HUMB C8

Neuron (nerve cell)

basic structural and functional unit of the nervous system

Neuroglia

major supporting cells of neurons

Axon

nerve fibre

Nerve

bundle of axons and their sheaths

Sensory receptors

separate specialised cells which detect temperature, pain, touch, light, sound, odour and other stimuli

Action potential

electrical signal that transmits information along neurons and allows communication between neurons and effectors

Effector

the organ, tissue, or cell in which an effect or action takes place

Interneurons

Connect sensory neurons to motor neurons inside the CNS

Synapse

junction of a neuron with another cell

Ganglia

collection of neuron cell bodies/somas outside CNS

Plexuses

extensive network of usually axons outside CNS

Nervous System Functions

Receive internal and external sensory input

Integrate information

Motor output

Maintaining homeostasis

Establish and maintain mental activity

Nervous system – Structural Classification

Central nervous system (CNS): decision maker, protected by skull and vertebrae

Brain

Spinal Cord

Peripheral nervous system (PNS): messengers that collect info and take it to CNS, allow for motor output

Sensory receptors

12 pairs of Cranial nerves arising from brain

31 pairs of spinal nerves arising from spinal cord

Ganglia

Plexuses

Nervous system – System Classification

PNS and CNS in constant communuication with each other

PNS divided into Somatic NS, Autonomic NS, Enteric NS - all with a sensory (afferent) and motor (efferent) component

Autonomic NS motor component produces sympathetic and parasympathetic responses

Nervous system – Functional Classification

PNS split inito sensory division and motor division

Only the motor division can be classified as somatic, autonomic, or enteric

Autonomic can be further divided into parasympathetic and sympathetic

Somatic NS

Enables voluntary contraction of skeletal muscles

Single neuron system, one synapse

Cell bodies in ventral horn

Myelinated axons

Autonomic NS

Involuntary control

Action potentials in motor neurons travel from the CNS to smooth or cardiac muscle, or glands

Two-neuron system, two synapses (preganglionic and postganglionic motor neurons)

Preganglionic are myelinated, postganglionic are unmyelinated

Preganglionic somas in lateral horn in CNS, postganglionic somas in autonomic ganglion in PNS

Target tissues stimulated or inhibited (organ activity can be increased or decreased)

Enteric nervous system - Functions

Controls digestive system:

Stimulate/inhibit smooth muscle contraction and gland secretions

Detect changes in content of lumen

Enteric nervous system - Anatomy

Sensory neurons connect digestive tract to CNS

Autonomic motor neurons connect CNS to digestive tract

Enteric neurons confined to enteric plexuses within digestive tract wall

Sensory/afferent division

Specialised (sensory) receptors detects external and internal environmental stimuli

Collects and transmits input as electrical signals from sensory receptors to CNS (nerves)

sensory neuron’s cell body is outside CNS (cluster forms dorsal root ganglion)

Motor/efferent division

Transmits electric signals from CNS to effector (e.g., muscle, gland)

Motor neuron’s cell body located inside CNS

Autonomic NS Motor Division - Sympathetic Response

Fight or flight

arises from thoraco-lumbar region

Primes the body to act in threatening situations

Shorter neuron pathway, quicker response

Complementary to parasympathetic

E.g: increased heart rate and respiratory rate, sweating, pupil dilation

Autonomic NS Motor Division - Parasympathetic Response

Rest and digest/feed and breed

Arises from cranio-sacral region

Relaxes the body inhibiting high energy functions

Longer neuron pathway, slower response

Complementary to sympathetic

Neuron

Structural and functional unit of NS

≈100 billion in body

Receive info, integrate it, and send appropriate response to target

Classified by structure and/or function

Highly modified cell

Electrically excitable

Unidirectional impulse flow

Neuron parts

cell body (soma) - contains all major organelles except centrioles, protein synthesis, energy production, signal integration

Dendrites - branched extensions of soma, Generate electric current when stimulated that flows from dendrites’ tips to soma

Axon - vary from mm to m, can be collateral (branched), often arise from axon hillock

Axon parts

trigger zone = axon hillock + inital segment,

presynaptic terminal/axon terminals (axon endings)

terminal boutons/synaptic knobs (swollen ends of axon terminals)

myelin sheath (in PNS sheath made of Schwann cells)

node of Ranvier (gaps between Schwann cells)

Neurons – structural classification

Multipolar

bipolar

unipolar/psuedo-unipolar

Multipolar neuron

One cell body

Many dendritic processes and an axon

E.g motor neurons, interneurons

Bipolar neuron

One cell body and two processes, a dendrite and an axon

Specialised sensory neurons in retina of eye and nasal cavity

Psuedo-unipolar neuron

One cell body and only one process, axon - this bifurcates into peripheral process (dendtrites) and central process (axon terminals)

