The Nervous System

2 categories in nervous system. Central nervous system (CNS) – brain, spinal cord Peripheral nervous system (PNS) – nerves outside CNS. 2 divisions of PNS – somatic (voluntary), autonomic (involuntary)

Autonomic divided into 2: 1 sympathetic (during stress), 2 parasympathetic (normal functioning)

Nerve Tissue 2 types of cells: 1 neurons (responsive cells that conduct impulses at fast speeds) 2 neuroglia (support, maintain neurons)

5 types of neuroglia 1 astrocytes (anchor neurons) 2 ependymal (form cerebrospinal fluid) 3 microglia (eat invading microorganisms) 4 oligodendrocytes (provide insulation around CNS – myelin) 5 Schwann cells (insulation around PNS)

Neuron made up of… 1 cell body (cytoplasm, nucleus, organelles) 2 dendrites (branching extensions from cell body – receive impulses) 3 axons (conducts impulse away from body)

Axons enclosed with Schwann cells forming layers rich in fat. Provides insulation – myelin. Insulation not continuous (gaps – nodes of Ranvier)

Gray matter is made of unmyelinated fibers – shorter with no myelin White matter is made of myelinated fibers – can be longer and have myelin

Types of Neurons Structurally, 3 different types. 1 Multipolar – many dendrites – carry impulses to skeletal muscle. 2 Bipolar – single dendrite, single axon – special sensory areas (ears, eyes) 3 Unipolar – one nerve fiber (from skin to spinal cord)

Functionally, 3 different types of neurons. 1 Sensory (afferent) – carry from body to CNS) 2 Association (interneurons) – links between neurons 3 Motor (efferent) – from CNS to body

Impulse Transmission Difference in voltage across cell membranes. Openings in membrane are called ion channels. They regulate movement of ions. Greatest influence – Na +, K +

Sodium-potassium pump – transports sodium out of cell, potassium inside. Causes concentration gradient – ions actively move across cell membrane through ion channels Every 3 Na + pumped out, 2 K + back in.

Outside of the membrane accumulates positive ions (potassium leaks out faster) Resting membrane potential – no impulses are transmitted

Potential on inside -70 mV. Change in membrane permeability to sodium – ions flow outward; inside becomes more positive – depolarization (+ 30 mV) Restored to normal – repolarization – potassium inward through ion channels, sodium channels close.

Depolarization followed by repolarization – impulse sent down axon. Nerve impulse – wave of ion reversals (changing charge of membrane)

Myelinated fibers- Conduct impulses faster than nonmyelinated fibers Node of Ranvier – gaps in axon of myelinated fibers Impulse jumps across myelin sheath from node to node – fastest conduction in body.

Types of Stimuli All-or-none – either impulse conducted or not. Threshold – minimum strength of stimulus needed for action potential. Subthreshold – no action potential. Series of subthreshold – summation (lead to action potential)

Gap between adjacent neurons - Synapse Neuron that sends impulse – presynaptic neuron; recieves impulse – postsynaptic neruon Axon of presynaptic – bulb with synaptic vescicles (contains neurotransmitters)

Synaptic Cleft – Small space between the terminal end of an axon and the next neuron or muscle.

2 effects of neurotransmitters 1 Excitatory – increase membrane permeability to sodium ions (cause action potential) – accetylcholine, norepinephrine 2 Inhibitory – lowers chance of impulse crossing synapse – endorphins, GABA (inhibit pain)

Central Nervous System 1 Spinal cord – from base of brain to 1 st, 2 nd lumbar vertebrae. Enters through foramen magnum of skull. Protected by vertebral column, fluid, and meninges (layers of membrane)

Direction of Impulses Ascending tract (up towards brain)– sensory information Descending tract (away from brain)– motor information Spinal cord also serves for reflexes – rapid response to emergency.

Reflex arc Reflex arc- Receptor (generates action potential) sends message along sensory neuron to CNS (spinal cord). Examples: Withdrawal reflex, patellar reflex, vomiting- smooth muscle reflex, heart rate- cardiac muscle reflex

The Brain 3 major regions: 1 forebrain, 2 midbrain, 3 hindbrain. Forebrain – cerebrum, diencephalon Midbrain – below diencephalon Hindbrain – pons, medulla oblongata, cerebellum. Brain stem – midbrain, pons, medulla

Cerebrum Cerebrum – higher brain – conscious thought, memory, learning. Divided into left and right cerebral hemispheres. Wrinkled structure (convolutions) – result from rapid growth during development.

Foldings project upward – gyri; downward – sulci. Deep grove – fissure; 2 major ones – longitudinal (divides hemispheres), transverse (cerebrum from cerebellum)

Peripheral Nervous System Nerves, ganglia, sensory receptors. Communication between CNS and other areas of body. Nerve – composed of more than 1 type of tissue; responsible for transporting nerve impulses.

Nerves with… sensory fibers – afferent nerves motor fibers – efferent nerves both – mixed nerves Ganglia – clusters of neurons outside CNS

Sensory receptors – respond to stimuli (changes in environment) Most endings of dendrites from sensory neurons. Also found in special sensory organs (eye, taste bud, etc)