Nervous System• Complex network of nerves and cells that carry messages

to and from the brain and spinal cord to various parts of the body

Nervous System

Neurons• Specialized nerve cells • Functional unit of nervous system • 3 parts – Cell body– Dendrites – axon

Neuron• Cell Body – contains nucleus and most organelles in cell– Synthesizes proteins, carbohydrates, lipids

• Dendrites– Tree like branches that send signals towards the cell body

• Axons– Single long, thin extension from cell body – Carry signals away from cell body– Contains axon terminals at tip of axon that enable signals to be

transmitted from one neuron to another

Neurons • 3 classes • Afferent neurons

(sensory) – transmit stimuli collected by sensory receptors

• Interneurons – integrate information, formulate a response

• Efferent neurons (motor) – carry response signal to effectors (muscle, glands)

Neural Signalling

• Communication by neurons

• Response to stimuli • 4 components1. Reception– detection of stimulus

(eyes, skin)

2. Transmission– movement of

message along a neuron

3. Integration– interpretation of

message

4. Response– output or action

Neuronal Circuit• Connections between axon terminals of one neuron and

the dendrites or cell body of a second neuron• receptor → afferent neuron (dorsal root) → one or more

interneuron → brain or spinal cord (reflex arc) → efferent neuron (ventral root) → effector

Reflex Arc• Simplest of neural circuits which does not require

coordination of brain

Neuron Support System • Schwann cells

– Form tightly wrapped layers of plasma membrane around axons – myelin sheaths

– Myelin sheaths – electrical insulators (high lipid content)

• Nodes of Ranvier– Gaps between Schwann cells – Expose axon membrane

directly to extracellular fluid• Glial Cells

– Remove dead neurons– Provide nutrition and support

to neurons– Remove dead pathogen

Nerve Signals• Use internal cellular

energy to generate current (ATP)

• Communicate across a synapse – Site where neuron

makes a connection with another neuron or an effector

• Two sides to a synapse– Pre-synaptic cleft –

axon terminal– Post-synaptic cleft –

dendrite or cell body • Communication

occurs in 2 ways– Chemically – Electrically

Chemical Synapse• Pre-synaptic

cleft and post-synaptic cleft are separated by a gap (25nm) – synaptic cleft

• Uses neurotransmitters to communicate between neurons

Electrical Synapse• Pre-synaptic cleft and post-synaptic cleft are in

direct contact • Current flows directly through nerurons• Gap junctions allow ions to flow • Provides rapid/synchronous transmission between

neurons

Conduction of Electrical Signals by Neurons• Membrane potential – difference in charge across

the plasma membrane (K⁺ Na⁺)– Resting membrane potential/action potential

• Sudden flow of ions across the plasma membrane via voltage-gated ion channels (Na⁺/K⁺) causes nerve impulses

Resting Membrane Potential• Neuron is not conducting a nerve impulse• Steady negative membrane potential

(-70mV)• Cell is polarized

Action Potential• Neuron conducts an electrical impulse • Temporary change in membrane

potential • Positive charges flow inside the cell • 6 phases

– Resting state– Threshold– Depolarization phase of action potential – Repolarization phase of action potential – Undershoot – Back to resting state

• Action potential is produced only if the stimulus is strong enough to cause depolarization to reach threshold – all or nothing principle

Stages of Action Potential• Resting State

– Both sodium and potassium channels are closed (voltage-gated channels)

• Threshold – Stimulus opens some Na⁺ channels– If threshold is achieved (-50mV), more Na⁺ are

opened triggering an action potential • Depolarization

– Na⁺ channels are open but K⁺ channels remain closed

– Na⁺ ions rush into the interior of the cell making it more positive

• Repolarization – Na⁺ channels close, K⁺ open– K⁺ ions leave the cell – Inside of cell becomes more negative

• Undershoot – K ⁺ channels remain open – The cell becomes hyperpolarized (membrane

potential falls below normal resting potential)

Na⁺/K⁺ Pump• Pumping of 3 Na⁺ ions out of the cell for every 2 K⁺

pumped into the cell• Net positive charge outside of cell

Refractory Period• Threshold elevated to ensure

a one way direction in neuron and gives channels time to reset themselves (resting period)

Factors That Affect Action Potential• Magnitude remains the same as it travels along the axis• The greater the stimulus, the faster the action potential

is• Rate of conduction increases with diameter of axon

Myelinated Axons• Saltatory conduction– Hopping of action

potentials over myelin onto nodes of Ranvier

– Na⁺ and K⁺ channels are crowded into nodes allowing for action potentials to develop

– Speeds of up to 130 m/s compared to 1 m/s in unmyelinated

– Allows for smaller sized and more tightly packed axons

Conduction Across Chemical Synapses• Action potentials cannot jump across synapses• Use of neurotransmitters • Transmission becomes delayed allowing neurons to receive

hundreds to thousands of axon terminals at the same time

Conduction Across Chemical Synapses• Neurotransmitters are

stored in synaptic vesicles in the cytosol of an axon terminal

• Action potential stimulates the release of Ca²⁺ into the cytosol

• Triggers a protein which allows vesicle to fuse with the plasma membrane releasing neurotransmitters into the synaptic cleft by exocytosis

