Nervous System

Nervous System

•Master controlling and communicating system of the body

•Functions• Sensory input – monitoring stimuli occurring inside and

outside the body• Integration – interpretation of sensory input•Motor output – response to stimuli by activating effector

organs

Figure 11.1

Organization of the Nervous System

•CNS – Central Nervous System•Brain and spinal cord• Integration and command center

•PNS – peripheral nervous system•Paired spinal and cranial nerves•Carries messages to and from the spinal cord and brain

Figure 11.2

PNS – Two functional Divisions

•Sensory (afferent) division• Sensory afferent fibers – carry impulses from skin,

skeletal muscles, and joints to the brain•Visceral affer

•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•Divisions

• Sympathetic – voluntary nervous system• Parasympathetic – involuntary nervous system

Figure 11.3

Nervous Tissue Histology

•Two principal cell types of the nervous systems are –•Neurons – excitable cells that transmit electrical signals•Neuroglia or glial cells– cells that surround and wrap

neurons• Provide supportive scaffolding for neurons• Segregate and insulate neurons• Guide young neurons to proper connections• Promote health and growth

Neuroglia of the CNS

•Four main neuroglia•Astrocytes•Microglial cells• Ependymal cells•Oligodendrocytes

Astrocytes

•Most abundant, versatile, and highly branched glial cells

•Cling to neurons and their synaptic endings, and cover capillaries

•Functions –• Support and brace neurons• Anchor neurons to nutrients• Guide migration of young neurons• Control the chemical environment

Microglia

•Small, ovoid cells with spiny processes•Phagocytes that

monitor the health of neurons

Ependymal cells

•Range in shape from squamous to columnar• They line the

central cavities of the brain and spinal column

Oligodendrocytes

•Branched cells that wrap CNS nerve fibers

Neuroglia in the PNS

•Schwann cells – surround fibers of the PNS

•Satellite cells – surround neuron cell bodies with ganglia

Neurons (nerve cells)

•Structural units of the nervous system•Composed of body, axon, and dendrites• Long lived, amitotic, high metabolic rate

•Plasma membrane functions in –• Electrical signaling•Cell-to-cell signaling during development

Figure 11.5

Regions of the Brain and Spinal Cord

•White matter – dense collections of myelinated fibers

•Gray matter – mostly soma and unmyelinated fibers

Neuron Classification

•Structural -•Multipolar – three or more processes•Bipolar – two processes (axon and dendrite)•Unipolar – single, short process

Table 11.1.1

Figure 11.2.3

Phases of the Action Potential

Threshold and APs

•Threshold – membrane is depolarized by 15-20 mV•Established by the total amount of current flowing through

membrane•Weak (subthreshold) stimuli are NOT relayed into action

potentials•Strong (threshold) stimuli are relayed into action

potentials•All-or-none phenomenon – APs either happen completely,

or not at all

Focus Figure 11.3

CENTRAL NERVOUS SYSTEMCNS

Gray vs. White Matter

•Gray matter – short, nonmyelinated neurons and cell bodies

•White matter – myelinated and nonmyelinatedaxons

Figure 12.3

Cerebral Hemispheres

•Form superior part of the brain and make up 83% of its mass

•3 basic regions – cortex, white matter, and basal nuclei

•Contains ridges (gyri) and shallow grooves (sulci)

• Longitudinal fissure – separates left and right hemispheres

•Transverse cerebral fissure – separates cerebrum and cerebellum

Major Lobes, Gyri, and Sulci of the Cerebral Hemisphere

•Deep sulci divides the hemispheres into five lobes –• Frontal, parietal, temporal, occipital, and insula

•Central sulcus – separates the frontal and parietal lobes

Figure 12.5

Diencephalon

•Central core of forebrain

•Consists of 3 paired structures –• Thalamus•Hypothalamus• Epithalamus

Figure 12.11

Thalamus

•Plays key role in mediating sensation, motor activities, cortical arousal, learning, and memory

Hypothalamic Function

1. Autonomic control center – regulates blood pressure, rate and force of heartbeat, digestive tract motility, rate and depth of breathing, and many other visceral activities

2. Emotional response – involved with perception of pleasure, fear, and rage

3. Body temperature regulation4. Food intake – regulates feelings of hunger and satiety5. Regulation of water balance and thirst6. Regulates sleep/wake cycles7. Control of endocrine system functioning

Epithalamus

• Includes pineal gland

•Secretes melatonin

•Regulates the sleep-wake cycle with the hypothalamus

Brain Stem

•Controls automatic behaviors necessary for survival

•3 regions –• Midbrain – cerebral peduncles (pillars to hold up cerebrum),

cerebral aqueduct (connects 3rd and 4th ventricles)• Pons (bridge) – conduction pathway between brain and spinal

cord• Medulla oblongata – location of several visceral motor nuclei

controlling vital functions such as cardiac and respiratory rate. • Also, vomiting, hiccupping, swallowing, coughing, and sneezing

Figure 12.13a

Cerebellum

•Located dorsal to the pons and medulla•Protrudes under the occipital lobes of the cerebrum•Makes up 11% of the brain’s mass•Provides precise timing and appropriate patterns of skeletal muscle contraction•Occurs subconsciously

Figure 12.16

Table 12.1

Table 12.1

Table 12.1

Table 12.1

Protection of the Brain

•Bone

•Meninges – dura mater, aranoid mater, pia mater

•Cerebrospinal fluid – cushions brain and is a delivery mechanism for nutrients and chemical signals

•Blood-Brain Barrier – maintains stable environment

Cerebrospinal Fluid (CSF)

•Watery solution similar in composition to blood plasma

•Contains less protein and different ion concentrations than plasma

•Forms liquid cushion that gives buoyancy to CNS organs

•Prevents brain from crushing under its own weight

•Protects CNS from blows and other trauma

Blood-Brain Barrier

•Protective mechanism that helps maintain a stable environment for the brain

•Bloodborne substance are separated from neurons by –•Continuous endothelium of capillary walls•Relatively thick basal lamina• Feet of astrocytes

Blood-Brain Barrier - Functions

•Selective barrier that allows nutrients to pass freely

• Ineffective against substances that can diffuse through plasma membranes

•Absent in some areas – allows these areas to monitor the chemical composition of the blood

•Stress increases the ability of chemicals to pass through the blood-brain barrier

Spinal Cord

•Enclosed in vertebral column•Protected by bone, meninges, and CSF•Epidural space – space between the vertebrae and dura mater filled with fat and a network of veins•Functions •Provides two-way communication to and from brain and

body•Major reflex center – reflexes are initiated and

completed at spinal cord

Spinal Cord

•Spinal nerves – 31 pairs attach to the cord by paired roots

•Cervical and lumbar enlargements – sites where nerves serving the upper and lower limbs emerge

•Cauda equina – collection of nerve roots at the inferior end of the vertebral canal

Figure 12.27