The Nervous System Fundamentals of the Nervous System and Nervous Tissue

Function of the Nervous System (1) Sensory Input: Monitors changes (stimuli) occurring inside and outside the body via sensory receptors (2) Integration: Processes and interprets the sensory input and decides what to do at each moment (3) Motor Function: Causes a response by activating effector organs (muscles or glands)

Homeostasis:  The nervous system is responsible to sense changes in the body (receptor), analyses the change (control centre), and cause a response (effector)  The nervous system has ultimate control over homeostasis

Divisions of the Nervous System  CNS: brain and spinal cord  PNS: outside the CNS; nerves that extend from the brain and spinal cord; spinal nerves carry impulses to and from the spinal cord and cranial nerves carry impulses to and from the brain

Peripheral Nervous System  Sensory/Afferent Division: convey impulses to the CNS from sensory receptors; (1) Somatic Afferent Fibers: from skin, skeletal muscles, and joints (2) Visceral Afferent Fibers: from visceral organs (ventral body cavity)  Motor/Efferent Division: transmits impulses from the CNS to effector organs, muscles, and glands (2 parts)

Peripheral Nervous System  Motor/Efferent Division: Somatic Nervous System: aka voluntary NS because we consciously control our skeletal muscles; motor impulses from the CNS to skeletal muscles Autonomic Nervous System: aka involuntary NS; visceral motor nerve fibers that regulate the activity of smooth muscles, cardiac muscles, and glands; 2 divisions

Peripheral Nervous System  Motor/Efferent Division: Autonomic Nervous System: (1) Sympathetic Nervous System: “Fight or Flight” (2) Parasympathetic Nervous System: “Rest and Digest”

Autonomic Nervous System  5 types of Adrenergic receptors Alpha one – causes??? Alpha two - TBA B1 –heart – causes ??? (three things) B2 –lungs – causes ??? Dopaminergic – renal causes patient to pee!

Receptors  If receptor responds to acethylcholine is cholinergic and two types = muscarinic or nicotinic  If receptor responds to norephinephrine or epinephrine = adrenergic Five types! ACH NOREPI EPI adrenergic muscarinic nicotinic

Nervous Tissue  2 principle types of cells (1) Neurons: the excitable nerve cells that transmit electrical signals Basic structural & functional unit of NS (2) Neuroglia: supporting cells that surround and wrap the more delicate neurons  Like connective tissue Together they form the structures of both the CNS and PNS

Neurons  Cell body  Nerve fibers: Dendrites receive info into cell body Axons conduct info away from cell body Axonal terminals at ends contain neurotransmitter to permit communication between neurons May be myelinated or unmyelinated

Neurons

Gray & White Matter in the CNS  Gray Matter: contains mostly nerve cell bodies and unmyelinated fibers  White Matter: contains dense collections of myelinated fibers and are primarily fiber tracts

Reflexes  Rapid, predictable, automatic responses to stimuli, in which a particular stimulus always causes the same motor response; unlearned, unpremeditated, and involuntary, considered to be built into our neural anatomy  Occur over neural pathways called reflex arcs that have 5 elements: (1) receptor (2) sensory neuron (3) CNS integration center (4) motor neuron (5) effector

Reflex Arc

Resting Membrane Potential  The potential difference between the inside (cytoplasmic side) of a neuron’s membrane and the outside; the value varies from –40 mV to –90 mV (millivolts); membrane is polarized  The resting membrane potential is generated by differences in the ionic makeup of the intracellular and extracellular fluids; primarily sodium (outside) and potassium (inside)

Graded Potentials  Short-lived, local changes in membrane potential that can be either depolarizations (i.e. less negative inside) or hyperpolarizations (i.e. more negative inside); called “graded” because their magnitude varies directly with the strength of the stimulus; the stronger the stimulus, the more the voltage changes and the farther the current flows; triggered by some change in the neuron’s environment (heat, light, etc)

Action Potentials  Only cells with excitable membranes (neurons and muscle cells) can generate action potentials, which are the way neurons communicate  In neurons, a transmitted action potential is also called a nerve impulse

Action Potentials  They are generated by changes in membrane permeability to Na+ and K+ which cause depolarization (inside the membrane becomes more positive) followed by repolarization (return to negativity on the inside of membrane)

Action Potentials  Threshold: when the membrane has been depolarized by 15 to 20 mV from the resting value, an action potential will occur (same size every time)  All or None Phenomenon: the action potential either happens completely or it doesn’t happen at all

Conduction Velocities of Axons  (1) Influence of axon diameter: the larger the axon’s diameter, the faster it conducts impulses  (2) Influence of a myelin sheath: if present, the myelin sheath dramatically increases the speed of transmission because myelin acts as an insulator to prevent leakage of charge from the axon; the electrical signal jumps from node of Ranvier to node of Ranvier (1mm) along the axon

Synapse  The functional junction between two neurons or between a neuron and an effector  In order to propagate nerve impulses from neuron to neuron, they have to meet; the junctions are the synapses; the gap between them is the synaptic cleft  Neurotransmitters are released into the cleft to facilitate (or inhibit) transmission

Happenings at the NMJ…..

Depolarization

Neurotransmitter Removal  The neurotransmitter must be removed from the synaptic cleft once its job is done  This occurs via diffusion, break down via enzymes or transport via special membrane proteins  Some drugs act on this area (E.g SSRI and MAOIs)