The Nervous System Overview The Nerve Nerve Impulse Synaptic Transmission CNS PNS

Overview The Nervous System Central Nervous System BrainSpinal Cord Peripheral Nervous System Somatic Nerves Autonomic Nerves Sympathetic Nervous System Parasympathetic Nervous System The coordinating center of the body’s mechanical and chemical actions The body system that coordinates actions, interprets stimuli, and transmits impulses The component that relays information between the CNS and other parts of the body Interprets all information received from nerves Relays information to and from the brain The system that controls the voluntary movement of muscle, bones, and skin The system that maintains involuntary control over the internal organs The system that responds to stress The system that restores the body after stress

Overview The function of the nervous system is to maintain homeostasis – Maintaining an internal environment It does this by using negative feedback to regulate various bodily processesnegative feedback – This is closely associated with the endocrine system

The Nerve The nervous system is composed of 2 major types of cells – Neurons The functional units of the nervous system – Glial Cells Non-conducting support cells of the nervous system A bundle of neurons is called a nervenerve

The Nerve The Neuron (Nerve Cell)Neuron – Dendrites Cytoplasmic projections that carry a nerve impulse to the cell body – Cell Body Houses the nucleus and organelles of the neuron – Axon Cytoplasmic extension that carries nerve impulses away from the cell body

The Nerve Myelin Sheath – Insulates the axon aiding in impulse transmission (it prevents the loss of ions) Schwann Cells – Secrete the myelin sheath Schwann Cells – Nodes of Ranvier Gaps between myelin sheath where nerve impulses occur Saltatory Conduction – The mechanism by which nerve impulses travel or jump from Node to Node Saltatory Conduction – Neurilemma Neurilemma A thin outer membrane surrounding the axons of some neurons Produced by Schwann Cells and repairs/regenerates damaged axons

The Nerve Axon Terminal – The point where a nerve impulse is passed onto another neuron

The Nerve Neurons are classified into 3 groups: 1.Sensory NeuronsSensory Neurons Carries impulses received by sensory receptors to the CNSsensory receptors – Temperature, Pressure, Pain, Touch Cell bodies are found in ganglia outside the spinal cordganglia 2.InterneuronInterneuron Found only in the brain and spinal cord (CNS) Connects to other interneurons and integrates /interprets sensory info and relays it to motor neurons 3.Motor NeuronsMotor Neurons Carry impulses to effectors (muscles, organs, glands) to produce a response

The Nerve The Reflex Arc A reflex is an involuntary and unconscious response to a stimulus A reflex arc is a simple nerve pathway that does not involve the brain

Nerve Impulse Nerve impulses involve the movement of an electrochemical charge along the length of the neuron electrochemical charge – The charge is due to the movement of ions (Na + and K + ) across the neural membrane through ion channels Ion channels ensure that ions are not able to pass down there concentration/electrochemical gradient – Gated sodium ion channels – Gated potassium ion channels The process involves a number of steps:

Nerve Impulse 1.Resting Potential (Polarized)Resting Potential High [Na + ] outside cell and small [K + ] inside cell The membrane potential is negative b/c the inside is more negative compared to the outside (-70mV) K + channels are open 2.Threshold (Depolarization)Threshold A stimulus arrives, resulting in some of the Na + channels opening – Na + enters the cell The membrane potential then becomes more positive If the membrane potential reached the threshold level (~-50mV) an action potential begins

Nerve Impulse 3.Depolarization All sodium channels open – Na + enters the cell in large amounts The membrane potential reaches the maximum positive value (~+40mV) 4.Repolarization Sodium channels close and potassium channels open at the most positive membrane potential – K + then leaves the cell The membrane potential then approaches the resting membrane potential

Nerve Impulse 5.Hyperpolarization The potassium gates are slow to close once the resting potential is reached – This results in excess K + being transported out of the cell The membrane potential then becomes more negative (hyperpolarized) The sodium-potassium pump restores the membrane to resting membrane potentialsodium-potassium pump – 3 Na + out, 2 K + in, 1 ATP  Active Transport The nerve impulse is best represented by a graph

Nerve Impulse Refractory Period The time taken to return the membrane to resting potential after repolarization – No action potentials take place during this time The depolarization propagates along the axon in a wave- like mannerwave- like manner It causes neighbouring sodium gates to open Saltatory Conduction – Occurs in mylenated axons – Ion channels are only found at the Nodes of Ranvier and the nerve impulse is able to jump from Node to Node – Nerve signal is transmitted along axon at a much faster rate

