NERVOUS SYSTEM INTRODUCTION BIO 137 Anatomy & Physiology I

Nervous System One of body’s 2 main control systems Detects a stimulus inside or outside of body & coordinates a response Acts by electrical signals, called nerve impulses Nervous tissue is excitable Controls most of body’s activities to help maintain homeostasis

Organs of the Nervous System Brain Spinal Cord Nerves Cranial Spinal Peripheral

Functions of the Nervous System 3 main functions Sensory function Integrative function Motor function

Sensory receptors are located at the ends of peripheral nerves Sensory receptors detect a change outside or inside the body Monitor internal, external and environmental factors Light and sound, temperature, oxygen concentration Information from sensory receptors is converted into a nerve impulse, which is transmitted from peripheral nerves to the CNS Sensory Function of the Nervous System

Integrative Function of the Nervous System The nervous system processes sensory information by analyzing it and making decisions for an appropriate response ‘Sensory information is integrated ‘

Motor Function of the Nervous System Conscious and subconscious decisions are made and acted upon Response is carried out by effectors through cranial and spinal nerves Muscles and/or glands

Divisions of Nervous System Central Nervous System, CNS Brain Spinal cord Peripheral Nervous System, PNS Peripheral nerves Cranial nerves Spinal nerves PNS connects CNS to body parts

Divisions of Nervous System

Peripheral Nervous System Divisions Sensory Division Sensory receptors in the PNS pick up sensory information from the body and deliver it to the CNS as a nerve impulse Motor Division Neurons carry information from the CNS to effectors

PNS: Motor Division Somatic Division Oversees voluntary activities Skeletal muscle contraction Autonomic Division Controls visceral activities Involuntary activities Including: Heart rate, blood pressure, breathing, body temperature

PNS:Motor:Autonomic Sympathetic Active under stressful situations ‘Fight or Flight’ Parasympathetic Active under normal, restful conditions Returns body to homeostatic levels following a stressful experience

Divisions of the Nervous System Ganglia are small masses of neuronal cell bodies located outside the brain and spinal cord, usually closely associated with cranial and spinal nerves. There are ganglia which are somatic, autonomic, and enteric (that is, they contain those types of neurons.)

Histology of Nervous System 2 main types of cells Neurons A single nerve cell Neuroglia Support and nutrition of brain cells ‘neuron helpers’ Found in both CNS and PNS

Neuronal Structure Dendrite(s) – receive information from within or outside the body Cell body – contains nucleus, organelles Axon hillock - Axon arises from here on the cell body Axon – transmits received information as a nerve impulse A neuron may have many dendrites but only 1 axon Axon may have branches towards its end called collaterals Ends of each axon collateral are called axon terminals which end at a synaptic knob Vesicles of neurotransmitter located here Synapse exists between end of an axon and what it innervates

Neuronal Structure

Flow of Information in a Neuron Dendrite(s) Cell Body Axon hillock Axon Collaterals Axon Terminals Synaptic knob

Axon Terminals Axons can terminate at three locations 1. Muscle 2. Gland 3. Another neuron The site of communication between two neurons or between a neuron and another effector cell is called a synapse.

The Synapse: Chemical Transmission  Neurotransmitter is stored in the synaptic end.  The synaptic cleft is the gap between the pre and post-synaptic cells.

Classification of Neurons Neurons can be classified structurally and functionally Structural classification Relationship of number of dendrites to the axon Functional classification Where information is carried Carry info to CNS, entirely in CNS or out of CNS

Structural Classification of Neurons Multipolar Many dendrites Most common in CNS; motor neurons in PNS Bipolar One dendrite and one axon extend from cell body Eyes, ears, nose Unipolar One process extends from the cell body Touch, stretch receptors

Structural Classification of Neurons

Functional Classification of Neurons Sensory (Afferent) Neurons Carry info from a body part to the CNS Detect change outside or inside the body Usually unipolar Interneurons Lie entirely in the CNS Link other neurons together (within CNS or CNS to motor neurons) - Integrators Usually multipolar Motor (Efferent) Neurons Carry impulses from the CNS to effectors (muscles, glands) Usually mulitpolar

Functional Classification of Neurons

Neuroglia Neuroglia do not generate or conduct nerve impulses. They support neurons by: Forming the Blood Brain Barrier (BBB) Forming the myelin sheath (nerve insulation) around neuronal axons Making the CSF that circulates around the brain and spinal cord Participating in phagocytosis

Neuroglia There are 4 types of neuroglia in the CNS: Astrocytes - support neurons in the CNS Maintain the chemical environment (Ca 2+ & K + ) Oligodendrocytes - produce myelin in CNS Microglia - participate in phagocytosis Ependymal cells - form and circulate CSF There are 2 types of neuroglia in the PNS: Satellite cells - support neurons in PNS Schwann cells - produce myelin in PNS

Neuroglia

PNS: Neuroglial Cells Schwann Cells Produce myelin on peripheral myelinated axons Satellite Cells Support neurons

CNS: Neuroglial Cells Astrocytes Star shaped, found between neurons and vessels Form scar tissue in response to brain injury maintain blood brain barrier Regulate nourishment of neurons

CNS: Neuroglial Cells Oligodendrocytes Form myelin sheath in CNS neurons Ependyma Line ventricles in brain and central canal in cord Cerebrospinal fluid circulation Microglia Support neurons and phagocytize bacteria and debris

Functions of Neurons Conduct nerve impulses Axonal transport Transport of biochemicals produced in the cell body to the end of the axon

Myelination of Axons Myelin sheath found around some large neuronal axons in PNS & CNS Myelin has a high lipid content that insulates axons and increases nerve impulse speed CNS: Myelin formed by Oligodendrocytes PNS: Myelin formed by Schwann Cells

