BMS165 Topic 5: The Nervous System

Part 1

Nervous System: General Functions

  • The nervous system is the master control and communication system of the body.

  • Every thought, action, and emotion reflects its activity

  • It communicates with body cells using electrical impulses, which are rapid and specific and cause almost immediate responses.


Nervous System: Primary Functions

To carry out its normal role, the nervous system has three overlapping functions:

  1. It uses its millions of sensory receptors to monitor changes occurring inside and outside of the body. This information is called sensory input.

  2. It processes and interprets the sensory input and decides what should be done at each moment. This process is called integration.

  3. It then causes a response, or effect, by activating muscles or glands via motor output.



Anatomical Subdivisions

There are two major anatomical subdivisions

  1. Central nervous system (CNS)

  • Brain and spinal cord enclosed in bony coverings

  1. Peripheral nervous system (PNS)

  • All nervous system, except the brain and spinal cord, consists of

  • Nerves: bundles of axons in connective tissue

  • Ganglia: swellings of cell bodies in a nerve


Function Subdivisions of Peripheral Nervous Syste

Sensory (afferent) division (receptors to central nervous system)

  • visceral sensory and somatic sensory division

Motor (efferent) division (central nervous system to effectors)

  • somatic motor division

  • effectors: skeletal muscle

visceral motor division (aka autonomic nervous system)

  • effectors: cardiac muscle, smooth muscle, glands

  • sympathetic division (fight or flight)

  • parasympathetic division (rest and digest)



3 Functions Classes of Neurons

  1. Sensory (afferent) neurons

  • detect changes in body and external environment

  • information transmitted into brain or spinal cord

  1. Interneurons (association neurons)

  • lie between sensory and motor pathways in central nervous system

  • 90% of our neurons are interneurons

  • process, store and retrieve information

  1. Motor (efferent) neuron

  • send signals out to muscles and gland cells

  • organs innervated by motor neurons that carry out responses are called effectors


Cell Body

  • also referred to as Soma

  • Contains the nucleus and most organelles

  • grouped in CNS into nuclei

  • grouped in PNS into ganglia


Axons

  • Process extending from the outgrowth from the soma called the axon hillock

  • May be covered in places with a substance called myelin

  • produced by a cell called a Schwann cell

  • Conduct the primary signal (action potential)

  • from initiation in the initial segment, to the terminals where neurotransmitter release occurs

  • Length from 1mm to 1m


Terminals

  • The end points of axons

  • Specialised structures for packaging and releasing neurotransmitters following an action potential

  • Very closely opposed to special areas (called post-synaptic densities) on the dendrites of their target cells

  • Terminal + post-synaptic density = a synapse (discussed later)


Dendrites

  • dendron = tree branch

  • Receptive area

    • transmits information (electrical impulses) to cell body

    • Receives this information from other neurons

    • Variable in number from 1000s to 1.



Types of Neuroglial Cells

  • Even though it is complex, nervous tissue is made up of just two principal kinds of cells

    1. Neurons

    2. Supporting cells

  • Supporting cells in the nervous system are called neuroglia

  • Neuroglia are found in both the central nervous system and in the peripheral nervous system

  • Each type of neuroglia has a specific function

  • Neurons need neuroglia in order to maintain homeostasis.



Types of Neuroglial Cell


Myelin Sheath

  • Insulating layer around a nerve fibre

    • Schwann cells make myelin in the Peripheral Nervous System, and oligodendrocytes make myelin in the Central Nervous System

    • Formed from wrappings of plasma membrane in a spiral fashion:

    • 20% protein and 80 % lipid (looks white)

    • The myelin sheath insulates the axon for rapid action potential conduction, as well as separating axons from surrounding extracellular components

    • All myelination completed by late adolescence

  • In the Peripheral Nervous System, hundreds of layers wrap axon

    • The outermost layer of the Schwann cell is called the neurilemma

    • Covered by basal lamina and endoneurium.

  • In Central Nervous System - no neurilemma or endoneurium

  • Oligodendrocytes myelinate several fibres:

  • Myelination spirals inward with new layers pushed under the older ones

  • Gaps between myelin segments= nodes of Ranvier

  • Less nodes of Ranvier in Central Nervous System compared to Peripheral Nervous System.