Dendrites connected to sensory receptors, axon terminals to CNS

Sensory neurons

Neurons – functional classification

Sensory neuron

Motor neuron

Inter-neuron

Sensory neuron

Conduct action potential (AP) from sensory receptors to CNS

Motor neuron

Conduct AP away from CNS towards muscles/glands

Inter-neuron

Conduct AP within CNS from one neuron to another

Neuroglia

10-50 times more abundant than neurons

Supporting cells

6 different types - Astrocytes, Ependymal, Microglial, and Oligodendrocytes in CNS, Schwann and Satellite in PNS

CNS Neuroglia - Astrocytes

Star shaped

Branched cytoplasmic processes with end feet

Functions - Scaffold CNS cells and capillaries, control blood-brain barrier permeability, synaptic support, and CNS homeostasis

CNS Neuroglia - Ependymal cells

Line brain ventricles and central canal of spinal cord

Ciliated to circulate cerebrospinal fluid (CSF), simple cuboidal and columnar

Function - Produce and regulate CSF, Protection, and hormone transport

CNS Neuroglia - Microglial cells

Oval body with many projections

Become active, mobile, & phagocytic in response to inflammation, otherwise resting

Function - Phagocytose foreign substances, necrotic tissue, and pathogens

CNS Neuroglia - Oligodendrocytes

Have cytoplasmic extensions that wrap around axon, forming myelin sheath

Each cell can enclose multiple axons

Function - Insulate CNS axons, saltatory conduction

PNS Neuroglia - Schwann cells

Have cytoplasmic extensions that wrap around axon, forming myelin sheath

Each cell forms part of sheath around one axon

Function - Insulate PNS axons, saltatory conduction of action potential (AP propagates by jumping from one Node of Ranvier to next)

PNS Neuroglia - Satellite cells

Surround soma in ganglia

Function - Provide support and nutrition to soma, Protects neurons from heavy metal poisons (Pb, Hg)

Myelinated axons

Schwann cells (PNS) and oligodendrocytes (CNS) wrap repeatedly around axon

Nodes of Ranvier

Saltatory conduction of electrical impulses

Faster nerve impulse conduction

Unmyelinated axon

Axons rest in invaginations of Schwann cells (PNS) and oligodendrocytes (CNS)

electrical impulses travel as continuous wave

Thinner

Slower nerve impulse conduction

Grey matter

Consists of somas, dendrites, axon terminals, unmyelinated axons, glial cells, and synapses

On periphery (outer cortex) in brain but in middle of spinal cord as H-shape

Forms ganglioins in PNS

White matter

White due to myelin sheath

Very few somas

Forms nerves in PNS and nerve tracts in CNS

On inner side in brain but periphery in spinal cord

Electrical impulses

allows responses to environment (external & internal), emotions, conscious thoughts, memory, and action of glands & muscles

Membrane potential

measure of electrical properties of cell membrane

Determined by:

Ionic concentration difference across plasma membrane

Permeability of plasma membrane

Cell membrane – ionic concentration

Higher concentration of Na⁺ and Cl^- extracellularly

Higher concentration of K⁺, proteins, and PO₄^3- intracellularly

if total number of cations and anions in extracellular and intracellular fluid is equal, cell is electrically neutral

Cell excitability depends on:

ion movement across cell membrane

differences in ionic concentration across cell membrane

Na⁺/K⁺ pump

Cell membrane – ion channels

Non-gated/Leak ion channels:

Ion specific

Always open

More K⁺ and Cl^- leak ioc channels that Na⁺ ones

Gated ion channels:

require signals to open

Ion specific: ligand-gated open when chemicals attach, voltage-gated open with charge difference, and other-gated open with temp, pressure, etc

Resting membrane potential (RMP)

difference in charge across cell membrane in a resting cell caused by leak ion channels and Na⁺/K⁺ pump

Measured in mV

RMP of neurons is -70mV

-ve if intracellular side is more negative

Establishing resting membrane potential

Intracellular fluid becomes more -ve because:

More K⁺ leak ion channels moving K⁺ out than Na⁺ ones moving Na⁺ in

Anionic proteins remain inside cell as they’re too large and charged to pass cell membrane

Na⁺/K⁺ pump moves 3Na⁺ out but only 2K⁺ in

Action Potential (AP) - Def and Steps

how neuron transmits info, as electrical signals, away from soma, down axon, to effector

Steps:

Resting membrane potential

Depolarisation

Repolarisation

Threshold

membrane potential a neuron needs to reach to trigger an AP

-50 to -55mV for neurons

AP - Resting membrane potential

All gated Na+ and K+ channels are closed

K+ leak channels open, moving K+ out

Na+/K+ moving 3Na+ out and 2K+ in

-ve RMP

AP - Depolarisation

Cell is stimulated

Na+ voltage gated channels open, moving Na+ inside

K+ voltage gated channels remain closed so less K+ leaves cell

Membrane potential becomes more positive

AP - Repolarisation

Na+ voltage gated channels close, preventing Na+ from entering

K+ voltage gated channels open, moving K+ out

Membrane potential becomes more negative

AP - Hyperpolarisation (afterpotential)

Na+ voltage gated channels close, preventing Na+ from entering

K+ voltage gated channels close slowly so K+ continues to leave the cell, producing afterpotential

