Chapter 9: Nervous System

Functions Sensors Processing Effectors Monitor external and internal environment Processing Receives information, integrates it, and decided what to do Effectors Carries messages to effectors and tells them what to do

Types of Nervous Cells Neurons Neuroglial Main cell of nervous tissue Relay and process messages Neuroglial Provide support to the neurons Several types known Ex: Schwann, microglial

Neuroglial Cells Microglial cells Oligodendrocytes Schwann cells Scattered throughout CNS Phagocytize bacteria or cellular debris Oligodendrocytes Along nerve fibers Provide myelin sheath (made of a fatty substance called myelin) around axon in CNS Schwann cells Same as oligodendrocytes but in PNS

Neuroglial Cells (cont) Astrocytes Provide connection between a neuron and a blood vessel Provide support, help regulate ion concentrations in tissue, make-up scar tissue after injury Ependymal cells Forms epithelial-like linings on the outsides of specialized parts or lining cavities within the CNS

Neurons Remember: It’s a cell! Body of neuron Cell Structures Cell Body – contains cell organelles Dendrites- carry messages to cell body Axons – carry messages away from cell body Cell Structures Large nucleus with easily seen nucleolus Chromatophilic substance – similar to rough ER Scattered throughout cytoplasm, membranous Neurofibrils- help support cell shape

Axons Can be myelinated or unmyelinated PNS CNS Schwann cells form myelin sheath Nodes of Ranvier- small breaks in myelin sheath CNS Oligodendrocytes form myelin Myelinated neurons form white matter Unmyelinated neurons form gray matter

Structural Classification of Neurons Multipolar Many small branched dendrites One axon Found in CNS Bipolar Two processes off of cell membrane (one axon and one dendrite) Neurons in special sense organs Unipolar One process off of cell body (one axon) Found throughout PNS

Functional Classification of Neurons Sensory (afferent) neurons Have sensitive dendrites that are stimulated by changes in environment Message is taken into CNS Usually unipolar or sometimes bipolar Interneurons Transfer, direct, and process messages within CNS Usually multipolar Motor (efferent) neurons Carries message out of CNS to effectors

A Neuron at Rest Inside the neuron Outside the neuron Result High in K+ High in negative ions Outside the neuron High in Na+ High in positive ions Result K+ tends to diffuse out Na+ tends to diffuse in Negative ions cannot cross

A Neuron at Rest (cont) Na/K pump - helps to restore concentration gradient across the cell membrane Resting potential - difference is electrical charge across the membrane Established by concentration gradients of various ions Inside of the membrane has a negative charge of 70 mv Membrane is said to be polarized

Potential Changes Stimuli cause changes to the resting potential by making the inside of the membrane less negative Once a stimulus happens: If stimulus is not strong enough to reach threshold potential = cell membrane will return to resting potential If stimulus is strong enough to reach threshold potential = start an action potential Summation - when additive effect of stimuli causes action potential

Action Potential Starts at trigger zone of axon Threshold stimulus open sodium channels Sodium moves into axon Because of the concentration gradient Because of the negative charge that attracts the positive ions Depolarizes the membrane as negative charge diminishes Potassium channels open and potassium moves out of the axon, repolarizing the membrane Animation #1 Animation #2

Action Potential Animation

Nerve Impulse Action potential at the trigger zone stimulates the next part of the axon to do a action potential Potentials spread along the axon like a wave Unmyelinated axons Wave continues uninterrupted; relatively slow Myelinated axons Wave goes through saltatory conduction (jump from one node to the next); very fast Animation

Neuron Responses All-or-nothing effect Neuron does not react until a threshold stimulus is applied, but once it is applied it reacts fully Stimuli greater than threshold levels don’t change the size of the response but changes its frequency Refractory period: After a action potential Brief period of time The nerve cannot be stimulated again.

The Synapse The connection between two neurons Don’t touch, separated by synaptic cleft One-way communication between axon of presynaptic neuron and dendrite of postsynaptic neuron Neurotransmitters are made in the synaptic knob of the axon, stored in synaptic vesicles, and cross the cleft when needed

Neurotransmitter Actions Excitatory Action: A neurotransmitter that puts a neuron closer to an action potential (facilitation) or causes an action potential Inhibitory Action: A neurotransmitter that moves a neuron further away from an action potential Response of neuron: Responds according to the sum of all the neurotransmitters received at one time

Neurotransmitters Acetylcholine Monoamines – modified amino acids Neuropeptides- short chains of amino acids Depression: Caused by the imbalances of neurotransmitters Many drugs imitate neurotransmitters Ex: Prozac, zoloft, alcohol, drugs, tobacco

Release of Neurotransmitters When an action potential reaches the end of an axon, Ca+ channels in the neuron open Causes Ca+ to rush in Cause the synaptic vesicles to fuse with the cell membrane Release the neurotransmitters into the synaptic cleft After binding, neurotransmitters will either: Be destroyed in the synaptic cleft OR Taken back in to surrounding neurons (reuptake) Animation