Nerve Impulse Threshold Levels The minimum level of stimulus required for a neuron to fire Different neurons have different threshold levels An intense stimulus will fire neurons with higher action potentials All-or-none response Once the threshold level is reached, the nerve or muscle will respond completely to a stimulus or not at all Higher intensity stimuli do not cause bigger action potentials They increase the frequency of action potentials

Nerve Impulse The brain interprets a stimulus by: 1. The number of neurons firing 2.Their respective threshold levels 3.The frequency of action potentials

Synaptic Transmission Nerve impulses need to be transmitted between neurons and from neuron to effectors Synapse The region between neurons or between neuron and effectors Involves a pre-synaptic and post-synaptic neuron A neuromuscular junction is the space between a neuron and an effector Transmission across the synapse involves neurotransmitters neurotransmitters Chemical messengers released by the pre-synaptic neuron that bind to receptors on the post-synaptic neuron

Synaptic Transmission The nerve impulse transmission occurs in a number of steps:steps 1.The wave of depolarization reaches the pre-synaptic axon terminal 2.Vesicles containing a neurotransmitter fuse with the axon terminal membrane 3.The vesicles release the neurotransmitters into the synapse (synaptic cleft) 4.The neurotransmitters diffuse across the synapse and bind to receptors on the post-synaptic neuron 5.Ions channels open in the post-synaptic neuron causing either depolarization or hyperpolarization 6.Enzymes in the synaptic cleft break down the neurotransmitter

Synaptic Transmission Neurotransmitters, such as acetylcholine, need enzymes to break them down to prevent constant depolarization Acetylcholinesterase breaks down acetylcholine Diffusion is the most time-consuming process of synaptic transmission More synapses = Slower signal Reflex arcs involve very few synapses, accounting for the rapid response

Synaptic Transmission Neurotransmitters can be both excitatory and inhibitory Excitatory neurotransmitters make the post synaptic neuron more permeable to sodium Causing depolarization and a subsequent action potential Inhibitory neurotransmitters make the post-synaptic neuron more permeable to potassium Potassium diffuses out, causing hyperpolarization, inhibiting any action potentials Your body has multiple neurotransmitters that cause either inhibitory or excitatory effects, or both

Synaptic Transmission Excitatory transmitters can interact to cause summation summation When two or more excitatory neurons (A & B) fire together, releasing excitatory neurotransmitters to another neuron (D), causing a depolarization in (D) Depolarization would not have occurred if the neurons fired individually

Synaptic Transmission Activation and suppression of inhibitory and excitatory pathways is what gives drugs their various effects Drugs are usually classified as either a stimulant or a depressant

Synaptic Transmission Parkinson’s Disease Not enough of the neurotransmitter dopamine Causes involuntary movements and tremors Alzheimer’s Disease Decreased production of acetylcholine Causes deterioration of memory and mental capacity

CNS CNS (Central Nervous System) includes the brain and the spinal cord It is surrounded by the meningesmeninges A 3 layered protective membrane Contains cerebrospinal fluidcerebrospinal fluid A circulating fluid found between the inner and second membranes of the meninges Acts as a shock absorber and transport medium for nutrients and wastes for CNS cells Meningitis A bacterial or viral infection of the meninges

CNS The CNS consists of grey and white matter Grey Matter Consists of non-mylenated interneurons White Matter Consists of mylenated nerve fibres

CNS The BrainBrain

PNS PNS = Peripheral Nervous System Relays information between the CNS and all parts of the body Divided into 2 main divisions 1.Sensory-Somatic System 2.Autonomic Nervous System

PNS Sensory-Somatic System Collects information from the external environment Touch, Taste, Sight, Hearing, Smell Sends information out to the skeletal muscles Consists of cranial nerves and spinal nerves Cranial Nerves Hearing, Vision, Balance, Taste, Smell, Facial, Neck and Tongue Movements Spinal Nerves Skeletal Muscle in the rest of the body Largely voluntary (ie. Under conscious control) Reflexes are an involuntary aspect

PNS Autonomic Nervous System The system that operates “automatically” (ie. Minimal conscious control) Responsible for internal homeostasis Controls smooth muscle, cardiac muscle, and internal glands and organs Operates via negative feedback Divided into 2 major divisions Sympathetic Nervous System Parasympathetic Nervous System

PNS Sympathetic Nervous System Prepares the body for stress Parasympathetic Nervous System Restores the body after stress Utilizes Vagus Nerve Vagus

PNS OrganSympathetic NSParasympathetic NS Heart+ HR- HR Digestive Tract- Digestive Processes+ Digestive Processes Liver+ Glucose Release- Glucose Release EyesDilates PupilsConstricts Pupils BladderRelaxes Sphincter (pee)Contracts Sphincter (no pee) Skin+ Blood Flow- Blood Flow Adrenal Gland+ EpinephrineNo effect