Schwann Cell Myelination in PNS Gaps between regions of myelin called Nodes of Ranvier Nerve impulse jumps from node to node in a myelinated axon This is called Saltatory Conduction

Multiple Sclerosis Disease resulting from destruction of myelin sheath in some neurons of brain & spinal cord Leaves scar tissue called sclerosis Nerve fibers may also be damaged Prevents nerve impulse transmission to/from brain Muscles atrophy without stimulation Possible causes *Autoimmune Viral infection (no direct link to one specific virus) Symptoms reflect specific neurons in the CNS that are affected

Regeneration of Neuronal Axons CNS neuronal axons can not regenerate PNS may allow regeneration of axons Nerve tissue regeneration is largely dependent on Schwann Cells Cell body damaged – no regeneration Axon damaged – may be regenerated Regeneration is slow and the axon may not always grow in correct location May still lose some function, but not all

The outer nucleated cytoplasmic layer of the Schwann cell, which encloses the myelin sheath, is the neurolemma (sheath of Schwann). When an axon is injured, the neurolemma aids regeneration by forming a regeneration tube that guides and stimulates regrowth of the axon. Neuronal Regeneration

Gray and White Matter White matter of the brain and spinal cord is formed from aggregations of myelinated axons from many neurons. The lipid part of myelin imparts the white appearance. Gray matter (gray because it lacks myelin) of the brain and spinal cord is formed from neuronal cell bodies and dendrites.

Neuronal Physiology Neurons are excitable cells that have an electrical charge Cell membrane is polarized (electrically charged) A difference in charge exists between inside/outside cell Inside of cell negative relative to outside The difference in charge is called membrane potential, measured in millivolts MP due to differences in distribution & permeability of Na +, K + and other anions between the inside and outside of the neuron

Resting Membrane Potential, RMP At rest, RMP is -70 millivolts Inside cell High potassium, K + Anions Outside cell High sodium Na + These ions can only move through ion channels in the plasma membrane Cell membrane impermeable to anions

- anions

Resting Membrane Potential, RMP RMP maintained by normal diffusion (80%) and Sodium/ Potassium ATPase pump (20%) By normal diffusion, what way would Na + travel? By normal diffusion, what way would K + travel?

RMP: Sodium-Potassium ATPase Pump 3 Na + out for every 2 K + in ACTIVE TRANSPORT Results in charge separation between inside and outside a neuron

Nerve Impulse When a neuron responds to a stimulus, it affects the resting membrane potential Begins on dendrites or cell body BUT effect is on gated ion channels in the axon cell membrane (K + /Na + ) Gated ion channels open in response to an electrical impulse Recall which way Na+ and K+ travel by normal diffusion

Membrane Responses to Ion Movement Hyperpolarized – inside more (–) than resting Depolarized – inside more (+) than resting Repolarized – return to -70mV

Stimulus - Depolarization 1. Stimulus is detected must cause RMP to reach threshold potential, -55mV (depolarized) 2. Voltage gated Na + channels open, Na + rushes into cell Inside more positive than RMP, up to +30mV Result is beginning of Action Potential - rapid reversal of membrane potential

Repolarization When the MP reaches +30mV, potassium (K + ) channels open (while sodium channels close) Potassium ions (K + ) flow out of the cell until RMP is restored back to -70mV Repolarization is required before the neuron can be stimulated again.

Action Potential Sufficient Stimulus Threshold reached Depolarization when Na + gated channels open, Na + enters cell Na + channels close, K + channels open K + exits cell Repolarization

Action Potentials Only occurs on the axon AP is an all or none response May be summation of many stimuli to reach threshold (graded potentials) Many AP per single axon AP goes in one direction AP can not be stimulated during absolute refractory period

Nerve Impulse A nerve impulse is the propagation of action potentials along an axon Unmyelinated axons Impulse conducted over entire surface Myelinated axons Impulse occurs only at nodes of ranvier

Saltatory Conduction Regions of axon undergoing AP simulate adjacent areas to reach threshold Impulse jumps from node to node Occurs through length of axon Saltatory conduction

The Synapse Nerve impulses pass from neuron to neuron at synapses Gap between the pre-synaptic neuron axon terminal and post-synaptic neuron dendrite called synaptic cleft Is a type of chemical transmission

Synaptic Transmission Pre-synaptic axon terminal synaptic knob contains vesicles that store chemical messengers called neurotransmitters Nerve impulse reaches knob, Ca ++ channels open, Ca ++ enters knob Neurotransmitter released into synaptic cleft NT combines with receptor on post-synaptic membrane Inhibits or excites post-synaptic neuron

Fate of Neurotransmitter Must be removed or inactivated 1. Diffuses away from synaptic knob 2. Inactivated by enzymes 3. Taken back up into synaptic knob

Synaptic Transmission Neurotransmitters (chemicals) are released when impulse reaches synaptic knob Nervous system produces at least 100 different neurotransmitters A single neuron can release one or more types of neurotranmitters

Neurotransmitters Both excitatory and inhibitory neurotransmitters are present in the CNS and PNS. The same neurotransmitter may be excitatory in some locations and inhibitory in others. For example, acetylcholine (ACh) is a common neurotransmitter released by many PNS neurons (and some in the CNS). Ach is excitatory at the NMJ but inhibitory at other synapses.

Commonly Known Neurotransmitters Acetylcholine Stimulates skeletal muscle contraction Epinephrine Adrenalin (↑ heart rate, ↑ blood pressure) Dopamine Involved in controlling fine motor movements