The Nervous System and the Brain

The master controlling and communicating system of the body Nervous System The master controlling and communicating system of the body Functions Sensory input – monitoring stimuli Integration – interpretation of sensory input Motor output – response to stimuli

Nervous System Figure 11.1

Organization of the Nervous System Central nervous system (CNS) Brain and spinal cord Integration and command center Peripheral nervous system (PNS) Paired spinal and cranial nerves Carries messages to and from the spinal cord and brain

Peripheral Nervous System (PNS): Two Functional Divisions Sensory (afferent) division Sensory afferent fibers – carry impulses from skin, skeletal muscles, and joints to the brain Visceral afferent fibers – transmit impulses from visceral organs to the brain Motor (efferent) division Transmits impulses from the CNS to effector organs

Motor Division: Two Main Parts Somatic nervous system Conscious control of skeletal muscles Autonomic nervous system (ANS) Regulates smooth muscle, cardiac muscle, and glands Divisions – sympathetic and parasympathetic

Concept Check What is the difference between peripheral and central nervous systems? Explain the difference between somatic and autonomic nervous systems… What are the two divisions of the PNS?

Central Nervous System Brain Spinal Cord Peripheral Nervous System Sensory (afferent) Motor (efferent) Autonomic Somatic Sympathetic Parasympathetic

Autonomic Nervous System Split into two parts Parasympathetic Sympathetic Responsible for our bodies ability to respond to stress, and recover from that stressful response Controls involuntary and visceral functions Respiration, digestion, cardiovascular, urinary, reproduction

Sympathetic The section that allows our body to respond to stressful environments or situations “Fight or Flight” Sometimes we can get stuck in this response, which causes long term health issues Physical Reaction Long-Term Impact Blood Pressure Rises Heart Disease Stress hormones Rise Anxiety, insomnia, addiction, weight gain Slowing of Digestion Gastro Intestinal Problems Growth and sex hormone levels decrease Premature aging, infertility Immune system weakens Cancer and infections Sticky blood platelets increase Plaque build up – heart attackts

Parasympathetic Controls our ‘vegetative’ functions Constantly opposing the sympathetic system Restores body to homeostasis

Concept Check What are some of the consequences of being at a high stress level for a sustained period of time? What are the major functions of the sympathetic system? What are the main functions of the parasympathetic system?

Neurons See How I work

Histology of Nerve Tissue The two principal cell types of the nervous system are: Neurons – excitable cells that transmit electrical signals Supporting cells – cells that surround and wrap neurons

Microglia and Ependymal Cells Microglia – small, ovoid cells with spiny processes Phagocytes that monitor the health of neurons Ependymal cells – range in shape from squamous to columnar They line the central cavities of the brain and spinal column

Microglia and Ependymal Cells Figure 11.3b, c

Oligodendrocytes, Schwann Cells, and Satellite Cells Oligodendrocytes – branched cells that wrap CNS nerve fibers Schwann cells (neurolemmocytes) – surround fibers of the PNS Satellite cells surround neuron cell bodies with ganglia

Oligodendrocytes, Schwann Cells, and Satellite Cells Figure 11.3d, e

Concept Check What is the difference between Schwann Cells and Oligodendrocytes?

Structural units of the nervous system Neurons (Nerve Cells) Structural units of the nervous system Composed of a body, axon, and dendrites Long-lived, amitotic, and have a high metabolic rate Their plasma membrane function in: Electrical signaling Cell-to-cell signaling during development

Neurons (Nerve Cells) Figure 11.4b

Dendrites of Motor Neurons Short, tapering, branched arms of the nerve They are the receptive, or input, regions of the neuron Electrical signals are conveyed as graded potentials (not action potentials)

Cell Body of the Nerve Factory of the neuron Produces the proteins for the neuron to function Connects the dendrites to the axon

Axons: Function Generate and transmit action potentials Secrete neurotransmitters from the axonal terminals Get information from the dendrites and pass it along to another nerve Movement along axons occurs in two ways Anterograde — toward axonal terminal (Normal) Retrograde — away from axonal terminal (abnormal)

