HUBS191 Module 3: Nerves and Muscles (neuroanatomy and neurophysiology)

Cells and organisation of the nervous system, functional information flow, action potentials, propagation and synaptic transmission, spinal cord and spinal nerves, meninges and ventricular system, Structure and layout of major brain areas; Sensory and motor pathways, Somatic sensation, Motor control

Functions of the nervous system

Detects internal and external environment, integrates information in CNS, produces coordinated motor responses, maintains homeostasis

Basic organisation of the nervous system

Central nervous system (brain and spinal cord), Peripheral nervous system (nerves and ganglia)

Brain and spinal cord

Components of the central nervous system

Nerves and ganglia

Components in the peripheral nervous system

Function of brain and spinal cord

Integration and coordination

Function of nerves and ganglia

Communication between body and central nervous system

Afferent

Sensory information travels into the central nervous system

Efferent

Motor output travels out of the central nervous system

Two main cell types in the nervous system

Neurons and glial cells

Neurons

Excitable cells which generate action potentials and transmit information

Glial cells

Cells which support neurons, providing nutrients, insulation and immune defence

Structure of a neuron

Dendrites, cell body, axon hillock, axon, axon terminals

Dendrites

Branch like extensions of a neuron which receive chemical signals from other neurons and contain receptors for neurotransmitters carrying signals towards the cell body

Cell body (soma)

Contains nucleus and organelles and is the site of metabolic activity, integrates incoming signals from dendrites and sends processed signals to axon hillock

Axon hillock

Junction between soma and axon and is the summation zone which determines whether threshold is reached to initiate action potential

Axon

Long, thin projection from neuron which conducts action potentials and carries signals away from the cell body

Axon terminals

End branches of axon which contain synaptic vesicles with neurotransmitters and release neurotransmitters into synapse converting electrical signal to chemical signal

Action potential

A rapid electrical signal caused by ion movement across a membrane

Functional zones of a neuron

Input zone, summation zone, conduction zone, output zone

Multipolar neurons

Many dendrites and one axon, integrating lots of input

Bipolar neurons

One dendrite and one axon, having specialised sensory roles e.g vision

Unipolar neurons

Single process branching into two, fast signal transmission

Anaxonic neurons

No distinct axon, integrates signals locally

Main types of glial cells

Astrocytes, microglia, ependymal cells, oligodendrocytes, shwann cells

Functions of astrocytes

Provide metabolic support, surround blood vessels, maintain extracellular environment, form glial scar after injury

Functions of microglia

Immune cells of central nervous system derived from myeloid lineage that engulf pathogens and debris

Functions of ependymal cells

Line ventricles and spinal canal, contain cilia which helps circulate cerebrospinal fluid

Functions of oligodendrocytes

Form myelin sheath in central nervous system, one cell myelinates multiple axons and increases conduction velocity

Functions of schwann cells

Form myelin in peripheral nervous system, one cell wraps one axon segment and supports nerve regeneration

Myelin sheath

Lipid rich insulating layer which prevents current leakage and speeds up signal transmission

Nodes of ranvier

Gaps between myelin allowing saltatory conduction and increase speed

Structure of a synapse

Presynaptic terminal, synaptic cleft, postsynaptic membrane

Process occuring at the synapse

Action potential arrive, vesicles release neurotransmitters, neurotransmitters diffuse across cleft, bind receptors and a new signal is generated

Role of neurotransmitters

Chemical messengers, bind to receptors, trigger response in postsynaptic cell

Grey matter

Neuron cell bodies in cerebral cortex or spinal cord

White matter

Bundle of axons, long fibre tracts in cerebral cortex or spinal cord

Nucleus and tract in CNS

Nucleus- group of cell bodies
Tract- bundle of axons

Ganglion and nerve in PNS

Ganglion- group of cell bodies
Nerve- bundle of axons

Types of information transmitted in the nervous system

Sensory (afferent)- information sent into CNS
Motor (efferent) - information sent out of the CNS to effectors

Somatic nervous system

A component of the peripheral nervous system that enables voluntary control of body movements via skeletal muscles and transmits sensory information to the CNS

Autonomic nervous system

A component of the peripheral nervous system that regulates involuntary physiologic processes, including heart rate, blood pressure, respiration, digestion

Somatic afferent pathway

Sensory information we are aware of

Somatic efferent pathway

Voluntary control of skeletal muscle

Autonomic afferent pathway

Internal sensory information, not conscious

Autonomic efferent pathway

Involuntary control of smooth muscle, cardiac muscle, glands

Anatomical organisation of the somatic efferent division

2-neuron pathway:
-upper motor neuron (CNS)

-lower motor neuron (PNS → muscle)

Upper motor neuron

Motor neurons that originate in the cerebral cortex or brainstem and carry electrical signals that initiate and modulate voluntary movement and muscle tone

Features of the upper motor neuron

-Cell body in brain (motor cortex)
-Axon travels down spinal cord

-Located in CNS

-Myelinated

Lower motor neuron

Nerve cells that connect the brainstem and spinal cord directly to skeletal muscles causing contraction

Features of the lower motor neuron

-Cell body in spinal cord

-Axon exists via spinal nerve

-Travels to skeletal muscle

-Myelinated

Somatic efferent neurons communication with effectors

-occurs at neuromuscular junction

-neurotransmitter = acetylcholine (ACh)

