Nervous System Divisions ► Central Nervous System and Peripheral NS ► Afferent Division and Efferent Division ► Somatic and Autonomic NS Autonomic: Autonomic: - sympathetic - sympathetic - parasympathetic - parasympathetic

Types of neurons ► Sensory (afferent)  Receives stimulus and sends info to brain ► Motor (efferent)  Carries signal from brain to effector muscles ► Interneuron  Connects sensory neuron with motor neuron; found in brain and spinal cord

Supporting Cells ► Reinforce, protect insulate and assist neurons. ► Do not conduct nerve impulses ► Out-number neurons. ► Unlike neurons, go through mitosis entire life Supporting cells of the CNS are called glial cells  Astrocytes  Microglia  Ependymal  Oligodendrocytes  Schwann cells which produce the myelin sheath for peripheral neurons.

Astrocytes: largest & most numerous Function: BBB, control of environment structural framework & repairs regulation of ions, nutrients, gases

Oligodendrocyte Smaller than astrocyte Cover neurons with myelin in CNS (white matter vs. gray matter!) Myelin improves the rate of impulse conduction

Microglial cells ► Smallest ► Phagocytosis ►  # during infection or injury

Ependymal cells Lining of ventricles & central canal Some regions ciliated Some specialized to produce and monitor CSF

Typical Neuron Structure ► Cell body or Soma with Perikaryon ► Dendrites ► Axon with axon hillock ► Synaptic terminals Fig 12.4

Cells of the Nervous System ► Neurons  Excitable cells  Transmit chemical and electrical messages from one part of the body to another.  Cell Body: nucleus, cytoplasm, organelles ► Dendrites  Convey signals to the cell body  Short, numerous and extensively branched ► Axons  Conducts signals away from the cell body –larger the diameter, the faster the conduciton  Wrapped in Schwann cells that create an insulating covering called the myelin sheath.  Extend from the axon hillock  Often have one or more side branches called axon collaterals  Distal tips of axons form branches called teledendria that each terminate in synaptic terminals that release neurotransmitters

► Myelin Sheath ► insulates the neuron ► fatty covering formed by Schwann cells ► Nodes of Ranvier  gap between Schwann cells  serves as points along the neuron for generating a signal  signals jumping from node to node travel hundreds of times faster than signals traveling along the surface of the axon.  allows your brain to communicate with your toes in a few thousandths of a second. ► Insulation permits the nervous system to exercise fine control over muscles. ► The reason that babies cannot smile or move precisely at birth is that the insulation for their nerve fibers is not completely developed. As the insulation does develop in a child, they can smile and move with greater coordination and precision.

► Synapse – gap between the synaptic terminals and dendrites of another neuron. ► Neurotransmitter – chemical that relays the nerve impulse across the synapse. ► Gray matter: parts of a neuron not covered with myelin (cell body, dendrites) ► White matter: covered with myelin sheath (axons, etc)

Nerve Cell

Neurons ► Nerves: bundles of nerve fibers of axons held together by several layers of connective tissue. (Nerves are called ► Nerves: bundles of nerve fibers of axons held together by several layers of connective tissue. (Nerves are called tracts in the central nervous system.) - endoneurium: fibrous connective tissue around EACH nerve fiber - perineurium: connective tissue that holds a fascicle (bundle of nerve fibers) together - epineurium: fibrous coat that holds together numerous fascicles and the blood vessels that supply them

Nerve structure Fig 12.16, p 333 A nerve is USUALLY both sensory and motor Similar to muscle terminology Epineurium Covers the nerve Perineurium Covers a fascicle Endoneurium Covers an axon

Classification of Neurons ► Classified according to number of extensions from the cell body: 1.Multipolar 1.Multipolar 2.Bipolar 2.Bipolar 3. Unipolar 3. Unipolar

Structural Classification of Neurons MultipolarBipolarUnipolar

Structural Neuron Classification cont... BipolarUnmyelinated Rare, but important in special senses Multipolar Most common All motor neurons

Structural Neuron Classification Unipolar Also called pseudounipolar Sensory neurons See fig

Reflex Arc ► Sensory neurons convey information from the external environment to the CNS.  Presynaptic cell - cell transmitting the signal (affector cell) ► Interneurons – integrate sensory input and motor output.  Located within the CNS ► Motor neurons convey impulses from the CNS to effector cells. Postsynaptic cell - target cell (effector cell) Postsynaptic cell - target cell (effector cell)  Effector cells – cells that actually carry out the response. ► Muscle or glands cells

The circuit fig 12.11

Na+/K+ Pump ► Active Transport ► Embedded in plasma membrane ► Pumps Na+ out of the cell (neuron) ► Pumps K+ into the cell (neuron) ► Ratio is uneven 3Na+:2K+ ► Need to keep a slight imbalance in order to maintain resting potential ► Membrane Potential – difference in electrical charge across the plasma membrane ► Resting MP - Only Na+ slowly diffusing through channels; no action potential yet

Membrane Potential ► Signal transmission occurs when the membrane potential is changed within a neuron. ► A stimulus causes the membrane to become permeable to sodium thus changing the membrane potential. ► A charge separation between the outside of the cell and the cytoplasm creates voltage across the membrane.  This voltage is maintained by the sodium- potassium pump.

► A solute potential also exists across the neural membrane.  Na + has a tendency to slowly diffuse into the cell.  K + has a tendency to rapidly diffuse out of the cell.  In general the cytoplasm of the cell is much more negative than it’s exterior. ► Resting Potential  Voltage across the membrane of a neuron that is not transmitting a signal.  It is about -70mV ► Negative sign means the cytoplasm is negative relative to the cell’s surroundings

Action Potential ► A rapid reversible depolarization of a neuron’s membrane near the point of stimulation that generates a nerve impulse. ► Originate from the axon hillock ► Is an “all or none” event  Magnitude of the voltage change is the same at each ”firing” regardless of stimuli strength.  Stronger stimulus increases the frequency of action potentials but NOT the strength of the response.

1. Resting Potential (-70 mV) 2. Stimulus triggers Na+ channels to open and allow Na+ into cell (Depolarization) 3. As threshold potential (- 59mV) is reached, more Na+ influx, membrane depolarized more 4. At action potential peak, Na+ gates close (+30 mV) 5. K+ gates open, K+ diffuses out (Repolarization) 6. Brief period of hyperpolarization (too much K+ outflow), membrane potential is restored with ions in resting position Mechanisms To Produce Action Potential

How is Action Potential (an impulse) Generated? ► Stimuli increase the membrane’s permeability to Na +. ► Depolarization  Some of the sodium channels open and Na + rushes into the cell causing the cytoplasm to become less negative.  This is known as depolarization.  If enough depolarization occurs then the cell will reach a threshold potential and additional Na+ will open.  If the threshold potential is reached then action potential is initiated and the impulse will travel down the axon towards the synaptic terminals.  During action potential the interior of the cell becomes positive.

Repolarization ► The cell has to be repolarized to prepare for another action potential. ► The K + channels slowly open allowing K + to leave the cell and the Na+ channels close preventing more Na+ from entering the cell. ► The cytoplasm becomes more negative as positive charges increase outside the cell.

Propagation of the Nerve Impulse ► Saltatory Conduction  The action potential ”jumps” from one node of Ranvier to the next, skipping myelinated regions of the membrane.  Results in faster transmission

Saltatory Conduction