The role of Myelin

  • Diameter of fibre and presence of myelin increases the speed (conduction velocity) that electrical signals (action potentials) travel along an axon:

  • Large fibres have greater conductance and less relative leak and capacitance

    • Myelin protects large axons

    • Myelin further reduces leak and capacitance by being an insulator.

  • Speeds

    • small, unmyelinated fibres = 0.5 - 2.0 m/sec

    • small, myelinated fibres = 3 - 15.0 m/sec

    • large, myelinated fibres = up to 120 m/sec

Multiple Sceloris (MS)

  • A complex neurodegenerative disease affecting the central nervous system (CNS).

  • The exact aetiology of the disease is unknown.

  • Observational research has shown that that it is associated with demyelination inside of the CNS whilst the axons are preserved.


Part 2


Resting Membrane Potential

  • A neuron that is “resting” has an electrical difference across its plasma membrane​.

  • This means that there are fewer positive ions sitting on the inner surface of the neuron’s plasma membrane than there are on its outer surface.

  • While the inside of the neuron remains more negative than the outside, the neuron will remain in its resting state. ​

  • The electrical difference whilst the neuron is at rest is called the resting membrane potential. ​

  • The cations and anions inside and outside of the neuron are different



Electrical Potential and Currents

  • An electrical potential is the difference in the concentration of charged particles between one point and another​

  • An electrical current is a flow of charged particles from one point to another

  • Living cells are polarized​:

    • resting membrane potential is -70 mV with a negative charge on the inside of membrane


Ionic Basis of Resting Membrane Potential

  • ​At resting state, Na+ (positively charged) is attracted by the electrical force into the -negatively charged cell

    • Since the concentration of extracellular Na+ is higher, it tends to be pulled into the cell by the concentration force

    • In the resting state there is almost no Na+ flow.​

  • K+ (positively charged) is attracted into the cell by the electrical force, but repelled outwards by the diffusion force

  • Protein molecules (negatively charged) tend to be driven out by both the electrical force and the diffusion force

    • These molecules are too big to pass through the cellular membrane

  • Cl- (negatively charged) is repelled outward by the electrical force but pulled inward by the diffusion force.​

  • Due to the overall unequal distribution of the charged particles, the inside of the cell is -70 mV relative to outside of the cell. This is the resting potential of the neuron.


Sodium-potassium Pumps


Local Potentials

  • Local disturbances in membrane potential ​

    • Occur when a neuron is stimulated by

      1. Chemicals

      2. Light

      3. Heat

      4. Mechanical disturbances

    • Depolarisation decreases potential across cell m​embrane due to opening of ligand-gated Na+ channels but not enough to stimulate the voltage gated Na+ channels at trigger zone​

    • Na+ rushes in down concentration and electrical gradients​

  • Na+ diffuses for short distance inside membrane producing a change in voltage called a local potential


Action Potentials

  • More dramatic change in membrane produced where high density of voltage-gated channels occur​

    • trigger zone up to 500 channels/mm2 (normal is 75)​

  • If threshold potential (-55mV) is reached voltage-gated Na+ channels open (Na+ enters causing depolarisation)​

  • Past 0 mV, Na+ channels close = depolarisation​

  • Slow K+ gates fully open​

  • K+ exits, repolarising the cell

  • Negative overshoot produces​ hyperpolarisation​

    • excessive exiting of K+


The Refractory Period

When -55mV is reached , this opens the voltage-gated sodium channels.

Sodium rushes into the neuron then the voltage-gated sodium channels shut. And it doesn’t matter what you do, you cannot open them again.

When they shut, they become deactivated(locked). Recovery from inactivation takes 1 -2 msec. This is the absolute refractory period.

From 2 msec onwards, some of the voltage-gated sodium channels start to come out of inactivation(unlocked) which means that they can be used again. This is the relative refractory period.


Impulse Conduction in Unmyelinated Fibres

  • Threshold voltage in trigger zone begins impulse​

  • Nerve signal (impulse) - a chain reaction of sequential opening of voltage-gated Na+ channels down entire length of axon​

  • Nerve signal (non-decremental) travels at 2m/sec

  • Voltage gated sodium channels are distributed evenly along the axon.

