CHAPTER 11Nervous System

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 components

Reception – detection of stimulus (eyes, skin)

Transmission – movement of message along a neuron

Integration – interpretation of message

Response – output or action

Neuronal Circuit/Reflex Arc Connections between axon terminals of one

neuron and the dendrites or cell body of a second neuron

Receptor → afferent neuron → one or more interneurons → efferent neuron → effector

Reflex Arc Simplest of neural circuits which does not

require coordination of brain

Neuron Support System Speed rate at which electrical impulses move along

axons Glial cells

Provide nutrition and support to neurons E.g. 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

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 ion channels (Na⁺/K⁺ active transport pump) causes nerve impulses

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

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

Refractory Period Threshold elevated to ensure a one way

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

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

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

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 Carries signals to muscle glands which act as effectors Divided into

Somatic system – communicates with skeletal muscles 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