Neuronal Pools Groups of highly interconnected neurons that work together in the CNS Convergence Axons from different parts of the nervous system connect to the same neuron combining their affects Divergence A message from one neuron is sent to many neurons at once; amplifies message

Divergence Convergence

Nerves Nerves are made of bundled axons, called nerve fibers Sensory (afferent)- carry messages to CNS Motor (efferent)- carry messages from CNS to effectors Nerves Same definitions hold true Most nerves contain both types of fibers and are called mixed nerves

Structure of a nerve A nerve fiber (axon) is surrounded with endoneurium Nerve fibers are bundled together and surrounded with perineurium to form a fascicle Fascicles are bundled together and surrounded with epineurium to form a nerve

Nerve Pathways Path that the message takes through the body Includes: Sensor Sensory neuron Interneuron Motor neuron Effector

Reflex Arc Simplest nerve pathway is a reflex Reflexes without pain Involve only sensory and motor neurons Ex: knee-jerk reflex Reflexes with pain Involve interneurons in CNS Ex: withdrawal reflex

The CNS Central nervous system Consists of brain and spinal cord Made of both gray and white matter Covered in meningies

The Meninges (Brain) Cranial Bone Dura mater Arachnoid mater First layer; tough, fibrous connective tissue Forms inner periosteum of cranial bone Folds into the cranium in some places to form division walls in the brain Arachnoid mater Web-like membrane over CNS Does not dip into crevices

The Meninges (Brain) Subarachnoid space Pia mater Below arachnoid layer Contains cerebrospinal fluid Pia mater Lower layer of meninges Forms a tight covering over brain Does dip into crevaces

Meningies (Spinal Cord) Same except: Vertebrae bones - protection Epidural space- filled with loose connective and adipose tissue All other are the same

Cerebrospinal fluid Flows through ventricles (spaces in brain) in the subarachnoid space, and through the central canal of the spinal cord Fluid is made by the choroid plexus

Spinal Cord Stretches from brain to intervertebral disk between first and second lumbar vertebrae 31 pairs of spinal nerves come of the cord Gray matter core surrounded by white matter

Spinal Cord (cont) Responsible for communication between brain and body and spinal reflexes Ascending tracts Nerves that send info to brain Descending tracts Nerves that send into to effectors

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The Brain Made up off about 100 billion neurons Four main sections: 1) Cerebrum Nerves for processing sensory and motor function Higher functions (like reasoning) 2) Diencephalon Processes sensory information 3) Brainstem Regulates certain body functions like breathing 4) Cerebellum Coordinates skeletal muscle movements

Cerebrum Divided into two hemispheres: right and left Corpus callosum Connects the two sides Other structures Convolutions - ridges Sulcus - shallow groove Fissure - deep groove

Areas of the Cerebrum Frontal lobe Parietal lobe Temporal lobe Occipital lobe Insula Each lobe has unique functions

Areas of the Cerebrum (cont) Cerebral cortex Thin layer of gray matter surrounding cerebral hemisphere; contain most of the cell bodies in the cerebrum Inner part of the cerebrum is mainly made of white matter

Functions of the Cerebrum Motor areas Primarily in frontal lobe Send information out to effectors Sensory areas Interpret information from sensors Area in parietal, temporal, and occipital lobes Association areas Analyze information from sensors Located in areas in all lobes mentioned above

Diencephalon Located between the cerebral hemispheres above the brainstem Contains: Thalamus Hypothalamus Pituitary gland Pineal gland

Functions of the Diencephalon Thalamus Routes sensory impulses to the correct region of the cerebrum Hypothalamus Monitors many internal conditions, link between nervous and endocrine system Limbic system Thalamus, hypothalamus, and basal nuclei Controls experience and expression (feelings)

Brainstem Connection between spinal cord and the rest of the brain Contains: Midbrain Pons Medulla oblongata

Midbrain Located between diencephalon and pons Contains some visual and auditory reflexs Serves as the main connection for motor neurons between spinal cord and upper part of brain

Pons Rounded bulge between midbrain and medulla oblongata Relays impulses between medulla and cerebrum or between cerebrum and cerebellum

Medulla oblongata Lowest part of brain, connect to spinal cord All ascending and descending tracts run through the oblongata Serves as a control center for many vital function like heart rate, blood pressure, and respiratory center

Cerebellum Located in the lower back part of the brain Structured liked cerebrum with inner white matter core and gray matter covering Controls posture and complex skeletal movements

The PNS Peripheral Nervous system Includes: Divided into: 12 pairs of cranial nerves 31 pairs of spinal nerves Divided into: 1) Somatic nervous system Controls conscious activities; connects to skin and skeletal muscles 2) Autonomic nervous system Controls unconscious activities; connects to internal organs or structures

Autonomic Nervous System Two branches: 1) Parasympathetic Control under more normal conditions 2) Sympathetic Control under stress or emergency conditions (fight or flight) Usually these have antagonistic effects Work to counteract each other (one increases, while other decreases) Either might be utilized to maintain homeostasis

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