Nervous System

Chapters 48 and 49

Nervous System Diversity

• Cnidarians – (hydra, sea stars), have nerve nets that control the gastrovascular cavity

• Cephalization – shows greater complexity of the nervous system

• Annelids/Anthropods – (segmented worms), have ganglia (clusters of neurons), have small brains

Nervous System Diversity

Info Processing

Info Processing: Sensory input, integration and motor output

Types of Neurons

• Sensory Neurons – transmit info from sensors (that detect internal or external stimuli) to interneurons (the CNS)

• Interneurons – either the spinal cord or brain, integrate the sensory input and send message the motor neurons

• Motor Neurons – send message from interneurons to effector cells (muscles or endocrine cells)

“Knee Jerk” Reflex

Describe the function of each in the reflex arc: sensors, sensory neurons, interneurons, motor neurons, effector cells

The Neuron

• Cell Body – contains nucleus• Dendrites – branches that receive signals• Axon – extension that transmits signals• Axon Hillock – conical region of axon where it joins cell body• Myelin Sheath – lipid layers around axons• Nodes of Ranvier – spaces between myelin sheath• Synaptic Terminal – branches of axon, send neurotransmitters

Neuron Diversity

Glia Cells

• Supporting cells of the neuron• Ex: astrocytes, radial glia, oligiodendrocytes and

schwann cells• Astrocytes: structural support, form blood-brain

barrier, stem cells• Radial Glia: form tracks for newly formed neurons

to move from neural tube, stem cells• Oliodendrocytes (CNS) and Schwann Cells

(PNS): form myelin sheaths which insulate axon and allow for faster impulses

Schwann Myelin

Multiple Sclerosis- autoimmune disease, T cells destroy myelin sheaths

Resting Potential = -70mV

The electrical potential difference between the outside and inside of a plasma membrane is called the membrane potential. A membrane potential of a cell at rest is -70mV

Resting Potential

• Resting Potential (when a neuron is not signaling) is -70mV

• The inside is negative relative to the outside• Maintained by the sodium potassium pump, which

pumps 3 Na+ out of the cell for every 2 K+ it pumps in, and K+ ion channels that allow for the diffusion of K+ out of the cell

• Na+ is not allowed in (the Na+ ion channels are closed)

Stimulating a Neuron

• The membrane potential changes from its resting value when the membrane’s permeability to ions changes – triggers signaling

• Types of Ion Channels: Stretch Gated, Ligand Gated and Voltage Gates Ion Channels

Depolarization

• When the resting potential becomes less negative due to the opening of Na+ ion channels (which let Na+ into the cell)

• Threshold – when a stimulus changes the membrane potential enough to cause a response, or action potential

Action Potential

Production of Action Potential

• 1. Resting potential: Na+ gates closed, some K+ gates open (move out) and Na-K pump active

• 2. stimulus Na+ channels open, causing depolarization• 3. When threshold is met, membrane is in rising phase• 4. The Na+ channels close and K+ channels open- falling

phase• 5. Because more K+ are open than usual, the membrane

potential is more neg – undershoot• 6. More K+ close returning the potential to normal

Production of Action Potential

Refractory Period

• Na+ channels remain closed during falling phase and undershoot, therefore a second stimulus could not trigger stimulation during this time

Conduction

• The Na+ changes at one part of the neuron stimulate the depolarization of the neighboring section (like dominoes)

• Because of the refectory period, the impulse can only move in one direction

Saltatory Conduction

The action potentials are not generated at the myelin sheath, only at nodes; causing action potential to jump from node to node

Synapses

• 2 kinds – electrical and chemical

• Electrical – gap junctions between two neurons that allow for direct flow of the electrical current from one neuron to the next

• Chemical – involve the release of neurotransmitters from synaptic vesicles

Chemical Synapse

Neurotransmitters

• Released by exocytosis through the synaptic cleft

• Can cause excitatory (excitatory postsynaptic potentials- EPSP) or inhibitory (inhibitory postsynaptic potentials – IPSP) effects