  • As the action potential allows sodium to depolarise a section of axon, the sodium channels adjacent become recruited and propagate the action potential continuously along the axon

  • The refractory membrane is where the sodium channels become closed (called inactivated) and cannot re-open for a period of time

  • This keeps the signal moving in one direction (after initiation)



Excitatory Synapse

  • Nerve signal opens voltage-​gated calcium channels ​in synaptic knob​

  • Triggers release of ACh, which crosses synapse

  • ACh receptors trigger opening ​of Na+ channels producing ​local potential​

  • When voltage reaches -55mV, triggers action potential in postsynaptic neuron


Excitatory and Inhibitory Synapses

  • There are two kinds of synapses, excitatory and inhibitory. ​

  • At excitatory synapses, neurotransmitters induce a depolarizing postsynaptic potential. ​

  • Excitatory neurotransmitters bind to protein channels that allow Na+ to enter the next neuron, this makes the post synaptic neuron more positive

  • At inhibitory synapses, neurotransmitters induce a hyperpolarising post synaptic potential. ​

  • Inhibitory neurotransmitters bind to protein channels that allow Cl- to enter the next neuron, this makes the post synaptic neuron more negative.


Stopping Postsynaptic Neuron Stimulation

  1. Diffusion

  • Some of the released neurotransmitter molecules diffuse away from the synaptic cleft​

  1. Enzymatic degradation

  • Enzymes break down neurotransmitters. Acetylcholinesterase breaks down Ach into acetate and choline with these products having no stimulatory effect​

  1. Reuptake of neurotransmitter

  • Many neurotransmitters are actively transported back into the presynaptic neuron by endocytosis


Part 3


Functions of the Spinal Cord

  1. Conduction​

  • Bundles of fibres (Australian spelling) passing information up and down spinal cord​

  • A fibre is another name for an axon.

  • Carries information that is afferent (comes in) and efferent (goes out)

  1. Locomotion​

  • Repetitive, coordinated actions of several muscle groups​

  • Central pattern generators are pools of neurons providing control of flexors and extensors (walking)​

  1. Reflexes​

  • Involuntary, stereotyped responses to stimuli (remove hand from hot stove)​

  • Involves spinal cord and peripheral nerves, brain informed afterwards


Structure of the Spinal Cord

  • Cylinder of nervous tissue within the vertebral canal (as thick as a finger)

  • Vertebral column in an adult extends to L1

  • Extends through vertebral canal from foramen magnum to L1​

  • 31 pairs of spinal nerves arise from cervical, thoracic, lumbar and sacral regions of the cord​

  • each cord segment gives rise to a pair of spinal nerves​

  • Cauda equinae is L2-S5 nerve roots – resembles a horse’s tail


Meninges of the Spinal Cord

  • There are three fibrous layers that enclose the spinal cord:​

    1. Dura mater (outer/superficial layer):​

    • tough collagenous membrane surrounded by epidural space​

    • epidural anaesthesia utilized during childbirth​

    1. Arachnoid mater (middle layer):​

    2. Pia mater (inner/deep layer):


Lumbar Puncture

  • The entry into the subarachnoid space is commonly described as feeling a "pop" sensation.

  • The needle insert is then removed, and CSF should begin to drip out.


Anatomy of the Spinal Cord

  • Central area of grey matter shaped like a butterfly and surrounded by white matter ​

  • Grey matter: consists of neuron cell bodies with little myelin and white matter consists of myelinated axons​

  • Pair of posterior (or dorsal) horns​

    • posterior root of spinal nerve is totally sensory fibres​

  • Pair of anterior (or ventral) horns,​

    • anterior root of spinal nerve is totally motor fibres​

  • Connected by grey commissure punctured by a central canal.

  • Horns: Grey matter of spinal cord: cell bodies​

    • posterior horn: cell bodies of interneurons​

    • anterior horn: cell bodies of motor neurons​

  • Roots: Bundles of fibres connecting spinal cord and spinal nerves​

    • posterior root contains sensory neurons​

    • anterior root contains motor neurons​

  • White matter/columns: Bundles of myelinated axons (fibre tracts) that carry signals up and down, to and from the brainstem



Spinal Cord Tract

  • White matter of the spinal cord is composed of myelinated axon tracts (or pathways)

  • If you think of your spinal cord as a tall building, these tracts are like elevators that go straight up to the top of the building.

