Structure and Function Nervous System Structure and Function

Functions Sensory input Integration Motor output

Classification Central Nervous System Peripheral Nervous System Brain Cranial Nerves Spinal Cord Peripheral Nervous System Spinal nerves Sensory receptors

Divisions of Peripheral Nervous System Sensory Division Motor Division muscles and glands Divisions of the Motor Somatic Autonomic

Figure 11.2 Central nervous system (CNS) Peripheral nervous system (PNS) Brain and spinal cord Cranial nerves and spinal nerves Integrative and control centers Communication lines between the CNS and the rest of the body Sensory (afferent) division Motor (efferent) division Somatic and visceral sensory nerve fibers Motor nerve fibers Conducts impulses from the CNS to effectors (muscles and glands) Conducts impulses from receptors to the CNS Somatic sensory fiber Somatic nervous system Autonomic nervous system (ANS) Skin Somatic motor (voluntary) Visceral motor (involuntary) Conducts impulses from the CNS to skeletal muscles Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands Visceral sensory fiber Stomach Skeletal muscle Motor fiber of somatic nervous system Sympathetic division Parasympathetic division Mobilizes body systems during activity Conserves energy Promotes house- keeping functions during rest Sympathetic motor fiber of ANS Heart Structure Function Sensory (afferent) division of PNS Parasympathetic motor fiber of ANS Bladder Motor (efferent) division of PNS Figure 11.2

How it works

Neurons = nerve cells Long lived, no mitosis, Cell body- developed Golgi Extensions outside the cell body Dendrites Axons Axonal terminals contain vesicles with neurotransmitters Axonal terminals are separated from other neurons or effectors (muscles or organs) by a gap called the synapse

Nerve Coverings Myelin- Lipid/Protein Schwann cells Nodes of Ranvier

rotates around the axon, wrapping its plasma membrane loosely around Schwann cell plasma membrane Schwann cell cytoplasm A Schwann cell envelopes an axon. 1 Axon Schwann cell nucleus The Schwann cell then rotates around the axon, wrapping its plasma membrane loosely around it in successive layers. 2 Neurilemma The Schwann cell cytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath. 3 Myelin sheath (a) Myelination of a nerve fiber (axon) Figure 11.5a

Classification of Neurons Multipolar neurons Bipolar Unipolar

Classification Cont.. Sensory Neurons afferent Interneurons multipolar most are unipolar some are bipolar Interneurons multipolar in CNS Motor Neurons efferent carry impulses to effectors, muscle or gland

Table 11.1 (2 of 3)

Neuroglial Cells: Support Cells in CNS Microglia Phagocytes that engulf debris, necrotic tissue, invading viruses or bacteria Astrocytes Have many processes – look like stars Perivascular feet wrap around and cover neurons and blood vessels Form the blood-brain barrier which allows only certain substances to enter neurons from blood vessels

Ependyma Oligodendrocytes Line the ventricles in the brain and central canal in spinal cord Form cerebral spinal fluid Oligodendrocytes Support CNS cells Have processes that form myelin sheaths around axons

Neuroglial Cells: Support Cells in PNS Schwann Cells-PNS Flattened cells, wrap around the axons Form the myelin sheath around the axon Satellite-PNS

Regeneration of Injury (if possible)

Principles of Electricity Opposite charges attract each other Energy is required to separate opposite charges across a membrane Energy is liberated when the charges move toward one another If opposite charges are separated, the system has potential energy

Definitions Voltage (V): measure of potential energy generated by separated charge Potential difference: voltage measured between two points Current (I): the flow of electrical charge (ions) between two points

Definitions Resistance (R): hindrance to charge flow (provided by the plasma membrane) Insulator: substance with high electrical resistance Conductor: substance with low electrical resistance

Role of Membrane Ion Channels Leakage (nongated) channels—always open Gated channels (three types): Chemically gated (ligand-gated) Voltage-gated channels Mechanically gated channels

Generating a Nerve Impulse polarized membrane: inside is negative relative to the outside under resting conditions -70 mV

Voltmeter Plasma Ground electrode membrane outside cell Microelectrode inside cell Axon Neuron Figure 11.7

Action Potential (AP) Brief reversal of membrane potential with a total amplitude of ~100 mV Occurs in muscle cells and axons of neurons Does not decrease in magnitude over distance Principal means of long-distance neural communication

The big picture 1 2 3 3 4 2 1 1 4 Resting state Depolarization Repolarization 3 4 Hyperpolarization Membrane potential (mV) 2 Action potential Threshold 1 1 4 Time (ms) Figure 11.11 (1 of 5)

Generation of an Action Potential Resting state Only leakage channels for Na+ and K+ are open All gated Na+ and K+ channels are closed

Depolarizing Phase Na+ influx causes more depolarization At threshold (–55 to –50 mV) positive feedback leads to opening of all Na+ channels, and a reversal of membrane polarity to +30mV (spike of action potential)

Repolarizing Phase Repolarizing phase Na+ channel slow inactivation gates close Membrane permeability to Na+ declines to resting levels Slow voltage-sensitive K+ gates open K+ exits the cell and internal negativity is restored

Hyperpolarization Hyperpolarization Some K+ channels remain open, allowing excessive K+ efflux This causes after-hyperpolarization of the membrane (undershoot)

The AP is caused by permeability changes in the plasma membrane 3 Action potential Membrane potential (mV) Na+ permeability 2 Relative membrane permeability K+ permeability 1 1 4 Time (ms) Figure 11.11 (2 of 5)

