NOTES - UNIT 5 part 1: Nervous System Organization

 The nervous system has three overlapping functions: 1) SENSORY: gather information from sensory receptors; detect changes in the external or internal environment 2) INTEGRATION: information from sensory receptors is interpreted and associated with appropriate responses (sensation, memory, perceptions, decisions)   3) MOTOR OUTPUT: conduction of signals from the integration center to effector cells (muscle cells or gland cells)

The Nervous System is divided into: CENTRAL NERVOUS SYSTEM (CNS) brain and spinal cord integration center/processing

PERIPHERAL NERVOUS SYSTEM (PNS) nerves communicate sensory and motor signals to and from CNS and rest of body made up of NERVES (ropelike bundles of neurons)

1 neuron (nerve cell)

Nervous Tissue-Two Main Types of Cells: 1) NEURONS (nerve cells): functional unit of the nervous system Specialized for nerve impulse (action potential) conduction Transmit signals from one location to another Provide most of the unique functions of the nervous system: sensing, thinking, remembering, controlling muscle activity, regulating glandular secretions CNS neurons: no mitosis/repair PNS neurons: some ability to repair themselves

The arrow shows the direction of the action potential message passing down one cell and then moving to the 2nd cell.

3 Types of Neurons or Nerve Cells: sensory neurons: detect many different types of stimuli & carry information to the brain Interneurons: integrates/processes sensory information; axons extend for only a short distance & contact nearby neurons in the brain or spinal cord motor neurons: carry information from the brain toward the spinal cord or out of the brain and spinal cord to muscles and glands

Neuron Structure: DENDRITES, CELL BODY, AXON, AXON TERMINAL, & SYNAPTIC END BULBS DENDRITES (“little trees”): receiving/input parts of a neuron CELL BODY: nucleus surrounded by cytoplasm and typical organelles (rough ER, mitochondria, lysosomes, golgi) nucleus

AXON: conducts nerve impulses away from cell body & toward another neuron, muscle fiber, or gland cell Axons end by dividing into many fine processes called axon terminals Axon terminals swell into synaptic end bulbs which contain synaptic vesicles storing neurotransmitters Axon terminal

synaptic knobs from more than 1 neuron synapsing with the dendrites of 1 neuron

many axons are enclosed by an insulating layer called the MYELIN SHEATH Many-layered covering composed of lipid and protein Increases the speed of nerve impulse conduction Schwann cells in the PNS and Oligodendrocytes in the CNS produce myelin sheaths by wrapping themselves around and around axons Axons with a myelin sheath: myelinated and axons without: unmyelinated

Myelin producing cell; is this CNS or PNS? Unmyelinated neuron

Gaps in the myelin sheath where the axon is exposed: Nodes of Ranvier

SYNAPSE: chemical junction between a neuron and another cell; found between: motor neuron & muscle cell 2 neurons sensory receptor & sensory neuron neuron & gland cell

Motor Neuron and Muscle Cell Real vesicles!!

electrical signal  chemical signal  electrical signal CHEMICAL SYNAPSES are separated by a synaptic cleft & a series of events converts: electrical signal  chemical signal  electrical signal Intercellular chemical signal: NEUROTRANSMITTERS

Synaptic Transmission Review… An action potential (the electrical signal) reaches the end of the axon/synaptic end bulb The action potential cannot cross the synaptic cleft Causes neurotransmitters to be released from vesicles in the synaptic end bulb The neurotransmitter reaches receptors in the cell of the postsynaptic membrane and causes a new impulse to be initiated

What Does the Neurotransmitter Message Say? Some will cause an action potential in the next cell (excitatory) Some will inhibit an action potential in the next cell (inhibitory) About 50 different neurotransmitters have been identified

Examples of neurotransmitters: Functions in: Acetylcholine (ACh) Neuromuscular junction; stimulates muscle contraction epinephrine/ norepinephrine attention & mental focus, cognition *epin. & norep. also function as hormones in “fight or flight” response dopamine/ serotonin dop. & ser. both affect sleep, mood, attention, learning; *LSD & mescaline bind to serotonin receptors* endorphins Decrease perception of pain by CNS (inhibitory); *heroin & morphine mimic endorphins* histamine Release in hypothalamus promotes alertness    

2) NEUROGLIAL CELLS (SUPPORTING CELLS) 10 to 50 times more numerous than neurons provide structure; protect, insulate, nourish, & assist neurons Even though glia do not generate or conduct nerve impulses, they are actively involved in nervous tissue

2) NEUROGLIAL CELLS-(SUPPORTING CELLS) continued In contrast to neurons, some neuroglial cells can multiply & divide in the mature nervous system Example of neuroglial cells: SCHWANN CELLS (PNS) and OLIGODENDROCYTES (CNS) Example of neuroglial cell : ASTROCYTES: form connections between neurons; protect neurons from harmful substances; responsible for blood-brain barrier

Astrocyte Nerve cells

Functions of the Blood Brain Barrier: Maintain a constant external environment for brain cells. Protects the brain from "foreign substances" in the blood that may injure the brain. Protects the brain from hormones and neurotransmitters in the rest of the body.