The Nervous System

Central Nervous System (CNS) – brain and spinal cord Peripheral Nervous System (PNS) – nerves that communicate to the rest of the body  Somatic NS – voluntary control  Autonomic NS – involuntary control Parasympathetic division – rest and digest Sympathetic division – fight or flight

Neurons Smallest functional unit of the nervous system (specialized cell) Consists of:  Cell body – nucleus/organelles  Dendrites – receive messages  Axons – transmit messages away Usually covered by myelin sheath  Insulates and speeds up impulse

Sensory neurons – transmits info from sensors to the CNS for processing Motor neurons – transmit signals to effectors like muscles and glands Interneurons – connect sensory and motor neurons Nerves – bundle of neurons

Nerve Impulse Caused by changes in the membrane potential of a neuron  Membrane potential – difference in electrical charge across a cell membrane At resting potential, the outside of the membrane is positive, while the inside is negative  Concentration of Na+ is higher outside and K+ is higher inside

Resting Potential

Action Potential Also called a nerve impulse An all or none response to depolarization of the membrane Threshold – minimum level of depolarization needed to generate an action potential

Action Potential A stimulus opens voltage-gated sodium channels and Na+ ions enter the cell, making the membrane potential positive  If threshold is reached, action potential is triggered Additional nearby channels open and the membrane potential is changed in the next region of the neuron

Action Potential Membrane potential is restored to resting by the inactivation of the Na+ channels and by the opening of voltage-gated K+ channels which increases K+ leaving the cell AP are propagated along the axon  Saltatory conduction – jumping of nerve impulse between nodes of Ranvier (non-insulated portions) Impulse animation

Synapse Junction between two neurons  Could be between neuron and muscle or gland Neurotransmitters – chemical messengers released from synaptic terminals (axon ends)  Diffuse across synapse and bind to receptors on the dendrites of the next neuron  Continues the impulse  May be excitatory or inhibitory  Examples: acetylcholine, dopamine, serotonin, epinephrine, norepinephrine

Synapse animation Synapse animation

Reflexes A simple automatic nerve circuit in a response to a stimulus  Called a reflex arc  Usually skips the brain and goes from sensory receptor to spinal cord and from spinal cord to the effector like a muscle

Sensory Receptors Mechanoreceptors – physical stimuli like pressure, touch or sound Thermoreceptors – heat or cold Chemoreceptors – solute concentration like taste and smell Electromagnetic receptors – EM energy like light, electricity, and magnetism Pain receptors – respond to damage or inflamed tissues due to extreme heat, pressure, or certain chemicals

Other Parts… Cerebrospinal fluid (CSF) – bathes nerve cells in fluid, removes wastes, and cushions brain and spinal cord Gray matter – mainly neuron cell bodies and unmyelinated axons White matter – mostly myelinated axons giving the “white” appearance

Glial cells or Glia Neuron supporting cells that don’t conduct impulses  CNS Astrocytes – provide physical and nutritional support Microglia – phagocytic for clean up and protection Oligodendrocytes – form myelin sheaths in CNS Ependymal cells – involved in creation and secretion of CSF  PNS Schwann cells – form myelin sheaths in PNS Satellite cells – help regulate external chemical environment

Parts of the Brain Brainstem – Homeostatic functions like breathing rate, heart rate and blood pressure  Consists of medulla oblongata, pons, and midbrain Cerebellum – motor coordination, posture, and balance

Cerebrum (cortex)  Higher brain functions like thought and action  Divided into 2 hemispheres with gray matter over white matter Connected by a bundle of axons called the corpus callosum

 Also divided into four lobes: Frontal – thinking, memory, behavior, and movement Parietal – language and touch Occipital - sight Temporal – hearing, learning, and feelings

Limbic System  Buried within the cerebrum  Contains the thalamus, hypothalamus, amygdala, and hippocampus  Controls basic emotions and drives