NERVOUS SYSTEM

NERVOUS SYSTEM 2 Main Divisions Central NS – brain and spinal cord Peripheral NS – all other nerves (cranial, spinal) CNS – median structure; all sensory impulses arrive there, all motor impulses begin there PNS -- nerves that lie outside (peripheral to) the CNS – convey all sensations and motor impulses to and from the CNS. -cranial nerves – originate from the brain and brainstem -spinal nerves – originate from the spinal cord both give rise to smaller branches.

PERIPHERAL NS 2 Subdivisions Somatic System (voluntary) – sensory nerves from the skin and sensory organs (ear, eye, nose) and motor nerves to skeletal muscles Autonomic System (involuntary) – controls blood vessels, gland further subdivided: Sympathetic – times of stress (injury, excitement, physical activity) Parasympathetic – conditions of normal organ functioning

NERVOUS TISSUE 2 Types of Nervous Tissue : Neuroglia – (glial cells) – 90% of the CNS -Small part of the PNS. -Provide structural framework for the neurons. -Non-conducting. Neurons – Structural and functional unit of the N.S. even though they are outnumbered by glial cells - Perform the functions of the N.S. – sense changes, integrate information, make a motor response.

TYPES OF GLIAL CELLS -Astrocytes (CNS) – most numerous – form webs that surround capillaries, allowing small molecules (O2 and CO2) to pass through, keep others out.   -Oligodendrocytes (oligodendroglia) (CNS) – help hold nerve fibers together – produce a myelin sheath, a protective covering -Microglia (CNS) – engulf and destroy microbes and debris in inflamed/damaged tissue -Ependymal (CNS) – line cavities of CNS – form and circulate CSF -Schwann cells (PNS) – form myelin sheath/neurolemma around peripheral nerves. Have high lipid content

NEURONS Classified according to the DIRECTION OF IMPULSE -Afferent (sensory) – toward brain and spinal cord -Efferent (motor) – toward muscles/glands/organs -Interneurons (AKA Association) – CNS only, connect afferent and efferent fibers

Classified according to STRUCTURE -Multipolar – many dendrites, single axon (CNS to skel. muscle) -Bipolar – single dendrite, single axon (special sensory areas) -Unipolar – one fiber extending from the body, it splits into two branches (one to the spinal cord (axon); the other to the peripheral body (dendrite) (sensory from skin to sp. cord).

NEURON ANATOMY Cell body- has nucleus, cell membrane At least 2 processes Dendrites –receivers (impulses from other neurons – go to cell body) Axon –senders (away from cell body) – 1 per neuron (may have branches, collaterals) Nissl bodies – extra protein synthesis (Rough ER)

Myelin sheath – around axon (fat) insulation – speeds up transmission (Schwann or oligodendroglia) myelin – (white matter) inside – medulla non-myelin (gray matter) outside – cortex Node of Ranvier- gaps in between that aid in impulse conduction   Neurolemma – outer covering of axons [Schwann] – helps in regenerating nerves – none in brain and cord *no regeneration Loses spindle fibers at about 3yo – no longer able to mitotically divide