  • Tracts can be ascending (go up) or descending (come down from) the brain ​

  • Some tracts cross over to the other side of the spinal cord and some do not

  • Some tracts travel from the brain to the spinal cord and some conduct impulses from one side of the spinal cord to the other


Nerves

A nerve is an enclosed, cable-like bundle of axons (the projections of neurons) in the peripheral nervous system (PNS). A nerve provides a structured pathway that supports the electrochemical nerve impulses transmitted along each of the axons. Each nerve contains many axons that are sometimes referred to as fibers. Within a nerve, each axon is surrounded by a layer of connective tissue called the endoneurium. The axons are bundled together into groups called fascicles. Each fascicle is wrapped in a layer of connective tissue called the perineurium. Finally, the entire nerve is wrapped in a layer of connective tissue called the epineurium.


Ganglia

  • A ganglion is a cluster of neuron cell bodies of nerves ​

  • Posterior root ganglion is sensory cell bodies ​

  • Fibres pass through without synapsing


Spinal Nerves

  • carry information to and from the spinal cord

  • are named for the region of the cord from which they arise ​

  • are formed when ventral root and dorsal roots combine ​

  • divides into two branches after formation.

The anterior and posterior branches of each spinal nerve contain both sensory and motor axons, thus damage to a spinal nerve or any of its branches results both in loss of sensation and in paralysis of the area of the body served.


Part 4


Reflexes

Although there are many types of communication between neurons, most of what the body must do every day is programmed as reflexes

  • Reflexes are rapid, predictable, and involuntary responses to stimuli.

  • A stimulus is any change that occurs inside or outside of the body.

  • Reflexes are like one-way streets, once a reflex begins, it always goes in the same direction.

  • You can think of a reflex as a preprogramed response to a given stimulus.


Somatic and Autonomic Reflexes

  • The types of reflexes that occur in the body are classed as either somatic or autonomic.

  • Somatic Reflexes

    • Involve skeletal muscles and are typically under voluntary control. Examples include the knee-jerk reflex and withdrawal reflex.

  • Autonomic Reflexes

    • Involve smooth muscles, cardiac muscles, and glands, and are generally not under voluntary control, such as the regulation of heart rate and digestive processes.


Reflex Arc

  • Reflexes occur over neural pathways called reflex arcs

  • All reflex arcs have a minimum of four elements: ​

    • A receptor (which reacts to a stimulus)​

    • An effector (the muscle or gland that is stimulated/responds)​

    • A sensory neuron (brings sensory information to the central nervous system)​

    • A motor neuron (delivers information out to the body from the nervous system)​

    • An integration centre (other neurons) are only seen in some reflex arcs.


Patellar Reflex (Somatic Reflex)

  • Our muscles must not be stretched too far because if they do, the cells making up that muscle may be damaged.

  • There are a class of reflexes (called stretch reflexes) whose job is to make sure that muscles do not stretch out too far

  • The patellar reflex (or knee-jerk reflex) is just one of many stretch reflexes

  • When you hit the tendon with the hammer it causes that muscle to stretch.

  • This muscle stretch activates sensory neurons in the muscle.

  • The sensory neurons synapse directly with motor neurons in the spinal cord.

  • The motor neurons cause the muscle to contract.


Withdrawal Reflex (Somatic Reflex)

  • A painful stimulus initiates the flexor or withdrawal reflex

  • This reflex causes automatic withdrawal of the threatened body part form the stimulus.


Salivary Gland Secretion (Autonomic Reflex)

  • Saliva in the mouth is a biofluid produced mainly by three pairs of major salivary glands

  • Salivary gland secretion is an autonomic reflex​

  • The volume of saliva secreted is dependent on the intensity and type of taste and on smell, chewing or touch stimulation


Non-Responsive Reflexes

  • Reflex testing is an important tool in evaluating the condition of the nervous system.

  • Reflexes that are exaggerated, distorted or absent indicate damage or disease in the nervous system.

  • Reflex changes often occur before a pathological condition becomes obvious in other ways.