Myelin Sheath Whitish, fatty (protein-lipoid), segmented sheath around most long axons It functions to: Protect the axon Electrically insulate fibers from one another Increase the speed of nerve impulse transmission

Nodes of Ranvier (Neurofibral Nodes) Gaps in the myelin sheath between adjacent Schwann cells They are the sites where axon collaterals can emerge

Unmyelinated vs Myelinated Axons Unmyelinated axons A Schwann cell surrounds nerve fibers but coiling does not take place Schwann cells partially enclose 15 or more axons Impulse must travel the entire length of the axon, slowing transmission Myelinated Axons Much faster Impulse can ‘jump’

Axons of the CNS Both myelinated and unmyelinated fibers are present Myelin sheaths are formed by oligodendrocytes Nodes of Ranvier are widely spaced There is no neurilemma

Regions of the Brain and Spinal Cord White matter – dense collections of myelinated fibers Gray matter – mostly soma (body) and unmyelinated fibers

Concept Check Why are myelin sheaths helpful to a neuron? What are the parts of the neuron? What are their functions?

How are Neurons work

Neuron Classification Functional: Sensory (afferent) — transmit impulses toward the CNS Motor (efferent) — carry impulses away from the CNS Interneurons (association neurons) — shuttle signals through CNS pathways

Neurons are highly irritable Neurophysiology Neurons are highly irritable Action potentials, or nerve impulses, are: Electrical impulses carried along the length of axons Always the same regardless of stimulus The underlying functional feature of the nervous system – this is what makes everything work Chemical Synapse

Definitions Synapse = the small space between the axon terminal of one neuron and the dendrites of others Neurotransmitter = chemical messages that allow neurons to communicate with other neurons across the synapse Receptor cites = areas on a neuron that connect and respond to neurotransmitters Acetylcholine = most common neurotransmitter Used in movement of peripheral NS and related to atterion and memory in the brain Neuropeptides = special class of neurotransmitters that regulate activity of neurons and systems in the brain Presynaptic terminal = site where the neurotransmitters are released Postsynaptic terminal = site that receives neurotransmitters

How does it work Our cells have proteins embedded in their plasma membranes These proteins create channels for ions (charged particles) to flow in and out of the cell Some require energy to open while others do not

Describes our membranes ‘electric charge’ Membrane Potential Describes our membranes ‘electric charge’ At rest, the inside of our cell is negative in comparison to the outside At rest Activation gates of Na+ are closed Inactivation gates of Na+ are open Voltage-gated K+ channels are closed Nerve Impulse

Types of plasma membrane ion channels: Role of Ion Channels Types of plasma membrane ion channels: Passive, or leakage, channels – always open Chemically gated channels – open with binding of a specific neurotransmitter Voltage-gated channels – open and close in response to membrane potential (change in charge) Mechanically gated channels – open and close in response to physical deformation of receptors

Operation of Sodium-Potassium Pump Chemical-gated channel – only works when the right Neurotransmitters bind to its receptor sites Active transport Moves the ions against their concentration gradient Steps – Na+ is inside and K+ is outside 3 Na+ and 1 ATP bind to the carrier protein inside the cell Protein changes shape and releases Na+ outside of the cell 2 K+ ions bind to the protein out of the cell Protein changes back to its original configuration and releases K+ into the cell Sodium-Potassium Pump

Operation of a Gated Channel Figure 11.6a

Operation of a Voltage-Gated Channel Example: Na+ channel Require a stimulus to open the channels Steps Nerve receives stimulus – opens sodium activation gates Sodium diffuses into cell causing depolarization Potassium channels open allowing potassium to diffuse out of the cell Depolarization Sodium is entering the cell faster than potassium can leave Voltage-Gated Channel

Operation of a Voltage-Gated Channel Figure 11.6b

Changes in Membrane Potential Changes are caused by three events Depolarization – the inside of the membrane becomes less negative Repolarization – the membrane returns to its resting membrane potential Hyperpolarization – the inside of the membrane becomes more negative than the resting potential

Check for understanding What is the job of the sodium potassium pump How does a synapse work What is the difference between and chemical channel and a voltage channel