The Nervous System

The Nervous System Overall Function COMMUNICATION Works with the endocrine system in regulating body functioning, but the nervous system is specialized for SPEED

Neurons A neuron is the functional unit of the nervous system Neurons are specialized for transmitting signals from one location in the body to another Neurons consist of a large cell body (contain a nucleus and other organelles), and neuronal processes Axons Conduct messages AWAY from cell body Dendrites Conducts messages TOWARD cell body

Neuron Structure

Communication Lines Stimulus (input) Receptors (sensory neurons) Integrators (interneurons) motor neurons Effectors (muscles, glands) Response (output) Figure 34.1 Page 579

Major Nervous System Processes Input The conduction of signals from sensory receptors to integration centers in the nervous system Integration The process by which the information from the environmental stimulation of the sensory receptors is interpreted and associated with appropriate responses of the body Motor Output The conduction of signals from the processing center to the muscle cells or gland cells that actually carry out the body’s responses to stimuli

Central and Peripheral Nervous Systems The central nervous system consists of the brain and spinal cord This is where integration occurs Made of interneurons The peripheral nervous system consists of the nerve cells that communicate signals between the CNS and the rest of the body Sensory neurons Carry info from the sensory receptors to the brain Motor neurons Carry info from the brain to effector cells (to do whatever the brain said!) Central and Peripheral Nervous Systems

Action Potentials In order to actually TRANSMIT a signal, the voltage (charge) across the membrane has to change A signal will cause the ion channels to open, letting some of the ions (Na+, K+) through, trying to achieve EQUILIBRIUM This depolarizes the membrane This causes the signal to be passed along the neuron, which is known as an ACTION POTENTIAL (like a wave of electricity)

Action Potentials Action potentials are either ALL or NONE Either there is enough change in the voltage to pass the message along, or there isn’t The neuron either “fires” or it doesn’t fire If your hand is on a 100˚C stove, the action potentials aren’t STRONGER than if you were on a 50˚C stove; they just happen MORE OFTEN

Structure of a Neuron dendrites INPUT ZONE cell body axon OUPUT ZONE TRIGGER ZONE CONDUCTING ZONE axon endings Figure 34.2 Page 580

Resting Potential Charge difference across the plasma membrane of a neuron Fluid just outside cell is more positively charged than fluid inside because of larged negatively charged proteins in the cytoplasm Potassium (K+): Higher inside than outside Sodium (Na+): Higher outside than inside Potential is measured in millivolts Resting potential is usually about -70mv

Action Potential A transitory reversal in membrane potential Voltage change causes voltage-gated channels in the membrane to open Na+ leaks in (depolarization): Inside of neuron briefly becomes more positive than outside K+ leaks out (repolarization): Inside goes back to being negatively charged compared to outside. Na+ / K+ pumps the Na+ and K+ back to original positions (refractory period)

Action Potential 1 2 3 4 Figure 34.5d Page 583 Na+ Na+ Na+ K+ K+ K+ K+

All or Nothing All action potentials are the same size If stimulation is below threshold level, no action potential occurs If it is above threshold level, cell is always depolarized to the same level

Recording of Action Potential +20 -20 Membrane potential (millivolts) threshold -40 resting membrane potential -70 1 2 3 4 5 Figure 34.6b Page 583 Time (milliseconds)

Chemical Synapse Gap between axon of one neuron and dendrite of adjacent neuron Action potential in axon ending of presynaptic cell causes voltage-gated calcium channels to open Flow of calcium into presynaptic cell causes release of neurotransmitter into synaptic cleft plasma membrane of axon ending of presynapic cell plasma membrane of postsynapic cell synaptic vesicle synaptic cleft membrane receptor Figure 34.7a Page 584

Neurotransmitters Neurotransmitters are substances that carry the “message” across the synapse Important neurotransmitters: Acetylcholine (bridges gaps between motor neurons & muscle cells), norepinephrine, dopamine, serotonin work in CNS

Synaptic Transmission Neurotransmitter diffuses across cleft and binds to receptors on membrane of postsynaptic cell Binding of neurotransmitter to receptors opens ion channels in the membrane of postsynaptic cell

Ion Gates Open neurotransmitter ions receptor for neurotransmitter gated channel protein Figure 34.7c Page 584

Synaptic Transmission Enzymes in synaptic cleft will degrade neurotransmitters after action potential is initiated on the post-synaptic cell. The neurotransmitters are recycled after they are broken down. Example: Acetylcholine is broken down by the enzyme cholinesterase

Nerve A bundle of axons enclosed within a connective tissue sheath myelin sheath nerve fascicle A bundle of axons enclosed within a connective tissue sheath Figure 34.10 Page 586

Myelin Sheath A series of Schwann cells Sheath blocks ion movements Action potential must “jump” from node to node; CREATES RAPID MOVEMENT OF ELECTRICAL IMPULSE Figure 34.11a Page 586

Multiple Sclerosis A condition in which nerve fibers lose their myelin Slows conduction Symptoms include visual problems, numbness, muscle weakness, and fatigue

Reflexes Automatic movements made in response to stimuli In the simplest reflex arcs, sensory neurons synapse directly on motor neurons; interneurons in CNS aren’t involved. Most reflexes involve an interneuron

Stretch Reflex Figure 34.12b Page 587 STIMULUS Biceps stretches. sensory neuron motor neuron Response Biceps contracts. Figure 34.12b Page 587

Central and Peripheral Nervous Systems Central nervous system (CNS) Brain Spinal cord Peripheral nervous system Nerves that thread through the body

Peripheral Nervous System Somatic nerves Controls skeletal muscle (voluntary) (Shown in green) Autonomic nerves _ Controls certain organs, smooth muscle & cardiac muscle (involuntary) (Shown in red) Figure 34.17 Page 591

Two Divisions of Autonomic Nervous System Sympathetic: “speeds up” heart, breathing, release of sugar into blood; fight or flight response Parasympathetic: slows heart, breathing rate; causes secretion of digestive enzymes, etc. Most organs receive input from both Usually have opposite effects on organ

Structure of the Spinal Cord ganglion nerve meninges (protective coverings) vertebra Figure 34.19a Page 593

Divisions of Brain Division Main Parts Forebrain Cerebrum Olfactory lobes Thalamus Hypothalamus Limbic system Pituitary gland Pineal gland Midbrain Tectum Hindbrain Pons Cerebellum Medulla oblongata anterior end of the spiral cord Figure 34.20 Page 594

Cerebrospinal Fluid Surrounds the spinal cord Fills ventricles within the brain Blood-brain barrier controls which solutes enter the cerebrospinal fluid Figure 34.22 Page 595

Anatomy of the Cerebrum Largest and most complex part of human brain (Responsible for thinking & higher level functions) Outer layer (cerebral cortex) is highly folded A longitudinal fissure divides cerebrum into left and right hemispheres Corpus collosum connects the two hemispheres

Lobes of the Cerebrum Parietal Frontal Occipital Temporal Primary somatosensory cortex Primary motor cortex Parietal Frontal Occipital Temporal Figure 34.25a Page 597

Limbic System Controls emotions and has role in memory (olfactory tract) cingulate gyrus thalamus amygdala hypothalamus Figure 34.36 Page 597 hippocampus

Other Parts of the Brain Cerebellum - Controls muscle coordination and posture Medulla oblongata- Controls heart rate & breathing rate

Variations in Nervous Systems Among Animals