Neurons: The Matter of the Mind Chapter 7 Neurons: The Matter of the Mind

Cells of the Nervous System Sensory (or afferent) neurons carry information toward the CNS from sensory receptors Motor (or efferent) neurons carry information away from the CNS to an effector

Cells of the Nervous System Association neurons (or interneurons) are located between sensory and motor neurons within the CNS where they integrate and interpret sensory signals

Types of Neurons in the Nervous System Sensory Motor Interneuron Figure: 07-01 Title: The nervous system. Caption: The nervous system consists of sensory neurons, interneurons, and motor neurons. Trace the pathway of the impulse from a sensory receptor that detects a change in the external or internal environment along a sensory neuron to an interneuron. In this diagram, only one sensory neuron is shown. An interneuron usually receives input from many sensory neurons. The interneuron integrates the information from the sensory neuron and stimulates a motor neuron. The motor neuron then conducts the information to a muscle or a gland (an effector).

Cells of the Nervous System Glial cells (also called neuroglial cells) are more numerous than neurons and provide structural support, growth factors, and insulating sheaths around the nerves They are able to reproduce, unlike neurons

Neurons (in red) and Glial Cells (in green) in muscle tissue

Function of Glial Cells Protection/selective uptake - provides blood brain barrier Guides neurons to proper destination (synapse) Developing embryo Adult Nurture - Secrete nerve growth factors

Function of Glial Cells (cont.) Clean-up - Mop-up chemicals & neurotransmitters Insulation of neurons Schwann cells in PNS Oligodendrocyte in CNS

Neurons Have Dendrites, a Cell Body, and an Axon Dendrites receive information from other neurons or from the environment Dendrites carry information toward the cell body of a neuron

Neurons Have Dendrites, a Cell Body, and an Axon A single long axon carries information away from the cell body

FIGURE 7.2 The structure of a neuron

Dendrites, Cell Body, and Axon A nerve consists of parallel axons, dendrites, or both from many neurons Nerves are covered with tough connective tissue

Dendrites, Cell Body, and Axon Most axons outside of the CNS are electrically insulated by a myelin sheath

Dendrites, Cell Body, and Axon Schwann cells (glial cells) form the myelin sheath, insulating it and allowing messages to travel faster as they jump from one node of Ranvier to the next in a type of transmission called saltatory conduction

FIGURE 7.3a The myelin sheath. (a) An axon protected by a myelin sheath. The Schwann cells that form the myelin sheath are separated by nodes of Ranvier—areas of exposed axon that allow for saltatory conduction. (b) The myelin sheath forms from multiple wrappings of Schwann cell plasma membranes. (c) An electron micrograph of the cut end of a myelinated axon.

How a Schwann cell wraps around an axon to create a myelin sheath FIGURE 7.3b The myelin sheath. (a) An axon protected by a myelin sheath. The Schwann cells that form the myelin sheath are separated by nodes of Ranvier—areas of exposed axon that allow for saltatory conduction. (b) The myelin sheath forms from multiple wrappings of Schwann cell plasma membranes. (c) An electron micrograph of the cut end of a myelinated axon.

Cross section of an axon surrounded by myelin FIGURE 7.3c The myelin sheath. (a) An axon protected by a myelin sheath. The Schwann cells that form the myelin sheath are separated by nodes of Ranvier—areas of exposed axon that allow for saltatory conduction. (b) The myelin sheath forms from multiple wrappings of Schwann cell plasma membranes. (c) An electron micrograph of the cut end of a myelinated axon.

Dendrites, Cell Body, and Axon The myelin sheath also facilitates nerve repair outside the CNS

Myelin Sheath on Neuron Multiple sclerosis involves progressive destruction of myelin sheaths within the brain and spinal cord.

Depending on where the nerve damage occurs, MS can affect vision, sensation, coordination, movement, and bladder and bowel control.

Dendrite Axon Synapse Impulsive The part of the neuron that carries impulses (info) away from the cell body is called …. Dendrite Axon Synapse Impulsive

Dendrites …. Carry information away from the cell body Carry information toward the cell body Receive information from other neurons

The myelin sheath Slows the conduction of nerve impulses Is formed by Schwann cells wrapping around axons Make saltatory conduction possible Has gaps in it called “Nodes of Ranvier”

The Nerve Impulse Is an Electrochemical Signal A nerve impulse, or action potential, is a bioelectrical signal involving sodium ions (Na+) and potassium ions (K+) that cross the cell membrane through the ion channels

FIGURE 7.4 The plasma membrane of a neuron provides two general ways for ions to enter or leave the cell: (1) diffusion through a channel or (2) active transport by a pump.

The Nerve Impulse Na+ and K+ are transported across the membrane by the sodium-potassium pump

FIGURE 7.4 The plasma membrane of a neuron provides two general ways for ions to enter or leave the cell: (1) diffusion through a channel or (2) active transport by a pump.

