Fundamentals of the Nervous System and Nervous Tissue 11 P A R T A Fundamentals of the Nervous System and Nervous Tissue

Structural units of the nervous system Neurons (Nerve Cells) Structural units of the nervous system Composed of a body, axon, and dendrites Long-lived, _ _ _ _ _ _ _ _, and have a high metabolic rate Their plasma membrane functions in: Electrical signaling amitotic PLAY InterActive Physiology ®: Nervous System I, Anatomy Review, page 4

Neuron Classification Functional: Sensory (afferent) — transmit impulses toward the CNS Motor (efferent) — carry impulses away from the CNS _ _ _ _ _ _ _ _ _ _ _ _ (association neurons) — shuttle signals through CNS interneurons

Neuron Classification/Components Reflex Arc

Neurons (Nerve Cells) Figure 11.4b

Contains an axon hillock – cone-shaped area from which axons arise Nerve Cell Body (Soma) Contains the nucleus Contains an axon hillock – cone-shaped area from which axons arise Concentrations of cell bodies termed _ _ _ _ _ _ in the CNS and _ _ _ _ _ _ _ in the PNS Nuclei ganglia

Armlike extensions for signal conduction Processes Armlike extensions for signal conduction Called _ _ _ _ _ _ in the CNS and _ _ _ _ _ _ in the PNS There are two types: axons and dendrites tracts nerves

Short, branched group of processes Dendrites Short, branched group of processes They are the receptive, or _ _ _ _ _ regions of the neuron Electrical signals here are conveyed as graded potentials (not action potentials) input

Axons: Structure Slender process of uniform diameter arising from the hillock Usually there is only one axon per neuron Axonal terminal – ending of an axon

Generate and transmit electrical signals (action potentials) Axons: Function Generate and transmit electrical signals (action potentials) Secrete _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ from the terminals neurotransmitters

Whitish, fatty, segmented sheath around most long axons Myelin Whitish, fatty, segmented sheath around most long axons It functions to: Protect the axon Electrically insulate fibers from one another Increase the _ _ _ _ _ of nerve impulse transmission speed

Formed by Schwann cells in the PNS A Schwann cell: Myelin Formation Formed by Schwann cells in the PNS A Schwann cell: Envelopes an axon in a trough Encloses the axon with its plasma membrane Forms concentric layers that make up the myelin sheath (jelly roll)

Myelin Sheath: Formation PLAY InterActive Physiology ®: Nervous System I, Anatomy Review, page 10 Figure 11.5a–c

_ _ _ _ in the myelin sheath between adjacent Schwann cells Nodes of Ranvier _ _ _ _ in the myelin sheath between adjacent Schwann cells Ion gates are concentrated here (important in development of action potential) gaps PLAY InterActive Physiology ®: Nervous System I, Anatomy Review, page 11

Conduction Velocities of Axons Conduction velocities vary widely among neurons Rate of impulse propagation is determined by: Axon diameter – the larger the diameter, the faster the impulse Presence of a myelin sheath – myelination dramatically increases impulse speed (saltatory conduction) PLAY InterActive Physiology ®: Nervous System I: Action Potential, page 17

Saltatory Conduction Electrical current passes through a myelinated axon only at the nodes of Ranvier Voltage-gated Na+ channels are concentrated at these nodes Action potentials are triggered only at the nodes and jump from one node to the next Much faster than conduction along unmyelinated axons

Saltatory Conduction Figure 11.16

Regions of the Brain and Spinal Cord White matter – dense collections of myelinated fibers Gray matter – mostly cell bodies and unmyelinated fibers BRAIN SPINAL CORD

Pathology: Multiple Sclerosis (MS) An autoimmune disease that mainly affects young adults Symptoms: visual disturbances (blindness), weakness, and loss of muscular control (paralysis) Myelin sheaths in the CNS become nonfunctional due to hardened lesions (scleroses) Shunting and short-circuiting of nerve impulses occurs

Pathology: Adrenoleukodystrophy (ALD) An X-Linked genetic disorder that affects males Myelin sheaths in the CNS are degraded due to build-up of VLCFAs Progressive disease leading to coma and death