitter releaser• Synapse: neuron junction

The Schwann (PNS) and Oligodendrocytes (CNS)cells help an impulse move along an axon at extremely fast speeds. The myelin that make up these cells are mostly lipids which are poor conductors of electricity.

The Synapsehttp://www.sumanasinc.com/webcontent/anisamples/neurobiology/synaptictransmission.html

Simple Nerve Circuit• 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

http://www.sumanasinc.com/webcontent/anisamples/neurobiology/reflexarcs.html

What is a nerve impulse?1. Neurons have Membrane potential (voltage

differences across the plasma membrane): All cells are slightly more negative inside than outside. Resting potential of a neuron = -70 mV

2. A nerve impulse occurs when the neuron depolarizes.Depolarization is made possible by specialized ion channels.

What is a nerve impulse?3. Types of ion channels:

-Chemically-gated ion channels: open or close when stimulated by a chemical.-Voltage-gated ion channels: open or close

when stimulated by a change in membrane potential.

4. Ion channels along the axon open and close, causing an “action potential” to move downalong the axon. The axonbecomes positive in sections.

How is the membrane potential maintained?

1. Sodium-Potassium Pump: By active transport,3 sodium ions are pumped out for every 2 potassiumions. This pump maintains membraine potential.

2. Some of the ions diffuse back but never reachequilibrium. They diffuse back through Na+ and K+ ion channels (remember “facilliated diffusion”?)

3. High concentration of anions inside the cell includeproteins, amino acids, sulfate, and phosphates.

How does an action potential senda nerve impulse?

How does an action potential senda nerve impulse?

1. Resting State: Na+ and K+ channels are closed.2. Threshold: Some Na+ open up. If the neuron

obtains a particular voltage (i.e. -50 mV), it will trigger depolarization.

3. Depolarization phase: Na+ channels open up very rapidly and Na+ rushes into the cell making it morepositive.

4. Repolarization phase: Na+ channels close, but theK+ slowly open. K+ ions leave the cell and thecell becomes more negative again.

5. Undershoot: As the K+ ion channels slowly close, K+ ions continue to leave the neuron causing it tobecome hyperpolarized. Refractory Period: when the neuron is insensitive to

depolarization; due to the closing of Na+ channelsduring undershoot.

Resting Potential in Neurons:

http://www.sumanasinc.com/webcontent/anisamples/neurobiology/signaling.html

Nerve Impulse; Depolarization:

http://www.sumanasinc.com/webcontent/anisamples/neurobiology/signaling.html

Saltatory Conduction

-In between each Schwann cell is an area of axon exposed to extracellular fluid. These areas are called “Nodes of Ranvier.” -Depolarization takes place at each node. -This propagates an impulse faster as action potentials jump from node to node.

Synaptic communication• 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 releaseneurotransmitters.

• Neurotransmitter: the chemical released that causes an impulse in the postsynaptic cell.

Synaptic communication

•The binding of the neurotransmitter to the receptor opens up Na+ channels Depolarization•The neurotransmitters are either degraded by enzymes or recycled by the presynaptic cell.•Two types of responses: 1. EPSP (Excitatory postsynaptic potential): excitatory response; action potential stimulus due to Na+ influx 2. IPSP (Inhibitatory postsynaptic potential): inhibitory response; due to K+ movement out of cell and Cl- movement in.

Neurotransmitters1. Acetylcholine (most common)

-Both excitatory and inhibitory

-Ex. skeletal muscle

2. Biogenic amines (derived from amino acids)

-Epinephrine: aka adrenalin; the “fight or flight” hormone; increase the heart rate, blood pressure, and levels of sugar and fat in the blood.

-Norepinephrine: excitatory and inhibitory; functions in the autonomic nervous system; low levels in the synaptic cleft is associated with depression and ADD/ADHD.

-Dopamine: excitatory; functions in the “reward system” of brain; low levels is associated with depression, ADD/ADHD, and Parkinsons Disease; high levels associated with schizophrenia; associated with feelings of pleasure, and motivation (food, sex); cocaine mimics dopamine.

Neurotransmitters -Serotonin: Inhibitory; regulates anger, body temp, mood,

sleep, sexuality, appetite; low levels associated with depression, aggressive behavior, OCD, migraines, bipolar, and anxiety disorders.

3. Amino Acids: -GABA (Gamma aminobutyric acid): main inhibitory neurotransmitter-Glycine: inhibitory action in spinal cord, brainstem,and retina-Glutamate: excitatory; associated with learning and memory-Aspartate: excitatory; could be associated with fatigue resistance.

4. Neuropeptides (short chains of amino acids):-Substance P: key excitatory transmitter that indicates pain.-Endorphins: natural pain killers; morphine and heroin mimic endorphins.

5. Gaseous Signals: -NO (nitric oxide): Example – male arousal causesNO to be released from neurons into the erect tissue of penis. NO also dilates blood vessels.

Evolution and Diversity of the Nervous System

Nerve net Cephalization Small brains & nerve cords

Vertebrate CNS1. Brain and spinal cord2. Spinal cord contains:

-White matter: bundles of myelinated axons-Gray matter: unmyelinated axons, nuclei, and dendrites.