-ACh→ binds muscle → contraction

Neurotransmitter used in the somatic efferent pathway

Acetylcholine (ACh) at all synapses

Anatomical organisation of the autonomic nervous system

3-neuron pathway:

-Neuron 1 (CNS)

-Neuron 2 (preganglionic)

-Neuron 3 (postganglionic)

Preganglionic neuron

Motor neuron that carries nerve impulses from the CNS to an autonomic ganglion

Features of preganglionic neuron

-cell body in CNS

-axon to ganglion (PNS)

-myelinated

-releases ACh

Postganglionic neuron

Neurons that carry signals from autonomic ganglia (outside the CNS) to effector organs

Features of postganglionic neuron

-cell body in ganglion (PNS)

-axon to effector

-unmyelinated

-releases ACh or norepinephrine

Effectors controlled by he autonomic nervous system

Smooth muscle, cardiac muscle, glands, adipose tissue

Autonomic neurons communication with effectors

-preganglionic neuron releases ACh at ganglion

-postganglionic neuron releases ACh (parasympathetic) or norepinephrine (sympathetic)

Two subdivisions of the autonomic nervous system

Sympathetic and parasympathetic

Function of the sympathetic nervous system

Prepares the body for stress responses, “fight or flight”

Effects of the sympathetic nervous system

increased- heart rate, sweating, pupil size, decreased- gastric motility, salivation

Function of the parasympathetic nervous system

Prepares the body for restful situations, conserving energy, “rest and digest”

Effects of the parasympathetic nervous system

Decreased- heart rate, pupil size, increased- gastric motility, salivation

Anatomical organisation of the sympathetic division

-thoracolumbar origin

-preganglionic: short axon

-postganglionic: long axon

-ganglia near CNS

Anatomical organisation of the parasympathetic division

-craniosacral origin

-preganglionic: short axon

-postganglionic: long axon

-ganglia distant from CNS

Sympathetic ganglia location

Close to CNS, in the sympathetic chain along spinal cord

Parasympathetic ganglia location

Close to or within target organs

Chemically gated ion channel

Open when neurotransmitters bind allowing ions to flow, converting chemical to electrical signals, located on dendrites/soma

Voltage gated ion channel

Open in response to membrane potential changes responsible for action potentials, located at the axon hillock and along axon

Mechanically gated ion channels

Open due to physical deformation such as touch or pressure and convert mechanical to electrical signals

Functions of chemically gated ion channels

Allow neurons to respond to neurotransmitters, generate local potentials, first step in synaptic communication

Structure of voltage gated ion channels

Two gates, three states: closed, open, inactivated

Local potential

A small graded change in membrane potential caused by ion movement through chemically gated channels which decrease with distance

Location of local potential

Dendrites and soma

Excitatory local potentials

Cause depolarisation bringing the membrane potential closer to threshold, often due to sodium ions entering the cell

Inhibitory local potentials

Cause hyperpolarisation moving the membrane potential further from threshold, due to potassium ions leaving of chloride ions entering

Summation at the axon hillock

Adds all incoming EPSPs and IPSPs, if the membrane reaches threshold (-60mV) an action potential is generated

Spatial summation

Inputs from multiple neurons simultaneously

Temporal summation

Repeated input from one neuron over time

Action potential

A rapid self propagating electrical signal along the axon cause by movement of sodium and potassium ions

Resting membrane potential

Maintained by ion distribution and membrane permeability, inside of neuron is more negative than outside- approximately -70mV

Depolarisation

Voltage gated sodium ion channels open → sodium ions rush into the cell → membrane potential becomes more positive

Na+ channel inactivation

Sodium ion channels inactivate after opening preventing further entry, marking the peak of the action potential

Repolarisation

Voltage-gated potassium ion channels open → potassium ions leave the cell → membrane potential returns toward negative

Hyperpolarisation

K+ channels close slowly causing the membrane to become more negative than resting (-90mV) and eventually returns to RMP

Refractory period

A period after an action potential where excitability is reduced

Importance of refractory periods

It ensures one-way propagation and prevents signal overlap

Absolute refractory period

A phase where no second action potential can occur because voltage gated sodium channels are inactivated and cannot reopen

Relative refractory period

A phase where a second action potential is possible but it requires a stronger than normal stimulus

Wave of depolarisation

How an action potential moves down an axon, where sodium influx in one segment triggers the next segment

Propagation at the axon hillock

A flood of sodium ions influx creates an electrical and chemical gradient that spreads to adjacent regions

Segmental conduction

Sequential activation of voltage gated Na+ channels along adjacent axon segments

Conduction in unmyelinated axons

Slow at about 1.5m/s, depolarisation occurs continuously along the membrane

Conduction in myelinated axons

Fast, myelin prevents ion leakage and allows saltatory conduction

Saltatory conduction

Action potentials ‘jump’ between nodes of ranvier greatly increasing conduction speed

Chemical synapse

A junction where a neuron communicates with another cell using neurotransmitters

Ca2+ role in synaptic transmission

It triggers synaptic vesicles to fuse with the membrane and release neurotransmitter

Occurs in the synaptic cleft

Neurotransmitters diffuse across the gap to the postsynaptic cell

Postsynaptic cell response

Neurotransmitters bind to chemically gated ion channels causing ion flow and a local potential