Role of Neurotransmitters• Diffuse across the

synaptic cleft and bind to receptors located on the post synaptic cell

• Binding opens gated ion channels

(Na⁺, K⁺, Cl⁻) • Causes

stimulatory/inhibitory effects

Neurotransmitters

Central Nervous System• Comprised of the – Brain, spinal cord

• Manages body activities by integrating incoming sensory information from the PNS into effective responses

• Control centre of the body

Protective Connective Tissue• Meninges – 3 layers of connective tissue that surround and protect the brain

and spinal cord• Cerebrospinal Fluid– Cushions the brain and spinal cord, nourishes and protects from

toxic substances

Cranial Nerves

• Originate from brain and brain stem

• Each carry different function

• Related to senses

• Motor, Sensory, Both

Spinal Cord• Carries impulses between brain and

PNS• Contains interneuron circuits that

control motor reflexes• Structures

– Grey matter – consists of nerve cell bodies and dendrites

– White matter – consists of myelinated axons

– Dorsal root – incoming afferent neurons

– Ventral root – outgoing efferent neurons

– 31 pairs of spinal nerves – Cauda equina – collection of spinal

nerves that leave inferior end of spinal cord

Anatomy of Spinal Cord

Cross Section of Vertebra

Brain• Receives, integrates, stores,

retrieves information • Interneurons generate

responses that provide the basis for– Voluntary movements,

consciousness, behaviour, emotions, learning, reasoning, language, and memory

• Contains – Grey matter, white matter,

meninges, cerebrospinal fluid

• Broken into– Forebrain, midbrain,

hindbrain

Structures of Brain• Medulla oblongata

– Involuntary behaviours – breathing, digestion, heart rate, blood pressure

• Cerebellum – Voluntary behaviours –

muscle contraction, balance, fine motor control

– Pons – mass of fibres that connects cerebellum to higher centres of brain

• Brain stem – pons and medulla – Connects forebrain to

spinal cord

Cerebrum• Controls most of the sensory and

motor activities• Makes up most of the brain• Cerebral Cortex– Surface layer of cerebrum– Thin layer of grey matter –

unmyelinated neurons – Carries out higher brain functions– Divided into left and right

hemispheres – capability of functioning separately

– Divided into parietal, frontal, temporal, occipital lobes

Left and Right Hemispheres

• Corpus Callosum – Thick axon bundles– Connect two hemispheres together and coordinates function – Recognizing faces, sense of time, recognizing emotions

Sensory Regions of Cerebral Cortex• Frontal Lobe

– Reasoning, motor skills, higher level cognition, expressive language

• Parietal Lobe– Processing somatosensory area -

touch, pain, temperature, pressure

• Temporal Lobe– Primary auditory cortex,

interpreting sounds and language

– Hippocampus – memory• Occipital Lobe

– Interpreting visual stimuli and information

Somatosensory/Motor Cortex• Regions of the cerebral cortex that are involved

with different functions• Form bands across the top of the brain

Thalamus, Hypothalamus, Basal Nuceli• Thalamus

– Receives sensory information and relays to appropriate regions of cerebral cortex

– Waking and inducing drowsiness or sleep

• Hypothalamus– Regulate homeostatic functions of

body • Basal Nuclei/ganglia

– Grey-matter centres that surround thalamus

– Moderate voluntary movements directed by motor centres in cerebrum

– Parkinsons disease

Blood Brain Barrier• Tight junctions (impermeable membrane between

two cells) that prevent most substances dissolved in blood from entering cerebrospinal fluid

• Astrocytes protect from viruses, bacteria, toxic substances

Peripheral Nervous System• Regulates both movement and internal

environment of the body

Efferent System• Made up of axons of neurons (efferent/motor) • Carries signals to muscle glands which act as effectors• Divided into 1. Somatic system– communicates with skeletal muscles

2. Autonomic system– communicates with smooth muscles and glands

Somatic System • Conscious and voluntary• Controls body movements • Carries signals from CNS to

skeletal muscles• 31 pairs of spinal nerves– 8 cervical, 12 thoracic, 5 lumbar, 5

sacral,1 coccygeal• Somatic nerves consist only of

axons• Some Involuntary contractions– Reflexes, shivering, balance

posture

Autonomic System • Works with endocrine

system to regulate the body in response to change

• Uses motor nerves• Controls involuntary

processes – Digestion, secretion,

circulation, reproduction, excretory, contraction of smooth muscle, breathing

• Broken into– Sympathetic – associated

with nerves of chest and abdomen

– Parasympathetic – associated with brain

Sympathetic/Parasympathetic• Always active• Have opposing effects on organs they affect (precise

control) one stimulates, the other inhibits• Uses two neurons– Dendrites and cell body in CNS– Ganglion outside CNS – enlargement of nerve where cell

bodies of neurons are located• Sympathetic predominates during situations of

stress, excitement, strenuous physical activity• Parasympathetic predominates during situations

that are quiet, low stress

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Sympathetic/Parasympathetic

Pain/Painkiller• Interpretation by the brain of

sensory input received by specialized cells called substantia gelatinosa (SG)

• Forms a band in the dorsal horn of grey matter in spinal cord

• These cells are unmyelinated• SG cells are stimulated by an

afferent nerve of PNS (stub your toe)

• SG sends signal to brain to release endorphins and enkephalins (opioids)

• Attach to receptors of SG and prevent communication