Vertebrate PNS1. Everything outside the CNS.2. Structure components of the PNS:

-Cranial nerves-Spinal nerves-Ganglia associated with the cranial and spinal nerves

Vertebrate PNS

Vertebrate PNS: Autonomic NS

Sympathetic:-Adrenal secretion-”Fight or flight” response

Parasympathetic:-Returns body to normal

Embryonic Development of the Brain

The Human Brain

The Human Brain1. The Brainstem “Lower brain”

-Consists of pons, medulla oblongata, and midbrain-Functions in homeostasis, coordination of movement, conduction of impulses to higher brain centers.-Medulla oblongata controls:

a. Breathingb. Heart and blood vessel activityc. Swallowingd. Vomitinge. Digestion

Relays information to and from higher brain centers

-Pons controls:a. Breathing

Relays information to and from higher brain centers

The Human Brain -The Midbrain:

a. Consists of two parts:1. Inferior colliculi: part of auditory system2. Superior colliculi: part of visual system

b. Functions in receiving and integrating sensory information.

2. The Reticular System, Arousal, and Sleep-Reticular Formation: group of 90 separate nuclei (groups of neurons) that passes throughthe core of the brain stem.a. Reticular activating system (RAS): regulates sleep and arousal. -Selects which info gets to the cerebral cortex; more info more alert

The Human Brain

Sleep and wakefulness produce different patterns of electrical activity of the brain. The patterns can be recorded in an electroencephalogram (EEG).

-Less mental activity produces more synchronous waves (b).

-Purpose of sleep?

The Human Brain3. The Cerebellum

-Functions in coordination, error-checking, learning and remembering of motor responses, hand-eye coordination. It manages multiple sensory info (visual, auditory, position of joints, muscles, etc)

to provide coordinated movements and balance.

The Human Brain4. The Epithalamus, Thalamus and Hypothalamus

-Epithalamus: a. Produces cerebrospinal fluidb. Contains the pineal gland (melatonin secretion)

-Thalamus:a. Main input center for sensory info going to the cerebrum and main output center for motor info leaving the cerebrum.b. All sensory info is sorted here and sent on to appropriate brain centers.

The Human Brain

-Hypothalamus: Most important brain region for homeostatic regulation. a. Hunger b. Body temperature c. Thirst d. Fight or flight response e. Pleasure f. Mating behaviors g. Circadian rhythms: internal biological clock

-Suprachiasmatic nuclei (SCN) – connectedwith the visual sensory organs; light is themajor external cue for circadian rhythms.

The Human Brain5. The Cerebrum: the most highly evolved structure

of the brain-Divided into right and left hemispheres, each of which is responsible for the opposite half of the body.-Two hemispheres joined at the corpus callosum;communication between the two sides.-Outer gray covering called the cerebral cortex -Neocortex – extra layers of cortex unique to mammals-Internal white matter-The cerebrum allows for cognition. Cognition is the process of knowing, awareness, judgement,learning, decision making, and consciousness.

-Regions of the cortex are specialized for different functions. -It is divided into four lobes:

1. Frontal lobe2. Parietal lobe3. Occipital lobe4. Temporal lobe

-The motor cortex of the frontal lobe controls motor function in response to the sensory neurons found in the somatosensory cortex.

Lateralization of Brain Function1. The Left Hemisphere:

- Specializes in language, math, logic operations, and the processing of serial sequences of

information, and visual and auditory details.- Specializes in detailed activities required for motor control.

2. The Right Hemisphere:- Specializes in pattern and face recognition, spatial relationships, nonverbal ideation, emotional processing, the parallel processing of information, and music.

What if you cut the corpus callosum and the twohemispheres could not communicate?http://faculty.washington.edu/chudler/split.html

Language and Speech

1. Broca’s Area: - Usually located in the left hemisphere’s frontal lobe- Responsible for speech production.- Damage to Broca's area causes Broca's aphasia, a

condition in which people have trouble producing grammatical language.

2. Wernicke’s Area:- Usually located in the left hemisphere’s temporal lobe- Responsible for the comprehension of speech.-Damage to Wernicke's area causes Wernicke's aphasia, a condition in which people can hear language being spoken, but cannot understand it.

3. Language is processed by multiple areas in the cortex.

Broca's Area

                        

 

Wernicke's Area

                        

 

Emotions

1. Limbic System: generates feelings called emotions.-Composed of the hippocampus, olfactory cortex, inner portions of the cortex’s lobes, and parts of the thalamus and hypothalamus.-Mediates basic emotions (fear, anger), involved in emotional bonding, establishes emotional memory.-Amygdala: involved in recognizing the emotional content of facial expression.

Learning and Memory1. Short-term memory is stored in the frontal lobe.2. Long-term memory is associated with the

hippocampus. The transfer of information from short-term to

long-term is enhanced by:-Repetition-Emotional states influenced by the amygdala-Influenced by association with previously stored information.

Human Consciousness

1. Consciousness: sensing, acting, and feeling in thepresent moment but also thinking about the pastand the future.

2. Are humans the only ones with a consciousness?3. Human consciousness was thought to be a topic

only considered in philosophy and religion classesbut new research shows neural activity that is associated with conscious thought and behavior.

Simple Nerve Circuit: Arc Reflex

The Human Brain