• Ex: acetylcholine, epi, dopamine, serotonin, nitric oxide

Central Nervous System

• Brain and spinal cord

• Filled with cerebrospinal fluid

• White matter – axons (myelin)

• Gray matter - dendrites

Peripheral Nervous System

• Gives and receives info from CNS – sensory and motor neurons

• Cranial and spinal nerves• 2 systems – somatic and autonomic• Somatic – carries signals to and from skeletal

muscles, responds to external stimuli• Autonomic – regulates internal environment,

controls smooth and cardiac muscles

The CNS and the PNS

Divisions of the PNS

Autonomic Nervous System

• 3 parts-sympathetic, parasympathetic and enteric

• Sympathetic – increases metabolism, ex: increases heart beat, etc.

• Parasympathetic – antagonistic to sympathetic, ex: slows heart beat

• Enteric- control organ secretions

Sympathetic and Parasympathetic Systems

The Brain

• Embryonic development – 3 parts of the brain: forebrain, midbrain and hindbrain

• Forebrain cerebrum, diencephalon

• Midbrain midbrain (brainstem)

• Hindbrain cerebellum, pons, medulla

The Brain Stem

• “lower brain”

• 3 parts: medulla oblongata, pons and midbrain

• Maintain homeostasis (breathing, heart beat, etc), coordination, and conduction of info to higher brain centers

Cerebellum

• Coordination-motor, perception and learning (cognitive function)

Diencephalon

• Epithalamus, thalamus and hypothalamus

• Epithalamus: pineal gland

• Thalamus: input sensory info to cerebrum

• Hypothalamus: regulates homeostasis

Cerebrum

• Outer gray, inner white• Analyzes sensory info, motor command and

language generation• Neocortex – cerebral cortex – more convoluted the

more intelligent the animal is• Corpus Callosum- band of axons that enables

communication between left and right brain (right brain : spatial, patterns “big picture”; left brain: language, math, logic)

Corpus Callosum

Cerebral Cortex

Limbic System

Sensory Receptors

• Mechanoreceptors – pressure, stretch, touch

• Chemoreceptors – solutes

• Electromagnetic Receptors – light electricity

• Photoreceptors - light

• Thermoreceptors – heat, cold

• Pain Receptors - damage

Hearing

• Convert the energy of pressure waves traveling through air into nerve impulses

• Three bones of the middle ear transmit the vibrations to the oval window, a membrane on the cochlea’s surface

• The vibration against the oval window creates pressure waves in the fluid

• Waves travel through the vestibular canal, pass around the tip of the cochlea and move through the tympanic canal and hit the round window

Transduction in the cochlea

Bending of the hairs increases the frequency of action potentials in the sensory neurons – the neurons carry sensations to the brain through the auditory nerve

Muscles

• Skeletal muscle• Muscle fibers are made up

of myofibrils which are made up of thin (actin) and thick filaments (myosin)

• Sarcomere – basic contractile unit of the muscle

• Muscle contraction is when the sarcomere shortens by the filaments slide past each other

Actin/Myosin

• 1. myosin binds to ATP

• 2. it changes ATP into ADP

• 3. myosin head binds to actin

• 4. myosin pulls the thin filament

• 5. Binding to ATP again releases the myosin head

Calcium

• Tropomyosin – regulatory proteins that blocks the myosin binding sites on the thin filaments

• Depolarization of neuron allows Ca+ in the cell.• Ca+ binds to troponin complex which controls the

position of the tropomyosin on the thin filaments, uncovering the binding sites – allowing contraction

The Eye

The Structure of the Eye

• Sclera- white outer layer, connective tissue• Choroid – thin inner layer• Conjunctiva- mucous membrane• Cornea – transparent sclera• Iris-color of eye, regulates light into pupil• Pupil – hole in center of iris• Retina – inner layer, photoreceptors• Aqueous Humor – liquid between cornea and iris• Rods – sensory receptor for light• Cones- sensory receptors for color

Opsin – contains retinal, found in cones – absorbs light

Rhodopsin – found in rods