Membrane potential becomes very negative

AP - Resting Membrane Potential

Na+ and K+ gated channels are closed

RMP reestablished by Na+/K+ pump actively (against concentration gradient) moving 3 Na+ out and 2 K+ in

Graded potential

short-lived, localised (occur in small region) changes in membrane potential that occur in dendrites or soma

if the stimulus, and thus graded potential, is large enough, AP fired

Can summate (add together)

Decremental (signal weakens as it spreads so cant transfer info over long distance)

All-or-none principle

If the stimulus, and thus graded potential, is large enough to cross threshold, the magnitude of AP is same every time

If threshold is not crossed, no AP fired

Stronger stimuli increase frequency not magnitude

Refractory period

Absolute refractory period - during depolarisation and most of repolarisation, no more AP can be generated

Relative refractory period - during late repolarisation and afterpotential, only very strong stimuli can generate another AP

Propagation of axon potentials

Only in unmyelinated axons

one axon membrane segment is depolarised, causes subsequent segment to be depolarised, then the initial segment is repolarised

Unidirectional (soma to axon terminal)

Saltatory conduction

only in myelinated axons

AP jumps from one Node of Ranvier to next

Faster impulse transmission

Synapse - Def and Types

Junction where a pre-synaptic neuron communicates with a post-synaptic cell (neuron, muscle fibre, gland celletc)

A neuron can have thousands of synapses

Electrical synapse – less common, electric signal transmits AP, in cardiac muscles & some smooth muscles

Chemical synapses – most common, uses neurotransmitter (chemical messenger) to transmit AP

Synaptic Cleft:

space between pre-synaptic membrane and post-synaptic membrane

Chemical Synapse

Pre-synaptic terminal has Ca²⁺ voltage gated channels in wall that open when AP recahes them

Ca²⁺ moves into cell and signal synaptic vesicles to move towards and merge with pre-synaptic membrane and via exocytosis, releases neurotransmitters into synaptic cleft

Neurotransmitters attach to receptor sites to ligand-gated Na+ channels, causing them to open and allow Na+ to diffuse into post-synaptic cell

If Na+ movement is enough to reach threshold, AP generated in next cell

Overstimulation avoided by enzymnes in synaptic cleft breaking down neurotransmitters, closing channels, to be reabsorbed into presynaptic terminal

Spinal cord

Extends from brain at foramen magnum to second lumbar vetebra

Shorter than vetebral column as it grows slower

Protected by meninges and bony veterbral column

Anterior median fissure (deeper, wider) and posterior median sulcus

Spinal Cord - Rootlets to Nerves

Rootlets arise posteriorly and anteriorly on sides

6-8 rootlets combine to form posterior and anterior root

Dorsal root ganglion

Posterior and anterior root join to form spinal nerve

Grey matter

On inside of spinal cord as butterfly/H shape

Posterior, lateral, and anterior horn

Gray commissure connects left and right sides, contains hole in middle called central canal where CSF circulates

White matter

Posterior, lateral, and anterior column

On outside of spinal cord surrounding grey matter

White commissure connects left and right sides

meninges

connective tissue membrane surrounding brain and spinal cord

Organisation of neurons in spinal cord

Sensory neurons (bipolar and pseduo-unipolar) enter through posterior root

Motor neurons (mulipolar) enter through anterior root

Spinal nerve is ‘mixed’ because have both sensory and motor neurons

Organisation of sensory neurons in spinal nerves

dendrites are located where sensory receptors are

axon travels along spinal nerve

soma is in dorsal root ganglion

central process of axon travels through dorsal root and into grey matter

collateral branches ascend and descend into white matter

synpases with motor neuron or interneuron in grey matter

Organisation of motor neurons in spinal nerves

Dendrites and soma in grey matter

Leave through anterior rootlet

Travel through anterior root and spinal nerve to reach effector

Reflexes

Automatic response to a stimulus, without higher brain centre involvement or mental processing

homeostatic (help maintain homeostasis)

Produces same stereotyped, immediate response from spinal cord

Rapid, predictable, and unlearnt

Somatic vs Autonomic Reflexes

somatic - skeletal muscle, protect from harm, e.g. pulling back when touching hot surface

autonomic - smooth muscle, cardiac muscle, and glands, maintain bodily functions, e.g. blood pressure

Monosynaptic Vs Polysynaptic

Monosynaptic - simplest, one synapse (sensory neuron to motor neuron)

Polysynaptic - slower, multiple synapses (involve interneurons)

Reflex Arc def

neuron pathway that facilitates a reflex

Reflex Arc

Sensory receptors detects stimulus

Sensory neurons conduct AP to spinal cord via posterior root

sensory neuron synapses interneuron or motor neuron

Motor neuron axon conducts AP to effector via anterior root

Reflex Arc - Brain

Sensory information sent to brain via ascending tract and returned via descending tract

Reflex does not need brain to occur, but can be supressed or exaggerated by brain

Reaction

Voluntary response to sensory stimulus

Slower than reflex

Involves brain and spinal cord

Reaction time improves through repetition

e.g catching/dodging a ball