The Nervous System

Chapter 9

Nervous System

• The master controlling and communicating system of the body

• Functions:– Sensory input – monitoring stimuli– Integration – interpretation of sensory input– Motor output – response to stimuli

Organization of the Nervous System

• Central nervous system (CNS)– Brain & spinal cord– Integration and command center

• Peripheral nervous system (PNS)– Paired spinal & cranial nerves– Carries messages to and from the spinal cord and

brain

Peripheral nervous system (PNS)

• Two functional divisions:– Sensory (afferent) division• Sensory afferent fibers – carry impulses from skin,

skeletal muscles, and joints to the brain• Visceral afferent fibers – transmit impulses from

visceral organs to the brain; Ex) feeling full

– Motor (efferent) division• Transmits impulses from the CNS to effector organs

Motor Division: Two Main Parts

• Somatic nervous system– Conscious control of skeletal muscles

• Autonomic nervous system (ANS)– Regulates smooth muscle, cardiac muscle, and glands

(unconscious)– Divisions:• Sympathetic – fight or flight system; increase in heart rate,

blood pressure, blood sugar• Parasympathetic – housekeeping systems

(resting/digestive); keeps functions going under normal conditions

Histology of the Nerve Tissue

• The two principle cell types of the nervous system are:– Neurons excitable cells that transmit electrical

signals– Supporting cells cells that surround and wrap

neurons (neuroglial or glial cells)• Provide a supportive scaffolding for neurons• Segregate and insulate neurons• Guide young neurons to the proper connections• Promote health and growth

Neurons (Nerve Cells)

• Lose their ability to divide• Very long lived• High metabolic rate – need astrocytes to get

nutrients from capillaries• NEED OXYGEN• No centrioles• Dendrites and Cell bodies receive graded

potentials

Neuron Structure

• Neurons vary considerably in size & shape, but they have certain features in common:– Cell body – Nerve fibers – Axon – Dendrites

Nerve Cell Body

• Contains the nucleus and nucleolus• The major biosynthetic center• Is the focal point for the outgrowth of

neuronal processes• Has well-developed Nissl bodies (rough ER)• Contains an axon hillock – cone-shaped area

from which axons arise

Dendrites of Motor Neurons

• Short, tapering, and diffusely branched processes

• They are the receptive, or input, regions of the neuron

• Electrical signals are conveyed as graded potentials (not action potentials)

Axons: Structure

• Slender processes of uniform diameter arising from the axonal hillock of the cell body

• Long axons are called nerve fibers• Usually there is only one unbranched axon per

neuron• Axonal terminal – branched terminus of an

axon

Axons: Function

• Generate and transmit action potentials• Secrete neurotransmitters from the axonal

terminals• Movement along axons occurs in two ways– Anterograde – toward the axonal terminal– Retrograde – away from the axonal terminal

Myelin Sheath

• Whitish, fatty (protein-lipoid), segmented sheath around most long axons

• It functions to:– Protect the axon– Electrically insulate fibers from one another– Increase the speed of nerve impulse transmission

Myelin Sheath & Neurilemma: Formation

• Formed by Schwann cells in the PNS• A Schwann cell:– Envelopes an axon in a trough– Encloses the axon with its plasma membrane– Has concentric layers of membrane that make up

the myelin sheath• Neurilemma – remaining nucleus and

cytoplasm of a Schwann cell

Nodes of Ranvier

• Gaps in the myelin sheath between adjacent Schwann cells

• They are the sites where axon collaterals can emerge

Types of Neurons and Neuroglial Cells

• Neuron Classification (structural):– Multipolar – three or more processes– Bipolar – two processes (axon & dendrite)– Unipolar – single, short process

Types of Neurons & Neuroglial Cells

• Neuron Classification (function):– Sensory (afferent) – transmit impulses toward the

CNS (stimulus)– Motor (efferent) – carry impulses away from the

CNS (reaction)– Interneurons (association neurons) – shuttle

signals through CNS pathways

Types of Neurons & Neuroglial Cells

• Classification of Neuroglial Cells– Microglial cells– Oligodendrocytes– Astrocytes: – Ependymal cells

Microglial Cells

• Small, oval cells w/ spiny processes• Scattered throughout the CNS• Support neurons & phagocytize bacterial cells

& cellular debris

Oligodendrocytes

• Occur in rows along nerve fibers• Form myelin within the brain & spinal cord• Insulation

Astrocytes

• Most abundant, versatile, & highly branched glial cells

• Cling to neurons at the synaptic endings & cover capillaries

• Functions:– Support & brace neurons– Anchor neurons to their nutrient supplies– Guide migration of younger neurons

Ependymal Cells

• Range in shape from squamous to columnar• Line the central cavities of the brain and spinal

column• Make a barrier for spinal fluid

Spinal Cord (structure)

• Consists of 31 segments – each gives rise to a pair of spinal nerves

• Cervical enlargement – in the neck region, supplies nerves to the upper limbs

• Lumbar enlargement – in the lower back, supplies nerves to the lower limbs

Spinal Cord (function)

• Has 2 main functions:– Conducting nerve impulses – Serving as a center for spinal reflexes • Nerve tracts provide a two-way communication system

between the brain and body parts outside the nervous system

Spinal Cord (function)

• Ascending tracts – carry sensory information to the brain

• Descending tracts – conduct motor impulses from the brain to muscles and glands

• Nerve fibers within ascending and descending tracts are axons