Resting Membrane Potential When a neuron is not conducting a nerve impulse, it is in a resting state The sodium-potassium pump uses ATP to transport sodium ions out of and potassium ions into the neuron The inside of a resting neuron has a negative charge relative to the outside

FIGURE 7.5a part 1 The resting state and the propagation of an action potential along an axon. (a) The sequential opening and closing of sodium-channel gates and potassium-channel gates produces the action potential. (b) The voltage across the membrane can be measured by electrodes placed inside and outside the axon. The graph shows the changes in voltage that accompany an action potential.

The Nerve Impulse When the neuron is stimulated the sodium gates open and sodium ions enter the cell The minimum charge that causes the sodium gates to open is called the threshold

Action Potential (aka, nerve impulse) Na+ channels open: sodium moves into the axon K+ channels open: potassium moves out of the axon Re-establishment of the Na+ and K+ gradients by the normal activity of the sodium-potassium pump

FIGURE 7.5a part 2 The resting state and the propagation of an action potential along an axon. (a) The sequential opening and closing of sodium-channel gates and potassium-channel gates produces the action potential. (b) The voltage across the membrane can be measured by electrodes placed inside and outside the axon. The graph shows the changes in voltage that accompany an action potential.

Figure: 07-05b Title: The resting state and the propagation of an action potential along an axon. Caption: Bottom: charge restoration.

FIGURE 7.5b The resting state and the propagation of an action potential along an axon. (a) The sequential opening and closing of sodium-channel gates and potassium-channel gates produces the action potential. (b) The voltage across the membrane can be measured by electrodes placed inside and outside the axon. The graph shows the changes in voltage that accompany an action potential.

The first and second events of an action potential are: Ca++ gated channel opens then closes Na+ pumped out and K+ pumped into the axon by the Na+/K+ pump Na+ moves in then K+ moves out of the axon Ca++ moves out of the SER and then is pumped back in

Axons…. Conduct impulses away from the cell body Conduct information toward the cell body Carry information both toward and away from the cell body

All-or-None and Self-Propagating Action Potentials Action potentials are “all-or-none” responses Action potentials are “self-propagating” Electrical current reaches threshold throughout axon during spread of the action potential

Synaptic Transmission Is Communication between Neurons Communication between neurons is by neurotransmitters; chemicals that cross the gap between two neurons

Communication between Neurons A synapse is the junction between the presynaptic neuron, which sends a message to the postsynaptic neuron

FIGURE 7.6a Structure of a synapse. (a) The synaptic knob at the end of the axon on the presynaptic neuron is separated from the dendrite or cell body of the postsynaptic neuron by a small gap called a synaptic cleft. Within the synaptic knob are small sacs, called synaptic vesicles, filled with neurotransmitter molecules. (b) An electron micrograph of a synapse.

FIGURE 7.6b Structure of a synapse. (a) The synaptic knob at the end of the axon on the presynaptic neuron is separated from the dendrite or cell body of the postsynaptic neuron by a small gap called a synaptic cleft. Within the synaptic knob are small sacs, called synaptic vesicles, filled with neurotransmitter molecules. (b) An electron micrograph of a synapse.

FIGURE 7.8a A neuron may have as many as 10,000 synapses at which it receives input from other neurons. Some synapses have an excitatory effect on the membrane of the postsynaptic neuron and increase the likelihood that the neuron will fire. Other synapses have an inhibitory effect and reduce the likelihood that the postsynaptic neuron will fire. The net effect of all the synapses determines whether an action potential is generated in the postsynaptic neuron. (The shape of the synaptic knobs shown in this electron micrograph is distorted as a result of the preparation process.)

FIGURE 7.8b A neuron may have as many as 10,000 synapses at which it receives input from other neurons. Some synapses have an excitatory effect on the membrane of the postsynaptic neuron and increase the likelihood that the neuron will fire. Other synapses have an inhibitory effect and reduce the likelihood that the postsynaptic neuron will fire. The net effect of all the synapses determines whether an action potential is generated in the postsynaptic neuron. (The shape of the synaptic knobs shown in this electron micrograph is distorted as a result of the preparation process.)

Communication between Neurons When an action potential (impulse) reaches the axon ending of the presynaptic neuron---- This causes the membrane of the synaptic vesicles to fuse with the plasma membrane and to release the neurotransmitter substances

FIGURE 7.7 part 1 Transmission across an excitatory synapse

Synaptic Vesicles …. Contain mitochondria Contain DNA Contain neurotransmitters Release neurotransmitters into the synaptic cleft

Communication between Neurons Neurotransmitters diffuse across the synaptic cleft to the other side where they bind with receptors on the postsynaptic cell causing the ion channels to open

FIGURE 7.7 part 2 Transmission across an excitatory synapse

FIGURE 7.7 part 3 Transmission across an excitatory synapse

Presynaptic neurons signal postsynaptic neurons across the synaptic cleft by: Electrons Ions ICBMs Neruotransmitters Texting

Communication between Neurons After the neurotransmitter crosses the membrane, it is quickly broken down or pumped back into the synaptic knob of the presynaptic axon