Peak of action potential Hyperpolarization Voltage at 0 ms Recording electrode (a) Time = 0 ms. Action potential has not yet reached the recording electrode. Resting potential Peak of action potential Hyperpolarization Figure 11.12a

potential peak is at the recording electrode. Voltage at 2 ms (b) Time = 2 ms. Action potential peak is at the recording electrode. Figure 11.12b

A&P Flix™: Propagation of an Action Potential Voltage at 4 ms (c) Time = 4 ms. Action potential peak is past the recording electrode. Membrane at the recording electrode is still hyperpolarized. PLAY A&P Flix™: Propagation of an Action Potential Figure 11.12c

Impulse Conduction

Coding for Stimulus Intensity All action potentials are alike and are independent of stimulus intensity How does the CNS tell the difference between a weak stimulus and a strong one? Strong stimuli can generate action potentials more often than weaker stimuli The CNS determines stimulus intensity by the frequency of impulses

Saltatory Conduction Appear the jump from node to node. Speed of impulses is much faster on myelinated nerves then unmyelinated ones. Speed also increases with increase in diameter. Ex.) 120m/s skeletal muscle .5m/s skin.

Conduction Velocity Conduction velocities of neurons vary widely Effect of axon diameter Effect of myelination Myelin sheaths insulate and prevent leakage of charge Saltatory conduction in myelinated axons is about 30 times faster

Nerve Fiber Classification Group A fibers Large diameter, myelinated somatic sensory and motor fibers Group B fibers Intermediate diameter, lightly myelinated ANS fibers Group C fibers Smallest diameter, unmyelinated ANS fibers

The Synapse Presynaptic neuron—conducts impulses toward the synapse Postsynaptic neuron—transmits impulses away from the synapse Axodendritic Axosomatic Some electrical, most chemical Cleft = gap

Axodendritic synapses Dendrites Axosomatic synapses Cell body Axoaxonic synapses (a) Axon Axon Axosomatic synapses Cell body (soma) of postsynaptic neuron (b) Figure 11.16

Chemical synapses transmit signals from one neuron to another using neurotransmitters. Presynaptic neuron Presynaptic neuron Postsynaptic neuron Action potential arrives at axon terminal. 1 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal. 2 Mitochondrion Ca2+ Ca2+ Ca2+ Ca2+ Ca2+ entry causes neurotransmitter- containing synaptic vesicles to release their contents by exocytosis. 3 Synaptic cleft Axon terminal Synaptic vesicles 4 Neurotransmitter diffuses across the synaptic cleft and binds to specific receptors on the postsynaptic membrane. Postsynaptic neuron Ion movement Enzymatic degradation Graded potential Reuptake Diffusion away from synapse 5 Binding of neurotransmitter opens ion channels, resulting in graded potentials. 6 Neurotransmitter effects are terminated by reuptake through transport proteins, enzymatic degradation, or diffusion away from the synapse. Figure 11.17

postsynaptic membrane that brings the neuron closer to AP threshold. An EPSP is a local depolarization of the postsynaptic membrane that brings the neuron closer to AP threshold. Neurotransmitter binding opens chemically gated ion channels, allowing the simultaneous pas- sage of Na+ and K+. Membrane potential (mV) Threshold Stimulus Time (ms) (a) Excitatory postsynaptic potential (EPSP) Figure 11.18a

hyperpolarization of the postsynaptic membrane and drives the neuron An IPSP is a local hyperpolarization of the postsynaptic membrane and drives the neuron away from AP threshold. Neurotransmitter binding opens K+ or Cl– channels. Membrane potential (mV) Threshold Stimulus Time (ms) (b) Inhibitory postsynaptic potential (IPSP) Figure 11.18b

Integration: Summation A single EPSP cannot induce an action potential EPSPs can summate to reach threshold IPSPs can also summate with EPSPs, canceling each other out

Neurotransmitters Most neurons make two or more neurotransmitters, which are released at different stimulation frequencies 50 or more neurotransmitters have been identified Classified by chemical structure and by function Some excite and some inhibit Can be nucleotides, gas, protein, amino acid, lipoprotein

Neurotransmitters

(a) Channel-linked receptors open in response to binding Ion flow blocked Ions flow Ligand Closed ion channel Open ion channel (a) Channel-linked receptors open in response to binding of ligand (ACh in this case). Figure 11.20a

AMP in this case) that brings about the cell’s response. Neurotransmitter (1st messenger) binds and activates receptor. 1 Closed ion channel Open ion channel Adenylate cyclase Receptor G protein cAMP changes membrane permeability by opening or closing ion channels. 5a cAMP activates specific genes. 5c GDP 5b cAMP activates enzymes. Receptor activates G protein. 2 G protein activates adenylate cyclase. 3 Adenylate cyclase converts ATP to cAMP (2nd messenger). 4 Nucleus Active enzyme (b) G-protein linked receptors cause formation of an intracellular second messenger (cyclic AMP in this case) that brings about the cell’s response. Figure 11.17b

Figure 11.22a

Figure 11.22b

Figure 11.22c, d

Clinical Application. Multiple Sclerosis Causes Symptoms myelin destroyed in various parts of CNS hard scars (scleroses) form nerve impulses blocked muscles do not receive innervation may be related to a virus Symptoms blurred vision numb legs or arms can lead to paralysis Treatments no cure bone marrow transplant interferon (anti-viral drug) hormones 10-29