Removal of Neurotransmitter from Synapse Enzymatic breakdown Re-uptake of neurotransmitter Example: Dopamine transporter (re-uptake protein) knockout mouse is hyperactive and unresponsive to cocaine and amphetamines Many drugs (legal and illegal), toxins, and venoms work at the level of the synapse

Communication between Neurons If neurotransmission occurs at an excitatory synapse and enough receptor sites bind with neurotransmitter substances to cause depolarization to threshold value, an action potential is generated in the postsynaptic cell

Communication between Neurons However, in an inhibitory synapse, the postsynaptic cell becomes more negatively charged and there is no action potential generated

Resting Membrane Potential, Graded Potential And Action Potential

Transfer of Information from Neuron to Post-Synaptic Membrane Post-Synaptic Response Release of neurotransmitter: graded potential achieved Effects of neurotransmitter is either: Excitatory: depolarize postsynaptic cell Inhibitory: hyperpolarize postsynaptic cell Role of postsynaptic neuron: integrates the relative amount of excitatory and inhibitory input

Points of Synaptic Regulation Compensation (up or down regulation) occurs at several levels: Release of Neurotransmitter Reuptake of Neurotransmitter Enzymatic Breakdown of Neurotransmitter Post-synaptic Receptor for Nt

What would you expect to happen in the synapses of a person who chronically takes cocaine, a drug that causes excess amounts of dopamine (DA) to collect in the synapse by inhibiting DA reuptake by the pre-synaptic neuron. There would be an increase in DA receptors on the post-synaptic membrane. There would be a decrease in DA receptors on the post-synaptic membrane There would be an increase in the enzymes that breakdown DA There would be a decrease in the enzymes that breakdown DA

Stimulus Quality How do we distinguish light from sound? Different stimuli (e.g, light and sound) are recognized as qualitatively different because they are sent to different areas of the brain.

Stimulus Intensity How do we perceive different signal intensities? Increased frequency of impulse signals increased intensity The number of neurons that fire increases when strength of stimulus increases

Neurotransmitters Different neurotransmitters play different roles Alzheimer’s disease - Acetylcholine Depression – Serotonin, Dopamine, Norepineph Parkinson’s disease - Dopamine

Alzheimer’s disease - Acetylcholine FIGURE 7.9 Former President Ronald Reagan died in June 2004 at the age of 93. He had Alzheimer's disease since at least 1994. His wife, Nancy Reagan, described Alzheimer's disease as "the long good-bye" because it robs people of their memories long before it takes their lives.

Parkinson’s disease - Dopamine FIGURE 7.10 Actor Michael J. Fox and former heavyweight champion Muhammad Ali have Parkinson's disease, a progressive debilitating disease characterized by slowed movements, tremors, and rigidity. The symptoms result from insufficient amounts of the neurotransmitter dopamine, caused by the death of dopamine-making nerve cells in a movement control center of the brain. The Michael J. Fox Foundation for Parkinson's Research (www.michaeljfox.org) funds research on the early diagnosis and treatment of Parkinson's disease.

Inhibitors that work at the post-synapse Curare (plant bark) Compet. Inhibitor of Ach for its receptor on muscle cell Alpha-bungerotoxin (black mamba venom) Similar mechanism to curare

Inhibitors that work at the pre-synapse Tetnus toxin Blocks inhibitory input to muscle; result is excess muscle contraction BOTOX (Botulism toxin) Blocks Ach neurotransmitter release (exocytosis) at neuromuscular synapse Antidepressants (SSRIs) Selective Serotonin Reuptake Inhibitors

Neurotransmitters Dopamine (DA) Generally excitatory; emotional responses, euphoria Re-uptake blocked by cocaine and amphetamines (see KO mouse) Parkinson’s disease: degeneration of substantia nigra area of brain leads to  DA Schizophrenia: Thorazine and clozapine are drugs that inhibit DA release from pre-synaptic terminal

Neurotransmitters (cont) Serotonin Involved in sleep and control of mood (eg, depression) Antidepressants/SSRIs (eg, prozac) inhibit re-uptake from synaptic cleft. LSD mimics serotonin

Neurotransmitters (cont) Acetylcholine Neurotransmitter in neuromuscular synapses Blocked by curare; a-bungarotoxin (snake venom), BOTOX, and nerve gas Norepinephrine (NE) Neurotransmitter used in pathways involved in arousal, regulation of moods such as pleasure and feeling good Cocaine augments the effects of NE

Neurotransmitters (cont) Substance P Signals perception of pain in sensory nerves Endorphin Binds to opiate receptor (same one that morphine binds to) & inhibits pain by inhibiting substance P release Thought to be responsible for “runners high”?

Neurotransmitters (cont) Gamma amino butyric acid (GABA) Tends to inhibit or slow down other pathways Target for anti-anxiety drugs such as Valium, which enhance the action of GABA

Action potentials are all-or-none Action potentials are all-or-none. Which of the following statements summarizes this principle? An action potential can vary in both strength and duration If a threshold stimulus is reached the action potential will always be of the same strength Several stimuli will cause a stronger response The effects of several stimuli are combined until the neuron fires

Which of the following carry impulses toward the cell body? axon dendrite glial cells Schwann cells